High comp. low RPM motor?

[BADASYJ]

Hey guys, I am curious as to why a high comp. low RPM motor has not been talked about. It seems everyone who builds a high comp motor also wants to get into high RPMs. Is there a reason why a high comp low rpm motor would not be feasible. Diesel motors are high comp and make a ton of torque with low RPMs.

[1bolt]

generally speaking higher compression will lead to more potential for detonation, that is the fuel exploding from compression, instead of catching on fire and burning in a controlled manner from spark...

Incidentally what gas engine guys call detonation, diesel guys call normal combustion... As you probably know Diesel ignites from compression.

The simple answer to your question is that with 93 octane and the practical aspect of variable quality of the fuel from station to station, somewhere around 10.5:1 compression is as high as you'd want to go in a Street 4.0/stroker.

Obviously this is not as high as some more modern engines (LS and Ford Mod for example) can go on the same 93 octane... Those engines and custom built performance engines you might see in magazines like Popular Hot Rodding; have tighter quench specs, and better combustion chambers. You'll see some of them approaching mid 11's static compression intended for high octane pump gas, but usually with a VERY lumpy cam to bleed off dynamic compression (which is really the important compression measurement, and could be called or thought of as "actual compression".

Needless to say a lumpy cam (high overlap) which makes more horse power, is not good for a low torque build. In fact that overlap period is bleeding off the exact thing that makes low speed torque... cylinder compression and ultimately BMEP or brake mean effective pressure on the tops of the pistons.

The trade off in a 4.0 stroker of running high octane for another point of compression (from 9.5 to 10.5) is usually not considered worth it versus the extra machine work needed to get the quench tighter. 9.5:1 CR is still higher performance than stock, and even that CR tends to Ping in strokers without mid grade or fuel and spark management. It is where most strokers end up so yeah most of us are running higher compression.

I would love to see a low torque build with 11 to 1 compression and a long stroke... you'd need very good quench, a good aftermarket fuel management system, run a bit on the rich side at all times, optimal combustion chamber and piston, and a cam that doesn't bleed off too much dynamic compression.. you might have to re-cam the sucker a couple times before you reached a good compromise between building as much pressure as possible and not pinging the engine into submission.

Of course with E85 instead of Gas you can throw a lot of that out the window and go even higher in compression but that is another thread and a subject that has been covered a couple times.

[BADASYJ]

Well I'm thinking somewhere between 12 and 14.1 comp with a high torque cam that will not bleed off cylinder pressure.I would also be running race fuel as this would be a mud bogging machine.
What I mean by low RPM is no higher than 5250rpms. It would seem to me that you could build the same power at lower RPMs with high comp. as you could with a motor that spins up to 6000 with a lower comp ratio. Maybe someone could run it on one of the those desktop dyno deals and see what numbers are possible with this setup.