Advanced Engineering Tech - Cams and supercharger

Hey guys, I `ve just been told you shouldn`t run highlift cams with a supercharger, is this correct, and for what reason ?

Thanks :)

why not... :jest: my new cam is a 226/232 .585 intake lift and .595 ex. lift. There really isnt any reason to go bigger to make huge HP. They have made 700rwhp on a stock Z06 cam 204/218 0.547/0.551lift. I'm shooting for 800rwhp.

Hey guys, I `ve just been told you shouldn`t run highlift cams with a supercharger, is this correct, and for what reason ?

Thanks :) The reason is because you do not want to bleed off boost. That is why you plan ahead. If you plan on going FI do not do a big huge cam that you later have to switch out to a blower friendly cam.

The cam was the last thing I did. Lost 2 psi of boost and stayed at the same rwhp level:).

yep, 0 valve overlap with my cam...

Thanks guys :)

It`s just that, I mentioned I was considering running a bigger cam with a s/charger, on my forum in the UK, and a couple of guys said they`d heard it wasn`t advisable, but didn`t know why.

Wow, I don't know about this advice here.... but you are running a Centrifigal blower correct?

You don't NEED as much lift in a blower situation, but if you can control everything it doesnt hurt. As for the overlap, I'm leaving that one alone.... one somebody explains to me how overlap bleeds off boost I will listen.

FWIW "boost" is actually the measurement of backpressure in front of the cylinder.

Bret

Wow, I don't know about this advice here.... but you are running a Centrifigal blower correct?

You don't NEED as much lift in a blower situation, but if you can control everything it doesnt hurt. As for the overlap, I'm leaving that one alone.... one somebody explains to me how overlap bleeds off boost I will listen.

FWIW "boost" is actually the measurement of backpressure in front of the cylinder.

Bret

you know, someday id like to meet you just so i can hear how sarcastic it actually sounds when you say something.... :jest: (and i mean that in a good way)

Wow, I don't know about this advice here.... but you are running a Centrifigal blower correct?

You don't NEED as much lift in a blower situation, but if you can control everything it doesnt hurt. As for the overlap, I'm leaving that one alone.... one somebody explains to me how overlap bleeds off boost I will listen.

FWIW "boost" is actually the measurement of backpressure in front of the cylinder.

Bret

Exactly, boost is merely a measurment of air pressure between the blower and the cylinder, measured in PSI.

A cam with overlap bleeds off boost because as the intake valve opens to let air in, the exhaust valve is still closing. Therefore whatever volume of air is pushed into the cylinder during that period, some of it is going to excape through the partially open exhaust valve. it will continue to escape until the exhaust valve fully closes. So if you were pushing 12PSI of boost before the cam, you might only be pushing 10PSI afterwards. Thats the reason its not reccommended that you run an overlapping cam with forced induction.

I bet large overlap cams with boost push fresh air/fuel past the combustion chamber like in a 2 stoke and mess up your O2 sensor readings.

You no longer need to try and use the momentum of the air/fuel charge to get more into the combustion chamber.

SStrokerace is going to have fun with this one :) .
A 110 LSA might not be so bad on a Blower car.
BTW Bret did you ever get my pm on the Morel lifters, not in a hurry by any means but wondering if you got them out to me yet?

Anybody ever consider that scavenging the clearance volume with boost might be a good thing? There is 10% more displacement to be had with a blown motor if you scavenge out the exhaust and fill the clearance volume. The overlap period is short and just gets shorter as rpm increases. If there is any work penalty from blowing down the chamber it will lessen with rpm. 1 psi of "boost" on an LS1 is roughly 40 cfm. That's quite a bit of air that would have to blow out of the exhaust during overlap and that's after the exhaust has vacated the premesis. Overlap on a blown engine effectively makes the engine larger.

Anybody ever consider that scavenging the clearance volume with boost might be a good thing? There is 10% more displacement to be had with a blown motor if you scavenge out the exhaust and fill the clearance volume. The overlap period is short and just gets shorter as rpm increases. If there is any work penalty from blowing down the chamber it will lessen with rpm. 1 psi of "boost" on an LS1 is roughly 40 cfm. That's quite a bit of air that would have to blow out of the exhaust during overlap and that's after the exhaust has vacated the premesis. Overlap on a blown engine effectively makes the engine larger.

