I am trying to find out when the intake valve should be closing on a Mopar purple shaft 292 duration, .508 lift cam advanced 4 deg. Also what would be the closing if installed as recommended at 108 degrees.

The dynamic compression ratio calculators want this info and I don't have a cam card for this cam.

I tried a search on this site but it will only allow 1 search before it tells you to wait a few minutes. (frustrating). I did a web search but came up empty.

The sales info (summit, jegs) only list lift, duration, and centerline.

Thanks

FastEddie

292/2=146-108=38+180=218-292=74


at 108 closing is 74 abdc

at 104 its 70 abdc

Perfect! Thank You very much!

Something seems wrong. Using the Wallace calulator

http://www.wallaceracing.com/dynamic-cr.php

is says my dynamic ratio is 8.74:1 and cranking compression is 178.21psi.

My cranking psi is 200 to 205.


Static compression is calculated at 11.5 using an assumption that shaving .080 off of J heads brings them down to 60 cc from 72 cc. Is this where my calculations are wrong? Are my chambers smaller?

Thanks

if you actually cut them .080 it would be about 18 cc's

stock small mopars are about .005/cc,, so .080 is 18 cc's

The Wallace calculator uses duration numbers at .050, you are probably off about 8 degrees m/l on your intake closing point using .050 numbers.

Thanks everyone. That makes it come out much closer. Changing the static compression to 12.5:1 from 11.5, and using intake closing point of 64 deg instead of 72 makes the cranking compression come out to 207.

It also helps explain why this motor wants so little timing. We initally set it at 34 deg but during tuning we raised it to 36 and slowed down. Bringing it back to 34 we picked back up and kept decreasing timing and picking up until we reached 28 deg. At 26 it slowed again so it likes 28 deg. Hard to believe but the numbers don't lie.

We are also rethinking the mixing of pump gas to race gas. We were mixing 3 gal 91 octane with 2 gal 110 to get an acceptable octane for 11.5 compression we thought we had. It now seems we have 12.5 with flat top pistons. We will probably go with straight 110 because the cost difference is only about $6.00 per 5 gallon tank.


The next time this motor is apart I will cc the heads so I will know exactly what they are.

Thanks again everyone.

Any other comments are appreciated.