Couple of reasons...

- As I said, you choose the valve events and let the duration fall out where it may. I know from experience that EVO isn't terribly influential on power, but if you make EVO = IVC + 10, it'll be pretty close. Depending on where you put overlap, you usually wind up with a conventional split pattern.

- INTMD8 wasn't impressed with his reverse-split.

- Cam Motion always recommends a reverse-split, regardless of backpressure-boost differential. Call them and ask why they do this. I did and wasn't impressed with the answer.

Mike

 

I agree that the duration/lsa/icl/ecl should be a function of the chosen valve events. The valve events should not be a product of a chosen duration/lsa/icl/ecl.

Vince, you still need to be more specific.

vehicle weight-
vehicle gearing-
et/mph goal at raceweight-
OR mph goal at raceweight-
transmission-
intended usage (street/strip/both)
desired driveability (must it be perfectly smooth or can compromises be made to driveability for increases in max power)
compression-
cyl heads-
cid-

Most important is the finished product. Does it meet your HP/ET/MPH/driveability goals? If not, you did somthing wrong. Pick an entire combination at the same time to meet those goals. Don't pick all of your parts and then say, hmmmm, what should I do for a cam now.

I will say that in my experience people should put more effort into planning an efficient vehicle (meaning engine/turbo system/gearing/converter/etc) and less time figuring out what camshaft would be the best crutch for thier less than optimal combination of parts.

 

I agree with you on people should spend more time on planning, but there is a lot of bogus information out there and so many claiming to be experts. The consumer is easily misled into following one shop and that shop can be completely wrong. Then the consumer is left with fixing the combo themselves or taking a risk with going with another shop and dropping more coin. This seems to be a cycle on the forums.. In the end the consumer usually compromises or sells the car. Very few on this very forum end up completely happy with the results and the ones that do have made compromises or have came to the conclusion you cant have your cake and eat it to. Many shops forget about the wants and needs of the customer and think more about filling their pockets by suggesting things the customer does not need or leading the customer down a road the customer did not initially want to travel. A friend said to me last night the Internet is the devil. LOL!!

OK for the APS C5 system on a stock motor..

vehicle weight-3200lbs
vehicle gearing-3.42
et/mph goal at raceweight-11.0
OR mph goal at raceweight-132mph
transmission-M6 or A4(3200stall)
intended usage (street/strip/both) both
desired driveability (must it be perfectly smooth or can compromises be made to driveability for increases in max power) Smooth
compression-stock LS6
cyl heads-LS6
cid-346ci

Built 346ci with twin 60-1's

vehicle weight-3200lbs
vehicle gearing-3.42
et/mph goal at raceweight-9.5
OR mph goal at raceweight-145mph
transmission-M6 or A4
intended usage (street/strip/both) both
desired driveability (must it be perfectly smooth or can compromises be made to driveability for increases in max power) Compromises can be made, but no surging below 1600rpm
compression-8.8:1
cyl heads-72cc cast that flow 300cfm at .600
cid-346ci


402/408ci with twin 60-1's

vehicle weight-3300lbs
vehicle gearing-3.42
et/mph goal at raceweight-9.5
OR mph goal at raceweight-150mph
transmission-M6 or A4(3200stall)
intended usage (street/strip/both) both
desired driveability (must it be perfectly smooth or can compromises be made to driveability for increases in max power) No surging below 1600 rpm
compression-8.8:1
cyl heads-72cc cast that flow 300cfm at .600
cid-402/408ci


427ci with twin 88mm turbos..

vehicle weight-3200lbs
vehicle gearing-3.42's
et/mph goal at raceweight-low 9's or high 8's
OR mph goal at raceweight-170mph
transmission- M6 or TH400(4kstall)
intended usage (street/strip/both) Both
desired driveability (must it be perfectly smooth or can compromises be made to driveability for increases in max power)Compromises of course
compression-8.8:1
cyl heads-72cc cast that flow 300cfm at .600
cid-427ci

