Stevil

Hey all,
I might have posted something similar before, but I know I never got a sufficient answer. According to Corky bells Maximum boost book, regarding exhaust, according to his graph;
For most of our 3rd gen cars, running maxed out twins or even smaller singles, producing ~430 bhp, for maximum exhaust efficency, the exhaust should be about 2.5" diameter. Why are all the aftermarket exhausts 3"? That is too big and therefore not efficient, by a large margin.
What is the stock exhaust size?
Would a 3" DP matted to 2.5" exhaust follow pretty close to the above, assuming straight thru?
I was thinking of using a 3 chamber, straight thru muffler instead of a midpipe(pretty much same thing, muffler is straight thru, just 3 chambers side by side, hopefully produce a better tone). Will this cause any problems? Example, exhaust running thru DP, then into above muffler, then being forced into 2.5" pipe. will this casue somekind of variance from having a straight pipe(not allowing the exhaust to enter a large chamber, muffler, then back into a constricted chamber?
Does this make sense?
Anyone have thoughts?
Thanks,
Steve

rotarygod

iTrader: (1)

When it comes to turbo exhausts no exhaust is the best exhaust. Obviously not true for naturally aspirated applications. Since the government and your local law enforcement agencies would highly disapprove of no exhaust the best thing to do is to put the most free flowing system on that you can. A 3" pipe can move alot of air. If you got the room go 4". No such thing as overkill on a turbo car. The less exhaust restriction the faster your turbo responds.

j9fd3s

iTrader: (3)

going from 2.5" to 3" on my t2 was like night and day

mike

protlewski

logic suggests the normal rule in modding (getting rid of restrictions) works no matter what the other mods are.
What ever book shoved that load of B.S. in your face is full of it to begin with. the less restriction the better.
The fact that the word efficency poped in there tells you that they are thinking scientificly not aiming for power. By no means is a 430Hp car efficent, not in the least.
That car is dumping fuel into the engine to not lean out, who cares about efficency when you have about 390 RWHP ripping the aspalt from underneth you.

rynberg

removing all restrictions kills low-end power which requires back-pressure, look at the dyno curves comparing with and without a main cat, you will see that midpiped cars make less power until the upper-midrange, at which point they take off compared to a stock cat car

I don't believe that no restriction is the best. At some point, you will hit an exhaust diameter that does not give you more power up top and is just killing your low-end power.

Yeah, a 4" exhaust might produce a higher hp number on the dyno but I would put money down that a 3" exhaust will have a much larger powerband. And will be more fun to drive to boot.

Stevil

Sorry if the word efficent was construed to have originated from the book Maximum Boost. I used this word. Corky Bell states, regarding exhaust diameter, "Larger is not always better". That there is an exhaust gas velocity that ought not be exceeded. He then set up a graph of exhaust diameter to bhp. This graph is for turbo charged engines. he does not specify between rotary and piston, however(if that really matters). 3" would be what is needed for 600-700bhp. He states max velocity should be 250ft/sec. So, I assume he means to stay as close to this figure as possible. Well, a 3" exhaust for 430 bhp would have have a number higher than 250 ft/sec, right? And this violates the rule. Am I correct? He does state that the exhaust dia should be bigger than the cooler intake side. He states this graph is a good guidline for selecting exhaust dia. He also says a tube diameter appro 10% larger than the turbine outlet diameter.
I am sure some of you have read his work. He, seems to know his stuff. But, I see all aftermarket at 3" dia. So, why was this standard chosen and not something different, such as 4"?
My MP has much less umph, than my High flow. I understand MP achieves more HP, but what about Torque thru the whole band.
The above exhaust dia, deals with just tube sizing. Of course no bends would be best as back pressure is an enemy to turbo cars.
I just put these book quotes in for those who have not read it.
I guess i could follow what is common(3") but I must question when I read a knowledgeable persons comments onthe whole affair and never have seen a thorough discussion on this forum regarding it. I know there are very knowledgeable people here, thus my asking. We must learn to question and not just aimlessly follow.