I think we are on to something here!

ALL ENGINES work on principals of pressure. Air/fuel moves from high to low pressure. If you can raise the high pressure side it will help fill the motor, if you can lower the low pressure side it will help fill the motor, it's that simple.

Bret

Ya well, when I bring this up in some circles they look at me like I'm blue with 3 heads.

It irks me when people say that boost is a measure of restriction. It is not. I posted this in another thread the other day:

"Boost is NOT just a measure of restriction. Even if the heads flow 400 cfm, you still want positive pressure in the intake and, more importantly, the cylinder. If you don't have positive pressure, you might as well not have FI.

Getting a better flowing set of heads will slightly reduce pressure in the intake and slightly increase pressure in the cylinder. If the heads, cam, and intake are good enough, you will have the same amount of positive pressure in the intake plenum and the cylinder BDC before compression, but POSITIVE pressure nonetheless."

Also, cases have been documented (by Larry Meaux, no less) where you CAN overscavenge even a naturally aspirated engine. If you can overscavenge a naturally aspirated engine, then you can d@mn sure overscavenge a supercharged engine. One case in particular was an NA Hemi. The motor was low on power and the measured VE% was abnormally high - way over 100. After doing some testing, it because evident that a well tuned intake runner, well tuned exhaust header, alot of overlap, and the layout of the hemi chamber design all combined to force alot of fresh air/fuel right through the cylinder and out the exhaust. Reducing overlap significantly increased hp and reduced airflow.

A supercharged engine with LT headers and a good exhaust can have the same affect. A well tuned header drawing down the pressure in the cylinder, combined with 12 psi in the intake runner, then add some overlap and you can over-scavenge.

Without some overlap how are you going to accomplish scavenge?
If you suck on a cylinder that has a closed intake valve how do you remove all of the combustion residuals?
Also I don't know why you went to the extreme with overscavenge. I was hoping for some comments on increasing the overlap period beyond what many hold as a sacred cow today. Overscavenge increases the BSFC dramatically, but the time is too short to loose the work your supercharger created. A turbo charged engine at higher boost can't take advantage of blowing the chamber down but a belt driven combo sure can.

I think we are on to something here!

ALL ENGINES work on principals of pressure. Air/fuel moves from high to low pressure. If you can raise the high pressure side it will help fill the motor, if you can lower the low pressure side it will help fill the motor, it's that simple.

Bret

Did i not say that above? Maybe not in those words, however i related it to VE.

Boost is generally classified a Density multiplier not a velocity "increaser"

But, like you said, boost will flow from high pressure to low pressure (basic fluid dynamics) therefore, in a naturally aspirated engine would had a decreased pressure differential between exhaust port and intake port. Therefore, for the same overlap, a greater percentage of air will flow into the exhaust port with a supercharged engine versus a naturally aspirated engine. Is this significant? Totally depends on what velocity the air is flowing. At lower engine RPM's the total event would last longer allowing less restriction/seal between the overlap. Since Air has viscosity, it also has resistace to movement, thus the more rapidly it is required to change direction/velocity, the greater its resistance to moving. This is also true in engines.

While Zero overlap may not be ideal.. less overlap than a naturally aspriated engine is typical. And we must also distinquish the difference between belt driven superchargers and turbochargers since exhaust manifold pressure also comes into play.

. . . I was hoping for some comments on increasing the overlap period beyond what many hold as a sacred cow today. . .

I have seen several supercharged combinations that gained little or nothing when removing the exhaust. My own was a 383 with a T-trim, running 13 psi boost. When I dropped the 3" Aerochamber cat-back, it gained less than 5 rwhp. I've always wondered whether the 3" Aerochamber is really that good, or perhaps it was maintaining some amount of backpressure, which served to prevent air/fuel from bypassing the combustion process. The cam was 224/236-114.

I'm usually the one to buck the common train of thought on camshaft selection, but in the case of supercharged engines, I really do think overlap needs to be kept low unless you're rev'ing the snot out of it.

I picked up a few books the other day from Amazon.com and one in particular is very interesting, "How to Build and modify chevrolet small-block v-8 camshafts & valvetrains" By David Vizard.