 

Great thread

And just to throw a single setup into the equation as well as the 4 twin combinations above....

vehicle weight - 3600lb
vehicle gearing - 3.7
mph goal at raceweight - 140mph
transmission - M6
intended usage - street/strip
desired driveability - compromised driveability and lumpy is good
compression - 10.4:1 or slightly lower with cometics
cyl heads - L92
cid - 364ci
turbo - single 80mm (ST80 or similar)

Should be some interesting replies

 

402/408ci with twin 60-1's

vehicle weight-3300lbs
vehicle gearing-3.42
et/mph goal at raceweight-9.5
OR mph goal at raceweight-150mph
transmission-M6 or A4(3200stall)
intended usage (street/strip/both) both
desired driveability (must it be perfectly smooth or can compromises be made to driveability for increases in max power) No surging below 1600 rpm
compression-8.8:1
cyl heads-72cc cast that flow 300cfm at .600
cid-402/408ci


Vince isn't that YOUR setup?

 

I normaly don't respond or reply to thread, I just read them, but I feel you and a few others have hit the nail on the head. Research and Homework.

Your best cam selection will come from a turbo shop or a seasoned cam shop. Over the phone, the above questions should be asked to include wheel size and tire size, rear-end gear, transmission, and so on. I hope you see where this is headed. Sorry if this was already stated. In my opinion, they really need the above and your heads to have them bench and flow tested, if you want the maximum output of you combination of selected parts. The cam(mechanical, like a traffic cop) is the brain in addition to your choice of engine management system(electrical) controls, they both moved things along in the best fashion possible.

Yes, you can get a non specific cam selection, but it probably will only maximize your potential to only ~70-90%, if that. Customize camswork better, especially for the car it was design for and it will work on yours, but you probably want get the same results, because no set-up is the same.

You must ask yourself, is it worth it and ultimately, what do I want to accomplish. Please refer to the above quote and don't bash me to bad, I had to post this one.

 

alot of good info in here so far...keep em coming guys!!

 

I used an NA cam on my current turbo car with a huge intake bias- Int 265, Ex 234 on a 106. The lift is also heavily biased to the intake. It came from my cam bin and I didn't have a lot of choices at the time. The power seems to be good (see graph) so it's not disappointing, even though with this cam I'm not sure what I was expecting.

A 260/260 with a 114 +4 is going in and I'm not sure if I should expect a huge increase in power even though this is suposed to be a more "turbo friendly grind".

I get the feeling that this is similar to my nitrous tuning days. If you had a cam that was well matched to your heads, then you switched to a nitrous specific grind you only picked up 5 horsepower. Should be back on the dyno soon to find out.

 

my car is lighter. I dont plan on running 150mph traps because my vette does not see the track. My current cam has no surging below 1600rpm. As for the turbos.. They are T3/T4's..

 

It would seem to me, and I am NOT an expert, that the cam's powerband would be the same as a NA cam vs. duration and split. The difference for a turbo would be entirely due to the backpressure/boost ratio. So we ought to be able to generalize and say that you start with an "average" NA cam for your goals and then adjust for your particular FI setup. I would think that you'd adjust one direction for a high backpressure setup, like an STS, and the other direction for a front-mount with dumps, to take the two extremes. What do folks think about this idea?

 

I think you're dang close to right. Problem is, we don't really know which way to go, as far as splits go, and where to start. For instance, does high backpressure need more exhaust duration or less? I could argue that high backpressure needs less exhaust duration since the exhaust stream is more dense so it will take less time to blow down. I could argue the other side by saying that the dP from the cylinder to the exhaust manifold is lower, so you need more duration to clear the exhaust out of the cylinder. Once you determine which direction is beneficial to which situation, then you have to determine where to start and stop. For instance, high backpressure might work best with 20 deg more on the exhaust and low backpressure might work best with a single pattern. Or, high backpressure might work best with a single pattern and low backpressure 20 deg less. Of course, this all breaks my rule about choosing valve events and let the duration fall out where it may. . .

