compression

[72starga]

hi all,

wondering what should normal compression be in a 2.7 rs motor that was just rebuilt?

[Chuck Miller]

Unless there is something suspect, you should put some miles on your brand new engine before you can actuarially test it's compression... the rings have to seat.

I heard something like 500-1000 miles.

I had a hot rod Chevy engine put together last year and it kept getting stronger for the first 3000 miles!

Another wacky thing about compression numbers is that they differ with elevation ... I just learned this, the lower the elevation the higher the compression. L.A is close to sea level, so higher compression numbers...

The other engine gurus could probably answer you more definitively, but I would guess a new, well sealed, broken in 8.5/2.7 RS motor would fall in somewhere between 125 - 165 lbs. ...

Of course consistency within that range is what's really important...

Ya don't want those numbers to vary much between cylinders.

Hope this helps,

[jayzoll]

Chuck, what do you get in your '73 S? I had someone tell me that, because of the cams in that engine, compression is normally as low as 120.

[Chuck Miller]

That is correct…

Check me if I'm wrong, but the way I understand it is:

The more duration in the cam, the less time to build compression …the lower the numbers


The last time my compression was taken was right after I zinged the motor at Willow in January…

The compression and leakdown was all over the map…
Besides the 2 slightly bent exhaust valves, there was everything else with 164,000 miles to give me low and inconsistent numbers.

Hopefully the next time I check it, it will be much better …

[jayzoll]

Ouch.