Here is a quote from him in the book.
"It is relatively safe to assume that at low rpm such an engine-supercharger combination will react to caming more like a normally aspirated engine. Thus, the opprotunity exists to cam the engine to better low-end output and let the usually high capability of a turbine-type supercharger boost take care of the top end. For a Street motor this is a good philosphy to adopt. As such it means selecting a cam with a TIGHTER LSA than would normally be used with a positve-discplacment-type supercharger. This, in effect, results in an ealier opening and closing of the intake valve compared to that given by a cam designed for us with a positive-discplacement-supercharger. The earlier opening and closing will allow the engine to breathe more effectively at low rpm, trapping more charge weight to enhance low-end output. Expected timing woud be similiar to a short-period cam timed in as for a normally aspirated engine. The amount of OVERLAP required would also be fairly similiar to that used for a Short-cammed, normally aspirated engine."

When I dropped the 3" Aerochamber cat-back, it gained less than 5 rwhp. I've always wondered whether the 3" Aerochamber is really that good, or perhaps it was maintaining some amount of backpressure, which served to prevent air/fuel from bypassing the combustion process.

Well that's a hypothesis but I'm leary of the conclusions made by just dropping the exhaust. In most cases the cars aren't retuned for that as its just a quick experiment. The Aerochambers are excellent mufflers by the way, for real.

I hope that you are considering my view of the performance potential that's available.

With no overlap, or absolutely minimal, the clearance volume or unswept volume will contain residual gasses from combustion. Let's say that volume is 4.5 cubic inches with a boosted 346. That's 36 cubic inches across 8 cylinders. In a centrifugally supercharged application this volume can be filled with an air/fuel mixture under boost to contribute to the output of the engine. This can only be the case if the residuals are purged from the cylinder. This requires some overlap. In effect increasing overlap increases the fillable displacement of the engine.

Now few would argue the popularity of the past 15 years of the "383" SBC. Personally my teeth hurt everytime I hear the phrase "stroker motor" but oh well. People spend $1000 on increasing their 350 by 30 some odd inches.
Why, because they've been told their no substitue for cubic inches (except for forced induction or nitrous).

I would assume that we might agree that the primary function of overlap on a n/a combination is to start the intake flowing and scavenging the chamber in the process.

The same overlap period with a S/C helps the intake get going at lower rpm and allows the clearance volume to blow down in order to make the most use of the air available from the supercharger and fill another 30 or so inches of volume.

I propose that the decrease in boost witnessed on a gauge from a tighter lobe sep cam and/or longer duration is from the more complete filling of the available space. There is such little time during the overlap period to scavenge the chamber and then short circuit the charge. These effects can be simulated on the PC.

Now how can it be that overlap is bad for power production on a supercharged engine? A person can plot out their overlap triangle and vary lobe seperation to see the effects of changing lobe sep 2-4-6 or 8 degrees. Regardless of the change if you consider the lift of the valves at that point there is no way you're going to dump 10% of the delivered airflow from your supercharger out the exhaust valve. How much time do you think is available at 6000 rpm for that to happen? With 94 degrees of overlap at .005" tappet lift its about .0026 of a second. At 3000 rpm its about .005 of a second. I would consider 94 degrees of seat to seat overlap extreme.

Just consider it.

Ben, Vizard doens't know anything. Tons of guys will tell you that!

andereck , give up man engineermike is just that a ENGINEER so he knows everything. You are beating a dead horse. Your not going to convice him that scavenging works or that you can do it on a blower motor, but oh well.

Either way I agree with you, customers will shit their pants when I send them a RACE blower grind. Then when they haul ass they really wonder WTF.

Bret

andereck , give up man engineermike is just that a ENGINEER so he knows everything. You are beating a dead horse. Your not going to convice him that scavenging works or that you can do it on a blower motor, but oh well.

Brett, I sure wish I knew what I did to piss you off. I even quoted one of your hero's this time around, but since it didn't go along with your logic-of-the-day, you attack me (again). There are 3 types of problems: technical, political, and emotional. Why do you always have to take a technical issue and turn it into an emotional one? Can you grow up? I, once again, am reminded why there are several highly respected guys who argue with you constantly on the boards. Perhaps it's because you attack them personally in the middle of technical discussions. Are you telling me that every time I post in a thread you are involved in, I will receive similar attacks?

Did I ever say that scavenging doesn't work on a supercharged engine? Actually, the whole point of my post is that it is easy to overscavenge a supercharged engine.