It really would take an extensive amount of dyno testing to prove anything. I talked with Geoff at Thunder about this a while back, and he agreed to supply the dyno time, but a turbo LT1 strapped to a dyno doesn't make the best platform for multiple cam swaps. This fall, I may have access to a 377 SBC with a single T76GTS (current cam is 230/230-114, making 930 hp at 18 psi) for dyno testing on an engine dyno. This would be much easier and cheaper to play with than my car.

Think we could start a collection to fund an end-all extensive turbo cam test?

Mike

 

If someone is serious about it I am down with dropping some money for this. Hell we might even be able to get the Forum owners to drop some money for this. We are all here to help the customer.

 

Yes, but when you "choose valve events" you're just trying to work with the pressure differentials and gas inertia throughout the system. What's different about turbos is that the pressure differentials are different (duh). Let's try and reason this out a bit, just talking it out amongst ourselves, and again keeping in mind that I am not an expert and could be completely off base.

Higher exhaust backpressure means that pressure in the exhaust system will exceed pressure in the cylinder earlier. For example, just pulling some numbers out of the air, let's assume we are running the "classic" 224/228/114/110 and have an exhaust system with low backpressure. Let's also assume this cam is optimized for this car. The exhaust valve opens (all numbers at .050, for convenience) at 52 BBDC, during the power stroke. Cylinder pressure is high, and the relatively lower pressure in the exhaust system causes the gases to rush out. The piston now passes BDC and pushes up, continuing to force the exhaust gases out. At 4 BTDC the exhaust valve closes, and let's assume that at this point cylinder pressure and exhaust backpressure are evenly balanced..

Now, assume we had higher backpressure in the exhaust. The equilibrium point would be reached when cylinder pressure is higher, which would occur earlier in the cycle. We might now need to close the exhaust valve at 8 BTDC. If we wait until 4 BTDC, pressure has been equalized for several degrees, which just means that the exhaust has been sitting in the cylinder, polluting the now-incoming fresh charge (because we are in overlap). So, we want to move the exhaust closing point earlier. We can do that by running a shorter duration, or increasing the split.

Let's see how this theory works when comparing two well-known and proven NA cams of similar powerband, one designed for "normal" built engines, implying headers, and one for completely stock, hence higher backpressure, exhaust systems. We'll use the aforementioned 224/228/114/110 (the "header cam") and compare it to the Thunder Racing CheaTR, which is a 214/230/117/113(?). The CheaTR doesn't have a published advance, but let's assume 4 degrees, which is typical for cams of this size on LS-1s. I don't have exact lobe specs for the CheaTR, so I'm going to substitute LSK lobes at 215/231/117/113, which is close enough for discussion.

Now lets compare valve events (at .050):

224/228
IVO 2 BTDC
IVC 42 ABDC
EVO 52 BBDC
EVC -4 ATDC
Overlap -2

215/231
IVO -5.5 BTDC
IVC 40.5 ABDC
EVO 56.5 BBDC
EVC -5.5 ATDC
Overlap -11

Ok, so what do we see? The CheaTR opens the exhaust valve earlier, and closes it earlier, due to its LCA. It also opens the intake later, and closes it earlier, due to its split.

The IVC I think we can ignore for the purposes of our discussion here. Yes, it may be the single most important event, but it has little to do with exhaust backpressure. We can choose IVC more or less the same as we would for an NA car.

The CheaTR's combination of closing the exhaust earlier and opening the intake later leaves less time for the higher exhaust backpressure to pollute the incoming charge, and I think explains why this cam works so well with restrictive exhaust. Applying this to turbo engines, the greater the boost, and/or the lower the exhaust backpressure (either leading to a higher cylinder/exhaust pressure ratio), the less we have to worry about this. Lower boost and/or higher backpressure, the more "pollution" effect we see and the more we have to move towards lower overlap. This would explain why relatively low boost/high backpressure street turbo cams tend to have much higher LSAs than racing turbo cams.

