Bad compression, good leakdown

[KoukiMonsta]

I have an sr with 500 miles since rebuild, it has always had low vacuum(12 in/hg with bc 264 cams) and now with over 500 miles I need to sort this issue out.

I performed a leakdown test, percentage wise the results were great. all cylinders less than 5%. I could hear air escaping through the crank case breather(rings?) and oil fill port(valves?) on the valve cover. Now the valve cover bit concerns me since the head was serviced by mazworx and i would think should seal very well considering how recently the job was done.

compression test (warm engine) on the other hand, not so good. (lets note this motor was built to a higher compression ratio of over 9.0:1 so values should be higher then standard DET)
dry: 146, 149, 145, 141
wet: 170, 181, 174, 172

so that seems to be an obvious ring issue. The block was bored/honed to 0.020 over. brand new cp pistons/rings installed. break in procedure involved lots of variation in rpm, load in higher gears, and decel in gear for lots of vac, and a non-detergent oil.

does anyone think it needs more miles to 'seat' the rings? I feel its probably 'too late' and it may need another hone and new rings

why does the leakdown show minimal leakage? nature of the test?(note it was done cold)

any thoughts/input is appreciated. thanks

[Protius]

Oil fill hole and lower crank breather are all a part of the crank case. What kind of quality is your compression tester? Bigger cams will make the compression reading lower and also not have as much vacuum under idle. I think you're worrying far too much.

[KoukiMonsta]

Oil fill hole and lower crank breather are all a part of the crank case. What kind of quality is your compression tester? Bigger cams will make the compression reading lower and also not have as much vacuum under idle. I think you're worrying far too much.

Thats a good point, I wasn't thinking that.

I've used 2 gauges. One have me readings of 120 psi, oem brand. The reading I posted were from a brand new OTC brand gauge. And I have since replicated the readings twice to verify the values.

I have mixed things on cams effecting compression, I'd like more input on this. Vacuum yes, but I have see. Identical setups in the 18in/HG region.

I may be worrying, better safe then sorry.

[Sileighty_85]

did you pull all the plugs and hold the throttle Wide open when doing the compression test?

[KoukiMonsta]

did you pull all the plugs and hold the throttle Wide open when doing the compression test?

Yes

And I did 12 revolutions. The needle stopped moving around 8. And a few more to be safe.

[Kingtal0n]

1. who did the bottom end
2. how did you break in the engine
3. what oil did you use to break in the engine
4. do you know your oil pressure at idle when hot
5. what kind of power does the engine make

6. that vacuum is too low for those cams. BC264s pull 15-17" of Hg at idle iirc. I can double check this weekend though.

good leakdown and low power/compression with low vacuum reading points to misadjusted camshafts. Which I highly doubt, because the compression is fairly decent to be off by that much. find your air fuel at idle, and your timing at idle. You should have 15-18* BTDC of timing and be idling around 14.7:1 at 850rpm with 15"+ vacuum.

power numbers would tell more. 9:1 compression with a good head those cams and 15psi of boost from anything larger than a stock turbo should put you around 320-350rwhp. At 12psi I was able to make 280rwhp with those cams before 6500rpm.

cranking speed is a factor when checking compression. your 150 could be 170 if you spun the engine +100rpm for instance. If it runs fine, makes power, has consistent compression across the board, doesn't smoke or leak, has good oil pressure, then you got nothing to complain about.

[KoukiMonsta]

thanks for the educated response

1. who did the bottom end
2. how did you break in the engine
3. what oil did you use to break in the engine
4. do you know your oil pressure at idle when hot
5. what kind of power does the engine make

6. that vacuum is too low for those cams. BC264s pull 15-17" of Hg at idle iirc. I can double check this weekend though.

good leakdown and low power/compression with low vacuum reading points to misadjusted camshafts. Which I highly doubt, because the compression is fairly decent to be off by that much. find your air fuel at idle, and your timing at idle. You should have 15-18* BTDC of timing and be idling around 14.7:1 at 850rpm with 15"+ vacuum.

power numbers would tell more. 9:1 compression with a good head those cams and 15psi of boost from anything larger than a stock turbo should put you around 320-350rwhp. At 12psi I was able to make 280rwhp with those cams before 6500rpm.

cranking speed is a factor when checking compression. your 150 could be 170 if you spun the engine +100rpm for instance. If it runs fine, makes power, has consistent compression across the board, doesn't smoke or leak, has good oil pressure, then you got nothing to complain about.


