427 so compression test readings

HSSS427 · #1 (**permalink**) 12-27-2004, 08:43 PM

Did a compression test yesterday on my 427 so and all the cylinders came out with readings of about 120 PSI. How do I tell if this is good, bad or indifferent? What type of readings should I be expecting? Bought the car from someone else and they indicated it had 9.5:1 compression. It has an Isky cam (250 duration, valve lift of .525), Shelby aluminum heads, TRW 10:1 pistons and Lemans rods and NASCAR crank. At least the readings are consistent, but am wondering what type of pressure I should expect.

Also, is there any easy way to verify the 9.5:1 compression ratio?
Thanks!
Scott

SCOBRAC · #2 (**permalink**) 12-27-2004, 09:30 PM

Well... The good news is they are all the same. The bad news is they are very very low. factory specs are 160 to 200. A good tight FE should run 180-195.

I'd go get a new or different gauge, warm it up, pull the plugs and try it again... Compression tests should always be done hot. but even cold they should be close to 175.

Is it possible the thing was set up for a paxton or a roots blower? I'd pull the heads and deck the block. It will likely be a huge miscalculation on which piston the builder used or the volume of the heads.

Is it new? it is feasable (but unlikely) you will see low compression for 1500 to as much as 3000 miles. But if it isn't a fresh rebuild someone messed up.

FYI Lemans rods are too narrow for a NASCAR crank journals the two are not interchangable

xlr8or · #3 (**permalink**) 12-27-2004, 09:53 PM

Give the cylinders a little squirt of oil and that will tell you if the rings are part of the problem. Are you sure the valves are closing completely?

mr0077 · #4 (**permalink**) 12-28-2004, 04:22 AM

Mine checks out at about 140 psi across the board, and put out 389 RWHP on a hot day here in Houston...I'm running 10.7:1 with the Comp Cams 292 camshaft. Compression readings are affected by a lot of stuff (compression ratio, valve overlap, cam indexing, etc.), not to mention mechanical condition of engine (rings, valves, etc.). Your readings sound a little low, try what Michael suggested, check it warm, all plugs out, throttles blocked wide open. Bear in mind that if you aren't REALLY fast, the motor will cool during the checking process, and that will affect your readings (so...check four cylinders, put the plugs in and warm it up, check the other four?) Nah, just note that if the last four start to drop off, it could be because of the cooling). If the readings are still low, squirt some oil in the cylinders and see if it bumps up to higher readings (if so, it indicates bad compression rings). Can you run a leakdown test as a backup?
Let us know what you find.

trularin · #5 (**permalink**) 12-28-2004, 05:16 AM

How many cranks on the crank shaft did you give to get the 120 PSI?

7 or 9 is suitable. If you did not do enough, it will read low.

However, Michael is correct and his advice is sound.

Cracker · #6 (**permalink**) 12-28-2004, 05:19 AM

Ken - 140psi at 10.7 Compression! Who built your motor? I've never heard of it being that low.

Scott - Your 120 readings are definetely low. typical set-ups and 10.5:1 compression should yield about 160 psi - give or take a few. Like SOBRAC, mine at 11:1 were all at about 190 - which is what you'd expect. Let me know if I can help you do a leak down test - just come up to the house as I still haven't gotten that ride.

A common mistake is putting the wrong pistons with 427 heads which have the 86 cc combustion chambers vs 72 cc . By putting 10:1 pistons (like you stated) your compression will be down in the 8's - probably 8:5:1 or so. To yield 9.5 compession your PISTONS would need to be 11.5:1 to offset the large chambers on medium riser heads (Shelbys are based on these). Good luck!

Ron61 · #7 (**permalink**) 12-28-2004, 05:24 AM

I don't have the 427, but my 428SJC reads from 170 to 180 all the way through when warm with all plugs out. I am running 11:1 compression.

Ron

HSSS427 · #8 (**permalink**) 12-28-2004, 05:58 AM

Thanks for all the prompt replies! Sounds like I may have a potential issue, but the car seems to run like a bandit. However, that's just running around on the street with no track time yet.

