X2 on the leak down test. You need compressed air for that, not a problem if you have a compressor, but you must know someone who does...if you don't.

A leak down and vacuum test will tell you a lot about the general health of the motor. Then you can decide your plan based on the results. Remember that if you do buy the aluminum vortec or any other performance head, you'll need a cam to realize the full potential.

 

When I test the compression in any car I usually wire the throttle wide open, remove all the spark plugs and connect a good battery charger. It is always better to start testing your compression with a fully charged battery. By doing this I am assured that my engine will spin at approximately the same speed for each cylinder. I also test my cars after letting them cool down for a couple hours but while the engine is still warm. I time each cranking cycle for about five seconds using my watch for consistency.

I am tired of some individuals saying that the C3's were/are a disappointment to drive. That may be true for some of the Corvettes made in the mid to late seventies with all their emissions equipment but it did not apply to the cars of the late sixties or to ALL of the C3's. My friends 1970 LT-1 motor is pretty exciting and it was installed in a C3.

When I bought my 1968 Convertible with it's 427 engine I was never disappointed. Yes when my car was made it was rated at 435 Hp and I doubt that it was ever really "boring".

Now the fact that C3's were made for so many years there are a lot of different engine personalities out there. My neighbors had a 1977 C3 and it was not GM's finest engine or Corvette. My 1968 C3 is not like my neighbors old car. I can reach down from above the engine and change every spark plug. My emissions system consists of a PCV valve.

If you are going to tell me that a Camry or Accord could outrun my C3 I would beg to differ. Back in it's day on F-70-15 tires it would still run low 13's.

Today my car is still a blast to drive and where ever I go people are interested in it. It is a 1968 L-71 Corvette and I am very proud that everything still works like it was designed to.

I get a thrill every time I have driven this car in the 26 plus years I have owned it.

Enjoy your Corvette because it is a "Corvette" and may it put miles of smiles on your face for many years to come!

 

If poor ring seal exists you'll turn the engine into an oil burner with increased compression without changing the rings. Plenty of pressure will push past the rings and create lots of blowby creating different problems for you.

The springs I believe you are referring to are used in conjunction with the available vacuum from the engine. With yours being stock the stronger spring would be used assuming higher vacuum levels. It determines when the metering rods will pull from the main jet allowing max flow of fuel. Weaker spring requires less vacuum to keep the rods in the main jet and visa versa.
If you think you need to go leaner, changing the jet is easier than getting different rods in my experience.
Get Cliff's book if you really want to know how the carb works. It's worth the $20ish dollars.

 

Cheap, fast and reliable.

Pick any two combinations.

 

I'm no expert, but I would say min. 140 psi on a good engine. And max. 5% difference between the cylinders.
If this was my engine, then I would say I had a problem.

I would make a leak down test. And I would take of the cylinder heads, and leak down test the short block and after the heads, to see where the problem is.

Just my 2 cents.....

 

That is a cool test plate! ^^

Did you make it, or did you buy it?

 

I made it myself.

 

I learned to use the leak down test when helping my airplane Mechanic and I quickly adapted it to my car applications. On aircraft they measure it when doing your annual maintenance (or 100 hour checks) and record it in the aircraft's logbook.

When I buy a car that is one of the first tests I do after checking the compression. I also make a record for the future, I save the oil sample results, compression and leak down results. I also test my cars when they hit a milestone like every 25,000 miles. It is nice to have long term records for the engine.

I may be going overboard but I like to know what is happening inside the engine and having records to compare to. You start to see trends that can help your maintenance program.

I have never seen anybody test a motor that was dismantled before. That is very creative and you deserve an "A" for effort on that one!

Good Luck keeping your Corvette running strong!

 

So...A couple of lessons learned.
I did another compression test with one I got as a loaner from Auto Zone, and the test came back with every cylinder at 150-155 psi.
phew, thank god.
Maybe the engine was rebuilt before.
The test from the auto zone compression tester has a screw in plug adapter that can be tightened fully with a socket wrench, instead of just a hand tightened. It made all the difference.
I also didn't hold the throttle open while doing the test before, and this made about 15 psi difference.
...Lesson learned.
Don't but the harbor freight compression tester; go even cheaper, and get the loaner from an auto parts store.

I feel really stupid, but ever so slightly smarter.

Thanks for all the great replies.

Now i just have to get this carb working properly.

 

Oh, that's somewhat on the harsh side. Stupid people are the ones that give up, you didn't. You sir, have moved your skill level forward.

 

Those numbers sound much better. So it was the tester that was at fault.
If you are still toying with the idea of higher compression heads then the leak down test that these guys have been discussing is a must before that decision should be made.

