Well, now you're talking about something different. Yes - TOO MUCH backpressure (too small a pipe or a very restrictive muffler, or combination of both) would create TOO MUCH backpressure & that would hurt flow! You know why the OEM GM chambered sidepipe exhaust was detrimental to flow vs. the undercar exhaust? THE FLOWPATH SIZE! Here's a photo of an OEM GM chambered C2 sidepipe internal - the diameter is 1-5/8"! I wouldn't run that on a 4-banger!

 

I agree with you. There has to be some backpressure in order to maintain some low/mid range torque. I've been through this too many times in the last 30 years or so, results are always the same. The problem is you have too many knuckleheads on these forums that think they have an understanding when they really don't. Can't hardly argue with them either. It's wasted time. I take it you do this professionally so you probably know what you are talking about.

 

You NEVER design a performance exhaust to achieve a certain restriction aka backpressure.. You design to a flow rate which compliments the engine and vehicle use. The restriction of the exhaust is a negative side effect of the design. In a perfect world the exhaust would flow at the speed you wanted without requiring any pressure to force it down the pipe. You don't want the engine having to create pressure to push the exhaust gasses down the pipes, you want the exhaust system pulling the exhaust gases out of the engine. And you want the pipes to create that effect in an rpm range that will give a desirable torque boost.

Only a completely brain dead knucklehead who has no clue what they are talking about would purposely want their performance exhaust system to be restrictive to the flow of exhaust gasses. Once again, watch the 30 minute long video of the guy with the qualifications and just try to find one time where he says backpressure is desirable. The whole time he discusses trying to achieve the flow rate with minimum restriction.

The problem is that too many people put a exhaust on their car which is suited to boost high rpm operation and then blame no backpressure as the reason it's faulty, seemingly believing that restricting flow would be of some benefit to the engine. Then you get writers and forum clown who go on about backpressure being required until it's a "thing". It's not. The failure was because the pipe design enhanced power in the wrong rpm range. Or, it was simply because most people can't or didn't properly tune their carburetor.

 

I can't begin to tell you my qualifications, or results of experiments I've seen. If you want to, run your 3" dual exhaust or even 3.5" dual exhaust on your small block Chevy with 320 HP...

 

Will this exhaust with 3.5" pipes give the correct scavenging to boost the low end torque of a 320hp engine?

Also, how many psi of backpressure at the manifold exit or header collector is required makes the best low end torque? If you've done so many tests that prove backpressure is required then you must have this data.

 

You are absolutely correct. Just got an education on this first hand. Just had Calvin Elston, a renowned header builder, build a system for my 614 horse 69. He never used the word back pressure either. Guess what size choke he used in my collectors? 2.5 inches. I was surprised. But the reason has nothing to do with back pressure. The term he used is reversion. You need to maintain speed through the system to keep it "clean" and keep it from contaminating the intake charge. All I know is I feel like I gained 50 lb.ft. With no loss in top end over my Hooker side mounts with 3 inch bullets inside.

 

Of course you don't design an exhaust system to achieve a certain back pressure. I mean, how are you going to (practically) measure the backpressure?

You can't get to quantitative when examining exhaust systems, you have to rely on empirical data (testing) such as chambered hints at. Experience trumps all...

 

Very informative video.

 

Apparently not, since the "experience" is claiming you want to have backpressure or restriction in the exhaust....

 

Well, you've got limited choices when picking a system diameter. Let's start at 2". Then 2.25", 2.5", 2.75", 3", 3.5", 4", etc. Any other oddball diameter & you will run into many problems such as there not being any tubing available in "odd" diameters, plus nobody will have tooling to bend the odd diameters. Notice in the video the guy from Magnaflow talks about going too big on the diameter out of the cylinder heads & creating a negative aspect for flow? You can try to get too technical & create your dream system using slide rules, computers, etc. When I talk about flow velocity in a gas system (as opposed to a diesel system), that's based on CID & horsepower, plus the intended use of the vehicle. If a vehicle is going to be drag-raced only, then you are considering the engine isn't going to see anything less than probably 4500 RPM, so you don't need to worry about bottom-end & midrange performance in the system. For a street car, it's a whole different situation. Also - what is the desired sound for the system? Do you want it LOUD? Or do you want some noise control, & if so, how much noise control? There are a LOT of variables that have to be considered in order to make the right choice for your application & desired results.

As far as a gas engine vs. a diesel, gas engines are fussy about the right diameter - not too much BP as to lose performance, but then not too large either or you will also lose performance. Now on a diesel truck for example - rule of thumb is diesels HATE backpressure, so if your Dodge Ram Cummins is 3" coming off the turbo & 3.5" downstream, if you go to a larger down pipe & say a 5" system diameter, fuel mileage, torque, & HP will increase (unless the engine computer defeats it).