Note that the rod and crank info isn't verified, but is what I believe I was told. I will recheck the Lemans Rods - NASCAR crank.

I did the test when the engine was cold (and I mean cold as the temps have been down in the 30s overnight). I had a remote starter, all plugs out, throttle plates blocked open. Cranked each about 4 times.

I'll try the oil test, and Tony that would be great if I could come by to do the leak down test. BTW - are you going to go to the AC Club gathering tomorrow at Hooters? 1:00 Holcomb Bridge location. I'm planning on going and we could figure out a time to do the test and you could drive it then. If not, I'm off work all this week so can do it another time as well.

This may tie into some concerns I originally had about oil usage, and Tony Radford's experience with a carb that was too rich keeping his rings from seating. Engine/car have just over 4000 miles on it and the carb was set too rich when I got it. Had it adjusted about 800 miles ago. Plugs show some black carbon on them and they're relatively new.
Thanks!
Scott

SCOBRAC · #9 (**permalink**) 12-28-2004, 07:51 AM

With 4000 documented miles you are well beyond typical break in. You mention oil use. How much are you using? Oil use and low compression suggests rings or cylinder wall issues.

If you warm up the car and do a compression check again do a second check after a squirt or two of 30wt oil (any kind really). If your rings / cylinder walls are not mating well you will see a pretty big improvement. If not it is likely a piston chamber miscalculation.

I had a 428 that sat for four years with contamination in #2 cylinder. (the engine in my car when I bought it) When I pulled compression I got 180-195 in all cylinders but #2 which was 160.
I pulled the head and found the cylinder pretty chewed up where the piston sat in a pool of fuel for a very long time. It used a quart of oil every 1000 miles.

The tricky thing about low cylinder pressure is you may not have been breaking in the cylinder walls as you would have with higher pressure. Another thing to consider is improperly adjusted valves. If you somehow got (all) your mechanical lifters set to a point they are not allowing the valves to completely close for their full duration, as opposed to .022"-.025" hot, You could create a condtion in which cylinder pressures are low, you are not allowing rings to seat and as a result you are using oil...

Good luck,

HSSS427 · #10 (**permalink**) 12-28-2004, 04:56 PM

This was definately not my day. First the coffee pot broke (well I shouldn't say coffee pot - it's a way too expensive automatic expresso machine.) Then I go warm up the cobra to adjust the valves and do another compression check. In the process of adjusting the valves I break off one of the adjustment studs on the #4 intake rocker arm. Get to a machine shop and it takes the guy about 2 hrs to get the broken stud out of the rocker arm. Now I can't seem to find anyone locally with a replacement part and the Comp Cams folks are doing inventory and say they can't even enter an order till Monday.

As such, the only update is that the lash on the valves I got to appeared to be around .22 and the Isky Cam calls for .018.

Also pulled one of the spark plug wire connectors loose as well, so open to any recommendations on a good brand of wires that will go with an MSD ignition.

I should have stayed in bed........

Scott

SCOBRAC · #11 (**permalink**) 12-29-2004, 07:12 PM

Unfortunately if your valves are set at .022 and your cam card reccomends .018 you are not going to gain anything (compression wise) by tightening them .004"... You may get a quiter idle. (trying to look on the bright side)

MSD makes excellent wires. www.summitracing.com

HSSS427 · #12 (**permalink**) 01-03-2005, 12:01 PM

Tony and I did a leak down check and it appeared tight as a drum. No significant leak down. We also did the compression test again with his gauge and came out with 140-150 readings. Perhaps I didn't let it crank enough times. Tony felt that based on "seat of the pants" driving measurement that I still probably didn't have the compression level that the engine needed to really produce that kick in the pants acceleration. Talking to Keith Craft to get his input on what I should do. BTW running an Isky cam with 250 duration at .050 and .525 lift. Car has the Shelby American aluminum heads with 82cc chambers. Don't have much info on the TRW 10:1 pistons so would appreciate any comment there. Hoping not to get into the short block except for maybe a new cam. Hoping to mill the heads and stage II port to see if I can bump up the compression and horsepower.
Thanks!
Scott

SCOBRAC · #13 (**permalink**) 01-03-2005, 12:59 PM

Don't confuse cranking pressure and compression. Different cam profiles / overlap have different characteristics. The fact that your leak down test was satisfactory is a good thing. I'd leave it alone at 9.5 you are going to make good HP and have good drivability.

