compression

[dirt fox]

just got new motor from engine builder. 4 valve relieve flat top pistons, 450 lift cam, 993 heads. it has 205 pounds of compression. our rules say 175 max. what are some things that i can do to make less compression?

[perfconn]

Tighten the valve lash if you don't want to pull the heads off.Put thicker head gaskets on it if you want to pull the heads.

[turner]

Your could put a cam in it with more duration, or say go from (example 108 to 113 etc.) Probally the easiest would be rhoades lifters. They will bleed down when they are checking, and wont hurt your performance. Just some ideas.

[DaveBauerSS6]

What size is the motor? And is the cam solid or hyd?? you will have to do a lot of work to pull down 30 poc.
On a 350 going from a 1003 gasket to a shim gasket will get 10-15 less.
Cam duration will drop more poc, but will change your torque curve. Its a lower lobe separation that will give you more overlap and less poc.
You can sink the valve seats to drop the poc.

[dirt fox]

sink the valve seats?











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[dirtman45]

Using a larger duration cam will knock it down quite a bit. Maybe your cam is too small. I have 2 valve relief flat tops, 60 cc chambers after milling, it calculates out to over 11:1 compression... and I have 145 lbs compression. Call a major cam company up and see what they think.

[merc123]

just got new motor from engine builder. 4 valve relieve flat top pistons, 450 lift cam, 993 heads. it has 205 pounds of compression. our rules say 175 max. what are some things that i can do to make less compression?

Not to be an rude...but ask your engine builder. He might even help you fix it for the cost of parts, especially if you told him the rules and he didn't build it to spec.

I told my engine builder which tracks I was going to run and he built the motor within those specs. If I'm protested and found to be illegal you can bet I'd be going to my builder and you'll see it posted everywhere if he doesn't make it right.

[stockcar5]

try another compression tester just to be sure before you get carried away changing stuff.

[DaveBauerSS6]

How do you lose compression by going to a thinner gasket?

Sorry, Ive got many excuses for screwing up the answer; but they are only excuses! Yes, thicker..

Cut in the valve seats puts the valve deeper in the head, making more room in the combustion chamber, lowers the CR. Its used to balance combustion chambers in your higher end motors.

[turner]

Are you sure going from 108 seperation to 113 sep wont decrease cylinder pressure Dave??

[perfconn]

If you don't change the duration then going from 108 lobe seperation to 113 will increase compression because you will have less overlap.

[DaveBauerSS6]

It's ok Dave. Just wanted to make sure.....sometimes the best way to learn new things is to do them by accident.

As far as this cylinder pressure goes....you can build 3 engines and have them "what you think" to to be identical and each one will read different on a screw in type pressure gauge. Especially if your using stock type components.

Agreed, I've ran 5 different gauges when the track wouldn't loan theirs out. The gauges can vary. Found out its just easier to build them a lot lower and not take the chance of a DQ.

Turner, the lobe separation angle is the time that the two valves over lap. When the exhaust is still open and the intake is opening, it draws less. This has everything to do with the duration of both also. Small overlap, big duration kills compression, but kills your bottom end power. Hope I described it right.

[DaveBauerSS6]

If you don't change the duration then going from 108 lobe seperation to 113 will increase compression because you will have less overlap.

Agreed, you type faster than me!

[turner]

I am pretty sure if you widen lobe seperation angle, from say 108 to 113 that is reduces cylinder pressure., and cranking compression.Are you sure Perf conn??? I thought the smaller the seperation angle, the higher the cylinder pressure, cranking compression and effective pressure.Am i lost on this?????

[perfconn]

Turner,
I am very sure.What you may be thinking is what the intake lobe is ground on.The lower the number the more torque you will have.Most small to medium size dirt track engines have the intake centerline ground on 106* and installed on 102*.This is a totally different aspect of cam profiles than the lobe seperation.

[turner]

I understand what you are saying Perf, but a 113 lobe seperation angle cam will have less cranking compression than a 108 lobe seperation angle cam. The smaller the lsa., the more cranking compression it will have, and more cylinder pressure.I guess i am confused.

[turner]

Perf. Isn't lobe seperation ground into the cam?(cant be changed). So the smaller lsa, the more the cylinder pressure, cranking compression and effective compression?Wider will be less cylinder pressure, cranking compression, and efective compression.Maybe i need to do more research. LOL. Just trying to figure out where i am going wrong.

[perfconn]

Turner,
You are right that the lobe seperation is a permanant thing and can't be changed without regrinding the cam and then you usually can't change it more than one degree.Think of the lobe seperation as two different lobes setting side by side. The further apart you move them(lobe seperation) apart the less overlap you will have.The overlap bleeds off compression and the more overlap you have the less comprssion you have,both on cranking and running.

[dirty white boy]

Turner,
You are right that the lobe seperation is a permanant thing and can't be changed without regrinding the cam and then you usually can't change it more than one degree.Think of the lobe seperation as two different lobes setting side by side. The further apart you move them(lobe seperation) apart the less overlap you will have.The overlap bleeds off compression and the more overlap you have the less comprssion you have,both on cranking and running.

the amount of over lap is effected by the size of the lobes too,..the bigger the cam,..the more lobe separation you'll need to achieve the same cylinder filling...or cylinder pressure,..lotta verabiles to a good cam profile!!

[perfconn]

Sure nuff.Thats the reason you can't expect a cheap cam from a dude that grinds half a dozen cams a year in his basement to give you the performance that the large companies that do lots of r&d do.