race fuel and CR vs compression psi

[T77911S]

i have a late model stock car with a chevy 350 in it. it has 64cc heads and I run 40degrees advance. and have read the 64cc heads make 10.4 to 10.7 CR.
I did a compression test and it was about 210psi.
do I need to run race fuel in it.

its an old car and I am thinking about turning it into a track car but don't want to spend $400 in gas for a weekend.

[masraum]

Originally Posted by T77911S

i have a late model stock car with a chevy 350 in it. it has 64cc heads and I run 40degrees advance. and have read the 64cc heads make 10.4 to 10.7 CR.
I did a compression test and it was about 210psi.
do I need to run race fuel in it.

its an old car and I am thinking about turning it into a track car but don't want to spend $400 in gas for a weekend.

The CR is not mainly based on the heads. The CR is mostly based on the pistons, and then once you've got that settled, then the heads come into play.

I wouldn't think mid 10s would be something that would require super high octane race fuel, but you'd probably have a problem running 87octane too.

Some others with more experience will doubtless chime in.

[JackDidley]

Originally Posted by masraum

The CR is not mainly based on the heads. The CR is mostly based on the pistons, and then once you've got that settled, then the heads come into play.

I wouldn't think mid 10s would be something that would require super high octane race fuel, but you'd probably have a problem running 87octane too.

Some others with more experience will doubtless chime in.

The heads effect compression ratio. Big chamber, low compression. Small chamber, high compression. Iron heads hold heat and are more likely to contribute to spark knock, Aluminum heads are generally safer. That said, the 10.7 compression ratio will get by on pump premium. Worse case you back the timing down a couple degrees. A bigger cam will reduce cylinder pressure and that effectively lets you get by with lower octane.

[masraum]

Originally Posted by JackDidley

The heads effect compression ratio. Big chamber, low compression. Small chamber, high compression.

Right, but the pistons are the biggest factor that you have to consider, right? If you buy high dome pistons they are going to give a range of compression that's pretty fixed based on the heads that you're using. There are other factors to consider too, deck height, head gasket thickness, etc...

For instance, these will approx yield the following CR with the given specs
https://www.jegs.com/i/SRP/867/140674/10002/-1
SRP Small Block Chevy High Compression Forged Pistons

Small Block Chevy (+.030")
Bore: 4.030"
Stroke: 3.480 (355ci)
Rod Length: 5.700"

58cc 14.0:1
64cc 12.7:1
70cc 11.7:1

But these are very different
https://www.jegs.com/i/SRP/867/138081/10002/-1
SRP Small Block Chevy Flat Top Forged Pistons

Small Block Chevy (+.030")
Bore: 4.030"
Stroke: 3.480 (355ci)
Rod Length: 5.700"

58cc: 11.1 to 1
64cc: 10.3 to 1
70cc: 9.6 to 1
76cc: 9.0 to 1

[T77911S]

yes, was thinking a bigger combustion chamber (70cc) to reduce CR if needed, that way I wouldn't have to change pistons.

I don't know what pistons are in it, assuming flat top pistons,.

since I wouldn't have any rules to follow I was thinking of the vortec head but it is 64cc head also.
right now it has some old turbo heads on it.

I could also pull off the restrictor plate and run a 4 barrel.


still in planning stages for the conversion. trying to put everything together that would have to be done. I can still run the car at the circle track here but don't know if that is what I want to do. if I wreck it I don't really have the tools to fix it as far as body and frame.

I think it would be a fun track car, also wouldn't worry about it on a road course that much.

big question right now is cutting out the chassis to put in a passengers seat, routing the exhaust to lower the floor pan and making leg room for passenger.
kinda sucks, just did a week end at NCM and I didn't need an instructor but that track is 11 hours away. wouldn't need to put in passengers seat.

[T77911S]

any way I can figure out what the CR is with engine together.

even at 12.7 and going to a 70cc head only brings it down to 11.7, that would still be race gas.

the car was built back in the 80's. don't know if it had a rebuild or when what was done to it.
I really doubt it had the hi comp pistons as I don't think the rules allowed those.

I suppose I could bore scope it and see whats in it.
if flat top then I am looking 10.3CR

[T77911S]

my father in law built the car and wrote the rule book for the track he works at.
his rules say flat top pistons and no more than 10.5cr, I cant imagine any other tracks allowing higher CR.

cant really ask him about it. he is strange/sentimental about the car so he does not mention it.
I did ask when I got the car from him, did not get much out of him, he mumbles and im deaf.
I did ask him about gas and he kind of mumbled and said something like yes, to be on the safe side.

so 10.5CR on 93 pump gas ok?

[masraum]

Originally Posted by T77911S

any way I can figure out what the CR is with engine together.

even at 12.7 and going to a 70cc head only brings it down to 11.7, that would still be race gas.

the car was built back in the 80's. don't know if it had a rebuild or when what was done to it.
I really doubt it had the hi comp pistons as I don't think the rules allowed those.

I suppose I could bore scope it and see whats in it.
if flat top then I am looking 10.3CR

I'm just thinking out loud here, but I would think you could get a rough idea by running the thing to TDC when the valves are closed, fill with fluid. Then run the piston to the bottom of it's travel, and fill with fluid and compare? I'm just not sure if there's a time when the piston is at the bottom of its stroke when both valves would be fully closed. I'm not sure exactly where the overlap occurs, but I'm thinking at the bottom of the stroke which would make it more difficult.

