I warmed the motor up, locked the carb wide open, removed all plugs, and let the motor crank over 4-5 times. I noticed it took this many rotations to get the gauge all the way up. It would jump to 165 or so after 2-3 revolutions, but it took 4-5 to get up to 175 -180.

I was pretty happy to see all the cylinders so close together. I was wondering about the psi it's self, this seems a little high for a street (pump) gas motor. Would this explain the motor detonating on crap 91 octane California gas.

I am going to a much bigger cam and single plane intake, I will post the differance between the two.

 

Well, it looks pretty healthy. The new cam you have will definitely drop it somewhat. But 170-180 isn't too bad. Seems like my 540 is around 175 or so but the old 427 was 190-195 with more compression and a smaller cam.

We can easily get 93 octane around here, 94 if you search, and they both run flawlessly on it with full timing. 91 might be right on the edge, so the larger cam and wider LSA should bleed off a little to help you out. Sometimes you have to do a little ignition curve tuning to get them responsive again.

Are you going to end up with anymore static compression when you go back together?

Compression is a good thing and you don't want to lose too much. Don't forget to get quench as tight as you can. Helps pump gas running.


JIM

 

I think the dynamic compression is going to drop a bit. I went on a site that supposedly calculates it out, but I wonder how accurate it is due to the fact it does not ask for head flow data. I may be wrong but I would think head flow info would effect Dynamic compression ratio. By this sites calulations the new cam will not work at all in the motor. Calulates dynamic conpression as low as 7.0 -7.2 ish with 10.25 static. I am not too worried because of the tests I have seen with a 10.0 to 1 motor making 600 hp with less head flow than I have with the same cam / size motor. I am going to see if we can mill the heads down and get 10.8- 11.0 static. The octane boost MM turned me on to seems to work real well, and is pretty inexpensive. I think I can get 5000-7500 miles out of a $96.00 case of it bumping the compression up to 93-94 octane. That will last me almost two years.


[Modified by 69 N.O.X. RATT, 12:20 AM 12/25/2002]

 

Take this for what its worth, but I will never trust a compression test again!

During my struggles with my 350 setup. I performed many compression tests. Hot/Cold you name it. Each time the compression test yielded results of +/-2psi between cylinders. I even tried different compression guages/setups. Still all the same.

Finally as a last idtch effort to put to bed my suspicions, I performed a leakdown test and got VERY bad results. Most cylinders were leaking 20% or more. How could this be I thought. So I contacted a VERY reputable speed shop in the area that has done work for my father. I told them I wanted to do some PM and see how my engine's leakdown was. I told them that I had done it, but wanted the results verified. I did not tell them what reading I got.

After leaving the car there overnight the next moring I contacted them to get the results. I very puzzled voice came on the phone. It was the guy that checked out the engine for me. He told me that he only checked the drivers side and said there was no sense in going any further.

He said that every cylinder was leaking at least 20% and #7 was leaking over 50%! He said the engine for racing purposes was long overdue for a rebuild/boring.

Hence the reason I am building a 406.

Anyways, the point i am trying to make is I do not put too much stock in the compression testing anymore. The speed shop said they don't even waste their time with it, the leakdown is a much more accurate test. Clearly seen in my engine.

One last thing. When I pulled the engine apart, EVERY top ring in the engine was busted in 2. The number 7 cylinder with 50% leakdown was in about 15 pieces. Yet that cylinder with the compression test, showed nothing out of the norm.

Sorry for the long post, but I spent much to much time banging my head against the wall this past summer, basing my engine condition on compression tests, when performance told me differently.
:cheers:

 

I agree for general motor health a leak down test is far more valuable. I did the compression test because of the detonation issues I have been having.

I should have done a leak down test before I tore the motor down, but I have had no problems that I know of that would warrant it.

 

ski_dwn_it -> You said your compression was +-2 psi but you didn't +-2 psi at what pressure. If it was +-2psi @50psi then you know you got a problem. I like to do both tests as I think you can get a better picture of what's going on, if you do a compression test and the cylinders are all low or some are low and some ore OK then you know right there you have problems, probably with the rings.

