I have heard that the size of your exhaust pipe(s) can truly affect the overall performance of the engine and think I may need to make some changes. Recently changed from a stock 302 to a 'Blueprint' 347 Stroker with EFI and it doesn't seem to perform like I thought it should. Currently running Hedman headers, dual 2-1/8"O.D. exhaust with some aftermarket mufflers.
What size exhaust pipe and muffler setup are folks running with a motor like this?
Don't want to hear the drone, but looking for something with a nice overall tone.
Thanks in advance for any insight.
2 1/8" exhaust? Is it possible it is a 2"? In any event, exhaust diameter is an important, but not sole, component of performance improvement.
Before we get into exhaust, you didn't say what transmission and gear ratio/tire height you have. Those are extremely important factors in the performance "feel".
It is likely that the power band moved way up (in RPM's) with the Stroker. Many do not make significant power gains until you crest the 27-2850 rpm mark. This can be a real problem in the "seat of the pants dyno" if you were already geared "tall" (numerically LOW gear ratio) and/or if you are running taller than stock tires. This will "feel" even worse with an automatic transmission. I won't bore you with the reasons, just trust me.
As long as the above factors are accounted for properly, here is the relationship of exhaust and "flow":
Your engine is really nothing more than a synchronized air pump. The more air that you can "suck" in (intake) combined with the more air that you can "blow" out (exhaust), the more power you will make. It is that simple. But, it's not that easy.
Many factors limit intake and exhaust. The most important things are the ones that you can control. So, we try to run higher CFM-capable carbs/throttle bodies/air cleaners to increase the INTAKE of air. Then, we add headers, "free-flowing" exhaust, and "open chamber design" mufflers to increase EXHAUST.
But, we have to remember that we need a certain level of "scavenging" of the exhaust gases AND we have to have a minimum level of backpressure from the exhaust system to keep the valve train from coming apart ("unshrouding", we used to call it...or, "floating the valves").
And, a Windsor-based Ford requires more back pressure and more scavenging than its bowtie counterparts.
At the end of the day, there is also a theoretical maximum number of CFM that the engine design can flow. Depending on who is yielding the calculator, an SBF can move anywhere between 475 CFM and 590 CFM...and that's the "range" that you need to focus on.
Too large of CFM on the intake side is of no benefit, and can actually reduce performance and create tuning nightmares. Too large of diameter pipe (or too long of a run at too large of a diameter) will reduce scavenging and not provide enough backpressure...again, hurting performance.
So, the answer?
Headers: (really whether shorties or long tubes) with 1 1/2" - (max) 1 5/8" primary tubes and a 2 1/2"-3" collector, with an appropriate reducer cone.
Exhaust pipe: 2 1/4"-2 1/2" diameter seems to be the sweet spot.
Mufflers: If you keep the length of the muffler around the 18-22" size, and match the inlet and outlet diameter to your pipe size, you can choose any of the big named mufflers and be happy.
Although you can run a "X-Pipe" or "H-Pipe" in a Bronco, it really makes servicing difficult for your drivetrain components.
Finally, make sure that the system is throughly sealed. Exhaust "leaks" can kill power and throttle response. Thick flanges on your headers; quality gaskets (like RemFlex), properly torqued; and a little sealant (paste) at all the connections and you should have a great sounding and excellent performing system.
BUT---Look at that gearing and tire size first. You have to operate in that powerband to "feel" those gains.
