cylinder pressures

ed hess

ed hess

race or get outta the way
I need to know how much cylinder pressures others have with 412 cam or any other cam other than stock.

Also need to know if motor was bored out.

and If you are running stock head or vortecs.

Also if you cut the deck or the heads:

cam:

Cylinder Diameter:

Heads:

deck or heads cut:


MY cylinder Pressures are running 180. And i am considering a diferent cam to possibly lower them.
 
S

Sy769

Donating Member
I had to read that a few times to make sure I wasn't seeing things. :eek:

Trying to reduce cylinder pressure with a camshaft change on a turbocharged engine just plain doesn't make sense.

Maybe I am missing something. A little more info please?
 
jwaller

jwaller

Evil Genius/SyTy Guru
ya I don't know how you can change the pressure by the timing of opening and closing valves. the pressure is created when both valves are closed so valve timing(ie cam) should have nothing to do with it.

cyl press to me is determined by deck height, CC of the piston, cc of the chamber, and head gasket thickness.

ed, bout the only easy way I see you changin the static cyl press is by using a thicker head gasket. and with you running copper you can get them really thick 0.120+. I would have to do the numbers but you should be able to change your CR by maybe half a point or so.
 
ed hess

ed hess

race or get outta the way
ok well how about letting me know how much cylinder pressures you all have with your 412 cams and vortec heads.

I wonder if i really told you why i REALLY want to know if i would get an answer. Dont think anyone understood that depending on when the valves close is goig to have an effect on cylinder pressure.

Just imagine motor one is 9-1 Cr and has cylinder pressures of 180.

no motor 2 is 11-1 cr and has cylinder pressures of 140.

Motor 1 runs high 11's with turbo.

motor 2 runs low 10's and high 9's.

now assume both use same turbo, same heads, but different cams.

I am not going to get to much deeper into this, but for right now i am just curiuos about what this most common combo, of vortec heads with 412 cam has for cylinder pressures.

ed
 
S

Sy769

Donating Member
ed hess said:
ok well how about letting me know how much cylinder pressures you all have with your 412 cams and vortec heads.ed
Click to expand...
Pressure generated with a compression tester will vary with the condition of the engine


ed hess said:
I wonder if i really told you why i REALLY want to know if i would get an answer. Dont think anyone understood that depending on when the valves close is goig to have an effect on cylinder pressure.ed
Click to expand...
I think I understand dynamic cylinder pressure pretty well. On a high compression normally aspirated (NA) it is common practice to have a long duration cam with lots of overlap that bleeds off cylinder pressure.Too reduce cylinder pressure you need lots of overlap.
Turbocharged engines however are a different animal.
Lots of overlap under boost can be detrimental. If you want we can discuss why.

ed hess said:
I am not going to get to much deeper into this, but for right now i am just curiuos about what this most common combo, of vortec heads with 412 cam has for cylinder pressures.ed
Click to expand...

Why not get deeper? You are the one asking for someone to do a compression test that has Vortecs.

ed[/quote]
 
ed hess

ed hess

race or get outta the way
actually wasnt asking for everyone to run out, pull plugs and check their motor. I figured most everyone would allready know what their cylinder pressures were.

ed
 
ed hess

ed hess

race or get outta the way
nightdiver said:
135 to 165psi with the 422 but the engine was too damn old with .0093 piston to wall clearance :oops:

Here is something for the cam tech:
http://www.iskycams.com/techinfo_index.html
Click to expand...

copying this from the above link says exaclty what i was leading towards.

that being that my compression seems to be higher than others using a 412 cam.

With a given static compression ratio, you will always have a higher reading on your compression tester gauge with a stock or low duration cam, because you will be closing the intake valve earlier on the compression stroke. The resultant longer effective compression stroke always delivers a higher gauge reading. Now switch to a longer duration cam. Your intake valve will close later, lowering your gauge reading because of the shorter effective compression stroke. Some people feel this is impossible, insisting that if it were true, why will you go faster with the bigger cam? The answer is, the bigger cam will have higher compression effect in the cylinder at higher engine speeds (where all that extra valve timing can do you some good), however at lower speeds and especially at starter-cranking speeds, the effect will be lower.

ed
 
BigAl...

BigAl...

Member
guys Ed is correct the cam specs do affect the cylinder pressure! what waller and the others are talking about is compression ratio two totally different things. Like sy769 said overlap is what really controls cyl pressure.

BigAl...
 
ed hess

ed hess

race or get outta the way
thank you Al.

What i am i beleiving and i could be sure is if you guys with the same cam and heads can tell me your cylinder pressures i will be able to confirm that something in my combination gives me a higher compression ratio. Since my pressure shows 180 and i have a feeling the rest of you wll be telling me pressures of 140-150.

ed
 
BigAl...

BigAl...

Member
Ed something to consider is, in order to get the same readings you'd have to do the test with the exact same conditions. same engine temp, same engine wear, same oil vescosity, exact same number of engine revolutions, same pressure gauge and the list goes on. you change some of these parameters and you could change the pressure 10psi on the same engine. are you getting 180psi across the board? cams aren't that expensive, I would order up a custom grind and do the swap. if it doesn't work the most it will cost you is the cam and a few hours worth of work but if it turns out to be the next greatest thing since sliced bread it would be worth it.

BigAl...
 
bigtime

bigtime

Sy-Ty Builder
Intake valve closing has a major effect on cranking compression/dynamic
compression. overlap will bleed off cyl. pressure while running. Dynamic
compression is very important for all engines. All compression
tests are affected by altitude. If you do the math for dynamic compression,
Eds cranking compression is correct (within 10lbs) We do not know his excact
compression ratio as he has not poured it. Using dynamic compression
formulas will help you figure cranking cyl. pressure for different camshafts before
purchasing. Helps me figure out cam profiles before I have cams ground.
There is allot more to a cam beside LSA,lift and duration.
 
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