Isuzu G-series Twin Cam

Just a quick question about what size exhausts people are running.

I was going to make up at 2.5" mandrel exhaust for mine, but the extractor collector is 2.25" - tossing up wether its worth modifying for a bigger one.

Ps - it will be revved hard. Real hard.

depends if your going to run a cat for engineering wether it high flow or normal and if you run straight through mufflers.

Its not being engineered and it wont be running a cat.

2.25 mandrel bent or 2.5 press bent will be fine just run 1 straight through muffler.

GeminiCoupe wrote:Just a quick question about what size exhausts people are running.

I was going to make up at 2.5" mandrel exhaust for mine, but the extractor collector is 2.25" - tossing up wether its worth modifying for a bigger one.

Ps - it will be revved hard. Real hard.

Remember to make the secondaries as long as possible. Isuzu OHV and DOHC make heaps more torque this way, Make the primaris 1\4 inch bigger than the exhaust ports then the collectors are bought in to balance the flow. I had this done on advice from an exhaust expert with 40 years in the trade. the pwer difference is awesome.

I'd stick to 2.25". You may gain a slight advantage at maximum revs with the bigger pipe but you'll lose a little torque off the lower rev pickup if the pipe is too big, you'll also have an annoying bark to the exhaust that attracts the wrong attention. Be honest, how often do you runs at 6000+ rpm? The length of the collector pipes should be calculated not just a stab in the dark guess.

some shonky advertiser wrote:Race headers, the best choice in exhaust because we just take stab in the dark guesses and make them as long as we can...

Um, big pipes alone do not induce better torque. You do need some containment of the exhaust for a street car. It does help give drive ability at bottom end engine speeds.

If you have ever run a car with an exhaust flange gasket blown you'll recall how it felt soft at low revs but (apart from the noise) once you stuck the foot in it felt responsive again. That's because you need that restrictive exhaust to assist in drawing the gas through at low engine rev ranges.

Poida, it will spend alot of time above 6000rpm - i drive my cars hard [within the confines of the law ofcourse ] and this car will be drag raced/motorkhana'd/circuited etc.

The extractors on it are a home-made looking job, looks very...based off SOHC specs for want of a better description.

I had a 2.5" mandrel on my SOHC with 4-1 pacemakers and beleive it or not, it lost no bottom end [this is including a lightened flywheel too] and gained heaps up top. And it was quieter then standard too!

For G200W - around 26" long secondary (from st'd manifold flange; 2:1 section) should provide above 350Nm from 2,000RPM.
The G200W will also start gaining again with about 4" exhaust (after dipping at 3"), but this is a bit impractical (splits & ovals are too tricky). And I think it's like going from DCOE45 to 48 - yeah, more power etc, but not really worth it.