Novusuhu

Hey everyone

I am going to replace my stock exhaust and headers with a custom dual one
​​​​(with kat removed).
​​​​​​The main goal is a louder, more aggresive sound and some extra hp.

My questions are:
-2,25 or 2,5 inch exhaust pipes for L48?
-Difference in sound between 2,25 & 2,5?
-Difference in sound between steel and SS?
-Recomonded header pipes diameter (collector and other)?

Some guys at exhaust workshops say the 2,5 inch has a negative effect on an L48 and that it would kill torque...

ntfday

If it's stone stock I wouldn't run bigger than 2" on the pipes and probably 1 7/8" on the header tubes.

The13Bats

I run hooker side mounts because i think they look cool and sound great,
But they do hurt my performance

Yeah, smaller tubes on yours will be better,
A cross over tube will lessen sounds a little so if you want old school banking noise dont run cross over

69427

That's nonsense, but for amusement ask them why. I'm betting most of us here already know what their answer is going to be.

PainfullySlow

Out of curiosity, do you know that they hurt your performance, like you have dyno'd the car with regular exhaust and again with the side pipes or is it something you were told? I am not calling you out, just genuinely curious since I have heard this a lot yet never been able to find any proof of it.

OP: FYI if you are already running headers you will not realize any significant horsepower gains with a new system however you will be able to get a different exhaust note by changing pipe size, mufflers, etc.

As far as pipe size goes I would challenge anyone to prove that a larger exhaust diameter hurts power (exhaust, not header/collector size...which actually does matter) in a street application. I am genuinely curious since this myth has gone on for a long time and I do not know if there is any validity to it.

As far as I know it is difficult in a street car to claim that too big hurts however too small can. While a 2" exhaust is likely sufficient for your stock motor, I would go with a larger diameter on the chance that someday you may want to bump up the power a bit. I would go with 2.5" system since they are readily available for the C3.

As for the sound, SS pipes tend to have a slightly more raspy or 'tinny' sound, mostly because the SS pipe is a thinner wall construction vs mild steel however it is very difficult to notice. Your muffler choice will have a much larger overall effect on the sound of your exhaust. I would go stainless steel simply because I would not want my exhaust to rust out so quickly.

Hope this helps.

ntfday

If you were around in the 60's when header manufacturing was starting to boom and dragstrips were king you'd know the answer and why. One hp per ci was still fairly significant and top fuel was making 1000hp or therabouts so making 300 hp or better out a sbc was a big deal. The guys running Hedman headers were consistently out performing the guys running Doug's, Hooker's and so on. Hedman used 1 7/8" primaries and the others were running 2" or better and that's the answer without flow tests, computer models or anything else. Been there, done that.

Raphiki

If you're looking for a more aggressive sound I'd go for chambered exhaust.

shenango

I have a 75 with a warmed over L48. Tri5 coated short tube headers, 2 1/2 all the way back to FloTech dual chambered exhaust. Sounds real good at idle, no drone at cruise and really barks at WOT. I am real happy with the sound but I couldn't say there is much HP gain or torque loss.

ntfday

FWIW, any internal combustion engine requires a certain amount of back pressure and too large diameter exhaust pipe can have a negative effect on both horsepower and torque.

BKbroiler

Not true.

69427

ntfday

Would you please explain why not?

Raphiki

My understanding has always been that a set of tuned headers actually scavenge exhaust gases from the other cylinders creating a low pressure situation and improving flow. So you end up with the exhaust gases actually being vacuumed out.

PainfullySlow

As I said, collector and header size DOES matter but we aren't discussing header dimensions. We are discussing whether or not pipe diameter can be too large and cause a reduction in performance. As far as I know, pipe size does not matter as long as it is large enough to carry the gas at velocity. Too small can be bad however I have yet to see actual evidence that shows that too large can be. This is my understanding.

I recently picked up a full set of Allens stainless chambered although I do not have it installed yet but from what I have heard on video, I am really looking forward to getting it done.

^ as far as I know.

No, please explain why? This is why I specifically asked for PROOF of this myth that is constantly spread. Please show me the factual data that you used to make this assumption.

If the backpressure myth is actually true, why do top fuel dragsters run ultra short exhausts with zero back pressure? Why do race cars run straight pipes without restrictive mufflers?

