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**MikeB** · 10-20-2018, 11:16 PM

I'm not convinced that a 2" or 2-1/4" down pipe bolted to an exhaust manifold makes more torque at any RPM than a 2.5" pipe. Even the best cast iron manifolds have so much restriction (therefore velocity), that a large pipe bolted to the manifold flange is nothing but a relief!

Now if we are talking about header primary tube size, that's a very different story because those pipes are bolted directly to the exhaust ports. But even with headers, once you get past the primary tubes, the bigger the better.

Someone above knows his stuff when he mentioned exhaust back-pressure being critical in a high performance two-stoke engine. It's the only way to keep the intake charge from getting sucked out the exhaust port at low-mid RPMs. Modern two-stokes use slide valves that seal off the top 1/3 or so of the exhaust port until RPMs build.

10-20-2018, 11:16 PM	#12
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,430	Re: 2.5" vs. 2" exhaust I'm not convinced that a 2" or 2-1/4" down pipe bolted to an exhaust manifold makes more torque at any RPM than a 2.5" pipe. Even the best cast iron manifolds have so much restriction (therefore velocity), that a large pipe bolted to the manifold flange is nothing but a relief! Now if we are talking about header primary tube size, that's a very different story because those pipes are bolted directly to the exhaust ports. But even with headers, once you get past the primary tubes, the bigger the better. Someone above knows his stuff when he mentioned exhaust back-pressure being critical in a high performance two-stoke engine. It's the only way to keep the intake charge from getting sucked out the exhaust port at low-mid RPMs. Modern two-stokes use slide valves that seal off the top 1/3 or so of the exhaust port until RPMs build. __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!