dynamic compression street cam ( cont...)

[als427]

Hi all , having debated changing cam or not I will first change intake from single ( very low rise) plane to dual plane for better street use / manners.
wasn't sure if cams too big but after car was sitting 3 years pulled the plugs , squirt of oil , hand turned engine then used starter to push some oil around.
That being done though Id do some compression testing. Hit 155-170 psi which I think tells me its a pretty good street cam?
ie not too big and losing too much compression through overlap bleed off!
Local engine guys ( Im in the uk ) said anything over 140psi good for a street car.
sound about right?
runing 93+ octane
fyi 230/235@50 114lsa with 4 degrees built in advance
thanks

[PackardV8]

said anything over 140psi good for a street car

Good for what? In what application? Economy or max power? On what fuel? 200 is about as high as iron heads will tolerate, but again, depends upon the quench/squish and chamber design.

jack vines

[F-BIRD'88]

Cranking compression is not bled off by valve overlap.

[F-BIRD'88]

Evaluate the cam's suitability based on how the car will be used. "too big". for ????

[als427]

iron heads , 93+ octane available here. street use. I was led to believe as an indicator of street use anything under 140 psi not so good , over 150 in the right area.Surely with a bigger cam / overlap there would be more bleed off and so a lower dynamic compression? Great for higher rpms but not so for lower rpms/ street

[frnkeore]

It's not the overlap that bleeds off compression, it's the IVC position. The later it closes, the less air is trapped.

[PackardV8]

As previously stated, overlap has nothing to do with the theoretical dynamic compression or the actual cranking compression; the IVC, the intake closing point is what determines that.

To get a better understanding of this, go to a cranking compression calculator. The same 10:1 static compression can be made to generate anything from 140 to 200 PSI by varying the IVC.

[als427]

ok fair enough , does a 155-170 gove an indication of engine health either way and suitability for the street? or is it all %^$£

[rebelrouser]

ok fair enough , does a 155-170 gove an indication of engine health either way and suitability for the street? or is it all %^$£

I think that you are wanting to use cranking compression, as the only indicator of if an engine will detonate on the street, and it is not that simple. I use calculated cranking compression for street builds, and the higher the cranking compression the more detonation is possible, but combustion chamber design and quench clearance piston to the head, the material of the head, etc. etc. all play a role, plus the ignition timing curve is one of the biggest. Detonation is more likely under part throttle high load conditions than at WOT. Buy yourself a good engine program and play with it and learn how an engine operates. And I like performance trends, the program predicts spark knock and calculates advance curves for different octane fuels, I have found it pretty accurate. Accurate enough that I can input the predicted timing curve into spark maps on electronic fuel injection and it is pretty much spot on.

[F-BIRD'88]

According to your engine health measurment yardstick you could advance the cam you have now and it will improve
the "health".. Try it.

[Stan Weiss]

ok fair enough , does a 155-170 gove an indication of engine health either way and suitability for the street? or is it all %^$£

That tells you nothing by itself about engine health. What is your CR and IVC (seat-to-seat @ the valve).

Stan

[als427]

Hi compression ratio 9.5-9.8:1 , IVC is 72.5 if I follow:
The intake valve closing point ABDC is calculated in the following manner. Divide the intake duration by 2, add that to the lobe separation and subtract out any ground-in advance. Lastly, subtract 180 degrees ( 285 adv duration , 114 lsa , 4 built in advance)