Exaust...too big..?

[Stallion]

I was watching this show, and it was a flowmaster representative, and he was saying that unless an engine puts out 400+ hp, then it really shouldn't have a 3" exaust on it. He was saying that it ruins lower-end horsepower.

Why is this? I thought what you want to get away from is back pressure. Can somebody explain this?

Thanks!

Stallion

 

[JohnZ]

It's a long technical answer that deals with reversion waves and extraction effects in high-speed hot gases, but what it boils down to is that excessively-large diameter primary exhaust pipes (as in header primaries) hurts low-end torque. For example, the same SB car with 1-5/8" primaries will have better low-end torque and throttle response than exactly the same car with 1-3/4" or 1-7/8" primaries.

Most header "hype" is meaningless in a car with a reasonably-stock exhaust system, as the extraction effects which headers are designed to enhance are essentially non-existent without a straight-through low-restriction (VERY loud) exhaust system.

 

[Stallion]

Okay, thanks for the explanation, John!!

TR

 

[78SilvAnniv]

...unless an engine puts out 400+ hp, then it really shouldn't have a 3" exaust on it. He was saying that it ruins lower-end horsepower.

Why is this? I thought what you want to get away from is back pressure. Can somebody explain this?

You need some backpressure. If your pipes are too big you loose low end torque and throttle response. If the pipes have some restriction (backpressure) the engine performs in an optimum manner.

How about this...
You swim, right? You must swim through water that provides resistance, but it is the resistance of the water against your hands, feet and body that propels you through the water. Backpressure is what 'propels' an engine.
If the consistency of the water was considerably less, it would require more effort from you to maintain your previous performance level, same for exhaust.
Now by going to a mid-larger exhaust you get better flow and performance...as you would if the water you swim in had only a little less resistance, it would be enough that you could swim nearly effortlessly because your body power would outweigh the resistance power of the water.
Heidi

 

[Stallion]

I understand, thanks Heidi.

 

[vettepilot]

JohnZ was right on the money with his reply, but I enjoyed Heidi's explanation better. ;worship

vettepilot

 

[Edmond]

Heidi,

Didn't you tell a story once about having a car that had almost no back pressure and blew the engine?

 

[78SilvAnniv]

Heidi,
Didn't you tell a story once about having a car that had almost no back pressure and blew the engine?

Why do you remember all the bad stuff? Good memory!

Yes, a 74 Camaro 350 engine had headers put on and ran them straight for several months through late winter and early spring before getting exhaust put on. The engine eventually melted a piston.

At that early age of my driving experience I can't remember what the power difference was between exhaust/straight headers...but the noise sure was great, like race day at the track!
I'd like to know what killed that engine...was it the straight headers having no back pressure or was it thermal cycles cooling too quickly through the uncapped headers...or what?
Heidi

 

[vettepilot]

It was probably the fact that the carb may have not been rejetted to enrich the mixture. Adding headers has the effect of scavaging the exhaust gases more efficiently by producing a low pressure area after each exhaust pulse from each tube into the collector. The exact reason people spend the money to have them installed.
This allows more combustible air to be introduced into the cylinder because of the valve timing overlap between the time the exhaust valve is still open, and the intake valve begins to open. Todays modern engine computer systems can compensate somewhat by reading the exhaust gases as they exit down stream. But carbs cannot compensate "on the fly", they need to be rejetted to produce a richer mixture that will not produce those very lean burns that we try to avoid.
I think that is probably what happened in your example.

vettepilot