Normal aspirated engines and back pressure.

[1989 406]

How much if any back pressure does a normal Corvette aspirated engine need to perform at it's best?

Current exhaust set up is headers running through 3 inch exhaust into 2 1/2 inch muffler. There is no back pressure except for the mufflers. I have a set of 3 inch mufflers I could put on it but I need a little help on deciding which would be the best set up.

Thank you.

 

[boomdriver]

How much if any back pressure does a normal Corvette aspirated engine need to perform at it's best?

Current exhaust set up is headers running through 3 inch exhaust into 2 1/2 inch muffler. There is no back pressure except for the mufflers. I have a set of 3 inch mufflers I could put on it but I need a little help on deciding which would be the best set up.

Thank you.

If you;re on the street, I'd keep the smaller 2.5"...
Complete scavenging of the exhaust is distorted by the kinks and bends of exhaust tubing past the header collector since the formula for a header describes exact lengths and diameters of the header tubes...The formula is out there online somewhere. Do a web search and run the numbers. You'll be surprised to see how far 'out of bounds' you might already be, even with big tubes to the rear. Its all about sound waves and pressure...Disrupt the waves too soon and they do nothing to help "pull" the next exhaust stroke/cycle out of the collector. Its complicated...way over my pay grade.

When I did mine and low end vanished I learned of something called "minimum cylinder pressure" when I did the exhaust mods and was trying to figure out what happened.. Lost all the low end,and had no guts until 3000 rpm. It pulled very hard from 3000 up to redline and felt as thought it would pull past that as well. The 3" exhaust all the way ended up being the problem. I went back to 2.5 in the rear and it got much better. If I had been running at daytona the 3" would have been fine with the 5500 rpm barrier on the L98 now broken... I suppose the goal or desired end result is the question.
Want hi revving HP or low end pulling power?

FSM says that there should NOT be more than 3 PSI at the collector @ 2000 rpm. Only 1.5psi back pressure @ idle.
Thats the basis for testing cats,. but 3 psi says something about what the stock system does and how the power band is effected.

 

[Hib Halverson]

The need for back pressure with a NA engine is a myth.

If you have 3-in pipes and this engine makes a more horsepower than stock, you need mufflers with which are 3-inch comin' and goin'.

 

[1989 406]

Wow,

Two opposite answers. I guess I will need to research more in detail.

Thanks guys.

 

[Hib Halverson]

I'll add that, given the rest of the exhaust remaining the same and the muffler design remaining the same, the switch from 2.5 in-and-out to 3.0 in-and-out may or may not improve performance. It depends on the engine's power level.

I'd say if you are at 350 hp or less, leave it at 2.5 because the extra work in installing the 3.0 mufflers won't gain much, but if you're at 400 or more horsepower, then 3.0 mufflers may help.

If we knew more about the mufflers, that might affect the opinions expressed.

What brand/type are your two muffler choices?

 

[93Rubie]

Here is an article you may find intresting...

Auto Exhaust Science

Keep things simple, an engine is an air pump. The easier air is in, and the easier it goes out=more power. Basically...

Keep in mind the velocity and volume of the air are important factors.

 

[boomdriver]

I want to be clear, I agree that bigger pipes will likely lead to more HP, but tend to shift the powerband toward the upper rpm range as I experienced.

My goal was to maintain the low end that is so much more usable on the street, and this is where the difference between torque and hp becomes meaningful. Given the desire to have the low end at the sacrifice of top end power, the slight restriction or build-up of back pressure from smaller pipes has a positive effect on low end power.

Knowing that torque is also the byproduct of mass and knetic energy, that means that the slower, lower rpm crankshaft is not totally responsible for the high torque in a L98 engine.It cannot be at slow speed, only higher rpm where it merges with raw hp that can maintain the load against it.
The generation of torque has to be assisted by some mild back pressure. Sure, this has a top end liability, but again, big top end pulling power is nice but its simply not practical in a street beast. Thats why I have 3" up front (to help scavenge the cyl) with the drop to 2.5" at the rear Y into the muffs.


NOTE from the LINK: interesting read... I brought a paragraph that supports what I've been saying...please read !


Headers -- Primary Pipe Diameters
Big pipes flow more, so is bigger better? Answer: absolutely not. Primary pipes that are too big defeat our quest for the all-important velocity-enhanced scavenging effect. Without knowledge to the contrary, the biggest fear is that the selected tube diameters could be too small, thereby constricting flow and dropping power. Sure, if they are way under what is needed, lack of flow will cause power to suffer. In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power.

At this point determining primary tube diameters is starting to look like a tight wire

 

[1989 406]

I would definately prefer to keep the low end torque in the car. I find that a blast. The car should be putting out to close to 400HP. The 3 inch mufflers I have is called TriFlow and is a circular design muffler. I had never heard of them before until now. They look like they are a straight through design. The ones on there now are a Walker QuietFlow. Typical square design, so I would imagine they are baffled. The po was an older gentleman and said the car was too loud for him with the TriFlow set up on there, this is the reason he went to the QuietFlow.

 

[boomdriver]

I would definately prefer to keep the low end torque in the car. I find that a blast. The car should be putting out to close to 400HP. The 3 inch mufflers I have is called TriFlow and is a circular design muffler. I had never heard of them before until now. They look like they are a straight through design. The ones on there now are a Walker QuietFlow. Typical square design, so I would imagine they are baffled. The po was an older gentleman and said the car was too loud for him with the TriFlow set up on there, this is the reason he went to the QuietFlow.

I think that this really comes down to personal preference. There are some "known" facts about exhaust but as in anything else for the consumer, you can find plenty of opposing arguements.

To me, it was GREAT just looking at the gas pedal and having that thing tach up to redline before I could blink !

BUT...

After driving in on the streets for a few weeks, I missed my tire squeelin, tire spinnin, jerk yer head off, low end torque.

Now, heres MY disclaimer...
I don't know for sure what the characteristics of your car truely are or how its going to react. All I can do is relate what mine did and how it behaved and it looks like there are others that act like that as well. You;s may be different with the cam or the intake...who knows. When dealing with voo-doo like exhaust systems.....basically you;re on your own !
Trial & error seem to be the only tried and true method of finding satisfaction with exhaust systems. The facts are only a guide since no 2 modified engines are the same.
Look at it this way, you can always try something different and do something else if you don;t like it...it's just money!......:ugh:ugh:ugh

(like I've got enough to experiment!)