Mike

. . .the opprotunity exists to cam the engine to better low-end output and let the usually high capability of a turbine-type supercharger boost take care of the top end. ...it means selecting a cam with a TIGHTER LSA than would normally be used with a positve-discplacment-type supercharger. ...The earlier opening and closing will allow the engine to breathe more effectively at low rpm, trapping more charge weight to enhance low-end output.

T/A KID, note the way this is worded. What Vizard is getting at is that centrifugal supercharged motors are notoriously soggy on low-end. Using a shorter cam on a tighter LSA will give you some low-end response, but the engine will still run well up top because of the boost curve. Notice that he never says that the tighter LSA will help hp or top-end.

Mike

With no overlap, or absolutely minimal, the clearance volume or unswept volume will contain residual gasses from combustion. Let's say that volume is 4.5 cubic inches with a boosted 346. That's 36 cubic inches across 8 cylinders. In a centrifugally supercharged application this volume can be filled with an air/fuel mixture under boost to contribute to the output of the engine. This can only be the case if the residuals are purged from the cylinder. This requires some overlap. In effect increasing overlap increases the fillable displacement of the engine.

[once again, keeping it technical] It is my contention that the pressurized air/fuel easily purges this residual exhaust gas from the cylinder during overlap. And, if you're not careful, some significant portion of the air/fuel exit also.

Regardless of the change if you consider the lift of the valves at that point there is no way you're going to dump 10% of the delivered airflow from your supercharger out the exhaust valve. How much time do you think is available at 6000 rpm for that to happen? With 94 degrees of overlap at .005" tappet lift its about .0026 of a second. At 3000 rpm its about .005 of a second. I would consider 94 degrees of seat to seat overlap extreme.

The short amount of time during overlap is enough time to accomplish some amount of flow, otherwise they wouldn't be able to use it in racecars to increase hp. Sure, overlap is only .0026 seconds at 6000 rpm, but the entire intake stroke is only .005 seconds and we manage to get plenty of flow in that timeframe.

A test was done some time back to compare superchargers and turbochargers. An engine made 8XX hp on a supercharger, then made 950 hp on a turbocharger, then made 1000 hp on pre-compressed air, all at the same boost level. So, let's just assume that this engine was running 20 psi boost. That means that the turbocharged version had 20 - 40 psi backpressure, but it reduced power by only 5%. I know if you add 20 - 40 psi backpressure to any naturally aspirated engine, power will drop by way more than 5%. A while back, I reduce backpressure from 9 psi down to 6 on a NA engine and gained 13% power. My point is that perhaps the added backpressure from the turbocharger prevented overscavenging (good), but increased pumping losses (bad) for a net loss of only 5%.

I really wish someone would run a once-and-for-all camshaft overlap test on both turbo and supercharged engines. It would be really great to see every thing else stay the same, but just altar overlap and see the results. The cams could be something like:

test 1: 240/250-106
test 2: 232/242-110
test 3: 224/234-114

Mike

Did I ever say that scavenging doesn't work on a supercharged engine? Actually, the whole point of my post is that it is easy to overscavenge a supercharged engine.

Mike

Mike, my problem with you is that you are usually so admitant that you are right when you are actually wrong, that's all. Re-read what I said and you will see that's what I ment. Thanks for the PERSONAL attack anyways.

Also if you can give me ANY idea of the difference in pressures of a cylinder, exhaust port and intake port during overlap of a supercharged vehicle you might undestand this. If you have seen anything work better back to back with high overlap vs. low overlap on a supercharged vehicle you might understand this, problem is you haven't and so you will continue to listen and live by logic that doesn't work. The only way you OVERscavenge a motor is when you have too much overlap for it, problem is you haven't SEEN too much before on a centrifigal blower motor and have PROVEN that is the exact problem. If you do run into this then you have another issue in the system that's holding it back, not the camshaft overlap.

Hell even you understand boost to backpressure ratios in turbos, all you have to do is extrapolate on that some more.

Well enough out of me, I'd rather do other things than share what I know about this if I have to argue these points.

