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Old 06-22-2008, 09:55 PM
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Supercharged exhaust question

I have a question. I have a 99 K2500 truck with a whipple-charged 350 in it. I just purchased one of those desktop dyno programs and thought I'd test it out. I currently have true dual exhaust, but the program says that I'd gain 43 RWHP by changing to a high-flow single exhaust??? Does that sound right? The exhaust was on it when I bought it, so I don't even know if it came with true duals or not.

There were a couple of parameters that I guessed at, like I assumed the HP rating of the blower was 100HP, assumed 50% efficiency (already entered by the program) and left the CFM rating at 0.

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Old 06-22-2008, 10:05 PM
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Couldn't tell ya of the accuracy of that program but I can tell you that Pro-Max on South Tacoma Way has a chassis dyno. That'll give you hard numbers with no assumptions.
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Old 06-23-2008, 06:46 AM
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Ya, but to do it, I'd have to have the exhaust changed. If I was wrong, then I'd have to change it back.

Turbo Technology is where I went the last time with my other truck. They are right down the road from Pro-Max. They were nice there, but I got the feeling that I was wasting their time because I didn't have any turbos.

Last edited by Pre-Tuner; 06-23-2008 at 08:51 PM.
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Old 06-23-2008, 08:52 PM
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Here's another thought. What if I created a restriction at the end of the tailpipe, like covering half of it up on each side? Would this work to tell me if it was worth having my exhaust redone? Any idea of what I could do to accomplish this?
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Old 06-23-2008, 09:14 PM
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garbage in = garbage out. Whipple blowers are not horsepower rated. But are rated for CFM per revolution and efficientcy.
7PSI of real boost will give you about 45% more horsepower with a whipple.
more boost (blower pulley drive ratio) gives more power.

Put it on a real chassis dyno and after dialing in the Air fuel ratio and spark timing under WOT power and boost, compare the power while running thru your exhaust and the power with open headers.

Last edited by F-BIRD'88; 06-24-2008 at 02:12 AM.
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Old 06-23-2008, 09:28 PM
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Uh, how does that help me decide to keep the dual or install single exhaust? If I do any dyno runs, it will already be too late. I can run a base line now with the dual, but then I'll have to commit to single and run it again. If it loses HP, then it would be too late.
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Old 06-23-2008, 10:35 PM
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I don't think a 43 rwhp change is realistic. You've done something fishy with the desktop dyno. I'm also highly skeptical that a single exhaust will suit you better than a good dual system. What size pipes do you have already and what kind of horsepower are we talking about? If you have a decent dual system with an H pipe, you'll be fine. If you don't already have an H pipe or X pipe, you might want to consider that.
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Old 06-24-2008, 01:40 AM
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Quote:
Originally Posted by Pre-Tuner
Uh, how does that help me decide to keep the dual or install single exhaust? If I do any dyno runs, it will already be too late. I can run a base line now with the dual, but then I'll have to commit to single and run it again. If it loses HP, then it would be too late.
What part of test it with open headers did you not get?
Forget the single exhaust.
Your inputs made the results of the dyno simulation meaningless.
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Old 06-24-2008, 01:10 PM
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I have dual 3" exhaust and the engine is completely stock except for the whipple and a PCM upgrade. I figure I probable have 350HP, but I haven't had it on the dyno.

F-BIRD'88, I don't want to be an a-hole, maybe you don't understand the problem. I think the dual exhaust may not be providing enough back pressure and could be hurting the scavenging. If I run open headers, that problem will only get worse, right?
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Old 06-24-2008, 01:25 PM
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Quote:
Originally Posted by Pre-Tuner
I have dual 3" exhaust and the engine is completely stock except for the whipple and a PCM upgrade. I figure I probable have 350HP, but I haven't had it on the dyno.

F-BIRD'88, I don't want to be an a-hole, maybe you don't understand the problem. I think the dual exhaust may not be providing enough back pressure and could be hurting the scavenging. If I run open headers, that problem will only get worse, right?

No such thing as not enough back pressure. Not enough or lack of back pressure is a +. An engine is just an air pump. Remove, reduce the resistance to pump and the thing will make more power.
The only time a reduction in "back pressure" hurts power is of you don;t compensate for the improved breathing by rejetting the carb. (if nessessary)
Back pressure hurts exhaust scavaging by supressing + and - wave pulses in the exhaust pipe. A super charged motor depends a lot less on tuned exhaust scavenging to make power. A super charged motor always makes more power with less exhaust back pressure. Take the cork out.
If you find that on the dyno your exhaust temps suddenly go way up when you open the headers and test, then the cam may need to be changed. A "stock cam" is not he kind of cam that gets you into trouble. A "stock" cam is not going to over scavenge on ya.
You're confusing exhaust back pressure with exhaust velocity.

