Low compression psi but everything in good shape, runs strong. Stumped!

crazypete · Nov 17, 2004

What would low compression psi numbers suggest? I installed some 1.7 RR's and gt-40p's home ported (valves relieved, chamber smoothed, runners smoothed, guides footballed) then tested compression:

I unplugged all 8 plugs, had poured oil in before installing heads (keeps cylinders from rusting), screwed in compression tester, propped carb open, turned it over 5-6 times and stabalized at about 108-112 psi per cylinder. This number stayed solid for 5 minutes before I moved on. This engine was ice cold.

This is low! Everything looked in really good shape (minus the carbon I scraped off the pistons) blockside and topside everything was new. It doesnt really burn oil so rings are not suspect. I replaced the RR's with stamped rockers thinking I have misadjusted the RR's. The new compression results were 100-105! ARG!

Did I lower compression by relieving the valves or something? Isnt it supposed to be 150-175 psi? By the way, it runs like a scolded dog (really well)

HardmanGT · Nov 17, 2004

well the test is supposed to be done on a warm engine first of all. Tolerances on the rings and pistons close as they come up to operating temperature.

crazypete · Nov 17, 2004

The replacement ignition system arrives today so I'll retest once the engine runs.

Do I need to pull all 8 plugs or can I just do 1 cylinder with the other 7 still plugged and the carb propped open?

Daggar · Nov 17, 2004

If you're using the starter to crank it and the coil wire is off, I don't see why you'd need to remove all the plugs. Just the ones for the cylinders you want to test.

Black1987Stang · Nov 17, 2004

Do a compression test without oil and one with oil it would tell you the condition of ur rings, if the numbers are the same its not the rings. mines runs strong also but i no the rings are shot.you can hook up air into the spark plug hole, (leak down test) turn on the air and see where the air is leaking, if its coming out of the exhaust u no its the exhaust valve, if its going out the intake then u no its the intake valve.

Ranchero5.0 · Nov 17, 2004

No info on a cam. Radical cams will lower it pretty bad. As will a plugged exhaust or closed throttle plate, or even a weak battery.

To do a correct test have the battery fully charged (on a charger over night if it hasn't run in a while), all the spark plugs out and the throttle plate wedged open and turn it over for 5 - 6 pulses on the gauge. Yours is pretty low so look for obvious problems with the test if the motor runs good.

Jamie

crazypete · Nov 22, 2004

Got it!

It must have been the cold engine. I ran it around the block for 15 minutes then pulled one plug but had the throttle pegged open with my foot on the gas and cranked. It's not a perfect test but it suddenly reads 160 psi.

Why would a hot engine react so differently? I though hot air was less dense than cold air!

DMAN302 · Nov 22, 2004

Heat expansion will have an effect on readings. You do need to remove the plugs and open the throttle, without oil if your readings are consistant from all cylinders within 10-15% I would be happier than just high inconsistant numbers. Example if 1,2,3,4,5,6,7,8= 130 is better than 1,2,3,4,5,6=160 and 7,8=140. Hope that makes sense.

autoXr1 · Nov 22, 2004

crazypete said:
Got it!

It must have been the cold engine. I ran it around the block for 15 minutes then pulled one plug but had the throttle pegged open with my foot on the gas and cranked. It's not a perfect test but it suddenly reads 160 psi.

Why would a hot engine react so differently? I though hot air was less dense than cold air!

I believe aluminum (pistons) will expand as they warm up, which tightens the seal against the iron cylinder bores.

Michael Yount · Nov 22, 2004

And the forged pistons in your 91 actually expand quite a bit. Cold piston slap isn't unheard of in those engines. You've got to run the test with the engine warm. The hypereutectic pistons which came later don't expand nearly as much - much more stable thermally than the forgings for a tighter cylinder fit. You probably wouldn't see as big a difference between cold and hot with the later pistons.

speedytang · Nov 23, 2004

What is happening to you is the reason auto manufactures originally went to Cast pistons from Forged. My Mustang when new in '89 would use a Quart of Oil every Oil change and it was because of cold start up and the oil going past the rings when cold. I changed to 20w50 and it keeps the cold blowby from using as much oil. I kept taking my car in and they would check the compression and it would come back Ok and now with over 150k miles it still uses a quart of oil and the plugs are clean and the compression still comes within 5% on every cylinder.

crazypete · Nov 23, 2004

The interesting thing is that the 110 psi reading would hold rock solid for quite a while. Isn't bad seal supposed to leak into the crankcase?

Yeah, I second the quart of oil thing. I avoid 5w-20 and synthetic like the plague. I shoot a quart out the tailpipes in a solid day of driving. 10w-30 might lose a 1/2 quart by the next oilchange.

Here's another interesting question:

I just got it running so I havent tuned my new 600 dp chokeless race carb yet. It has 4 corner idle that was a biznatch to get running, even after 2 years of holleying it on a 2 corner idle avenger. It has the holes drilled through the butterflies.

This holley runs insanely rich cold. I mean it makes your eyes water and leaves 2 black soot marks on the ground behind it (REALLY! No exageration!) and a really neat smoke screen. 3-4 minutes later, the smoke changes to white and now there's moisture running out the back. 5 more minutes later, the tailpipes are clean. I run it around the block, looking at the A/F guage, it reads dead lean and a tiny bit of constant throttle power variance. When I pulled the plug for the comp test, it was carbonless and a little bit white (so it's lean).

Why is the combustion process that bad when it's cold? Go from tears in the eyes to crispy lean?

Michael Yount · Nov 23, 2004

How, or is, the carb choked?

crazypete · Nov 23, 2004

Now that would all make sense but there is no choke whatsoever. There isnt even a choke horn! The mixture is slowly changing as the engine heats up. Wierd, aint it?

Luckily, even in below zero weather with this carb, it's like 1 turnover and it kicks in. It starts even easier than it did with efi!

Michael Yount · Nov 23, 2004

Just wanted to confirm that "chokeless race carb" really meant that. I'd contact the carb manufacturer's tech guys and see what they tell you.

Just curious - why would you install a chokeless race carb on what I think is a street car?

crazypete · Nov 23, 2004

Well, I went carb 2 years ago and got a vacuum secondary 670 street avenger with electric choke. I hated that choke. The engine simply refused to run at all unless the choke was set so agressively that it was basically holding the plate open a crack while it's cold and rotated open within 10 seconds. I had to take the pegs out to rotate it almost 2 turns. So I got fed up and pulled it off and drove around chokeless for 2 years, winter included. I set it to run a bit rich at idle and everything was cool after 5-10 seconds of throttle blipping. So when the time came to replace it (my poor 670 avenger...RIP) I decided to go with a chokeless carb "80540-1" to get the milled choke horn effect and some neat nicknacks like adjustable airbleeds, 4 corner idle and double acc pumps.

Like I said, she starts up even better than my efi ever did ice cold after 1 or 2 turnovers. It's creepy how easily it starts! I bet the hi-6 ignition contributes a bit too.

I'm probably going to get like 5 mpg when I get to testing it

Low compression psi but everything in good shape, runs strong. Stumped!

crazypete

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crazypete

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Low compression psi but everything in good shape, runs strong. Stumped!

All my crevices are greased.

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