Please help with Low Compression
 

Last year I put a turbo on my 3.2. This year I just don't seem to have the power that I had when I first put the turbo on. I pulled all the plugs and I apparently have some detonation that I need to figure out. Not overly surprised as I don't have an intercooler yet.

I ran a compression test on all cylinders and got strange results. I measured 110 PSI (+/- 3 PSI) on all cylinders. It seemed low so I ran a leakdown test and measured 8-10% on all cylinders. I did some research here and it seems that most are measuring way over this. I tested the gauge with another gauge to be sure this wasn't the issue.

The car runs well, just doesn't seem to have the full power it once had when I first put the turbo on. Today I thought about the cam timing. I have recently pulled both of the chain cases to resolve leaks which involved, of course, retiming the cams. I tried marking the gears, but the chain apparently slipped a tooth on the intermediate gear and had to time them. I am wondering if I did something wrong that could be causing my low compression numbers. I followed the procedure in the Bentley manual.

Any thoughts. What I don't understand is how I am getting detonation with low compression.

I appreciate any ideas or suggestions.

EDIT: I did some more research about cam timing. A guy posted that he did his cam timing with mechanical tensioners and when he put the oil tensioners back in, the timing went from 1.25 to 1.5. I assume that under oil pressure, those oil tensioners would stiffen up and give a similar timing as the mechanical. When I did my timing, I just used the oil tensioners and didn't use any supplementary tension as I am now reading that I should have. Following the logic, this would leave my timing retarded and logic tells me, along with research, that the later closing intake would show me low compression readings. It also tells me that my low/mid-range would be decreased and upper range increased in performance. This makes sense with what I experience. Anyone know how retarded I would have to be to get these compression readings? Should I get back in there and time them again with proper tension. SHould this get my performance back.\

Thanks,

Larry

you don't need mechanical tensioners. You time it with the ones you have. Having oil in the tensioners does not give it more tension. But it may do you well to do the cam timing again. Uniformly low compression likely has something to do with cam timing being off.

Before going to all the trouble of re-timing the cams, put the dial indicator on and see what it says.

No point guessing.

did you ever do a comp test before you put the turbo on? what compression ratio are you running? what dizzy are you using? most turbos back off the timing when the boost comes in. you say you are having detonation problems? how much boost?
i would not mark the gears and try to put it back together. always re-time it.
how do you know you have detonation problems? the plugs melted? what heat range are the plugs? they should be no hotter than WR4.

make sure you have good tension on the chains when you line up the timing marks. IE, dont back up the pulley. slowly come up to the mark to keep the tension on the chains.
after, i go around several times and recheck.



there is a tool you can get that keeps tension on the sprockets while doing the cams.

are you reading the dial in the CCW direction? this dial reads .32mm, not .68mm.

make sure 1 and 4 valves are adjusted properly. use the timing marks on the pulley, not the dizzy. just some things i have seen others do.

http://forums.pelicanparts.com/uploa...1292852763.jpg

Was the throttle 100% open during the compression test? If not that would give low readings.

Any visible sign of detonation is a VERY bad thing. What exactly are you seeing? It is possible that you have damaged all six pistons and your low compression is a result of that damage. Did you make changes to your induction when you added the turbo? A lean turbo motor will run like a rocket for a few minutes. Tell us more about your build. I think a stock 3.2 would show 135-140, not 110.

Lindy

I never did do a compression test on the motor before putting the turbo on. However the P/O did a leakdown and all were in the 8-10% range similar to what I observed.

I put the turbo on as well a Megasquirt system to control fuel and timing. I ran .4 bar boost during testing and dialed it up to .6 last summer. I have been planning an intercooler, but haven't added it yet (doing it this winter). When I pulled the plugs to do the comp test, I noticed light silver speckeling on the plug insulator and bar telling me that I am experiencing detonation. I will try to post a pic later. however, it is almost impossible to see the speckling without magnification. This made me believe that the detonation is moderate and not severe. I have never heard audible detonation, but have now dropped the timing down and taking steps to figure out when it is happening. I can't imagine that I have lost enough enough piston material to lose compression as I just haven't put that many miles on the engine since going to .6 bar. I run AFRs of 12 or less on boost.

The plugs were stock (Bosch WR7) and I think that may have been part of my issue. I have gone 1 step colder to NGK BPR7 plugs (NGK 6 = Bosch 7).

I will pull a valve cover and check the cam timing. It seems that is the best thing to do to figure out exactly what I have. While I would turn the engine backwards during testing, I always went clockwise to creep up on the measurement to keep tension on the chain. I am familiar with dial indicators and it's not likely I read it wrong. However, I struggled to get the indicator solidly on the retainer and slippage is a possibility.

Thanks for all the help here guys. I'll try to check at least one cam tonight and post results.

Larry

Just to verify, can someone tell me the amount of valve lift I should measure at #1 TDC for cam timing on a '88 3.2?

Thanks,

Larry

I checked the timing on the #1 side tonight and came up with 1.42mm. According to Bentley the target is 1.25 and the tolerable range is 1.1-1.4. If I am thinking about this correctly, this leaves me a bit advanced. I would expect an advanced cam to actually increase compression.

I am beginning to question my gauge. I test it by attaching it to my leakdown tester and it measures within 5 PSI. It does seem a bit slow to get there. I'll investigate further.

Any other thoughts.

Larry

thats not that much off. not enough to worry.

WR7 is WAAAAAAY to hot. you want at least a WR4. i think the 6= 6, i never can keep it straight. so i think it would be a BPR8. its better to be too cold than too hot. hot can cause engine damage, too cold may only foul.
when i was younger, i thought hotter was better, just because it was hotter.


the boost timing is what you have to look out for.

stock compression ratio? +.6 boost ?????

what brand of leak down tester?

I have been following the model that Procomp used for the 3.2 with 9.5:1, which was .5 bar w/o intercooler and .7 with. Several have run this config without issue. Going to .6 bar without the i/c was probably not a good idea. Thought I could address it with a rich mixutre and timing reduction. the detonation is moderate and I think it is coming on the boost ramp up, as I sometimes get lean spots on the boost ramp up. It is tough to manage on a speed density system as the MAP tends to fluctuate some.

The leakdown tester was a harbor freight model, but I tested each gauge to confirm accuracy and created my own .40 orifice, as there's didn't have one. I'll give BPR 8's a try. I did some research on the 930 forum and some guys were running these successfully.

Larry

i have one of the HF ones too. i would not trust it. its better to have one that puts at least 100psi in there. the HF one is only about 15psi.

an update on my progress with detonation

http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/577542-turbo-recommendation-2.html#post5762315

I never did get a good compression reading, but engine is running well with new intercooler, so I think I have a bad gauge and power fall off was from high IAT's

Larry