Bret

If you have seen anything work better back to back with high overlap vs. low overlap on a supercharged vehicle you might understand this, problem is you haven't and so you will continue to listen and live by logic that doesn't work. The only way you OVERscavenge a motor is when you have too much overlap for it, problem is you haven't SEEN too much before on a centrifigal blower motor and have PROVEN that is the exact problem.

So you're saying you have done an overlap test on a centrifugally supercharged motor while holding all other variables the same, even EVO and IVC? By all means, I have asked for this data before and no one ever seems to have it. If you have it, then please, post it. If I am wrong, perhaps you can show me the error of my ways. Or, perhaps your test will be completely unrelated to the point at hand because of the nature of the build. Or, perhaps what you call alot of overlap and what I call alot of overlap may be 2 totally different numbers.

Well enough out of me, I'd rather do other things than share what I know about this if I have to argue these points.

That goes back to my point above. You'd rather post comments like "engineermike, David Vizard, and Joe Overton are all stupid and I know everything", rather than share some valid data that other people might learn from. That makes the more inquisitive people doubt the existence of said data.

uhh... yeeea, so anyway.... back to the guy who started it

Hey guys, I `ve just been told you shouldn`t run highlift cams with a supercharger, is this correct, and for what reason ?

Thanks :)

peak lift is one of least critical elements of a camshaft... within reason, lift is almost irrelevant in this situation.

what is much more important is timing... thats what everything is debated about above.

one of the basic concepts of a performance camshafts involves opening the exhaust while the intake valve is still closing.... this period while both intake and exhaust valves are open is the overlap...

the concept they're arguing about above, is how much (if any) overlap is ok for a supercharged motor... some argue that with both valves open, the incoming pressurized air will go right out the exhaust.
others argue that it works the same as a NA motor, and it doesnt matter, nothing weird happens..
and most of the smart ones are in between those two extremes... although there are alot of people who think any overlap is a "bad" thing on a boosted motor.


its one of those things where 100 people can do their own testing 100 times, and every one of them will draw their own conclusion..

but to sum up your original question.... the LIFT itself is not effected by the supercharger... but the overlap (and therefore the duration and LSA if you're going by basic cam specs) can be... so you most likely dont want a "huge cam"

Well I really wanted this to be civil and thought provoking, but its neither which is unfortunate. I will continue to experiment with my own stuff in this area and hopefully learn from it while still enjoying myself. There is nothing presented here that counters my philosophy besides just what the status quo says. I certainly don't believe that anybody has put together a test that included 1300+ cfm of compressed air and was able to maintain that flowrate. I don't see that happening.

Peace all.

one of the basic concepts of a performance camshafts involves opening the exhaust while the intake valve is still closing.... this period while both intake and exhaust valves are open is the overlap...




First, who do you think you are being reasonable on this board?

Second, you have the above backwards and probably know that but I didn't want anyone to get confused.

How much is to much overlap in a centrifugally supercharged motor? Say a 403 with a F1 ati blower. What cam would be good?

you know, someday id like to meet you just so i can hear how sarcastic it actually sounds when you say something.... :jest: (and i mean that in a good way)
The apple doesn't fall far from the tree. You probably wouldn't like to meet me, Dude. ;)

Ben, Vizard doens't know anything. Tons of guys will tell you that!
Sometimes you shouldn't take things too literally, but it may be hard to know when not to. Oh, well...

andereck , give up man engineermike is just that a ENGINEER so he knows everything.

NOt all of us are like that. I'm an engineer and if engineering has taught me anything, it's taught me how little I truly know! :lol:

I'd like to toss in a few points that I picked up from some of you that have
not been mentioned in this discussion:

- LSA does not determine overlap alone

- Overlap at the lobe, is not equal overlap at the valve (pre-load, rocker ratio, floating...)

- Compression can't begin until both valves are closed. Cylinder pressure
is determined by IVC, and VE (< all that encompasses cylinder filling).

- Overlap is tuned to work within a small range of RPM. You can't grind a
a camshaft to equal efficiency over the entire operating range...especially
in a non-variable environment.

In a race motor, does it really matter if some of the charge is wasted outside
of the tuned area?

First, who do you think you are being reasonable on this board?

Second, you have the above backwards and probably know that but I didn't want anyone to get confused.


oopsie... you're right.

it should say:

...opening the intake while the exhaust valve is still closing..


not the other way around.. good catch.