Again, I am not saying that it is not true. I do not know myself but what I do know about fluid mechanics (which is similar to gasses) says that it is likely that it is not. This is why I would like to see evidence proving it one way or another.

69427

Backpressure causes a reduction in exhaust flow efficiency. This reduced efficiency results in residual exhaust gas mass remaining in the cylinder when the exhaust valve closes. On the intake stroke the cylinder can't fill to 100% (theoretical) due to there already being a non-combustable gas (exhaust) taking up some of the cylinder volume. This lower volumetric efficiency results in less horsepower developed by the engine.

Additional crank horsepower is lost due to the piston having to fight against the backpressure during the exhaust stroke.

Lakeside49

Modern vehicles with a functional emissions system are calibrated with several inputs, including a signal from the EGR. I recall as a Ford Zone Manager of some complaints about engine performance reductions with exhaust system upgrades -because the ECM calibration tables were based on an expected amount of EGR-based back pressure. The reduced back-pressure messed up the ECM's calibration, and was (is?) the reason why non-stock exhaust systems were illegal in my former State of Colorado with their emissions compliance laws. This applies to all OEMs whose emissions systems are government certified - a tall order on a model-by-model basis on a good day (Dave McLellan, C3-C4 Chief Engineer says in his book `Corvette from The Inside' that certifying the late C3's gave him nightmares). I suspect this awareness and communication by certified Technicians is the reason for the valid concern. EGR back-pressure is an important signal (or, it was just a few years ago before I moved to a new assignment). I know it sounds counter-intuitive, but the emission calibration algorithms are
pretty sophisticated, and if there is a signal that is not possible in the table, it will try to over or under compensate something else, reducing efficiency. It can also trip the emissions light, or worse case - especially if there are already other factors putting it on the margin - it can send the engine into LOS ("limited operation strategy") to allow it to limp to a repair shop.

Bottom line, if there are no electronic emissions systems on a vehicle such as pre-81, I suspect that there are limited or no issues and mostly positives in helping an engine to breathe as efficiently as possible. As an aside, most people think that the `82 gained it's extra 10 HP over the `81 due to it's electronic fuel injection. Dave McLellan said in the above mentioned book that it was actually created by slightly enlarging the exhaust system, and by using a smaller, easier breathing cat converter. It mostly had nothing to do with the crossfire fuel injection setup.

leadfoot4

I'm puzzled, as to why you would say this. WAY back, in the fall of 1976, I bought a brand new, 1977 Pontiac T/A. It had a low compression 400 cu/in engine, backed by a 4 speed transmission, and a single exhaust with a catalytic converter (just as the OP said his car currently is equipped).

Shortly after I brought the car out of hibernation, in the spring of 1977, I removed the OE exhaust system, and installed a Hooker "side mount" header set-up....4 tube headers off the engine, merging into 4" sidepipes, with the glass pack muffler inserts.

Installing that system was like taking a cork out of the exhaust. Even before I re-tuned the carburetor, to compensate for the difference in exhaust capability, the car was SIGNIFICANTLY quicker......like "night and day" quicker.

Sadly, the headers rusted out, where the primary tubes met the collectors, after only 2 summers of driving. Fun while they lasted, but the worst waste of money I ever encountered.

BKbroiler

Sorry for the late response here, but raphiki and 69427 covered the answer pretty well. It boils down to moving intake charge in and exhaust out as much and as quickly as possible. When the exhaust valve opens, the less pressure on the port side of the valve, the faster and more completely the exhaust will flow out.

JoeMinnesota

On my 1970 355 I am running a very traditional 1-5/8" long tube header (Doug's) with 3" collectors and dual steel 2.5" exhaust, small H-pipe behind the crossmember (to kill some drone) and Flowmaster 40's. Sounds great, in my opinion. Some authority and announces arrival, but not obnoxious and okay when you're in the car for an hour.

Bone stock will always sound a little different, though, as more compression usually sounds quite a bit tighter and ferocious in any exhaust system.

leadfoot4

One time, just for grins, when I had my T/A, and had the headers off for refinishing (just before they fell apart), I put them back on, and left the side tubes/mufflers off, and took a short drive. Of course, the car was loud, but in more of a "motorboat" sort of way. It was nothing like the "snap and crackle" of the high compression race engines I was used to....