VaporiZe

I just did a warm/dry compression test on my 91 GT and was wondering what the compression should be knew?

I got 1-160 2-170 3-173 4-162 5-160 6-175 7-165 8-165 with 130k miles if anyone was wondering.

jcorenman

Seems a bit low and uneven, but trouble would be one cylinder reading 30. So absent other symptoms it seems fine.

Did you hold the throttle wide-open while cranking? Charger connected so that the cranking doesn't get slower towards #8? Same number of cranking revolutions per cylinder? Good gauge with a screw-in fitting with a tight seal? (Fuel-pump fuse pulled so you aren't squirting fuel into the cylinders? EZK fuse pulled so you don't damage coils?)

VaporiZe

Did four jumps of the needle of each cylinder. I did the cylinders in the order of 8, 7, 6, 5, 1, 2,3, 4 but did not charge the battery

VaporiZe

What should the compression be? I always thought if it was within 10% of the original value it was good

ROG100

If they are all within 10% they are pretty good.

daveo90s4

As to what the readings should be: I had always understood that the rule of thumb was 16 times CR, or in this case 16 times (about) 10 or 160. Which is what you have. I have no idea why '16' is the proposed constant in my mind, but that's what I've always understood.

FredR

My S4 typically hits 185 psig on my test rig and I do not get any noticeable difference cylinder to cylinder.

To get good consistent readings the motor needs to be warm and as Jim says, the throttle has to be opened. If your battery is in good condition then sequence should not factor in as a problem.

Consistent readings are a better performance indicator than gauge pressure readings unless you have a rig calibrated against a known pressure.

Your average pressure is 166 psig and the spread is covered by about 5% deviation so nothing excessive there but I would want to know why the test rig is showing numbers that superficially appear to be a bit on the low side.

Rgds

Fred

mark kibort

185 for stock GT. that seems kind of low.

terry gt

perhaps Mark its the amsoil building up in the cylinders .... sorry I tried to be good but there was too many other options

kmascotto

When I checked mine (cold) I had 200psi to 205psi on the DS and Zero on the PS...but that was because my cam chain let go and the intake valves were not opening on the PS. I have about 110,000 miles

davek9

I see around 210 to 220 on S3's with fresh Valve jobs and warm, however the CR is a bit higher on S3's.
Without doing the heads/valves 150 to 180 range.

the 16 # doesn't seem to work here with 10.10:1 CR listed for S3's
Unless this chart is incorrect?

