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... 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.

 

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

 

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

 

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.

 

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.

 

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 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."

 

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 **** their pants when I send them a RACE blower grind. Then when they haul *** they really wonder WTF.

Bret