Excaliber · #14 (**permalink**) 01-03-2005, 01:12 PM

Mine was 12.5 to 1, cranking pressure was 230-240 lbs.

I'm going with new custom Arias pistons with my high riser 72cc heads and shooting for 9.5-.8 compression this time (street use).

You DID hold the throttle wide open while doing the cranking compression test I hope? If not the engine will "starve" for air and you'll get a low reading.

coyled · #15 (**permalink**) 01-03-2005, 01:17 PM

Scott, how much did each cylinder leak down in the test? If there is no leak down the cylinders are fine. A compression check will only tell you if you have a dead cylinder, weather its 140 or 200, it does not matter much. Scott

DAVID GAGNARD · #16 (**permalink**) 01-03-2005, 01:54 PM

Do not confuse a "compression test" (cylinder pressure) with overall compression........Cam overlap has a lot to do with cylinder pressure at low rpms,I've seen one 351-W with 10.5 to 1 calculated compression,run 120 psi on each cylinder in a "compression test",doing the oil sqirt in each cylinder gave NO higher reading,leak down test showed everything was sealing and within specs,cam had a lot of overlap and at low rpm cranking did not show a lot of cylinder pressure,motor runs fine and goes like hell........

If your compression test per cylinder is within 5 to 10% of each other that is good,if you use the oil sqirt test and gain nothing,then the rings are good,if you do a leak down test and everything shows good,then the valves are sealing and that is good.....if it runs good,then I would not worry too much about it,run it for a while and see how you like it before going in the motor........

David

HSSS427 · #17 (**permalink**) 01-03-2005, 02:10 PM

Well, my summary of what I'm taking away from all this is that there isn't a problem with the rings or valves - YEAH!

I may have a lower (9.5:1) compression rating due to the combination of big chambered Shelby heads at 83ccs and a relatively low compression piston - TRW 10:1, but that's what that combination will produce.

I think I'll put it on a chassis dyno next and see what HP I'm getting to the wheels. If I feel that number is low, then I'll move on to doing the other stuff.

Of course, I get the feeling that no matter what the HP I get, at some point it will seem "too low," and I'll have to go get more anyway. I think it's like lots of things - they can never be "too big"

Thanks!

Excaliber · #18 (**permalink**) 01-03-2005, 02:52 PM

The consensus of opinion around here is:

You can never have to much horse power!

For me though, I DID have to much! Or, should I say at what cost high horse power? Mine was so radical I couldn't "cruise" Waikiki for fear of flooding or over heating of some issue with the motor! 6 mpg on a GOOD day. Running 3rd gear on the FREEWAY to keep the revs up and the plugs clean. But it DID scorch the 1/4 mile!

When I'm done I expect to have "only" around 500 horse power left, that WILL be enough for me!

HSSS427 · #19 (**permalink**) 01-03-2005, 07:28 PM

....but I figure if you have too much you'll run out of room on the island before you can get out of third! And, somehow overheating while crusing Waikiki just doesn't sound as bad as it does in Atlanta traffic ;-)

Out of curiosity, how many members in the Hawaii Cobra Club? Boy I'd sure like to go for a cobra drive out there.
Thanks!
Scott

Excaliber · #20 (**permalink**) 01-03-2005, 08:09 PM

Well we don't actually have a "club" per se. Some of us hang with the Shelby Mustang club which is pretty big and VERY active here on the island.

The "Cobra" guys really came together about a year ago, a core group of perhaps 8 to 10. We met weekly for quite some time. We sponsored and got passed into law a "bill" that allowed us to legally register our cars here. Before that we were "closet" Cobra owners that came out at night mostly...