928 Engine Types & Performance Data
928 Model Engine Size/Type HP (SAE) Torque ft/lbs (SAE) Comp. Ratio Fuel Injection System Top Speed MPH 0-62 mph (sec) 1/4 mile (sec) Weight (lbs)
77-79 - 3-auto (US) 4.5L-16v M28/04 219 245 8.5:1 K-Jetronic 140 8.5 16 3373
77-79 - 5-man (US) 4.5L-16v M28/03 219 245 8.5:1 K-Jetronic 143 7.5 15.5 3285
77-79 - 3-auto (Euro) 4.5L-16v M28/02 229 250 8.5:1 K-Jetronic 140 8 16 3197
77-79 - 5-man (Euro) 4.5L-16v M28/01 229 250 8.5:1 K-Jetronic 143 6.8 15.5 3197
80 - 3-auto (US) 4.5L-16v M28/14 220 265 9.0:1 L-Jetronic 140 8.5 16 3385
80 - 5-man (US) 4.5L-16v M28/13 220 265 9.0:1 L-Jetronic 143 7.5 15.5 3351
81-82 - 3-auto (US) 4.5L-16v M28/16 220 265 9.0:1 L-Jetronic 140 8.5 16 3385
81-82 - 5-man (US) 4.5L-16v M28/15 220 265 9.0:1 L-Jetronic 143 7.5 15.5 3351
80-82 - 3-auto (Euro) 4.5L-16v M28/10 229 271 10.0:1 K-Jetronic 140 7.7 16 3197
80-82 - 5-man (Euro) 4.5L-16v M28/09 229 271 10.0:1 K-Jetronic 143 7.2 15.5 3197
80-83 S - 3-auto (Euro) 4.7L-16v M28/12 300 263 10.0:1 K-Jetronic 152 7.2 15.5 3197
80-83 S - 5-man (Euro) 4.7L-16v M28/11 300 263 10.0:1 K-Jetronic 155 6.6 15.2 3197
83-84 S - 4-auto (US) 4.7L-16v M28/20 234 263 9.3:1 L-Jetronic 143 7.2 15.5 3385
83-84 S - 5-man (US) 4.7L-16v M28/19 234 263 9.3:1 L-Jetronic 146 6.8 15.2 3351
84-86 S - 4-auto (Euro) 4.7L-16v M28/22 310 295 10.4:1 LH-Jetronic 150 6.7 15.5 3308
84-86 S - 5-man (Euro) 4.7L-16v M28/21 310 295 10.4:1 LH-Jetronic 155 6.2 15.2 3308
85-86 S - 4-auto (US) 5.0L-32v M28/44 288 302 10.10:1 LH-Jetronic 152 6.8 14.9 3439
85-86 S - 5-man (US) 5.0L-32v M28/43 288 302 10.10:1 LH-Jetronic 155 6.3 14.2 3351
87-88 S4 - 4-auto (US) 5.0L-32v M28/42 316 317 10.0:1 LH-Jetronic 162 6.6 14.7 3550
87-88 S4 - 5-man (US) 5.0L-32v M28/41 316 317 10.0:1 LH-Jetronic 165 6.0 14.5 3505
87-88 S4 - 4-auto(Euro) 5.0L-32v M28/42 316 317 10.0:1 LH-Jetronic 165 6.3 14.5 3484
87-88 S4 - 5-man(Euro) 5.0L-32v M28/41 316 317 10.0:1 LH-Jetronic 168 5.9 14.2 3484
89 S4 5-man 5.0L-32v M28/41 316 317 10.0:1 LH-Jetronic 168 5.9 14.2 3505
89-91 S4 - 4-auto 5.0L-32v M28/42 316 317 10.0:1 LH-Jetronic 165 6.3 14.5 3550
89-91 GT 5-man 5.0L-32v M28/47 326 317 10.0:1 LH-Jetronic 171 5.8 14.1 3505
92-95 GTS - 5-man 5.4L-32v M28/49 345 369 10.4:1 LH-Jetronic 171 5.7 13.7 3593
92-95 GTS - 4-auto 5.4L-32v M28/50 345 369 10.4:1 LH-Jetronic 171 5.9 14.1 3593

FredR

1990 S4 motor actually.

My test gauge is a US marketed item [i.e. made in China I expect]. Quality looks good but I expect these things are like dyno's- potentially all over the place to some extent. Thus I reckon they will give consistent readings but not necessarily exact spot on numbers- appreciate you have a different world view of dyno's!

That being said, I do believe I have some "missing performance" and have beleived this since I purchased my original 90 S4 some 17 years ago. When we put this motor into my current GTS chassis the valves we lapped in with two inlet valves replaced on No 7 taken from another cylinder head. I have always assumed they indexed the valves correctly apart from the two replacements.

The research I did suggested that 185 psig was a fair number, whether a thorough valve job would enhance these numbers a bit I have no idea. All I can say is that the values were more or less identical and leak down values were also very good.

At the end of the day my motor is now 26 years old and more or less as it left the factory but with some 100k+ miles under its belt in somewhat hot conditions. I suspect the numbers are a fair reflection of reality.

Regards

Fred

GeorgeM

I've found that a critical piece of equipment for a good reading is a hard tube or pipe rather than a rubber hose to ensure a good seal at the spark plug hole. Not many off the shelf kits have this accessory.

merchauser

anyone know where to get this??