What I'm trying to tell you is to compare your big dual exaust system on the dyno against open headers. (you may want to add a open collector extension pipe to the open headers)(torque below the peak torque point) You'll probabily find that the exhaust system you have is not affecting the power much. If the exhaust system is good the power and optimium jetting will be close to open headers.
If the exhaust system is not good there will be a big difference in power and will require a larger change in carb jetting to optimize. This (big difference) tells you the system needs work. Why make changes if you don't need to?
2 into one exhaust systems are much more critical to build. The Y pipe is critical.
If you keep thinking "I need back pressure to make power" Instead of I need the correct exhaust flow and velocity to aid/optiminze net pumping efficientcy and power, you will make less power than you could (dual ex or 2 into 1single ex)

The old back pressure myth comes from people taking a stock motor/car with a very restricted stock exhaust (With a stock carb that is jetted for that)
Then they open the exhaust and it now runs lean cause the stock carb and fuel system cannot keep up to the engines new found "lungs" (it now has some effective exhaust pulse wave scavaging) and the thing runs bad and hot at WOT. (can even burn the ex valves) They assume that the change in exhaust back pressure is the cause of the problem. It is not. It is the fact that the carb was not re-adjusted to flow the now increased required fuel for the change. Once the carb and fuel system is adjusted, the motor makes a lot more power and runs normal combustion and exhaust temps at WOT cause its not starving for fuel. (Lean WOT AFR burns slow(er) dumping fire into the exhaust port overheating the ex valve and cylinder walls)
Same on some stock OEM EFI motors. (speed density) they won't self tune to the change in volumetric efficienty when you open or make a big change the stock exhaust and/or camshaft
Once you remap the EFI fuel curve and up the fuel delivery volume to keep up, it runs like it should.

Last edited by F-BIRD'88; 06-24-2008 at 01:52 PM.
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Old 06-24-2008, 08:06 PM
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I couldn't agree more with F-bird on this one.
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Old 06-24-2008, 09:47 PM
406cu.in. of tire smokin' fun
 

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Well that makes sense. Thanks for the detailed explanation. I guess I'll save my money and work on the other truck. Thanks.
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Old 06-25-2008, 12:32 AM
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I have seen well designed single exhaust headered cars make more power than a dual setup, particularly on turbo'ed/supercharged cars. Many dirt track cars run single exhaust setups with adjustable restriction mufflers for this reason.

The reason...exhaust pulsing is symetrical in a single exhaust and hence tuning for air fuel ratios from cylinder to cylinder is simplified.

It is correct that an engine is just an air pump...but it is an air pump with a controlled series of explosions that have to occur at the correct time individually and with the added complication of individual cyl plumbing differences. Single exhaust systems simplifies the system and allows for added tuning by changing relative lengths of cyl plumbing to be changed for specific circumstances.

Look at your firing order and you will see separated dual exhausts systems do not have symetrical exhaust pulses and one cylinder gets the short stick. Balance pipes added to dual systems improve the situation but on pressurized induction systems where mismatches on fuel air mixture can spell disaster, from cyl to cyl it is easier to tune a single exhaust engine at the edge of detonation.

Now you know why the guys with dual turbos have trouble getting a good balance from side to side in fuel/air mixtures and typically have to tune for the lean side. Usually adding a single exhaust outlet evens out the problem and allows the tuner to fine adjust the system to the ragged edge of detonation better than a dual exhaust setup.

Mainly dual exhaust systems just look cooler from a rodding standpoint, they don't necessarily add more power than a well designed single exhaust system.

BTW backpressure is something that cannot be dismissed as not necessary or desirable, some back pressure is needed to build low end torque at low speeds where cam overlap will bleed cylinder pressure. If all your looking for is a peak figure then a open exhaust will always outperform a closed exhaust system, however as most experienced rodders know its the area under the curve that counts and that means from idle to peak rpm. If adding a little restriction to the exhaust adds low rpm power with little loss up top then you have a net gain which adds up on the chronometer at the end of the strip.

Just something to think about.
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Old 06-25-2008, 01:03 AM
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Great Thread. (loud Applause In Background). :d
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Old 06-25-2008, 07:07 AM
406cu.in. of tire smokin' fun
 

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Damn, back to square one.
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