NOt all of us are like that. I'm an engineer and if engineering has taught me anything, it's taught me how little I truly know! :lol:

OldSStroker, my oldman is one as well... most GOOD engineers are not like that from what I have seen.

Mike, i'm not giving out the data for free... sorry.

Bret

Mike, i'm not giving out the data for free... sorry.

How convenient for you to not have to back up anything with data. . .

Can you at least share what you think is not enough, just right, or too much overlap? At this point, no one has even defined the amount of overlap we're talking about. Who knows, maybe we all agree in the end. . .

Mike

How convenient for you to not have to back up anything with data. . .



Who in their right mind would give out that kind of information for free? Especially the kind that cost THEM time, effort and $$$$ to gather?

Seriously, search through their posts and pick up on the crumbs and start doing research. You'll learn A LOT, you won't become an expert but at least you'll gain an appreciaton for this stuff.

Who in their right mind would give out that kind of information for free? Especially the kind that cost THEM time, effort and $$$$ to gather?

INTMD8 has tested 4 cams in his turbo setup and post full results from each. Also, I've been privy to just about all the cam testing that Thunder Racing did to develop cams like the TRex and CheaTR. Last time I checked, Thunder and LS1Speed were still in business and doing well.

INTMD8 has tested 4 cams in his turbo setup and post full results from each. Also, I've been privy to just about all the cam testing that Thunder Racing did to develop cams like the TRex and CheaTR. Last time I checked, Thunder and LS1Speed were still in business and doing well.

:lol: if you've been so "privy" to TR's testing why don't you go ask them, have them give you the answer? Have them foot the bill.

if you've been so "privy" to TR's testing why don't you go ask them, have them give you the answer? Have them foot the bill.

I really hate pointing out the obvious, but Thunder has done tons of R&D on naturally aspirated LSX camshafts, but none on turbocharged LTX or SBC engines. Believe me when I say that I've learned lots from the R&D that Geoff shares with me.

Mike

And here I am thinking I had already answered this question. :jest:

The cam was the last thing I did. Lost 2 psi of boost and stayed at the same rwhp level:).

Care to share the spec's of the old and new cams?

I really hate pointing out the obvious, but Thunder has done tons of R&D on naturally aspirated LSX camshafts, but none on turbocharged LTX or SBC engines. Believe me when I say that I've learned lots from the R&D that Geoff shares with me.

Mike

Yes, I already knew that. You should also know a lot of people aren't going to be as open or very open about the information they collect. Especially with competitors out there. That should be pretty obvious too.

I would like to add a few things not brought up yet. We have theory's of boost bleed off and which valve events are better for FI setup's.
Now my question is for the guy's with FI like the Vortech setup's on the F body's.
With a blower that is very small and not capable of pushing much CFM's I belive cam shaft design might be even more critical.
But I have no proof of that. I just feel that with a vortech setup you need to try and get everything as efficient as possible to help that little head unit work.

And the boost is a measurement of restriction saying....
The last Vortech F body we did made 7 psi, when we changed out to a fast 90/90 and a set of ported heads his car made 5psi. More HP? Yes but less boost.
We took the restriction's away and less boost was "read" on the boost guage. More air was geting into the motor however and the proof of that was the additional HP, and the fact that the tune required more fuel to get the car back to the same A/F ratio.

Let me also add that we all have oponion on what works best.
If you look at different FI setup and try and compair one setup to the next it is tough.
However on this board alone I have seen a huge number of different guys that have boosted cam setup's. Many have different cam's. Not once have I seen a cam go in and the car loose power over stock. There are even a few guys running G5X2 and G5X3 cams in their FI setup's. Is it the best cam for that setup? Probally not, but does it work? Yes. It clearly makes more power then a similar setup with a stock cam.
Remember like 3-4 years ago and still today when people say that a 114+LSA cam with a 4 or more degree split in duration is a "nitrous cam"
then a guy comes in with a TR 230/224 on a 112 that makes 400 NA and 545 on a 150 shot. We can argue what we believe all day, but the proof is in the results.
I for one don't believe that if your spraying a 150 shot the cam is real improtant, and also feel that if your making power with a blower and any cam then the cam must work no matter who says it won't.

Care to share the spec's of the old and new cams?


From stock to 224/232-581/595.

