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Discussion Starter #1
ok a few moths ago i had my car at McNews for some work and a new 10psi dyno. they said they will be putting the car on the dyno soon. so a few hours later i get a call, they said the car only made 1 hp and 5 foot lbs at 10psi.(form the original 8psi) so they told me they did a compression test, and it was 105 on 2 and 3 and 115 on 1 and 4.....ok.
so i scared the engine would loose all compression, and i took off the PC

yesterday i did a compression test just to see how much the compression went down and to figure out when i would do a rebuild. to my surprise the compression went up! it is now 140 130 145 135. now if im not mistaken thats pretty damn good. i even checked the the compression tester for accuracy using an air compressor set at 150.

my question is what the hell went wrong, i even checked the pulley size and it was the 2.55 (10psi). im not saying McNews did anything wrong, they are great guys. im going to call them tomorrow to see if we cant figure out what went wrong

any one got any ideas?
 

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Yeah......compression gauges all return different numbers compared to each other. Mainly you have to be concerned if the pressure between the cylinders differs greatly.

Techniques between different mechanics also differ......how many times did he crank each cylinder, how hot was the engine, did he open the throttle, etc.
 

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Discussion Starter #3
i will ask him. i did mine when the motor was warm, 4 cranks, and with it at WOT. as the directions said.

even if our gauges were totally off, i think i could have made more than 1 hp.
 

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WWTPD?
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The variable intake cam will mess with your compression results - normal SVT's will show low 100's for compression if the cam is fully retarded. Sometimes though the cam is in a different spot (adv/ret) and your results will be different.
 

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Discussion Starter #5
so how can i get the true compression of the cylinders?
 

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Fins Up!
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the readings being close to each other is more important than the actual compression reading.
 

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Here's one way.....take your gauge and measure the compression of some completely stock engines of the same model. Then take the same gauge and measure your own compression.

Now you can figure your own actual compression (fairly closely). You can find the stock compression of that engine. Say it's 10-1......and your compression is 10% higher than stock. Your actual compression would be 11-1. Simple enough...[cool]
 

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Discussion Starter #8
the compression ratio of a stock svt engine is 10-2-1. i didnt do any internal upgrades so it cant be any higher than that.

i want to know why i didnt make any power with the additional 2 psi.
 

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Discussion Starter #11
boost gauge in the car and a second gauge both gave a 11psi peak
 

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bluefront said:
Here's one way.....take your gauge and measure the compression of some completely stock engines of the same model. Then take the same gauge and measure your own compression.

Now you can figure your own actual compression (fairly closely). You can find the stock compression of that engine. Say it's 10-1......and your compression is 10% higher than stock. Your actual compression would be 11-1. Simple enough...
i want to know why i didnt make any power with the additional 2 psi.
PSI doing a compression test is almost completely unrelated to physical compression ratio.

It's called static compression and dynamic. You can alter your dynamic compression by changing cam lift/duration/separation angle as well as cam timing.

Like was said, have your reading be similar to each other (I believe the rule of thumb is 10% high to low) is more important of a test than actual readings.
 

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True....but. An engine rated at 11-1 actual compression ratio (stock) will normally read higher on a compression gauge (about 10%) than the same engine with less radical pistons that is rated at 10-1. Of course gauge comparisons become unreliable when you modify cam duration.

I use two different compression gauges.....one is very short, hand-held. You just push it against the spark plug opening. The other is a Snap-on, has long hoses and screws into the plug hole.....used on engines that have deep plug wells. The Snap-on never reads nearly as high as the other (on the same engine), even if you crank the engine a long time.

It's the differences between cylinders that you judge.....You can have an engine with 20% difference between cylinders that will run just fine, and you certainly wouldn't want to fix anything unless you were racing.
 

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Discussion Starter #14
so what you guys are telling me is that ill never get a true compression test reading, its going to vary from tester to tester...ok. im with in 7% of each other, so im good there. but why did i only make 1 hp with 2 more psi?

i would love to put this procharger back on, do you think it would be a bad idea?
 

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I AM DA FOCI FANATIC
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As long as everything is fine I don't see a problem with it. Your not burning oil are you?
 

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Discussion Starter #16
i check my oil all the time and its always right where it should be, and there is no smell of oil what so ever.

so no, not burning oil.....lol
 
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