Huckster's '84 TA

just theoretically speaking, how would i go about building a engine with a higher top rpm without worrying about torque. like with motorcycle engines, do they combine a small bore with a longer stroke or a large bore with a shorter stroke? i know this is a stupid question, but just humor me. thanx.

Sitting Bull

Yeah, you're right, it is a short stroke engine like a 302. It has a 4 inch bore and a 3 inch stroke. Some would rev well over 7000 rpm and were (in fact, ARE) great road racing engines. For instance, the Trans Am racing series has a 5.0 liter engine size limit, which is where Chevy drew its 302 engine size from back in the 1960s for the original Z28.

Last year Lou Gigliotti ran a 302 in his Corvette and won the Long Beach Trans Am event. He gets about 700 hp out of them!

ede

it's hard to make it that simple, a small bore will have less weight,an important factor, but not the only one. back in my go karting days we took the 5hp b&s engines to 6 with a 2" bore, so if mass was the only consideration a 2" pistion would spin twice as fast as a 4". the fact is the single biggest factor is how effecient an air pump the engine is, the more you can suck in and the more you can push out the higher the rpms will be. if other limiting factors like valve springs work with the overall combination.

Jim85IROC

iTrader: (2)

True, but with a shorter stroke your piston speed is much slower. With a slower piston speed you have less inertia which makes it easier to reverse direction. When you're running ultra high rpm, then I'd say that piston weight due to its size will become an issue, which is why IRL/CART/F1 all have such small displacement motors. But, like you mentioned, airflow is key. Those motors have substantial airflow. With a typical 2 valve head, I'd say your best bet is going to be to stick with a big bore & short stroke.

RB83L69

Past a certain point, it doesn't matter how large the bore-to-stroke ratio is. If you start out with a stroke that's twice as large as the bore, you'll have a pig; as you shorten the stroke you'll lose torque (something the youngsters who all think a 302 Chevy must be better than a 350 seem to forget, never having had a 302) but gain some RPM capability. The first incremental change you make will have the largest effect. Each succeeding increment will have less and less effect. In engines of the sizes and RPM ranges we commonly deal with, once you get past a ratio of about 1.15 bore to stroke, further changes make relatively little difference. The same goes for our favorite competitor: that's why the really fast ones use 351s... same bore as their (or the Chevy for that matter) 302, but a 3½" stroke, like our 350. Big increase in CID with very little if any penalty in RPM capability.

The things that give an engine high RPM capabiity are the cam, optimizing valve timing for max volumetric efficiency at heigher RPMs, and heads (FLOW!!! FLOW!!!), and other induction system parts for the same reason as heads.

Huckster's '84 TA

thanx everyone