Compression Test results / What are REALLY acceptable differences?

[Nick.M18]

I just had a compression test done and the results were:

Left side front to back:
165
170
170
150

Right side front to back:
175
155
170
175

Are these acceptable numbers? The mechanic who performed the compression test said everything was good and that there's nothing to worry about, but doing my research I have found anywhere from "a 20% difference is fine" to "I have a 10% difference, I guess it's time for a rebuild" on these forums. I know these numbers aren't optimal, but for a 74k 03 cobra these aren't bad either. Thoughts? Experiences?

Thanks

 

[Rambro]

When I did mine a few years ago number seven was low at 155 and all the others were in the 175 range. I've been driving it ever since and besides a little cold start piston slap it runs fine.

 

[cj428mach]

When I did mine a few years ago number seven was low at 155 and all the others were in the 175 range. I've been driving it ever since and besides a little cold start piston slap it runs fine.

Is that a stock motor?

Hard to believe the stock setup could have piston slap considering the tight piston to wall clearance these motors have stock.

 

[Nick.M18]

How much do you drive yours Rambro? I'm a new owner, but don't plan on putting more than 5k a year on it. So just out of the lack of driving, I think it will last me a number of years before anything becomes an issue, if at all.

 

[Rambro]

My motor is stock. I just figured it was a piston issue because it makes a slight slapping noise untill it gets up to temp then it's smooth as silk. I probably put less than 2k miles a year on it, but it's not completely pampered. This past year it was dyno tuned for the port/e85, ran on the street and the track. This year it's getting a 2.3 whipple so we'll see how that works out.

 

[hotcobra03]

Was this on cold engine?

 

[speeddemon2000]

They look fine to me. But compression checks can be deceiving. You need to look at the leak down test too.

Is there an issue you are trying to chase down or you just doing this for piece of mind?

 

[Bdubbs]

Is that a stock motor?

Hard to believe the stock setup could have piston slap considering the tight piston to wall clearance these motors have stock.

I thought some piston slap was normal on these motors? I've found threads in the past where members said that Ford dealerships said its normal. I thought piston slap is also partly because of forged pistons.

 

[jrgoffin]

There should be no piston slap in these engines. The PTW clearance is about 0.001" (4032 Al for stock pistons), and with the offset wrist-pin, they are meant to be quiet. My billet aluminum pistons that are in the new Aluminator have a PTW of around 0.004" (2618 Al) with a centered wrist pin and there is no slap there either.