azbluemach1

Hey guys - wanted to see what you think of these results. Car is all 365hp parts (intake, exhaust, assume heads) but the guy who rebuilt the engine somewhere between 79-90 told me he put 9:1 pistons and hydro bump stick in. Plugs were black/grey with white on the tips (I have pics if you need to see them).

Results:


160psi 4 B 4 180psi
165psi 3 L 3 182psi
160psi 2 O 2 180psi
162psi 1 C 1 180psi
K
Front Of Engine

By the way - running a compression test SUCKS!!! My gawd, I had NO idea how freaking hard it is to get the plugs out - and even worse getting the compression guage tube in!!! An old time racer buddy of mine who had a 66 back in the day said he cut holes in his wheel wells so he could have a straight shot at his plugs. Now Im thinking that is an excellent idea! I invented some interesting profanity combos tonight.

Black60

Odd the difference from one bank to the other. Consistency in the readings from cylinder to cylinder is more important than overall compression however I have rarely seen what your motor displays.

You had the primary butterflies in the carburetor wide open during your recordings for each bank?

Nevertheless, your readings should leave you with a level of comfort knowing the motor is pretty healthy compression wise. There are many other considerations when determining engine health but for an engine rebuilt 20 years ago I would not be disappointed.

Ralph

Allcoupedup

Nice consistency on each bank. My explanation to the difference on one side vs the other is a difference in compression ratio.

Could be caused by a number of factors

- one head milled more than the other resulting in different combustion chamber volumes on one head vs. the other. If your heads are not matched, this is common. Any cahnce you know about the combustion chamber volume on each?

- deck height higher on the side with lower compression numbers. My deck was .010 higher on one side - this is fairly common.


Brian

67L36Driver

I'd be looking at your cylinder head casting numbers to make sure you have a matched pair of heads. Off hand it looks like a half point of compression ratio difference.

They are nice and consistent along each side.

rgs

I agree with the others who question the matching of the heads. It would appear that one has more volume than the other. They could be the same part number with one milled more than the other. Another test of value similar to the compression test, is a leakdown test used for aircraft engines. Instead of cranking over the motor to push air, a compressed air source, regulated at 100 psi at the tester, is used and air is put into the cylinders. Two gauges separated by a specific size orifice then show the drop in pressure. The volume of the cylinder head will not have any affect on the leakdown results. Leakage can also be isolated to its source by listening for escaping air.

If a leakdown test shows consistently the same readings across all cylinders, then I would definitely question the matching of the heads based upon your compression readings.If that is the case, I would start looking to get them matched. This much mis-match in compression will result in noticeable power imbalance and resulting vibration. It won't be terrible, but could be made much better.

BTW, if you use an air fitting on a short piece of pipe to the spark plug adapter, and the mating fitting on both the compression gauge and leakdown tester, you can do both on each cylinder without fighting the hoses. It isn't any more difficult to install than a spark plug then.

Plasticman

Did you perhaps do the "high reading" side first? The compression (and leakdown tests) should be done on a warm engine, and a fully charged battery in the case of the compression test. If the high reading side was done first, then it is possible that decreased engine temp and/or battery state could cause lower numbers (doubt that it would be so even side to side, but why panic until you know what the probelm is?).

Recommend redoing it the other way around (unless you find something obvious like the previously mentioned "different" cylinder head numbers).

Good luck!
Plasticman

Cobra66

[QUOTE=azbluemach1]

160psi 4 B 4 180psi
165psi 3 L 3 182psi
160psi 2 O 2 180psi
162psi 1 C 1 180psi
K
Front Of Engine
Interesting, as I have similar compression readings, and my heads are recently installed, new, Pro Top Line aluminum units.

166 176
164 178
168 180
160 179
front of engine

The block, a 350 cu.in., was allegedly rebuilt in 2003 by a previous owner, and I know just about nothing about the internals other than the cam seems quite aggressive.

Car runs fine, so I've just accepted the differences in compression from one bank to the other.

azbluemach1

You know - I was wondering about that. I did do the drivers bank first (the one with the higher reading) - with the engine hot. The passengers side was a total pain in the *** - and it took me quite a while to do it. Just for a sanity check though, I went back and did the first 2 cylinders on the drivers side, and the readings matched what they were when the engine was hot.

I will double check the casting numbers tonight - Pretty sure I did, and they matched, with dates matching nicely with the build date of the car.

The battery is a brand new optima red top.

I am trying to decide if I want to pull the engine now and put a small crate engine in her and then rebuilding it back to stock, or work on beefing up the driveline and other stuff this summer while saving up for a really nice crate motor and 5 speed. I am pretty torn on where I want to go with the car...

w1ctc

Has the head gasket been changed to a thicker one on the low side? Take a look a where the head meets the block.

DansYellow66

Wait until you run a leak-down test (which is more useful than a compression test alone). They're a load of fun.

Dan

MikeM

My advice is to quit yankin' on the hood release when there's nothing wrong or you'll be writing in wanting to know how to get your hood open when the cable wears out and breaks.

If it runs good, I wouldn't even pursue why the difference bank to bank. Why worry about it? Now if you were posting 90's on one bank and 85's on the other, that's cause for concern.

azbluemach1

Lol, who said it runs good?

Casting numbers are the same, but the cast dates on the heads are 5 days apart. Is that normal?

MikeM

Very normal.

azbluemach1

I re-ran the compression test, and the passengers bank came up to 180 avg with the engine warm. I guess I just worked too slow the first time around!! Thanks for all of the input though guys!

Plasticman

Congrats! And thanks for letting us know the outcome.

Plasticman