1965 Corvette 327 300hp 10.5 to 1 compression ratio from factory.

I just bought this car which appears to be original 49000 mile car, but I'm not sure if the mileage is correct or if the motor has ever been rebuilt.

1) Is there a way to check compression ratio without taking the motor apart?

2) Is there away to check if it has hardened exhaust valve seats without taking the heads off?
Dave

 

1.) You can check with a local NASCAR facility and ask if they can check the compression ratio. I know they can check the CID and I think they can check the CR, too.

2.) No way I know of, unless the head casting number becomes a "giveaway". Of course if it does have the hardened seats, your "original 49000 mile car" goes out the window.

RACE ON!!!

 

Thanks for the reply. I just spoke to an engine builder and he said do a compression test and if the compression is 165lbs or more the compression is too high for street use. Dave

 

Find another engine builder. While cranking compression is an important number, It has more to do with cam cam timing than it does with the mechanical compression ratio. My 23 year old, 165,000+ mile, never had a head off, raced weekly for years, 9.0:1 compression, oil burning, stock, crossfire engine, has cylinders that range between 165 and 175 psi cranking compression.

So therefore he thinks a 9.0:1 "compression ratio is too high for street use"? Maybe he knows something about screwing an engine together, but he doesn't know jack about spec'ing them or the cause and effect of compression ratio vs cam timing as related to cranking compression pressures. I can't believe that someone who thinks of himself as an engine builder could say such a thing. Amazing! Imagine! My stock, worn out 9.0:1 compression ratio engine has a "compression ratio is too high for street use".

RACE ON!!!

 

He did mention compression bleed off due to cam choice and or cam timing, but not knowing what cam or cam timing is in it, he said due the compression test to see where your at. He didn't say 165lbs = 9 to 1 or anything like that. The last street motor he did for me was 9 to 1 and it ran fine on 91 octane. I did speak to a 2nd. engine builder and he said the same thing. Short of taking the motor apart do the compression test. His number's were "140lbs your good to go and at 200lbs the compression is too high for pump gas" (weather it be from cam timing or the actual compression ratio). The car is not stored at my place so I have not had a chance to check head casting numbers yet to see if they have been changed as you suggested. I will do this the first chance I get. I'm just looking for an easy way to find out if this motor will run well on pump gas or not. Dave

 

But he did say "165lbs or more the compression is too high for street use" And that is SO wrong!


If it had any kind of a cam, it would probably have run on turpintine. What did the compression on that motor test at?


I don't know where you live, but if you have all these local experts, why are you posting questions here? What is the point in asking for advice and then act as if you are smarter than, and start arguing with those kind enough to respond? Idiots!

RACE ON!!!

 

First off I'm not arguing with anybody here, I'm just trying to learn something and I never claimed anyone to be a expert. I've just repeated what I've been told by others and looking for input to their statements to which you have given, thanks. I'm sorry if you have taken this the wrong way. I never did a compression test on my last motor so I don't know what it would be. Dave

 

Ok my answer to #1 is basically no as static c.r. measuring should be done with heads off eng. And a compression test can be helpful to diagnose and as a relative indicatior but can vary depending on camshaft. For Ex. my 8.8 c.r. engine measured 180psi-190psi with the wiped Crane Cam, burned regular unleaded and no detonation in 100*F weather driving. Compustion chamber shape and quench can make a big difference preventing detonation. IMHO u would spend more time experimenting trying to measure c.r. than just to pull the heads and measure directly. Heck u can also install some hardened vlv seats while your heads are off and add a vlv job too.
Now if you find your c.r. too hi while heads off you can fix it by replacing heads (upgrade) or have the cambers opened up, maybe even contoured.

BTW Dave if you simply get the stamped block number and head casting number for us its likey we can look up the stock head chamber size and more than likely the stock c.r. too. This would also help answer question #2 for hardend seats. And hardend seats aren't as big an issue as most think. Many owners and head rebuilders post that unhardend seat heads work just fine. And a leakdown test will tell u if a vlv job needed - then install those hardend seats once heads are off if vlv job needed. A leakdown test is as simple as a compression test and will better tell you the condition of the cyl(s) - right away.

If u are really worried 'bout detonation try a MSD knock box that senses detonation - sells <$150 and installs in the block wtr jacket plug hole.

Yea, don't let the turkeys lurking here get to you Dave. They have more fun ruining your thread than they do by helping you. And challanging a turkey's expertise to get an straight answer really burns them. Just put'm on your honor (ignore) list where they belong.

Hope this helps.
cardo0

 

One turkey to the ignore list, it's too bad that you have to do this sometimes.
I think a leak down test would be a very good idea to see what condition this motor is in before pulling the heads off. The car is in the body shop for a paint job, so it will be a while before I can get the car to somewhere that I can get a leak down test done. I went to the shop yesterday to get the head & block numbers but the car was all papered & taped up for paint. I'll have to wait.
What is a Knock Box? How does it work, retard timing under knock conitiions? Dave

 

Turkey, eh? OK. Even Cordo0, (I've got oil pumps in my lifters), was doing well before his last paragraph. Look at posts #2 and #4, I answered you your questions honestly and correctly, to try to help. It was only after you challenged my statements, based on the ramblings of some misguided so called local experts that you took offense at me taking offense. Enjoy your endeavors and I hope and pray that your cranking compression pressures never reach 165#.

RACE ON!!!

 

Come on gentlemen you know that 165 lbs is not the compression ratio it`s only how tight the cyl seals. And camshaft to cut compression? The valves are closed on compression, not open except on overlap. CC the heads if you want the correct numbers. Dont go by someones computer program with magical consequences. 'CFI' merely stated what are obvious untruths thats all.

 

Well u can go to the MSD w/s and read up on the Knock Alert part #8964. And our supporting vendor Jeg's has them at a competitive price of $180. I have the sensor in my block but not hooked up yet. But u can just velcro the box anywhere to monitor and tune with then once your set up unplug from the sensor and store it. Hey if u have more than 1 hot rod all you need is extra sensors to monitor and tune them all.

IMHO if u find c.r. too hi and detonation, a head change or chamber modification is the easyest and cheapest fix. But that leak-down test can tell u if cyl wear/rings are a problem or not.

Good luck.
cardo0

 

YES! Valve timing has a greater effect on cranking compression than the compression ratio. When the piston approaches TDC on the compression stroke, both valves are closed (as you said). However, the stroke immediately preceding the compression stroke, is the intake stroke. The intake valve does not close at or before BDC. The intake valve closes AFTER BDC, into the compression stroke. Compression cannot begin until the cylinder is sealed. Therefore, in two identical engine, the cranking compression will be higher in the engine with the camshaft (milder) that closes the intake valve closer to BDC

If you have ever heard the terms "static compression ratio" and "dynamic compression ratio", and wondered what the difference is? Here you go. Simply, the static compression ratio is the comparison of the volumes between when the piston is at TDC vs when it is at BDC. The dynamic compression ratio takes valve timing into account and reflects the "effective" compression ratio.


See above. Overlap occurs at the end of the exhaust stroke and into the beginning of the intake stroke. The length of, or a difference in length of, the overlap period has no effect on cranking compression.

And as a follow up, no one that knows engines would say that 165 pounds of cranking compression is too high for street use.

RACE ON!!!