Thread: What's with my compression test???

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....Hi All, I pulled all my spark plugs, pulled the coil wire & wired the carb wide open. Although I DIDN'T warn the engine up to operating temperature. My compression readings were as follows; 185, 190, 210, 180, 190, 182, 180, & 180.... I cranked the engine over 7 times for each plug. On the one that measured 210 I re-did it & got 190 by just cranking it 5 times..... So what's up with the 210??? Why this high??? And isn't 180 to 190 high on my others??? I have a 413 small block Chev, 10 to 1 compression, AFR 195 street heads with 74 cc chambers, cam is 230/236 @ .050. w/ .512 lift {don't know if you need all this}..... Oh, even though I didn't warm the engine up to operating temperatures the directions said if you have aluminum heads to let the car cool for 10 minutes before pulling plugs. Why is this??? Thanks, Bill

Don't know why the variation in pressures, or why that one is so high. The warning about pulling plugs is because a hot aluminum head expands, grabbing the plug hard enough to damage the threads if you attempt to remove them.

Could be excess fuel in that cylinder and on the second test it was starting to disipate. Have you tried testing it again?

Hey Bill, it's a chebbie!!!! Looks like you have one good cylinder!!!!!!

Originally Posted by Dave Severson

Hey Bill, it's a chebbie!!!! Looks like you have one good cylinder!!!!!!

....LOL, good one Dave!

....No, haven't tried it again but I will later today.... ok, it damages the head if you try to take a plug out when the engine is hot. That makes sense.... THANKS Bill

Hey Denny, I thought them chebbie cams were made that way!!!! You mean they're all supposed to be the same size?????

....THANKS! .... well I am considering going to a roller cam so this makes it easier!!! {should have used on to start with .... Ok, it's within 10% so that's ok...... As soon as I get my carb issues worked out I'm going to have my car dyno'ed on a rear wheel dyno. This will give me an idea {& a base line} of how my motor is running..... Bill

I really like using the chassis dyno. A friend of mine has one in his shop and he is a very sharp operator and print out analyzer... By far the best tune up tool I've ever used. We usually rent it for a couple hours, run a base line, make our changes, run again and see if we are going the right way. Beats the heck out of going to the track and guessing wrong and missing your dial in by a tenth!!! We've even hung out there all day, made runs with different ambient temperatures to note how the weather effects our tune up. The data is all there and once you understand the information getting the most improvement for the money becomes easy... Hate to ever be without access to one again, even given some thought to going partners with a guy and buying our own. Market isn't bad on some of the used ones.....

when u get that dyno done. get a vid and post it on street fire and put up the link

.....Dave, the local chassis dyno doesn't tune your car, they just run it 3 times on the dyno for $150. I know it put out 497 TQ & 460 HP on a flywheel dyno but I don't think it's running right now. So I was thinking if it put out 390 or so HP & 425 or so TQ on the chassis dyno then that's pretty much as good as it's going to get.....

383Chev, If I can get my scanner working I will. Is that www.streetfire.net that your talking about???? Bill

this is correct

Originally Posted by billlsbird

.....Dave, the local chassis dyno doesn't tune your car, they just run it 3 times on the dyno for $150. I know it put out 497 TQ & 460 HP on a flywheel dyno but I don't think it's running right now. So I was thinking if it put out 390 or so HP & 425 or so TQ on the chassis dyno then that's pretty much as good as it's going to get.....
Bill

I doubt it will be that high. Horsepower loss through the drivetrain is typically around 20% with an automatic trans....

Most dyno operators don't do any tuning, Andy is a friend and just pitches in cuz he's a nice guy!!!!

.... Dave, 20% loss? Then I'd want to see TQ in the 390 ish range & HP in the 360 ish range.... But then now that I think about it the engine was dyno'ed without the altenator & a/c set up on so would it be even more than a 20% loss? My flywheel HP/TQ of 460/499 was GROSS not net! No wonder my car doesn't seem that fast! THANKS a bunch! Bill

....VERY interesting Denny! Thank you! ;0 Bill

Some are not that high on parasitic loss. The transmission eats up a lot of it, automatics are worse then manuals, and so does the ring and pinion. The horsepower used to spin all the components before the rear wheels is not measured at the rear wheels. This is also one of the reasons I prefer a chassis dyno, more accurate real world numbers of the power that is actually making it to the ground where it counts!!!!