Looking at events, again: with a boosted intake, we can use fewer degrees to get sufficient cylinder fill. IVC still sets our power peak, so we set IVO later in order to keep overlap where we want it. On the exhaust side we want EVC earlier, again to reduce overlap, but, we need sufficient exhaust duration to allow efficient cylinder evacuation. But if we place EVO too early, we lose power we could otherwise have used.

Hmmm. Putting all that together I think what I'm seeing is reduced intake duration (higher split), and increasing LCA, to reduce overlap.

What might an example of that look like? What if we wanted to take the classic 224/228 above and set it up for a turbo? How about shortening the intake 4 degrees to 220, which shouldn't hurt much due to the boost. Then widen the LSA to 116 to reduce overlap. We'll keep the 4 advance. We get:

220/228/116/112
IVO -2 BTDC
IVC 42 ABDC
EVO 54 BBDC
EVC -6 ATDC
Overlap -8

You can see we've kept the same IVC, so should see the same peak power RPM. We've delayed the IVO by 4, and advanced the EVC by 2, decreasing the overlap by 6. We've also advanced the EVO by 2. All this would, I think, result in the power "points" being more or less the same, but with the turbo spooling faster (due to the earlier EVO and hence higher initial exhaust pressure) and the engine making more power due to reduced intake charge dilution.

What do you think?

My apologies for the length of all this, and again I am NOT an expert by any means. Just trying to think it through on a Sunday afternoon.

If I'm all wet, please say so!

 

vehicle weight = 3080lbs
vehicle gearing = stock Z06
transmission- Z06 tranny ratio
intended usage (street/strip/both) Mostly street
desired driveability = Choppy is fine with me
compression = 9.1
cyl heads = 225 72cc AFRs
cid = w2w forged 347
turbo kit = PTK twin turbo (equal length)


I have a 224/228 .581/.588 114 LSA and also a Z06 cam.
Should I go with the 224/228, Z06 cam or something different?
Car is mainly a fun/street with mild strip use.

 

The CheaTR is only +1 advance

You have to look at overall combo not just 1 area of the wide spectrum.

Just to throw an exemple on a 346:

224/222 113-1 LSA

 

Here is one that we can results to in the next few months

vehicle weight-3100 race weight
vehicle gearing-3.42
et/mph goal at raceweight-8.50
OR mph goal at raceweight- XXX
transmission- 1.8 PG, converter to be spec'd by Niel Chance converters
intended usage- Entire Strip
desired driveability- Not an Issue
compression-8.8:1
cyl heads-Massaged AFR 225's
Motor-408ci
Valvetrain- T&D Shaftmount/ Crower Hippo Lifters (8000rpm limit due to wet sump)
PCM- BS3
Boost- 25/30 psi
Turbo's Twin BB TC76MPS turbos

 

Good job of picking apart the valve event, Gannet. Here are my thoughts. . .

During the power stroke, you start out with higher cylinder pressure and more combustion volume than NA. This means that cylinder pressure is higher and continues to do work later in the stroke than a high compression NA engine. By this logic, you would tell you to open the exhaust valve later so you can extract more of this useful energy from the gas. However, you also have up to double the mass of exhaust that has to get out of the cylinder. It is at a higher pressure, so it takes up less space, but it's also more viscous too, so it's less likely to flow through the small opening. This would make you more likely to open the exhaust valve earlier. So, which way do we go? I suppose you could just leave it alone, but tackle the low-lift flow of the exhaust port instead... The bottom line for EVO remains, that you want blow-down to be essentially complete the same number of degrees ABDC as EVO is BBDC. Lucky for us, we can vary this quite a bit without dramatically affecting power.

