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Discussion Starter · #1 ·
What should normal compression be on an SVT when doing a compression test. 2003 with under 30k miles. 150-160, correct????
 

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thats not too bad I think it should be closer to the 190-200psi, you can do a wet compression testyou should get slightly higher numbers with that, if your cylinders are washed out you will have lower numbers like a crank no start but its spraying fuel you will lose some compression past the rings
 

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The Librarian
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Don't forget, we have hotter cams than the base model, with more valve overlap.
That overlap will actually drop compression when doing a test.
I don't doubt an SVTF engine will show lower numbers than the base model because of that.
In fact, I'll bet the numbers come out to between 120-140 max.

Don't be so concerned with getting high numbers.
It's consistency you're looking for.
All cylinders should be within +/- 10% of each other.
^^^That's what you should be looking for. [;)]
 

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Discussion Starter · #4 ·
Actually, my compression numbers were 80, 90, 90, 90. in cylinders 1, 2, 3, and 4 respectively. I tested several times with 3 different testers. That seems a bit low to me. I was just thinking it should be 150-160. Question is are my numbers TOO low? It's an 03 with under 30K.

When I asked Tom for his advice he wrote: "The SVT compression is hard to do a compression test on because of the VCT..."

That seems possible to me, and I trust Tom as a knowledgable expert, but I have other "knowledgable experts" telling me that me that the VCT would not have any effect on a compression test.

My lack of knowledge concerning the VCT system makes either answer possible to me.

I just want to cover all possibilties before I tear into my motor for no reason.

Any advice would be appreciated!!!
 

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The Librarian
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That does seem a tad low.
Did you have the throttle plate open fully when doing the test???
If not, you're not pulling any air (well, very little) into the engine, which would explain the lower numbers.

I can't imagine the VCT having any effect at cranking rpm, and very low oil pressure.
 

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Discussion Starter · #6 · (Edited)
Yep, throttle wide open, then turn key. You make a good point about low oil pressure / no VCT.

BTW, just warmed the motor and did another compression test and got 89, 90, 90, 91.
 

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I may sound dumb, but what do you use to test the compression? Ive been wanting to see what im running at.
 

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Discussion Starter · #8 ·
It's a special tool that can be purchased at most parts stores. It consists of a pressure gauge, attached to a rubber hose with threads at the other end that you screw into the spark plug hole. I think you can just ask for a compression tester or compression checker and they should know what you need.
 

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Without oil pressure holding the VCT in position, it will sometimes go to full retarded as it strains to open the valves. This WOULD make the compression low as the intake cam closes way after BDC.

I wouldn't worry about the compression #'s as long as they are consistant (as WD-40 said) and the car is otherwise running well.

Edit to correct BDC for TDC. Early morning retardedness on my part.
 

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Without oil pressure holding the VCT in position, it will sometimes go to full retarded as it strains to open the valves.
^^^That's a good observation, and maybe that's that what Tom (1turbofocus) was referring to.
Thanks.
 

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My pleasure! And to expand just a bit, I believe that the VCT can only be advanced by the oil pressure pushing on the piston in the hub. The drag of valve opening retards it when the pressure is reduced. To work otherwise would require OP on both sides of the piston and I don't think it has this.

I've worked on a couple of Contours with VCT (though on the exhaust side) where someone had installed a filter without drainback valve. They ran REALLY crappy when first started until the OP came up and the cam was put in the proper position. Even set the CEL with a Cam Position Code when this happened.
 

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WD40 said:
Don't forget, we have hotter cams than the base model, with more valve overlap.
That overlap will actually drop compression when doing a test.
I don't doubt an SVTF engine will show lower numbers than the base model because of that.
In fact, I'll bet the numbers come out to between 120-140 max.

Don't be so concerned with getting high numbers.
It's consistency you're looking for.
All cylinders should be within +/- 10% of each other.
^^^That's what you should be looking for. [;)]
Yup, tested my compression about 8 months ago and they were all about 130psi.
 
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