Exhaust system question for the exhaust Gurus

Mike Puckett

SCCoA Member
For the past 8 years or so I've been running a split 2.5" to 3" to 2.5" exhaust with a Masterflow resonator, cats, and Magnaflow mufflers. I've acquired a true dual 2.25" with X pipe crossover, cats, no resonator, and Borla stainless steel mufflers. I was thinking that it was a 2.5" true dual and didn't realize that was a 2.25 until last night after I already had the newer 94 gas tank installed. So, my question is considering my power levels, currently 320rwhp and 389rwt, am I making a mistake? Or, is this system going to flow just as well as the older setup?
 
To give you a comparison, I took my system off a Corvette C6 and that was 2.25" dual but that was around 400 hp NA motor, considering your not NA and making a bit less, I think your right on. I say leave it. What headers are you using?
 
The headers are the shorty stainless type I bought from Bill Hull about 7-8 years ago. I'm thinking that when I build my new motor and pick up another 100 hp I'll need to upgrade it to a 2.5" true dual setup for sure.
 
If the 2.25" is crush bent I would not use it. For someone who races a lot, that is going to generate excessive back pressure which is bad for detonation. I'm thinking if you put that on you'll see higher boost than you did before.
 
The true dual exhaust has a cross sectional area of 7.952 all the way back (3.976 in^2 each 2.25" pipe), and your current exhaust at it's smallest point (the 3" midpipe) has a cross section of 7.06 in^2, but your dual 2.5" is a lot bigger than than the 2.25, obviously. So this system at its lowest point has a cross section better than your current lowest point, but the rest of your system flows fine. I think you should stick with what you have and possibly upgrade to a 3.5" midpipe if you ever up your power significantly.
 
For the past 8 years or so I've been running a split 2.5" to 3" to 2.5" exhaust with a Masterflow resonator, cats, and Magnaflow mufflers. I've acquired a true dual 2.25" with X pipe crossover, cats, no resonator, and Borla stainless steel mufflers. I was thinking that it was a 2.5" true dual and didn't realize that was a 2.25 until last night after I already had the newer 94 gas tank installed. So, my question is considering my power levels, currently 320rwhp and 389rwt, am I making a mistake? Or, is this system going to flow just as well as the older setup?

Go bigger.

David
 
After swapping out the gas tank last nite I woke up this morning at 3:30am with this bad feeling that the true dual pipe looked too small, so I got up and checked it. Sure enough, it was only 2.25".:eek: Back to sleep and I dream about getting lost going home from the exhaust shop without my car.:rolleyes: So, that combined with everyone's excellent advice, I think I'll do just that and keep my present system in place upgrading the new straight center section to 3.5". The car it came off of had badly blown headgaskets and would easily peg the boost gauge so I think that's a good confirmation it's going to be too small. I already had my old setup sold so hopefully my friend won't mind getting a true dual instead. Thanks guys.
 
I cant see the dual 2.25 being such a bad thing. Certainly not restrictive enough to peg a boost gauge and blow gaskets. And as been said two 2.25 is greater then one 3". Also one has to wonder just how restrictive having a resonator an dtwo mufflers is compared to no resonator.

I'd see how it fares before freaking out
 
Boosted cars are more sensative to exhaust size. If you can you should try to get the same size pipes through out the entire system. The exhaust is only as efficient as it's least efficient part.
 
Boosted cars are more sensative to exhaust size. If you can you should try to get the same size pipes through out the entire system. The exhaust is only as efficient as it's least efficient part.

Not exactly true. Turbo cars and supercharged cars are two entirely different things. The more accurate truth is that a supercharged car is not sensitive to exhaust size as long as it doesn't start to represent a major restriction. We made 390rwhp with an M90 through a dual 2.25" crush bent system but boost was about 2 psi higher than it should have been. Higher than normal boost levels due to intake manifold/port restrictions are one thing, but higher than normal boost levels due to exhaust restriction (particularly on a car that is going to be raced) is going to generate excessive heat in the cylinder which leads to detonation. In this case in order to run the car at the drag strip effectively we had to increase fuel and retard timing significantly to control detonation resulting in the inability to maintain 390rwhp all the way down the track and instead had to tune for about 340rwhp.

In Mike's case the 2.25" pipes might have made nearly as much HP as the 2.5" but durability and longevity of the motor would be another issue.
 
I never said that 2.25 wasn't enough pipe, what I was trying to elude to is the efficiency of the pipes themselves, necking up and down in the system can cause issues in turbulance and flow.
 
I'm gunna have to agree with damon here that its a better setup. Not to mention the comparison between the whole X/H pipe ordeal, bein that the X pipe will get you a lil more HP as well, due to the natural scavenging effects?, am I thinkin of the right term here?

Anyways, I've always been a fan of duals with some sort of crossover pipe on any vehicle with a V-shape engine myself. Read enough articles on power differences with v6/8 motors with different exhaust systems to know that true duals usually yields the best results.

