Exhaust diameter effect on power band

Tell me, gentlemen, what effect does the diameter of the exhaust have on the RPM range of the powerband? Does increasing the diameter move the power band up in the RPM range, extend the power band in both directions, extend only the top or the bottom end, or basically do nothing to the power band?

Also, what diameter exhaust is required on a T1 of, say, 2110cc or 2275cc displacement?

Thoughts?

Thanks,

Paul

The key issue with exhaust systems is exhaust gas velocity and restrictions in the system. It's best to keep gas velocity high (smaller primary tubes) and the primary tube length (before it joins with another tube) is also a factor. The diameter and length that leads to the best gas scavenging at lower RPM will be best for a street car, but an all-out race engine that doesn't run much below 4,000 RPM will require larger/longer tubes to move the scavenging effect up to where peak power occurs.

Too small a system can cause restrictions and overheating.

A "low output", low RPM 2,110cc VW engine (say 120 BHP @ 5,500 RPM) would probably be fine with a 1 1/2" primary system and 2 1/2" muffler. A mid-range 2,110 (say 145 BHP @ 6,000 RPM) would probably require a 1 5/8" primary system with the same muffler. An all-out racing 2,110 (say 195 BHP @ 8,000 RPM) would need a 1 3/4" primary system and 3" muffler.

There is a book, "The Scientific Design of Automotive Exhaust Systems", that discusses all of the issues prettyr thoroughly. If you have the time and money trial and error will tell you what works best on your engine, or someone with a similar engine may have a system that works well and would work well for you.

exhaust diameter affects cylinder head flow...If it can't get out, more cannot get in.

Restrictive exhaust backs heat up in the heads, and leads to damaged valves and seats, and hard to tune carbs and poor economy.

Exhaust makes a bigger difference on my dyno than any other single change, exhaust is everything and must be designed around the engines flow ratings.

The main thing is that you MUST have the exhaust system no smaller than the size that your cylinder heads have been opened to...if it is, the resistance in the port will be way high, and the results are disasterous.

(Message Edited 3/20/2003 11:40:33 AM)

I understand the impact of an exhaust that is too small or restrictive but can the exhaust be "too large" and/or free-flowing?

Bruce: Yes. As I mentioned, exhaust gas velocity is important; if you make the system too large exhaust gas velocity drops.

Most exhausts are designed to have a negative pressure effect in the combustion chamber as the exhaust valve is closing and the intake valve is opening. With a proper exhaust system and the right valve timing for the engine build this increases the pressure differential between atmospheric and the cylinder and helps the initial flow of input air/fuel mixture to overcome inertia and begin flowing into the cylinder (some may even flow out the exhaust valve before it is fully closed).

Correct me if I am wrong here, but too large diameter of an exhaust system will cripple your mid-range, which for a street driven car is your day to day sweet spot, i.e. bigger isn't always better. As Jake says, all the components should match each other.