I've personally seen the effects of running too much overlap with a restrictive exhaust. I had an NA combo running the GM 847 cam (14 deg overlap) and 9 psi backpressure. When I reduced the backpressure down to 5 psi, power went up by nearly 50 hp.

However. . . in almost every case that I've ever seen of adding overlap with a turbo car, power increased. In INTMD8's testing, the one consistent theme was that overlap=power even with a 2/1 backpressure ratio (I think he was around 40/20 psi). Though, he never used more than -2 deg @ .050. I think it's generally accepted that anything up to +5 deg @ .050 is good for power, but we're not sure what happens beyond that. It would be great if we had some sort of rule that said (hypothetical numbers). . .

With backpressure-boost < 5 psi, use 15 deg overlap.
With backpressure-boost 5 - 10 psi, use 10 deg overlap.
With backpressure-boost 10 - 15 psi, use 5 deg overlap.
With backpressure-boost 15 - 25 psi, use 0 deg overlap. (though mine seems to work well with backpressure-boost=17 psi, and +2 deg overlap)

Of course, idle quality considerations could override these depending on customer.

Even though the intake is boosted, the objective of intake valve events is still the same: to fill the cylinder (maybe ever overfill the cylinder) as much as possible. I wouldn't say you can tolerate less intake duration because we're still trying to get as much of the stuff in the intake manifold into the cylinder as possible. Let's look at how the gas properties have changed: the intake air/fuel is now roughly twice as dense. So, for any given cfm, you get twice as much lb/min. However, with the density twice as high, the gas in the intake power should resonate at a lower frequency and offer greater mass for ram-tuning, and it should also be more viscous and resistant to flow. This would lead me to more intake duration (to compensate for the viscosity) and a later ICL (to compensate for the lower natural frequency on the opening side and allow for more ram-tuning on the closing side).

The problem with all this is that a turbo motor already has a low DCR and has turbo lag to deal with. More intake duration and higher ICL both both lead to an engine thats soggy down low and slow to spool. I recently advanced my cam 4 deg to get some low-end response back and the difference was noticeable. BTW, when the cam was advanced 4 deg and 25 cid added, peak power only dropped 100 rpm.

ANYWAY. . . the point is that the intake cam duration and ICL is just much of a compromise as a turbo torque converter (which needs to be loose to allow spool-up then tight so it won't slip too much at 1000 ft-lb of torque). My advice here would be to use an aggressive lobe design and a port that flows alot at low and mid-lift (hello L92?). Choose IVC to get your low-end response and pre-boost torque where you want it. If you're not worried about low-end, then pick it a little later than you would NA. If the turbo is going to peter out on top end (like my T76/388 combo), then move it earlier to get peak power to an rpm where the turbo is still happy.

Mike

 

Bump since LS1tech didn't do it for me after the last post.

 

This is a very interesting thread. Its really got me thinking about my application...midframe 88 (GT47) on a 366 ci engine. Previous N/A testing with the Vic Jr manifold (at 346) suggested I wanted an IVC of ~46* ABDC to peak at 7200 rpm. Now that I've bumped up 20 ci, I think I can add ~2* to that value to keep me in the ballpark, so lets assume 48* ATDC. I know that the turbine side of the GT47 is a bit small, so 2:1 (maybe 2.5:1) pressure ratios are quite possible. So I probably want to minimize my total overlap at .050. For this argument, lets assume 0*. EVO of 56* seems to be a typical number for larger higher rpm cams (Trexish), so I'm going to keep it since these motors seem pretty forgiving when it comes to EVO. So if I want to close the exhaust a few degrees BTDC (4* for instance) gives me the following specs:

232/232 116+4

Any opinions?

 

Very interesting information guys!!

Fireball,
You kept the same stroke but increased bore right? What is your target boost level with the new setup and where does it shake out on the compressor map and consumption chart for your motor? I think that cam looks pretty good but you might peak a little higher with the 48ivc versus 46 provided the turbo is still in its efficiency range and backpressure not too high.