Thanks again,
Steve

KevinK2

at the 350-400 rwhp level, several with 3" exhausts, no MP, have measured backpressures around 6-8 psi. velocity is no good if it comes with that baggage.

don't recall others noting big lowend loss with MP. Many point to comparo dyno charts at wael's site, but don't know that the pre-transition run with mp was with a very bad spark/ign problem.

aka K

..."he does not specify between rotary and piston, however(if that really matters). .."IMO it does, in boingers you need some back pressure to help close and bed the valves. I can see no reason why you would want any backpressure in a rotory.
my 2 cents
Cheers
CRAIG

swoop

On a N/A engine, When the exhaust valve or port opens a pressure wave moves to the end of the exhaust system. Along the way it joins into a collector which is attached to other header pipes, and creates a scavenging effect which helps pull exhaust from another cylinder or rotor. The size of the header and collector and exhaust pipe affect its flow, thus the timing (tuning) of that exhaust system.

For a turbo motor, the pressure wave exits the valve or port and travel down the header until it hit a big obstruction, the turbo. Then it reverses, so it doesn't travel to the end of the entire exhaust. Therefore, exhaust tuning isn't as important on a turbo car. Back pressure is what you want to eliminate, so a 3 in. pipe behind the turbo work better than a 2.5 or 2.75.

Hope that helps.

Stevil

Ya,
Corky does specify for a turbo(and he does not diffierentiate between piston and rotary) any backpressure in the exhaust is bad. Ok, we all agree upon that. He just specifys about the velocity of gas exiting. He does not exactly detail why 250 ft/sec should be max velocity. Nor, what exactly happens to power or torque above or below this number. If anything. Of course his exhaust chapter is on creating the cleanest running, acceptable noise level, lowest backpressure system.
So, would a smaller diameter exhaust just create more back pressure? Now theoretically speaking, if say a 2.5" pipe(we will say it is straight all the wat thru) is large enough to allow a car w/ 400bhp to have its exhaust gases escape w/ nearly no backpressure(not sure if it would, just saying) then a larger pipe(say 3") would do what? Now I am told be exhasut shops, too big a pipe leaves little torque. So, is this all due to backpressure and nothing else in the system?
It sounds like a 3" exhaust or larger is what a turbo wants, for no backpressure, and you have all stated it, then why does Corky Bell have a max exhaust gas velocity that should not be exceeded? Why? Would it help reach his two other conditions for an exhaust; cleaner buring and low noise? There this last question sums up my whole topic.

Thanks for all the input.
Steve

protlewski

But in the end exhausts are not (most of the time) straight through they have bends and twists to acomedate the various shapes and components under the car. If you take those into account if a 2.5" pipe is pefect straight then 3" is better accual exhaust because there is now more backpressure from the bends and the muffler (if any) Unless you have side pipes this logic applies.

Stevil

Yup, I see the logic there, thats what I was thinking. But that is in reference to backpressure. Any ideas of why a specific velocity? If you did not have any exhaust pipe, and it dumped into atmosphere out side turbo exhaust port, which would be ideal, what velocity are we talking about?

swoop

More velocity does make a given exhaust system quieter as it exits the exhaust system.

Stevil

Ok, Im sure some of you are tired of this topic, but I have something useful to add, yet again.
After reading alittle more, I have come to find out why there is a maximum velocity that should not be exceeded in exhaust and intake. The intake velocity is allowed to be greater(450 ft/sec max compared to 250 for exhaust). But corky Bell says max velocity, above, should not be exceeded for reasons of rapidly increasing drag and consequent flow losses.
we know curves increase drag, but moving too fast through the pipe also increases drag? Hmm. He does state it just is a larger volume for the system which is not really good(he does say this in the intake chapter, but makes reference to it in the exhaust chapter, saying the same rules apply).
So more info and yet another question. So , can anyone explain how the tube size/velocity/drag relationship works. I am just curious.