It all comes back to what kind of motor you have or how you want to use it. Overlap does work for FI. Look at the import cars. When they're doing high rpms, they're using buttloads of overlap. Then, when they're driving around town they change to their primary lobes, and now the overlap decreased. With pushrod engines, you don't have that luxury to change profiles(unless they come up with something better than the L92), so you're fixed to one profile, and that leads to fixed power band. It's either low, mid, or high. Your choice. With a high overlap cam, you might be bleeding down low with FI, but when that sucker starts revving up, it's really going to pull. Again, it's to a point. You don't want a ginormous amount of overlap to make your motor rev 10K+, you just want enough for whatever you want your powerband to be, even if it is 10k(good luck with that one in the streets, unless you have nascar money).

I say screw the overlap and go for a later IVC, that way you'll pass smog easier and rev high, especially with positive displacement FI. Sounds almost like a MILLER cycle, if you choose to have a longer intake duration than your exhaust. I think IVC is the biggest factor for rpm. I could be wrong.

I think IVC is the biggest factor for rpm.

Precisely.

Turbo and Blower cams are TWO DIFFERENT WORLDS IN TERMS OF OVERLAP the results CAN'T be transfered from one to another.

Who in their right mind would give out that kind of information for free? Especially the kind that cost THEM time, effort and $$$$ to gather?

;-)

Bret

Exactly, boost is merely a measurment of air pressure between the blower and the cylinder, measured in PSI.

A cam with overlap bleeds off boost because as the intake valve opens to let air in, the exhaust valve is still closing. Therefore whatever volume of air is pushed into the cylinder during that period, some of it is going to excape through the partially open exhaust valve. it will continue to escape until the exhaust valve fully closes. So if you were pushing 12PSI of boost before the cam, you might only be pushing 10PSI afterwards. Thats the reason its not reccommended that you run an overlapping cam with forced induction.

Flow is induced by a pressure difference. Do you think that 12psi of boost is greater than the exhaust pressure? :)

I think the arguement here has missed a few key points that would determine what type of cam would work best in a setup. Those would be exhaust/intake pressure at valve events, residual gas, and compression ratio. IMHO there are some situations where a "moderate to big" cam would work better than a tight cam and vice-versa.

Bump...

SO how many people have tried some cams with more overlap ??? or tighter LSA's in a blower application ?

From reading the Aussie LS1 site, some of their top guys would suggest cams around the 230/240 with LSA's of 110 and tighter can work exceptionally well in a high power build.

Why would the thinking be so different on each continent ?

Bump...

SO how many people have tried some cams with more overlap ??? or tighter LSA's in a blower application ?

From reading the Aussie LS1 site, some of their top guys would suggest cams around the 230/240 with LSA's of 110 and tighter can work exceptionally well in a high power build.

Why would the thinking be so different on each continent ?

I have done a fare bit of camshaft testing on differant types of F/I over the years and found, as has been somewhat stated here, that differant types of F/I require differant camshaft designs. I have personally used camshafts from 112 to 121 LSA depending on the requirement, we recently used a 118 LSA cam on a 6.5 litre Twin turbo LS2 that made 760rwhp (aussie dyno dynamics dyno) on 14.8psi and pump fuel with only 16 degrees of timing (far from being maxed out), the car drove like a stocker, got awesome gas mileage and passed an engineering test for legality with 238 degrees at 0.050 on the exhaust! We would have tuned it up more if we could keep the heads on, (L92 for interest sake) and with a LSA like that you might think massive revs, however it made that under 6500 rpm.

Other testing has been based around Twin screw blown LS engines, you can have two camshafts with 114 LSA and have two completly differant results in power, the fact is a camshaft designed for NA use is probably not going to work great in a PD blown engine, the converse is true with Centrifugal however... :cry:

So, it aint an easy bang it and go from there, my suggestion would be to buy a tried and proven cam of one of the vendors that has the runs on the board, or go small, you cannot go wrong.

Matt.

Agreed with Matt.... I've used some forced induction cams with LSA's down in the range of what you would see on a circle track car.

