I plan to replace my spark plugs next month. My engine is stock other than the lemans cam. The engine runs great - only 7 or 8,000 miles since a complete rebuild many years ago, but I thought I would do a compression test on it anyway when I changed the plugs. When I do a google search on 289 hipo compression I see a range of between 130 - 170 lbs. That sounds like a wide range to me for a good engine. What range would the knowledgeable engine guys on this forum suggest I should be looking for / expect to see ?
It really depends on who did the re-build, how they did it and what they used. Do the compression test after getting the engine up to temp and throttle fully open. It'll probably be on the high side but whatever values you get, the variance shouldn't be more than +/- 5 psi.
Standard bore - just cleaned up. Stock pistons and heads. I realize the cylinders should ideally all be +/- 5 lbs or so - just wondering where within the 130-170 range is most likely. To me 130 sounds low and tired.
To me, I'd expect readings around 135 or so. The cam timing events are going to keep the readings way down from 170's, and they should.
I think that you are likely worrying about nothing.
In a perfect world all of your compression test readings should be within 5psi of each other, regardless of where that number is.
It's still possible, not highly likely though, that your compression rings are still "seating" or maybe better said, not "normalized" yet?
You can see that on a bleed down test, and it can depend on the rings you used and the finish that was honed into the cylinders.
I'd rather have the readings on in the 135 area then the 165 area. The 165 is where you will get detonation, pinging and hard hot re-starting.
As a matter of fact, if you did have 165's I'd think that the engine was drastically under-cammed with something like an "RV" cam in it?
OK, thanks. I'm not worried - the engine runs great. Just wondering what would be considered normal.
Actually I'd say a 125 reading is more common.
+1 with that cam , "cranking compression" will be lower , but "mechanical compression" will not change. The engines were advertised as 10.7 "in the day" but assembly line variations had it barely at 10-1. The important point of cranking compression is equality among all 8 as previously mentioned.
Randy
I think the 130 to 170 is the range because some people will do the Tests Dry and then do them Wet.
As an example, the last time I tested a stock HiPo the range was 150-160.
An increase in camshaft duration WILL lower cranking compression. This is often referred to as "dynamic compression". Advancing the camshaft also changes cranking compression numbers slightly higher.
Quote from: gt350hr on April 20, 2021, 10:44:31 AM
An increase in camshaft duration WILL lower cranking compression. This is often referred to as "dynamic compression". Advancing the camshaft also changes cranking compression numbers slightly higher.
I've done this a lot more then a couple of times. Expect 125 to 135. 160 is an RV cam and IS too high. It may read high before initial engine break in and drop down some.
You want the 289hp to be lower at cranking. There is NO advantage to a higher reading. It is just stressing out the starting system if it's higher. That static reading IS not effecting the performance of the engine.
ACTUAL cr of a stock 289hp or even a 4v is going to be right around 9.8. That's with cc'ing the heads, calculating the volume in the ring lands and valve pockets in the heads.
You need to shave the heads and use a thinner head gasket to get to an actual 10.7 AND for your work, it's going to "smoke" the starter on a hot restart so if you aren't racing, leave that stuff alone. It's only worth 20 to 25hp and isn't worth the additional aggravation of it acting like a race engine on the street.
Thanks for all the replies and great info.