Timing, carburation and exhaust can make a significant difference in your performance. Also making sure the Q-jet has a nice 1/4" isolating gasket between the intake and the carb to help prevent heat saturation of the carb bowl and fuel boling problems.
Have you seen or read Lars' papers?

 

I love this thread! I need to do this and the leakdown test but, I'm honestly scared of what I'll see.


I have to "second" the point that was made a few times to do this with a hot engine- the ring seals are designed to seal at temp -ESPECIALLY on the forged L82 motors that I'm going to guess had wider ring gaps because the alloy expands more from cold to hot.


I'm also going to ask a question: there's a lot of comments that this should be done at wide-open throttle, but my understanding is that cylinder pressures will be highest @ your max torque RPM, not at max HP RPM... Anyone want to opine on this?


I'm also going to throw out there for discussion the issue of stock timing chains stretching out and further retarding the already retarded cam timing on the smog era C3s. -These things came 3 degrees retarded from the factory (many/ most after market cams you buy today come with 2-3 degrees ADVANCE ground in). Plus the OEM timing chains DO stretch over time, so I absolutely would expect a 30 year+ old C3 to have lower-than-expected cylinder pressure from both the cam timing and then the timing being further retarded as the chains stretch. -Do a leak down test @ temp to determine how leaky the rings are.

If the leak-down test is decent but cylinder pressures are low (for your CR), then focus on ignition timing, and replace that timing chain with an adjustable one and install it straight-up or slightly advanced and test your cylinder pressures again! ---CHEAP FIX!

How bad would you feel if you had your bottom-end bored and honed and bought new pistons and rings and spent piles of $$ and then found out that you just needed a timing chain and the ignition timing tweaked?!?


Adam

 

I don't quite understand. If the compression test came back with relatively good pressure ratings, couldn't I assume the rings are at least as good as the measurement taken, 150 psi?

Aren't piston rings testing at 150 psi good enough to get upgraded vortec heads?




I would upgrade the cam at the same time as the heads, maybe even put in a hydraulic lifter conversion, and those kits usually include new cam chain and sprockets, so I'm hoping this upgrade would take care of all the issues you are speaking to.
Of course, a lot more research and learning is going to happen before I get to that point.

 

As a rule your maximum BMEP (Brake Mean Effective Pressure) will occur around the torque peak. Think of is this way, you have a piston with a certain area, with a certain pressure above it pushing on a lever that is a certain length. Pressure times area times mechanical advantage will give you a Pound-Force that it is exerting. There are other things that can affect pressure, and Horsepower is calculated based on torque and they always match at 5252 rpm. Horsepower goes up as torque goes down because rpm is part the calc. RPM x Torque / 5252.

Not sure if I cleared anything up, let me know if this helped, Chris

 

There are a lot of options out there and it sounds like you have a decent short block, just based on the consistency of the numbers you told us. If your goal was to up HP, a cam and heads is a great way to do it. I would look at what it will take to do the conversion, new intakes and valve covers add up fast.

My street rod has a very mild Goodwrench 350, with a decent exhaust, well tuned ignition system and a good fuel system it has quite a bit of zip.

Sounds like a good project, enjoy!

 

Maybe. Lots of variables in a compression test. It can tell you the general health of the cylinders as compared to each other.

A leak down test is going to measure the % pressure leaking out of the cylinder. If the valves are sealed good then it's measuring the leakage of the rings (assuming the head gasket is not leaking of course).

Raising the CR of the engine from say 7.6:1 (likely the real CR of your engine based on a 17 cc piston and a combustion chamber that may be as large as 81cc's) to 9.0:1 is a pretty good boost. Since the cylinder pressures will increase so will the likelyhood of excessive blowby. The rings seal only as good as the pressure put to them during break in. So in a 7.6 CR engine the seal is not as tight as a 9.0:1 engine. Once break in is complete there is no reasonable way that I know of to get the rings to seal better.

I like to see less than 5% leakage. 10% may be acceptable but definitely pushing the limit. More than that and you will probably want new rings before upping the CR.

Edit: Occurred to me that I may not have been clear on the % of leakage. 5%-10% is what I would consider for a compression bump or a higher performance engine. For a street driven engine with no planned bump in compression you may see 20% or even 25% leakage and still be ok if they are consistant to a reasonable degree amongst the different cylinders and oil consumption is not an issue.

 

 

Translation: The above is a very geeky and long-winded way of saying "Yes, peak cylinder pressure will occur at your peak torque RPM."


Adam

 

There, or real close....

 

Not sure what your point is here. Compression testing is done at cranking rpm (~200rpm). Having the throttle closed will always limit cylinder pressures (closed throttle plates restrict intake flow, therefore less pressure in cylinder). Peak torque will be at wide open throttle too...