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Old 12-30-2008, 12:50 AM
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Raising Compression on my 350 Olds?????

Alright guys... I've been thinking some more on how I am going to want to build this olds 350 that I found. So I was wondering, how can I raise the compression on the 350? This may sound like a stupid question to some of you, but what modifications or what parts would be needed to raise the compression on the motor? Also, with those modifications, would I run into any problems causing further work to be needed? Let's say I wanted to get to 10:1 compression.... what would I have to do, and how much would it cost me for the work/parts (estimate?). Sorry guys, I am just clueless

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Old 12-30-2008, 02:45 AM
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I'll try to help you out here. When you're figuring static compression ratio, you need 5 figures.

Cylinder volume (.7854 times cylinder bore times cylinder bore times crankshaft stroke times 16.387 to find cc's)

Cylinder head combustion chamber volume (usually given in cc's)

Head gasket volume (.7854 times bore times bore times compressed thickness times 16.387 to find cc's)

Deck height volume (deck height is the distance from piston crown to block deck with piston at top dead center) (.7854 times cylinder bore times cylinder bore times deck height times 16.387 to find cc's)

Piston crown volume (normally given in cc's by the manufacturer)

When the piston comes down in the bore, all 5 of these volumes are filled with air/fuel mixture. When the piston comes back to the top of the bore to compress, the mixture is compressed into all the volumes except the cylinder. So for instance, if you were drawing in 10 parts of mixture and you compressed them into 1 part, the motor would be 10.0:1 static compression ratio.

Changing any of the volumes will change the static compression ratio.

Boring the cylinder larger will increase the cylinder volume and increase the SCR.

Using a head with a smaller combustion chamber or having a machine shop take a cut off the deck of your heads will decrease the volume and will increase the SCR.

Using a thinner gasket that has less volume will increase the SCR.

Cutting the block decks to decrease the piston deck height or using a piston with a taller compression height to decrease the piston deck height will increase the SCR.

Changing the piston to one with less volume in the crown will increase the SCR.

Let's run through this and you'll see how to figure this stuff for yourself. This will be an elementary lesson, assuming that you know nothing.
The 350 has a bore and stroke of 4.057" X 3.385".
Get your calculator in front of you. Clear it.
Press .7854
Press X
Press 4.057
Press X
Press 4.057
Press X
Press 3.385
Press X
Press 16.387
Press =
You should have gotten 717.06584
This is the number of cc's in a 350 Olds cylinder. Drop the last three digits and call it 717.06, so now we have the cylinder volume.

Let's say the head you have is a ID Code 8, casting number 411929. This would be a smog head with 79cc chambers, so now we have the chamber volume.

Let's say you measured a used head gasket out of your motor and it had a 4.100" bore and was compressed to 0.040".
Clear your calculator
Press .7854
Press X
Press 4.100
Press X
Press 4.100
Press X
Press .040
Press X
Press 16.387
Press =
You should have gotten 8.65
Now, we have the gasket volume in cc's.

Let's say that you measured the piston deck height at 0.037".
Clear your calculator.
Press .7854
Press X
Press 4.057 (the cylinder bore)
Press X
Press 4.057
Press X
Press .037
Press X
Press 16.387
Press =
You should have gotten 7.83
Now, we have the piston deck height in cc's

Let's say the piston is a flat-top with no valve reliefs, so there is nothing to figure. If it had valve reliefs (notches cut into the piston crown), or was a dished piston, we would have to either look up the information or pour the notches/dish with a cc burrette and fluid.

Now, we have our 5 values. (the piston crown value is 0) Add all of them together. You should get 812.54 cc's. That's the total amount of mixture drawn in on the intake stroke. Now, remove the cylinder volume from the total. 812.54 less 717.06 = 95.48 cc's. That's the volume into which all of the drawn-in volume is compressed. Now if we divide 812.54 by 95.48, we will find a 8.51:1 static compression ratio for this motor.

OK, you should now be armed with enough information to figure the compression ratio on any motor without the aid of an online calculator.

The whole point of this is that you have to know where you are before you know where you're going. If you begin changing parts around without knowing how they will impact the outcome, you might just as well take up knitting and leave the hot rodding to someone else who will take the time to learn and figure it out for themselves.