local machine shop honed the motor, after inspection they said it was previously bored .020 over and all it needed was a hone. I assembled it, with the help of a friend who has successfully done multiple engines. Following FSM procedure exactly. torque specs, ring orientation etc.

break in procedure involved lots of varied rpm driving, load in higher gears, keeping rpm below 5500. lots of time decelerating in gear for max vac.

oil for break in was (still is) straight 30 weight non-detergant. generic brand from parts store. its cheap for doing multiple oil changes in the early stages.

oil pressure hot at idle is 13psi or so. i recall checking it by the 10 psi per 1k rpm rule and it was good. this is at idle of 800 rpm.

engine makes 253 whp at 16-17 psi with s14/s15 t28 non-BB on a dynojet, tuned by enthalpy.
The power level is what provoked me to do a comp test. It did not make the planned power, the fact that vac has always been low solidified that.

my timing is dead on 15*, afr at idle is in the 14.5-15.0 area. it likes to bounce around while at low rpm.

here is my mechanical timing
https://farm4.staticflickr.com/3907/14214403498_d565f20410_c.jpg (https://flic.kr/p/nE5wWo)
https://farm4.staticflickr.com/3854/14401026265_3e4b666fd9_c.jpg (https://flic.kr/p/nWz2oi)
https://farm4.staticflickr.com/3883/14214367319_51cd1d74f1_c.jpg (https://flic.kr/p/nE5mbB)

[Kingtal0n]

1. do you see how torque is falling after 5500rpm?

That means either the turbo is tired, (out of breath could be due to a boost leak or too many miles) or, the ignition timing is wrong in those areas. The fact the air fuel creeps up makes me think the ignition timing is wrong. You are seeing it go lean either because the tune isnt right, the maf is dirty/old, or fuel pressure is dropping, or unburnt oxygen is entering the exhaust system, due to wrong timing (previously stated, which means EGT would be going UP after 5500rpm, but you do not monitor EGT do you?) Or the process of combustion is incomplete due to (wrong plug gap, poor ignition system (old coilpacks) / poor grounding)

Order of operations at this point (from cheapest to most expensive)

1. check for boost leak by FILLING the plumbing with compressed air to 18psi
2. new plugs, run the OEM huge gap at first, ONLY gap down IF she blows out spark (gap as necessary)
3. check grounds / investigate wiring
3. barrow/rent fuel pressure gauge
4. try lower boost on the dyno and see if that serious lack of power remains after 5500rpm (that would rule out the turbo being the culprit and point to ignition timing / spark / boost leak)
5. EGT gauge to verify ignition timing is too retarded

other stuff to try:
A: you can try advancing the timing at the crankshaft 2-4* and see if that mess after 5500 clear up AFTER you do the boost leak check
B: try raising the fuel pressure to see if it brings the A/F's down, if it does not, that indicates fuel pump cant keep up (unlikely) (not a power output issue)
C: try a new maf hot-wire (for the A/F line, not a power output issue)
D: Try new coil packs

Ordered from most likely to least likely:
1. boost leak
2. turbo is maxed (my money is here if it isnt the boost leak)
3. spark plugs
4. maf (not a power problem, just A/F problem)
5. fuel pump (also nothing to do with power output but worth mentioning)
6. ignition / grounds
7. cam grind / setting (very unlikely)


Or, the ignition timing is wrong, could be #1 or #7 I gave you several ways to verify that already but keep in mind, if it is in the computer, could even be a bad computer, bad dwell settings, bad coils etc... and you will be chasing your tail all day long.

And finally, last on the list, the engine was built poorly. What that means is just, the clearance somewhere is incorrect, making it difficult for the engine to make good power. your low vacuum and low compression numbers seem to support this theory, however, that nice peaky torque curve up until 5500rpm seem to say everything is just fine. In other words, if you brought me this car, and show me that graph, and never told me that engine was rebuilt; I would never suspect the problem lies within the internals. The late response of the stock turbo (that is just a stock t-28 right?) also point more towards a boost leak, or exhaust leak, or improper ignition timing, or high mileage compressor than anything else. If you push 4* additional timing at the crank and the turbo spools 600rpm sooner and you straighten out that torque curve after 5500rpm you have your answer. On the other hand it might ping so I would also add some race fuel. Honestly I just think the turbo is out of breath, you will not make 300rwhp with a stock t-28 unless its brand new out of the box and even then it would take more than 16psi.

Oh also, make sure your PCV is set up correct, like stock.