[masraum]

I found a link that talks about using a manometer.
https://www.autoblog.com/2016/02/23/how-to-determine-compression-ratio/

But it doesn't say what "15" is, I would assume inches of water.

Method 2 of 2: Use a manometer

This method is ideal for those who have an assembled engine, and want check the vehicle's compression ratio through the spark plug sockets. You will need assistance from a friend.

Materials Needed

Manometer
Spark plug wrench
Work gloves

Step 1: Warm up your engine. Run your engine until it gets to normal temperature. You don’t want to try this when the engine is cold because you won’t get an accurate reading.

Step 2: Remove the spark plugs. Turn off the ignition completely and disconnect one of the spark plugs from the cable connecting it to distributor. Unscrew the spark plug.

Tip If your spark plugs are dirty, you can use this as an opportunity to clean them.

Step 3: Insert the manometer. Fit the nozzle of the manometer into the hole where the spark plug was fixed. It’s important that the nozzle is fully inserted into the chamber.

Step 4: Test the cylinder. While you’re holding the manometer, have a friend start the engine and accelerate the car for about five seconds so you can get a proper reading. Turn off the engine, take out the manometer nozzle, and refit the spark plug with the proper torque, as listed in the manual. Repeat these steps until you’ve tested each cylinder.

Step 5: Perform a pressure check. Each cylinder should have the same pressure, and they should match the number in the manual.

Step 6: Calculate the PSI to compression ratio. Calculate the PSI to compression ratio. For example, if you have a manometer reading of about 15 and your compression ratio is supposed to be 10:1, then your PSI should be 150, or 15×10/1.

[asphaltgambler]

Do you run mufflers on it? If so you should hear if the engine is pinging on hard acceleration if the octane is not enough. Also 210psi is fairly stout cranking compression - MO / experience is you're a little tall with that amount of advance. You should back that off to 38 total and then see the result. With 40 degrees in you're very likely loosing power in the upper range.

[T77911S]

40 to 38, yes
was thinking that too.

just read an article in hot rod, the guy said 200psi is his limit for 93 octane.

I will have to look at my numbers again,
don't remember if it was 200 or 210.

[asphaltgambler]

^^Yes in a conventional iron headed small block - that's about it^^^

[masraum]

Originally Posted by T77911S

40 to 38, yes
was thinking that too.

just read an article in hot rod, the guy said 200psi is his limit for 93 octane.

I will have to look at my numbers again,
don't remember if it was 200 or 210.

If that's no good, then you can just stick a bigger cam in with lots of overlap. That'll fix it!

OK, right, it doesn't have to be bigger to have more overlap. Also, I think overlap will drop some of the low end power.

[JackDidley]

I would say keep the compression and run 93. Slowing down the timing to 36 or 38 is way better than buying race gas. Do the rules dictate the heads and cam? As stated by someone above a cam with overlap will kill cylinder pressure and thats what it is about. Static compression can be high but the right combination of heads and cam can change the cylinder pressure big time.

[wdfifteen]

You can get thicker head gaskets. Felpro says their .05” gasket will reduce CR by half a point. They apparently publish gasket volumes for their various gaskets too, so you can calculate how much a given gasket thickness will change the combustion chamber volume.

[wdfifteen]

Originally Posted by afterburn 549

If you have ever worked with one of these you will know five thou either direction will not do much of anything.

Good to know. How thick do you think the stock gasket is, .045?”
I’ll listen to Felpro, I’m pretty sure they know more about it than you do.

[JackDidley]

Originally Posted by wdfifteen

You can get thicker head gaskets. Felpro says their .05” gasket will reduce CR by half a point. They apparently publish gasket volumes for their various gaskets too, so you can calculate how much a given gasket thickness will change the combustion chamber volume.

Originally Posted by afterburn 549

If you have ever worked with one of these you will know five thou either direction will not do much of anything.


https://pitstopusa.com/c-134528-tools-equipment-engine-tools-cylinder-head-tools-cylinder-head-cc-kits.html

.05 is 50 thou not 5. A stock steel shim gasket from 1970 would have been around .020. So ballpark numbers could take you from 11 to 1 down to 10.5.

[masraum]

0.05 = 0.050 = 0.050000 (discounting significant digits, otherwise, mathematically, they are the same number)

Just like 1 = 1.0 = 1.00

.1 - tenths
.01 - hundredths
.001 - thousandths
0.005 = 5 thousandths
0.050 = 50 thousandths or 5 hundredths = 0.05
0.055 = 55 thousandths

[masraum]

Originally Posted by afterburn 549

.05 leaves a lot up for grabs, MM?
0.050 defines and specifies it and takes the guess work out.

I would not want to leave .05 up for grabs on a print..., that would leave room for guesswork
To each their own.
Head gaskets come in many thicknesses and can even be custom ordered and made.
Cometic will make you most anything you want at very little extra charge .
0.050 will definitely drop the static CR .

Talking about a SBC, I believe everything is usually in inches.

[wdfifteen]

Originally Posted by masraum

0.05 = 0.050 = 0.050000 (discounting significant digits, otherwise, mathematically, they are the same number)

Just like 1 = 1.0 = 1.00

Yes, mathematically, but if you are measuring something those significant digits are significant.