Personal preference, True duals. I've got 2.5 duals w/magnaflow dual in/out x-pipe res and borlas. Idles like an 8, deep low tone, almost no drone. I love it and many ppl seem to think I have an 8 til I open the hood.
 
Right now my max boost is 14 psi and an increase in boost level will likely peg the gauge. Since this is a fairly worn motor and this is the last last year I'll race it, I don't want to overboost it. I've got about a dozen or more events to run this year and I want to be sure I get thru the whole season. My exhaust shop doesn't handle 3.5" pipe but I can get a 4' length and the reducers for $45 from Jegs if he can make it fit. Otherwise I'll just replace the 3" center section for the new gas tank and stick with the present setup since I know it works good. With the new motor I want to go to a 2.5" true dual setup since it'll have a bit more power.
 
Whats the collector size on the headers? There's really no point in going any bigger than that on one side. In fact, going too big suddenly can actually hurt performance because of the sudden deceleration of the exhaust pulse. Its like putting a 2.5" pipe off of a stock manifold... besides not being able to seal it which will cause tuning issues, the sudden deceleration of the exhaust pulse will kill any scavenging affect.

You also have to consider the peak HP rpm range of our motors. The higher the RPM of the motor the bigger the pipes you want. Our motors are not exactly high reving, so you need to focus on maintaining the velocity of the exhaust pulse in the lower rpm region to maximize scavenging etc. Putting a 3" pipe on one side on one of our motors that is still basically stock and peaks HP at 4500 rpm's is leaving power on the table. If the same motor was capable of making peak HP at 6500 RPM's then a bigger exhaust pipe is needed to maximize output and take advantage of scavenging at that rpm.

Bigger is not always better....

Fraser
 
DamonSlowpokeBaumann said:
I think though the point is dual 2.25 is better then a single 3 inch
I disagree. Although dual 2.25" pipes have slightly larger cross-sectional area compared to a single 3" pipe (7.95 square inches vs. 7.07 square inches), the dual pipes have a 50% increase in surface area and thus greater frictional drag on the exhaust gases.

Frictional losses from drag are a significant part of the equasion. Here is what the "doctor" had to say on the subject years ago...

fred 4 pt 3 said:
Re: Exhaust Theory 101!

I think that the "pervnert" (aka pv=nrt) analysis was a good start at getting a handle on the relative pressure. Some time ago I did a Bernoulli / Reynolds model for Spiro, in which I concluded that the center section for an exhaust is best served at 3". Some interesting data for Dookie -

header temps in a stage 2 mill are as high as 1550°F

You need to consider as much as 1400 kg/hr of moist air at that temperature, plus the gasses created by combustion.

Inertia and drag forces play a bigger part than pressure, which is more of a (negative) symptom than anything else. I tend to think of things in terms of median pulse velocities.
 
Paul I think there is more to it then just that. Thats really only touching the tip of the iceberg so to speak.

But if you really want to analyze the exhaust system as a whole there must be MAJOR frictional losses going through the catalytic converters as they are nothing more then a multitude of small diameter openings no? And how good can having a two in one out resonator be and all thats going on in there?

Personal experience has shown me not much a loss with a high flow cat..However I wonder just how restrictive running a resonator is.

And as far as frictional losses. When do they outweight one another. Is dual 2.25 better then a single 3 inch. Are dual 2.5's better then a single three inch? And if so why not just go single 3 inch to the back? or Single 3.5..You could probably even get the single oval 4 inch back there.

But I think the main question here is

Going from a dual 2.5 with cats to a single 3 inch back into dual 2.5 then into two mufflers

When compared to dual 2.25 into an x pipe back into dual 2.25 and into two 2.25 mufflers

I'd have to say that #2 would flow better.

At this point it would be interesting to see what the outcome would be just swapping exhausts
 
Going from a dual 2.5 with cats to a single 3 inch back into dual 2.5 then into two mufflers

When compared to dual 2.25 into an x pipe back into dual 2.25 and into two 2.25 mufflers

I'd have to say that #2 would flow better.

Then that would put you in disagreement with more experienced engine builders who maintain that a single 3" will support more power than dual 2.25" pipes.

2.25" crush bent pipes are way too small for a 350rwhp SC. Dual mandrel bent 2.5" pipes are just about right but don't take my word for it, do the math. :)
 
The collectors are 2.5" as are my downtubes and cats and I normally shift at around 5600-5800 rpm. My thoughts were also that the added surface area along with the crush bends of the dual 2.25" would undully restrict the flow over the 3" I have now. A friend needed a new exhaust for his and I went ahead and sold it to him so it's gone now. I may just keep the center at 3" for now and go to a 2.5" true dual for the new motor next year. There are a couple of other exhaust shops in the area and if either of them can do 3.5" I may have them do it instead of my regular guy. I've got time before the season starts.
 
Dave but you are not doing the math at what an extra single resonator and two cats would add to the equation. If the 2.25 are crushed down to 2 inches or so in some areas well...That of course is not such a good thing.

But just how much friction/turbulence do those componants add compared to having none at all but withdual smaller pipes
 
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