As you arrive at each answer on your calculator, write it down on a sheet of paper. When you have all 5 values written down, draw a line under them. Add them on your calculator. Draw another short line under the total and write down the total of the 4 values (the entire total less the cylinder volume). Then write an = sign to the right of the 4 values total. Do your division like I instructed above and write the static compression ratio to the right. Like this....
717.06 cyl
79 cham
8.65 gask
7.83 deck
0 pist
--------------
812.54 total drawn in
----
95.48 total compressed = 8.51

Now, I've spent an appreciable amount of time doing this for you, but I did it because others helped me when I was 16 and I figure it's my turn to give some back to a youngster. Say thank you.

Last edited by techinspector1; 12-30-2008 at 03:09 AM.
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Old 12-30-2008, 12:16 PM
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Quote:
Originally Posted by hemizach
Alright guys... I've been thinking some more on how I am going to want to build this olds 350 that I found. So I was wondering, how can I raise the compression on the 350? This may sound like a stupid question to some of you, but what modifications or what parts would be needed to raise the compression on the motor? Also, with those modifications, would I run into any problems causing further work to be needed? Let's say I wanted to get to 10:1 compression.... what would I have to do, and how much would it cost me for the work/parts (estimate?). Sorry guys, I am just clueless
The first problem is, where are you starting from? Prior to 1973, all small block Olds motors used the same size chambers in the heads (about 64 CC). Compression ratio was varied by changing the volume of the dish in the piston. The W-31 motors used flattop pistons and got a 10.5:1 CR with the stock steel shim head gaskets. The lesser "high compression" engines used a small dish and got about 10.25:1 CR. The low compression engines used a large dish and got 9:1, which dropped to about 8.5:1 for 1971. If you have the 64 cc heads (casting numbers 1 through 7 and including 7a - though not the 7A heads used on the 307 motors) then you need new pistons with the correct dish volume. If you have later heads with the 80cc chambers, you can swap the early heads and raise compression without changing the pistons.

One other consideration is that the stock head gaskets had a compressed thickness of around 0.014". The more common FelPro blue head gaskets seal better but have a compressed thickness of around 0.040". This results in about a 1/4 to 1/2 point drop in compression.
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Old 12-30-2008, 02:07 PM
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First off, techinspector1, that helped GREATLY. Thanks for running through all that for me. I didn't expect anyone to take that much time answering my post, but it's appreciated

joe_padavano, I'm starting with a 1971 motor, I do know that much... so what I am getting from this... Is that one thing I should be looking at doing is getting some new pistons that would be correct for what I want.

Anyone, how much material is commonly taken off of a head when getting it milled (is that the right word anyway, bah....). I've head of guys taking their heads in and getting all kinds of material taken off but what is a safe amount? I think I have read .024 would be safe and wouldn't mess with your intake any... but is this true? I have also heard that taking off to much material will give you valve issues.... Help again please. I thought i know stuff about engines before I started to think about building one... turns out, I got a long way to go
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Old 12-30-2008, 02:19 PM
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Zach, are you planning on a complete rebuild of this engine? If so then you should tear the whole thing down (if you haven't already) and find out exactly what you are starting with. If you tear it down and it's determined that the cylinders need to be bored then new pistons are a must anyway.

If you decide to keep the short block intact then at the very least pull the heads and look in the holes to get some idea of what you have. You may or may not be able to bring your compression up with just a head swap.

Either way, you need to figure out what compression you have now because if you don't you could end up with too much and you won't be able to drive it on gas you get from your neighborhood gas station. And at 16 you're probably not going to like having to pay for a full tank of race gas every time you fill the car up. That stuff is not cheap.
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Old 12-30-2008, 02:32 PM
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yeah... yer right... I will be looking at everything eventually. I guess I'm just getting a little far ahead of myself. Just asking questions for future reference is all. And racing fuel isnt cheap... yer right... I think it was like 5 bucks a gallon last summer....
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Old 12-30-2008, 05:24 PM
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What are your goals for the vehicle?

If the cylinder walls are in good shape yet (somewhat unlikely since the engine is 38 years old) you might get by with reusing the pistons.....but you probably won't get much compression with them. Your best bet would be to bore the block .030 over and get smaller dished pistons, or flat-tops.

I have successfully milled small block Olds cylinder heads .060" with no issues. The intake did have to be milled to fit, though.

As far as combustion chamber sizes go, I have found the #7 and #7a heads to be around 69-71cc's from the factory. My #7 heads were milled .060" and after a three-angle valve job, they were 60cc's.

As for head gasket options, Corteco makes a reasonably priced composite gasket that measures .028" thick compressed as opposed to the Fel-Pro .039".
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