would cam gears that has been adjusted affect your compression test?

i bought a 12:1 lsvtec that comes out 210 all across on 2 diff gauges. i seen a few other test at this compression at 270psi? does cp pistons even make a set of pistons that does 12:1 for lsvtec?


i wish i had one of those scopes where i could see in the cylinder to see if they have domes on the piston..

 

Thats a hard one its kinda hard to tell what your compression is without knowing what parts you got you know im sure cp pistons make a set that has high compression you would need a part number to verify that which would be on the bottom of the pistons.

 

yes, adjusting cam gears can change your comp readings, especially if you are running cams with larger than stock primaries.

Also a 12:1 rated piston means very little if you don't have the head and piston (in the bore) cc'd so you can figure out the actual compression ratio. When a piston is rated at a certain ratio, it's based on a number of assumptions. Any area (in your setup) that deviates from those assumptions will alter YOUR final number.

 

they are just ctr cams. i guess i will retest my comp with the cams at 0,0 before i install sk2 pro1

 

http://www.webcamshafts.com/pages/degreeing.html

2nd to last paragraph/sub section

 

I doubt you'll notice much if any difference after adjustment.

 

Depending where the ears are currently set.

Sounds like you bought a built motor from someone, My advise is to ABSOLUTELY find out more information on the motor before installing the P1 cams. 210 sounds like a stock LS pistons and that would be a mess with the P1 cams.

Hate to say it but you HAVE to find a way to identify the pistons, whether you are able to scope through the top (I doubt you will see anything through the carbon) pull the oil pan and find a number on the bottom of the piston (not all are identifiable from bottom) or last pull the head off and check (Best way but also toughest). How well do you trust the person you purchased the motor from?

 

1st thing, you can not compare compression numbers. The gauge you are using versus someone else could be very different. Also, where the cam centerlines are set on one motor versus another can be very different. Further, some motors are built loose others are tight, ie PTW and ring gap.

For instance, on my cheapy compression tester my motor will only push 212 per cylinder. But I have relatively loose PTW (0.0025") and ring gaps 0.022" top ring and 0.018" 2nd ring. However, the motor conssistantly makes 195+ whp 139 ft-lb tq after more than 50 dyno pulls, 38K+ miles, and 20+ 1/4 mile trips.

2nd, those compression numbers should only be used to check for differences between each cylinder. In my judgement they should only vary a 1 or 2 psi.

As for the construction of the motor, unless you know the person who built it and you trust them then you should perform some due diligence ie pull the head off, clay the motor, degree the cams, and check to see if the head had been milled. No bore-o scope is going to give you that information.

 

it has forge pistons/eagle rods for sure since i can see it form the bottom but didnt get the item number when i did it. its on the same tune as it was tuned by jeff evans with the listed specs. i think it might be my rings leaking, since im burning like 1/2-3/4 qt every 500-750miles. not even sure where its going, no smoke is coming out of the exhaust.
i tried the cap of oil into the cylinder to see if the compression goes up. i did cylinder 4 and it went up to like 240ish but the strange thing was when i did cylinder 1. it dropped compression to 170 instead? it kept on reading 170 until i started the car for a min to get the oil out of the cylinder.

going to order a leakdown test this week to make sure its not the rings. if its not going to pull the head and hope for low comp pistons. so i can boost this one too

 

oh , i just remembered he said he built the motor on the loose side. that probably would explain the low numbers.

yea the motor still make good power. does 192/148 on with stock bore,itr intake manifold, ctr cams and ebay header. motor has 28k miles on it

 

mar778c, I have never seen a compression gauge that is accurate to 1 psi, I feel that 5% is a reasonable amount. 1 psi on a 250psi motor would be .25%.

Plain and simple if your using that amount of oil it needs rebuilt. If its leaking that amount it should be evident on the undercarriage of the car.


Your low reading compression numbers in CYL 1 sounds like carbon deposits on your valve or seat area.

 

NA, I was not refering to accuracy but precision. Your guage should be able to reproduce the same number on a consistent basis. For instance, if you test cylinder 1 and the value is 225 on day X, then if all things are held constant you should be able to get the same number +/-1 psi on another day. If I were looking for accuracy then the tester and the test would have to change and become a lot more expensive.

While 5% is good for a factory motor, for a hand assembled motor with consistent PTW, properly set ring gaps, etc, then 1 to 2 psi should be the goal.

 

i redid the cap of oil test on cylinder 1. it came out fine with the rest of the other 3.

230psi straight across with a cap of oil on each cylinder. also i guess im not really burning that much oil too. i notice i have to wait like 30min or so before all the oil drains down the oil pan.

going to pull the oil pan again this week and check on the piston part number

 

The piston part number may not be present on the bottom.

For reference CP piston looks like this

 

its cp pistons for sure
it had hte cp logo on one side
the other side had this number 79-2? what does that mean

for a lsvtec to be 12:1 with a b17a head that havnt been milled. wouldnt it require the piston to have a slope dome not flat all around?