Optimum exhaust sizes.
Discussion
Is there a general rule of thumb about the ideal exhaust bore for particular engine power ?
The man that built my Rover V8 (fitted in a Griffith but not quite a TVR engine now) says the standard rear pipes where they leave the silencer are too small for the power I want. An exhaust man says the section from the Pre cats to the main cat is too big and possibly the headers are too big and the way they all meet with very unequal lengths is obviously less than ideal.
Does anybody on here have a definitive answer about the optimimum header and main pipe bore ?
The man that built my Rover V8 (fitted in a Griffith but not quite a TVR engine now) says the standard rear pipes where they leave the silencer are too small for the power I want. An exhaust man says the section from the Pre cats to the main cat is too big and possibly the headers are too big and the way they all meet with very unequal lengths is obviously less than ideal.
Does anybody on here have a definitive answer about the optimimum header and main pipe bore ?
Pumaracing said:
The answers were in articles on my website but the person who was hosting that for me has apparently stopped doing so. The wayback engine has copies of the site stored in its archives.
Unfortunately, not ALL articles from your site are archived. "Diagnosing Engine Problems" is not available, nor are your articles on cylinder head modifications. There are plenty of free places to put up websites these days, google has a nice one: https://sites.google.com/?pli=1
Exhaust design is a seemingly simple topic that is anything but! So many variables. How much power are you making? Also, how sensitive a particular engine is to exhaust restriction will be governed, to a large extent, by the cam spec (overlap, reversion, etc). If you're running a relatively low output motor with a mild-ish cam, it's probably not worth worrying too much about the exhaust. If, however, you're running a higher output engine with a fairly radical cam with a lot of overlap, the exhaust can make or break how the engine performs in the chassis. The original side pipes on my Cobra replica were costing me ~100bhp... I know this as I had the engine dyno'd, then had it put on a rolling road once it was in the chassis with the ehxaust hooked-up. The original spec was 1.75" headers (which is fine), but side pipes with a 2.25" bore and baffle plates to reduce noise; this was causing the exhaust gas to follow a snaking path down each side pipe. The original pipes were also just using a cone affair to collect the exhaust from the headers into the side pipes. I had new pipes fabbed-up to my spec using proper merge collectors and a 3" straight-through side pipe bore. Haven't installed the system yet (still tweaking to fit), but I expect it to liberate a lot more power. My engine is a 383 SBC which dyno'd a hair under 500bhp on an engine dyno with a single plane intake and quite a radical street solid cam.
One thing you'll probably hear a lot is the exhaust is a system and it's not sufficient to simply concentrate on bore size without considering all the other aspects... equal vs unequal length headers, length of primaries, collector configuration, system length, etc, etc.
One thing you'll probably hear a lot is the exhaust is a system and it's not sufficient to simply concentrate on bore size without considering all the other aspects... equal vs unequal length headers, length of primaries, collector configuration, system length, etc, etc.
Edited by ian_uk1975 on Tuesday 21st August 23:22
Try maxracesoftware.com. Purchase Larry Meaux's PipeMax software. Remembering the old software addage...... 'garbage in/garbage out'. Get the required inputs correct for your engine/type, and his calculated dimensional outputs for the exhaust system will give you best power across your required rpm band. -84Dave-
Very, very rough rule of thumb here, as everyone has said, it's a system, and it depends on the entire design, details, engine setup, cams, etc, but....
Manifold primaries:
1.50" ~30-35bhp per cylinder
1.75" ~35-45bhp per cylinder
2.00" ~45-60bhp per cylinder
For long tube lengths assume the low end, for short lengths the higher end.
For an N/A engine, main tubing size for a well made system is roughly:
1.50" ~100bhp per tube
1.75" ~145bhp per tube
2.00" ~185bhp per tube
2.25" ~220bhp per tube
2.50" ~250bhp per tube
But, if you want to refine it down to your exact setup, a lot more legwork will be needed!
Manifold primaries:
1.50" ~30-35bhp per cylinder
1.75" ~35-45bhp per cylinder
2.00" ~45-60bhp per cylinder
For long tube lengths assume the low end, for short lengths the higher end.
For an N/A engine, main tubing size for a well made system is roughly:
1.50" ~100bhp per tube
1.75" ~145bhp per tube
2.00" ~185bhp per tube
2.25" ~220bhp per tube
2.50" ~250bhp per tube
But, if you want to refine it down to your exact setup, a lot more legwork will be needed!
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