Compression

#1

Was going to do some compression tests to see how my car is doing. What compression should I look for as ideal?

#2

There is a lot of variance in compression test results because there’s so many variables. Engine temp, batt state of charge, testing hardware, RPM that starter runs. My #'s from perfectly sound engines always come in lower then most other folks, for example, and I’ve no idea why.

I get 150ish and Rich Bratton gets, IIRC, 170’s and 180’s out of a number of engines that were all equally sound. So some variable is different and I don’t know what.

I wouldn’t bother with compression tests unless you have reason to believe the engine is not sound. Then it’s a pretty good test to compare the cylinders to each other.

I drag raced you several times at RA. Your engine is fine. Sign up for dyno time at RA in early Aug. There will be other SpecE30’s on the dyno so you’ll have a good measuring stick.

#3

A leak down test is a better indication of engine condition.

#4

Ranger,
It never hurts to look and check out engine health in my mind. I had 5 cylinders within a pound of 155 and cyl6 at 162. I would guess that is a little off but not horrible. The engine is stock with 170k miles on it and then a complete refresh with stock parts 3yrs, but only 7 race weekends run on it. I would say it’s not the least power, but no where near what some of the front runners had. Of course my skills are no where near what most peoples are either. I will definitely sign up for the dyno when I know where and when I can do that. How will dyno’s be performed at Nats? Random or everyone since we are a HP limited/controlled class.

cwbaader,
I agree that having both compression and leak down is best, but I hardly ever do both unless I think I have a reason to. I haven’t done a lot of tests over the years but all engine problems I’ve had, I have known I had an issue from a compression test. Cranking over the engine bult pressure almost exactly the same and held it the same on each cyl during compression test. Only cyl6 was different, which leads me to my question.

Does anyone think I have an issue and need to investigate further or does that cursory look seem normal? I’d like to make it through this season as is and the do a .020 over build during December/Jan.

#5

Dang, you remind me so much of me. I mean that as a criticism.

Your engine is fine and I wouldn’t do anything to it unless you later get clear indications that the engine is weak. At the end of my first year I had a perfectly serviceable motor but I decided that if I only had a couple more ponies I’d have much more fun, rule the paddock, the world, and the hearts of umbrella girls everywhere.

Sign up for dyno time at RA in early Aug and you’ll see where you sit compared to the other e30’s that dyno. I don’t now how the dyno will work at Nationals, but it will be the same dyno so your result in early Aug should be pretty comparable to the results from those that get dynoed at Nats.

The amount different of #6 is within measurement noise. No worries there. Rod problems often start at #6. Reduced cooling in the head at 3 and 4 can stress the headgasket there. No telling which valve seals will wear faster.

If the engine issue is killing you, one guy to talk to is Rich Bratton. He usually shows up for our Savannah area beer and lies events but he’s been too busy with work lately.

You busy next Saturday? If not, come to Roebling and hang out with CWBaader and I. Chuck knows more about E30’s then I’ll ever know, particularly engines and engine management. He’ll likely keep us well fed all day in exchange for being his SCCA umbrella girls.

#6

I should be available on Saturday next week, if we are talking the 28th.

You confuse my reason for wanting to sort the engine however. I want everything to be perfect and 100%. It’s a never ending chase to ensure I have all things ready and as best as they can be. It ends when I get to the track, but outside of that I like to make sure I do all I can to make sure everything is sorted. It’s a lot of the fun for me.

[quote=“Ranger” post=77415]Dang, you remind me so much of me. I mean that as a criticism.

Your engine is fine and I wouldn’t do anything to it unless you later get clear indications that the engine is weak. At the end of my first year I had a perfectly serviceable motor but I decided that if I only had a couple more ponies I’d have much more fun, rule the paddock, the world, and the hearts of umbrella girls everywhere.

Sign up for dyno time at RA in early Aug and you’ll see where you sit compared to the other e30’s that dyno. I don’t now how the dyno will work at Nationals, but it will be the same dyno so your result in early Aug should be pretty comparable to the results from those that get dynoed at Nats.

The amount different of #6 is within measurement noise. No worries there. Rod problems often start at #6. Reduced cooling in the head at 3 and 4 can stress the headgasket there. No telling which valve seals will wear faster.

If the engine issue is killing you, one guy to talk to is Rich Bratton. He usually shows up for our Savannah area beer and lies events but he’s been too busy with work lately.

You busy next Saturday? If not, come to Roebling and hang out with CWBaader and I. Chuck knows more about E30’s then I’ll ever know, particularly engines and engine management. He’ll likely keep us well fed all day in exchange for being his SCCA umbrella girls.[/quote]

#7

Scott, all. I will not be racing at RR the weekend of the 28-29th. I got tied up in relocating my seat once again and had forgotten all about inviting y’all to attend and I apologize for any inconvenience. So,Scott, no free beer/food this weekend.

A leak down test will tell you where you need to correct any issues. Usually, on these motors, re-lapping the valves will be all that is needed.

#8

So got better and more thorough results.

190-195 on all cylinders except 5 which was 185 with 8% leakdown out of the exhaust valve.

#9

I had Dyno time in Long Island on a dynojet a week before Nats. HP for the three pulls were 128,130, 133. Graphs were relatively flat. Engine was close to operating temp. Oil would been cool though. I absolutely got raped on the back straight at RA. It was borderline embarrassing. The engine came out of a car with 230k on the odometer. I doubt anything gad been rebuilt. I just did a compression test. The engine was cold and I hit the key until I see 3 compression strokes. I record that number. I reset the gauge and do it exactly the same again. The second number is always different than the first. I reset the gauge again. This time I hit the key until the gauge tops out. Usually this would take 7 compression strokes. Then I record that number. My numbers are not terrible, but interesting to the point I think I should follow up with a leak down test.

1: 160,170,192
2: 165,154,185
3: 172,160,200
4: 155,137,175
5: 165,156,198
6: 135,140,150

Id say there’s most likely an issue with 6 and 4

Ray