Here's one for you to ponder - & this one is firsthand from a good friend & program manager on the Viper program from a few years ago. The Dodge Boys were experimenting extensively trying to get a better exhaust note on the funky V-10 sound. They tried many different mufflers, many aftermarket cat-backs, etc. Then they decided to do a 3" dual straight-pipe (no mufflers at all) to see what it would sound like. First off, it did not sound good to anybody because there was no "tuning" in the system. Secondly & to the surprise of everybody present, the engine lost about 35 HP, plus the engine temp. rose 30-degrees. They determined this on a dyno of course. After that & everybody scratching their heads, they let everything cool down, then spliced a couple mufflers (expansion chambers) into the system, & dynoed it again. Everything came back into normal parameters. That story illustrated something I never learned in a book or could calculate on a slide rule...

 

So,,, on a stock L48 what would be the best overall choice for the muffler/baffle in the side pipe? My choices are both from Hooker. One is the glasspack and the other is the reverse flow. They cost the same. I am looking for a performance sound. I have read mixed reviews on both. Keep in mind.. I have a stock motor.

Also... I assume that I don't need the alternator bracket from Hooker because it states the bracket is for alternators that are mounted to the factory exhaust manifold. Mine isn't. Its a 77 and it has a stand alone bracket mounted to the front of the motor on the drivers side. I don't have AC so I don't need that bracket.

 

You are correct. You won't need an alternator bracket.

 

I wouldn't go with either of those for the sound you're looking for.

I would get the STS baffle inserts ( I have and like) or get the chambered exhaust insert. I've heard some videos and they sound pretty good.

http://www.classicchambered.com/classic/products.html

 

I'm pretty sure this subject was covered in the old "How to Hotrod SBCs book." Exhaust pipes aren't long enough to tune at low-mid RPM, so back pressure and other techniques (like a crossover) are needed. In addition to reading about it, I've experienced this effect with numerous cars and motorcycles. Chop the pipes down to 2' long on a four-cylinder motorcycle and it'll fall flat on its face coming out of every stop light. Zoomies on a street rod do the same thing, but nobody cares when the whole car only weighs 1,800 lbs. Bottom line, if you want low end torque, you can't open up the system like you would on a high RPM dragster.

 

I agree with you 100%. But some others seem to think that zero backpressure won't result in a loss of low/mid range torque.

Can lead a horse to water, but can't make 'em drink....

 

If memory serves, this effect has to do with the speed with which the partially-combusted mixture leaves the port when the exhaust valve begins to open. Backpressure causes the mixture to remain in the cylinder just a little longer, extracting more torque from the expanding gasses. If you add to this the correct pipe lengths and connections for effective mid-range scavenging, you'll have a street car with real torque where it counts.

You know, a lot of hot rod enthusiasts didn't understand these principles when I left the hobby in the early '80s. It's sad to see that the situation hasn't changed much since then.

 

What's so difficult to understand? An expansion chamber muffler will flow better than a straight piece of pipe. The transition from inlet pipe to the muffler change works somewhat like a collector and can cause a scavanging effect on the upstream pipe.


Exactly, people hung-on on the backpressure myth just refuse to let that complete nonsense go.

You do realize that switching between a full length exhaust and zoomies is doing more than just changing backpressure?

Have you ever seen the general rule of thumb that you lose 1-3hp for every inHg of backpressure. There is 30inHg per psi. So 1psi of backpressure means you lose 30-90hp. Why would you intentionally want backpressure?

One part of designing a performance exhaust requiring maintaining a certain exhaust gas flow rate in the pipe. This creates a problem because the exhaust gas is flowing down a pipe where the inside surface of the pipe isn't moving. There is friction between the exhaust gases and the pipe which puts a drag on the exhaust gases. An exhaust engineers perfect piping would be one where this friction doesn't exist. Unfortunately, just the perfect piping doesn't exist. Still, the desire to have minimum restriction is why they use mandrel bends and ensure the joints are smooth on a performance exhaust system.

This isn't rocket science, it just requires people to let go of their wrong beliefs and actually learn something new. A performance exhaust system is NEVER designed to have backpressure. Any backpressure or restriction that exists is the undesirable result of the system design.

 

Apparently you never understood it back in the 80's. You're trying to claim that you want a restriction so the exhaust gas can keep putting downward pressure onto the piston instead of exiting the open exhaust port? Exactly how much of a restriction or how much backpressure are you adding to cause that to happen?

 

i have Huron side pipes which are 1 7/8" primary and 4 1/2" secondary pipe. engine is 408" all forged components with CNC and modified AFR heads,Lunati roller,long block built by world champ.the car sounds outrageous and you will not burn yourself at point of door exit (98 degrees).inserts are reverb stainless steel.

 

BTW. has anybody ever seen a SCCA c-3 race car that DID NOT have side pipes?