turbostreetfighter

boy that book really screws everyones mind up, no disrespect to Corky but on a turbo car as Rotary God said no exhaust is the best exhaust..i dont car what argument you come up with! look at Norwoods 4000h.p. funny car is uses 6.5" exhaust!!! i use 4.25 no my race car and going from 3" to 4.25" gave me 1000 rpm quicker spool up and 47 more RWHP and i had to add 12% more fuel to get the A/r back to where it was!! now the reason that some peoples car seems to gain more midrange with a smaller exhaust could be because of a lean condition when a larger exhaust was added, if you dont compensate for the added flow you will not benefit. just my thoughts flame if ya wanna

RICE RACING

I do not agree with many thing in that book (maximum ego, sorry boost ) he alianated every mechanical engineer on the planet with his "own opinions" on water injection, showing total disreagard to engineering facts and history.

The guy is a moron of the highest order in my books, any text that is full of little tip bits of inspired information, as displayed by Mr Bell is just a step up from a Hype magazine such as Turbo, Import tuner, Fast fours etc etc, they realy are a waste of trees, printing ink, transportation, plastic cary bags......

As far as the DP/exhaust is concerned, the largest physical size you can fit the better, so long as noise is not an issue. You simply want to duct away the exhaust gas from the chassis with as little restriction possible, exhaust tuning is not applicable after the turbine exit, changing flow from a spiraling helix to laminar for best flow is, but thats another topic. By the way which is covered well in any real turbo text books.

End of story.

RICE RACING

In responce to a commen further up, the least restrictive DP/exhaust combination in my experience increases mid range power, it does not decrease it once the engine parameters are set for the incresed air flow.

In EVERY instance for a turbo charged vehicle you gain increased turbine responce/flow when there is no back pressure or as little back pressure in this section !

For a classic example of how important this principle is look as Diesle trucks, they have very specific post turbine exhaust back pressure limits that must not be exceded, otherwise output & responce are reduced significantly, some two stroke diesle turbo engines have turbine exhaust back pressure limits that are very very low, In the region of 2 to 3 psi !!!

Just look at a 450BHP truck and ask yourself why it runs twin 4 or 5" vertical stacks with near straight thru mufflers. It's because the turbine/turbo responds the best and flows the most when there is very little or no back pressure on it.

The same is very true of pure turbine engines, stationary or vehicular, or pure turbo prop muilt satge turbine (no exhaust thrust) gas turbines have realy big post turbine exhaust systems (not for thrust !!!) we are talking staionary/vehicular gas turbines here . Some of these range in out put from 300 to 1000 bhp and their exhaust system pipe sizes are up to 12 or 15" diameter !!!!

Once again Corky fails to back his statements with real engineering facts, and examples.

turbostreetfighter

Very well said Rice, i would have tried to explain more but everytime i say anything that someone disagrees with i get flamed!! BTW i figured out my EGT delima, there was a small vacuum leak betwen the intake manifold and block. i fixed it and they went down to 750c
i also own a F350 CC dualie with 5" exhaust!!! love tha torque especially when pulling 6k lbs of trailer!!


MWW

Stevil

great thanks, all sounds good to me.
3" exhaust it is.
Ok, since we are on topic, how about the rest of my very first question;
Depending on noise levels, I might trade out my resonated mp with a straight thru muffler. The muffler has 3 chambers side by side, so is straight thru(hopefully quites things down). The muffler opens up quite a bit after entrance and then flows back into 3". So, will the muffler volume cause any negatives in the exhaust flow? A mp is a single pipe straight thru, while the muffler opens up then tapers down. Muffler dimensions: 3" inlet into 4" thick by 10" wide to 3" outlet. Again, muffler has really no restrictions, just 3 straight thru chambers.
Just wondering.

Thanks for all the input. Everything said by everyone made sense, except for Corky Bell. And yes, I wondered when he said water injection was bad idea. After all that I read on the topic, it seemed like a very good idea.