compression test with cams - SRT Forums

nutz · 08-11-2009, 06:22 PM

will a cam make the compression test lower, i have 18's (and a 5 layer hg)and my compression is a little over 120 (all 4 same) and i know stock srt's have 170-150 average

can anyone help me

nutz · 08-11-2009, 07:38 PM

wow its slow today

hercfe · 08-11-2009, 07:51 PM

i've seen cams affect compression a lot, and i've seen cams not affect compressino much at all. i'd say with your 18s(im assuming with a complete valvetrain upgrade) that you would see a pretty decent change in compression. thats why i ALWAYS recommend doing a compression check BEFORE a cam swap. otherwise, there is really no reference.

oh, and the thick head gasket will also affect compression.

nutz · 08-11-2009, 08:13 PM

my car is sooooo solw at 12 psi

nutz · 08-11-2009, 08:23 PM

i checked my cam timing and its good, i opened the oil cap for blow by and no pressure at all it kinda sucked then pump, a tiny but

i guess its just my cam ohh well

nemiro · 08-11-2009, 09:27 PM

Cams certainly have an effect on measured compression! It is the difference between Static Compression Ratio and Dynamic Compression Ratio. There are detailed explanations of this out there, and there are even software applications to show you how a certain cam might affect measured compression.

nutz · 08-11-2009, 09:59 PM

thanx my tune just needs 100% rework also, at 1000rpm (idle) i am getting 13-14in vacuum at 18*, before i swaped turbos it was more like 15-16in

nemiro · 08-12-2009, 10:29 AM

13-14" is not always bad. I am running a ported head and Crane 0016 cams. 14" is max idle vacuum for me. But then again, I am at almost 7000' elevation. At sea level, I would expect to see 17-19".

**04blusrt_4** · 08-12-2009, 01:03 PM

Might sound stupid, but you did have the throttle full open when you did the test, right?

GreaseMonkey69 · 08-13-2009, 01:32 AM

i was bored and i wanted to check my compression. 150 all across. now that was throttle closed with out the table spoon of oil for the wet test. im sure with the valve overlap from larger cams the 5 layer head gasket diff pistons your indication would be different.

hercfe · 08-13-2009, 01:49 AM

Quote: Originally Posted by 04blusrt_4

Might sound stupid, but you did have the throttle full open when you did the test, right?

having the throttle WOT during a compression test is NOT necessary. Generally, throttle closed will yield results about 10-15 psi lower than with throttle open. It can be done either way, just expect lower results with the throttle closed.

Nutz, did you do your comp test warm or cold? Did you do a wet test afterwards? Did you do a leakdown?

GreaseMonkey69 · 08-13-2009, 01:58 AM

see that is what i was wondering.. thee was alot of factors like throttle open or closed.... warm or cold motor. i have seen up to like10- 20 psi change in a cold motor from a warm motor. also the wet test like i stated earlier

chuck's_srt-4 · 08-13-2009, 02:03 AM

Taken from the last thread you posted in regards to your compression results and the question of whether cams have an effect on measured compression or not:

Quote: Originally Posted by chuck's_srt-4

Actually, cams can and DO change the pressures in the cylinder.

Valve opening and closing rates change with cams, as do the timing events of the opening and closing.

Take the following cam card specs for example:
Crower S2 cams
INTAKE: Duration: 257º Lift: .387
EXHAUST: Duration: 251º Lift: .382

INTAKE (@.050") Opens: -6.0 ATDC(aka, 6*BTDC) / Closes: 34.0 ABDC

EXHAUST (@.050") Opens: 39.0 BBDC / Closes: -17.0 BTDC(aka, 17*ATDC)

The timing event you want to look at is Intake opening and closing degrees.

As you can see, the intake valve reaches .05" open @ 6* BTDC (before top-dead center). While the piston has yet to reach TDC, the valve is beginning to open in preparation to draw air volume in.

You will note that the valve doesn't close to .05" until 34* ABDC (After bottom-dead center). That means that the piston has already reached BDC, and is already on it's way back up the bore before the intake valve has had a chance to fully close.

The larger the cam (the more duration it has), the more overlap like this you will see. If the cam had an extra 10* of intake duration, well, that 10* has to be spread out between the opening and closing points. It will have to open sooner than 6* BTDC, and close later than 34* ABDC. (example, opens 11* BTDC, and closes 39* ABDC)

When the valve has overlap like this, some of the air volume that has been drawn into the cylinder during the downward stroke of the piston is being pushed back out the intake valve before it is finally shut. If the valve were to shut sooner (say BDC exactly), the cylinder pressure would be higher since it would not have pushed any of the air volume out.

That is why you will hear terms such as "Static Compression Ratio" and "Dynamic Compression Ratio".

Static measurements are a measurement of compressed volume alone, not taking into account any loss due to overlap.

Dynamic measurements are measurements taking all variables into account.

GreaseMonkey69 · 08-13-2009, 02:18 AM

shit my brain kinda hurts chuck.

hey i learned something more about dynamic compression. i knew cams,valve springs, duration,overlap had something to do with the low compression,but thanks for the technical answer!

nutz · 08-13-2009, 02:38 AM

you thought that hurt

Cam Timing vs. Compression Analysis