If it's any help my 355ci was 230psi in all cylinders with about 25,000 miles and I run on 92-94 octane pump gas at 36 degrees advance ( @3000+ RPM) with no pinging.

 

:confused: Whatever happened to doing a wet/dry compression test and looking at the differance in the 2 tests?? :thumbs:

 

ski_dwn_it. I agree with you. I don't do compression testing anymore but do a leak down once a year. With a simple adaptor and even just a high pressure air line applied to the cylinder you can really hear a leaking cylinder. I have a very accurate leak down gage but after doing a few cylinders just attaching the air line to each cylinder you quickly can hear a bad one. A good cylinder hardly leaks any air and the motor turns over. A bad cylinder will not turn over and the compressor keeps working to keep the cylinder pressure up..
A compression test is just good to get an idea of cylinder pressure but the cam has a large effect on this reading.

 

I don't know that much about big blocks, but i've heard that they react the same as a SBC to octane and DCR. I had a 10.7 iron head and smallish cam that would rattle going down the freeway at 70 mph. That's when Calif. still sold 92-93. I always had to run octane booster and or retard the timing. I had a Unilite adjustable vac advance. I finally had it recourved to all in at 3000 rpm. I set the vac advance on the full manifold vacuum port so any throttle change would drop the advance out. Then i set the total mech advance to something like 34 degrees with 4-6 degrees of vacuum additional possible.

I was able to run that combo without octane booster. It's really a waste to not be able to use full advance so I later broke down and just went with a bigger cam.

 

My pressure was ~160 +/- 2 psi. Yet the rings were completely shot and #7 came out in about 15 pieces. All the rest were just busted in 2. This was with the 219 lingenfelter cam. Yes it was down ~20 PSI from that of others with similiar combos, but I would expect with the engine being in that kind of shape a compression test would have yeilded a little more convincing results. I would doubt those reading with have troubled even the most experienced mechanics.

But the leakdown was a whole different story. It sounded like an open ended air line. Its just my personal experience and may be an isolated case than showed these results, but if it takes just as long to screw in a leakdown tester as it does a compression tester, and to me, I will always feel more comfortable with a leakdown tester in my hand.

Granted, if you built an engine and it had XX psi compression after the breakin, and yyyy miles later it had XX-25 psi, I would think something is up, but for the instances you don't have a before reading, it could trip you up, not using a leakdown tester. :cheers:

 

Factors that play into your compression test are piston ring gap and piston configuration with combustion camber cc's and piston top style as you know. Also, doing the compression test with a hot or warm motor should increase compression ratio since the pistons and rings had a chance to warm up and expand in the cylinder walls, so these may be a little low depending on what configuration you are running.

I just got done checking my SBC 383 TPI motor that is still relatively fresh, with only a couple of thousand miles on it. I am running DSS GTX racing pistons 4032 material with large steel ring gaps (.028) for high power addler with inverted dome pistons running about 10.5:1 compression on a 3.75 stroke and 6" rods with 60cc alum heads. I have a oil drip occasionally on my front china wall coming under head and a tad of oil on my #6 plug and decided to do a compression test to see if the rings checked Ok, which they did, so probably a bad valve guide seal. And oil on this plug may have come from oil routing its way around the front china wall down the head and just dripping or a bad valve guide seal, anyway got to pull heads and want to go to SCE Spartan head gaskets for next stage of power addler project after motor break-in and excess possibilities further on break down of motor.

I did my compression test DEAD COLD and saw 2 cylinders @ 170psi - 3 cylinders @ 180psi - 3 cylinders @ 190psi on a split 240 duration roller cam @ .586 lift and surprising to see the cylinder with oil on plug washer was one that was @ 190psi.
I would expect under normal operating temp the 4032 pistons and rings would expand in cylinder walls bringing me up over 200psi on all cylinders. Also, a few more miles on break-in could increase psi as well yet.