Bret

This thread has some great info! I'm surprized there isn't more actual examples of people trying two cams, one with a good amount of overlap and one with less to none. I'm not going to change my N/A set-up untill I see what it does with the supercharger on. I'm not near as experienced as some of you but it seems to me it can't hurt to try. The way I see it, with a supercharged engine overlap can't make that much difference in power. I can see why it may make less boost but I think the power would be comparable. My example: my car makes 408rwhp n/a with a F14 232/234 112lsa. I'm going to add a supercharger and plan to have around 8 psi. Let's say it makes 600rwhp. Now I change out the cam with one with a lot less overlap. I now get 10 psi but still make the same 600rwhp due to the new cam making less power before the boost. Is this not reasonable thinking? It will be much easier to debate once the supercharger goes on but unless I make a considerable amount less hp then my goal of 600, I see no reason to change the cam.

Three points...

1) Lift is not something that most folks should belabor in threads like these, because most folks are not running combos that would merit going over 600 lift anyway. If you are building a beast of a car with heads that flow 350 at 650 lift well you are probably getting great advice already on cam selection (one would hope). If not, you can do some research. If you are running some good heads like say AFR 225's, your cam guy would want to look at the lift numbers before spec'ing a cam. If for example they hit 325cfm at 625, that's great, but if they hit 320 at 600 and then don't pick up much from ther, most cam guys will just grind a 600 lift cam since they won't necessarily advocate a higher lift cam that might be (a) less stable and (b) require changing the springs much more often. I've run as much as 630 lift with the blower, but am currently running around 600 lift.

2) Andereck's comments mirror others I have seen by Intmd8 and some other guys who do a lot of tuning, that overlap is important. I ran a 236/246//114 cam with my YSi and my power curve was straight across, which was the goal. Guys that race competitively in NMRA running blower will always run at least a 10 degree split and sometimes a bit more, though perhaps the amount of intake duration that they run is also reflective of other restrictions in their combinations. My combo went 9.40@143 at close to 3500 raceweight, and the next fastest similar combo went 9.25@145 or so but he's almost 250 lbs lighter than me. That's telling me my combo was making some good power. I dynoed 680rwhp a week before I went 144mph in July of that year.

3) My turbo combo is 10 times more soggy down low than my blower setup. My blower setup was like a 600 cubic inch n/a setup. My turbo setup feels like an Evo I test drove a few years ago, nothing down low but would push you back in your seat at 4000 rpms.

Bret, posting in a thread, and then 10 posts later saying you won't share data does not work well on message boards, maybe you should not even post. The whole point of message boards from a user standpoint is to do research and interact with a large community of fellow enthusiasts. And I agree with EngineerMike (gasp) about making comments personal, there is no need this is tech site.

Andereck, I'm running a 234/234//113.5 or 114 camshaft, what do you think of that with my turbo setup?

Andereck, I'm running a 234/234//113.5 or 114 camshaft, what do you think of that with my turbo setup?

John, sorry for not replying sooner. The fact is I just don't know. I don't feel qualified to evaluate your cam choice because I just don't have any experience with turbocharged engine combinations and their special requirements due to exhaust backpressure.

My feeling is that a turbocharged engine will not benifit from positive intake pressure scavenging due to the exhaust pressure being as high or higher under boost conditions negating flow in the right direction. This doesn't mean that some overlap would hurt however. Under light or partial throttle some overlap will help with cylinder fill, even with a log manifold.....I think.

I would probably set the intake c/l around 107-108 for good bottom end and midrange when you're not on the pipe. No reason the car has to be sluggish off boost.
With the basic numbers you've given I think it would be hard to get into trouble regardless of what you do. I do however think most cams of that nature will call for a later intake c/l than what feels right to me.

Take it for what its worth, as I said I'm not comfortable in that area.

I'm bringing this post back up because it's the most recent on this highly debateable subject. For guys like me who search and then read through all of these posts in hopes of getting a better idea of what would work with our combinations this thread is still open for business.

After I read this I did some searching on Comp's article list for any relavent testing of a supercharged engine with camshafts of different overlap and found a decent one. It supports EngineerMike's position quite well. On a BBC with a large traditional roots blower with relatively low boost running more overlap overscavenged the heck out of the cylinders down low but once the engine was screaming gained significant power. My conclusion of this particular instance would be your overlap on a positive displacement engine should reflect your purpose for it. On a street engine keep the overlap small but for all out race you can improve top end with more overlap. Remember, you can't drive a dyno.

Here's the link to the article:
http://compcams.com/Community/Articles/Details.asp?ID=-749654412

Vernon