DirtRacer9s Posted October 30, 2012 Report Share Posted October 30, 2012 Can someone explain to me how cylinder compression can be figured out. Sport mod rules say 175 pounds per cylinder with one plug out and five revolutions. How can I figure out what's the best possible engine I can build and have that compression per cylinder? Not sure if I asked that so people understand. Link to comment Share on other sites More sharing options...
jwmbishop Posted October 30, 2012 Report Share Posted October 30, 2012 You cant really "back into" that with math. Cranking PSI is effected by mechanical compression, cam, throttle size, cranking speed etc... A 10:1 with fairly hefty cam can make 175-185 in many cases - as you go down on comp ratio you can go up on cam aggression and vice versa. Link to comment Share on other sites More sharing options...
DirtRacer9s Posted October 30, 2012 Author Report Share Posted October 30, 2012 What's a hefty cam to you? Link to comment Share on other sites More sharing options...
jwmbishop Posted October 30, 2012 Report Share Posted October 30, 2012 What's a hefty cam to you? Depends on whats under it! For a large port 2.02 over a 355 @ 9.5:1 - a roller with 300I\308E duration and .565/.575 lift, 108 sep is pretty hefty. Link to comment Share on other sites More sharing options...
poorboy Posted October 31, 2012 Report Share Posted October 31, 2012 i give you an example of my engine in my sport mod. I have 2 valve releif pistons about 10 thousanths deck with last years legal hydraulic spreen cam and EQ heasds not milled , Felpro .041 thick gasket, it had 140 lbs compression with throttle open. I replaced the cam with a spreen solid cam and milled the heads 20 thousanths and my compression dropped to 125. It's kind of a guessing game. I think we cut another 30 off the heads and its back to 145. I'm sure Greg Spreen can help you get close. Link to comment Share on other sites More sharing options...
jwmbishop Posted October 31, 2012 Report Share Posted October 31, 2012 i give you an example of my engine in my sport mod. I have 2 valve releif pistons about 10 thousanths deck with last years legal hydraulic spreen cam and EQ heasds not milled , Felpro .041 thick gasket, it had 140 lbs compression with throttle open. I replaced the cam with a spreen solid cam and milled the heads 20 thousanths and my compression dropped to 125. It's kind of a guessing game. I think we cut another 30 off the heads and its back to 145. I'm sure Greg Spreen can help you get close. did you degree the replacement cam? Same lifter preload? both drastically effect cranking psi. Also when it was milled were the valves deepened to come back to chamber volume? Remember that when you crank - its still an air pump so although your single rotation PSI can go down - the five rotation value could go up as it overcomes the PSI from the previous rotation easier... A moving target either way. Link to comment Share on other sites More sharing options...
poorboy Posted October 31, 2012 Report Share Posted October 31, 2012 Yes everything was done right I was just trying to give him an example of how things change with different cams,By the way the hydraulic cam I was referring to was a "vacuum rule"cam and the solid cam was a non vacuum rule cam, Link to comment Share on other sites More sharing options...
jwmbishop Posted November 1, 2012 Report Share Posted November 1, 2012 Yes everything was done right I was just trying to give him an example of how things change with different cams,By the way the hydraulic cam I was referring to was a "vacuum rule"cam and the solid cam was a non vacuum rule cam, Gotcha - wasn't saying anything was done wrong - just brainstorming all things that effect the outcome! I prefer vacc rules. A lot easier to project the outcome. Link to comment Share on other sites More sharing options...
4cylfordz Posted November 1, 2012 Report Share Posted November 1, 2012 I agree completely compression rules are among my most disliked rules, right up there with no-capscrew rods. it is pretty much impossible to tell what the cranking compression of a motor will be. especially since not only is it effected by bore size, head cc, gasket thickness , camshaft overlap and even valve lash but it will also change throughout an engines life cycle. they start of low, after initial break in of the rings they gain a few psi and as the rings fade they'll lose psi. figuring out what an engine will be is a shot in the dark if you don't have a starting point. though what I can tell you should get you close for 175psi is around .025 off the block and .025 off a 76cc head with a 2 vr piston 3.48 stroke X 4.040 bore and a .039 gasket running a hydrolic cam around .477/.480 268/278 dur on a 110 lobe sep. should get you +- 10psi from your goal as long as the cam is set straight up and it is around 90 degrees outside at less than 100 ft above sea level. keep in mind .060 will be greater a .030 will be less psi. about .055-.060 is about all that can be taken off total (decking the block and flat surfacing the heads) before the intake doesn't fit without modifications to the bolt holes and possible gasket issues. Link to comment Share on other sites More sharing options...
DirtRacer9s Posted November 2, 2012 Author Report Share Posted November 2, 2012 Thanks for the help. Link to comment Share on other sites More sharing options...
drdan Posted November 2, 2012 Report Share Posted November 2, 2012 As mentioned earlier, there's lot's of things that can affect cranking compression numbers. However, with a little practice you can get fairly close to a predictable number. If you've settled in on a given camshaft profile, then during engine mock up, one can compute what the compression value is at the point of intake valve closure. What you're really trying to do here is establish the relative trapped area ( piston position ) when the intake valve closes. It's sort of the opposite of what you might think. In other words the higher the computed compression # is at intake closure, the lower the cranking compression # is going to be as the piston is higher in the bore, thus trapping less area, making less cranking compression. This is done with a lot of cam profiles that leave the intake valve open a little longer to assist in filling the cylinder a bit more for higher engine speed operation. Reaching back from a foggy memory, it seems that if we computed a compression ratio of about 7.0 to 1 to a 7.2 to 1 ratio ( #'s may not be accurate, but is a good example ) when the intake valve closed we came real close to 170 lbs cranking with throttle plates open. If we had a compession ratio of say 6.0 to 1 at intake valve closure, ( piston lower in the bore, trapping more area ) we'd bust out every time. This method helps in taking into consideration bore size, head gskt. thickness, & in theory how much could be taken off the cylinder head. I intentionally wouldn't calculate crevice volume above the top ring into the math just to leave a small safety fudge factor for the cranking compression #, and yes if you're really on the edge then cam phasing & lash adjustments can help to a minor degree, barring any inaccuracies from the compression gauge that the tech guy is using. ( That would never happen would it? ) Link to comment Share on other sites More sharing options...
jwmbishop Posted November 3, 2012 Report Share Posted November 3, 2012 So if I am understanding you Dan - rather than using total compression ratio using total swept volume - you use a "sub-calc" ratio using the swept volume of piston stroke remaining after intake valve closure? I think I can see how this would get the target into a smaller margin. And to expound - wouldn't this also be a good indicator of the cam's VE? Link to comment Share on other sites More sharing options...
drdan Posted November 4, 2012 Report Share Posted November 4, 2012 Yes sir. That's the idea ! Whew, V.E. is quite a subject unto itself. Respectfully, -- Dan Link to comment Share on other sites More sharing options...
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