#Camshaft compression calculator driver#
Car weight is 3900 pounds with driver and full tank, 2.93 gears, TH400 trans, 25.5 inch diameter tires.Īlso, does anyone know of a reference for the maximum dynamic compression feasible without detonation on 91 octane fuel? My reference guide gives a figure of 160-170 psi for 93 octane, but 91 octane is as high as it gets where I live. I am after smooth idle more so than top-end power. My planning factor is 5 additional pounds of dynamic compression for every 4 degrees advance of intake closing. My intent is to determine how much sooner (in degrees) I can have an intake valve closing event, and thus decide what camshaft will give me the highest compression without detonating (minus a safety factor). 030 over 400 with #64 heads - estimate 9.2:1 static CR, with cranking compression an average of 138 psi using my neighbor's gauge). Is it just a matter of using a cylender compression guage while the engine is running? I understand that dynamic compression is tied to the intake closing event, and thus my measured dynamic compression is only valid for the cam currently in the car ('041 grind in a. The low compression engine didn't like the extra overlap.Does anyone know of a reliable way to measure dynamic compression? I can measure cranking compression at 30 RPM, but this is probably lower than dynamic (about 2000 RPM) compression. Swapped cams and regained bottom end power. The Racer Brown had a little more lift and duration than the Isky. I was running a Racer Brown 42R cam and the last time I ran the 8.5:1 engine it had an Isky 505T. I put it in the car and it ran okay, but didn't have any bottom end power. At the time I was running a 377 engine and didn't have a fresh 350 short block. I was dabbling in USAC and they limited CID to 355. The engine actually ran pretty well and we ran it part of one season. The guy I got it from ordered the wrong part number and got an 8.5:1 short block rather than an 11:1 setup. Second cam story I had an LTI short block engine for the sprint car. It didn't have as much low end power as it had previously, but it ran real good once you wound it up. We installed one of the Crower cams and it never knocked again on premium. It knocked badly on the premium fuel of the time. When I worked for the Dodge dealership, we traded for a 440 Ramcharger that the original owner had put 12.5:1 pistons in.
They were using overlap to bleed off cylinder pressure. Markīack when we couldn't get good premium fuel, I believe it was Crower who made a cam for large displacement, high compression engines that made them basically a lower compression engine. I am sorry to make this so long but I hope this helps. At low RPM the flow isn't sufficient to cause this to happen and poor filling of the cylinder will cause running compression to be lower so mechanical compression (compression ratio) needs to be higher or as someone commented earlier the engine will be a real dog at low RPM. A big overlap cam does the same thing (holds both valves open for a duration of time) but at high RPM this overlap will cause the intake to fill more effectively, as the exhaust is flowing out at a high velocity it causes a low pressure which will draw the intake charge into the cylinder. The higher pressure (exhaust) would flow towards low pressure (intake) accomplishing the same thing as the EGR valve did thus lowering cylinder pressure during combustion. Once variable cam timing came along the EGR valve went away because you could accomplish the same thing with valve overlap, or opening the exhaust and intake valve at the same time. To overcome this too high pressure they used to use exhaust gas recirculation or EGR for short which is just displacing some of the oxygen that goes into the cylinder with exhaust gas from the EGR valve into the intake manifold. Too much cylinder pressure causes NOx, a binding of nitrogen and oxygen molecules which in turn causes brown smog so this is a no-no. On new dual overhead cam engines they have variable cam timing on both cams on most engines. Okay, I know this is OT but it might help explain.
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