Calculating compression ratio

[1low4x4]

So, Last time I called Comp cams, they wouldn't or couldn't recommend a cam to me, even with all my engine specs, without a compression ratio. My guess is they don't really know without the computer telling them, and the computer requires a compression ratio. So..

I looked up a compression calculator and then searched for the numbers Id need. Everything was pretty straight forward until the "Deck Clearance". Is there a way to measure this with the head on?

For deck clearance, I found 0.025 as a good number for a stock 350, but I have flat top pistons with 4 valve relief, so Im not sure if that changes the number. At any rate, I plugged in these numbers and this is what I came up with. basically 11:1 compression. It sounds high, for what the engine is, but Im not sure.

The engine is just a stock bore and stroke (4.00-3.48) 350, with 64 cc vortec heads, The dome volume number of +6.10 was pulled from the sheet on the pistons I bought, the compressed gasket of .041 was also pulled from the literature provided with the gasket.

Anybody tried this before?

 

[1low4x4]

ANd another calculator says 9.56:1.

 

[HotRodPC]

Keep in mind there is a difference in Static Compression Ratio, and Dynamic Compression Ratio. That calculator pic you have there should be your Static or advertised compression ratio.

Now you're learning a bit. So now you know when someone says, I have 10:1 compression pistons they're full of ****. Ok, so you have 10:1 pistons? 10:1 at what, what, what and what?

 

[chengny]

I have flat top pistons with 4 valve relief, so Im not sure if that changes the number.

The problem isn't the deck clearance value - you have entered the correct clearance for an undecked 350 (.0025"). The error is with the piston dome value (PDV) you're using.

Pure flat top pistons (rarely used) have a PDV of 0. Dished pistons have a negative PDV and domed pistons will have a positive PDV.

Since your pistons incorporate 4 valve relief (i.e. machined recesses in the crowns) they essentially fall into the same category as dished pistons.

Change the "Piston Dome Value" field to - 6.10 and see if you get a more realistic number.

 

[1low4x4]

Keep in mind there is a difference in Static Compression Ratio, and Dynamic Compression Ratio. That calculator pic you have there should be your Static or advertised compression ratio.

Now you're learning a bit. So now you know when someone says, I have 10:1 compression pistons they're full of ****. Ok, so you have 10:1 pistons? 10:1 at what, what, what and what?

fucckkkkkk

 

[1low4x4]

Pure flat top pistons (rarely used) have a PDV of 0. Dished pistons have a negative PDV and domed pistons will have a positive PDV.

Since your pistons incorporate 4 valve relief (i.e. machined recesses in the crowns) they are essentially dished pistons.

Change the Piston Dome Value field to - 6.10 and see if you get a more realistic number.

Yeah, it changed to 9.5:1

 

[1low4x4]

Man, If Id have known buying 4 relief flat top pistons was basically dished pistons, I wouldn't have got them. lol

 

[SkinnyG]

The 4-valve reliefs are not likely dished - the valve reliefs are "cut outs" that cut into your compression ratio. They are a fact of life, and not worth stressing.

64cc heads, with flat tops (yes, with four valve reliefs), pistons .025" in the hole with an 0.041 head gasket would be around 9.5:1. That's realistic.

But,

If you can run a steel shim headgasket, you will have better squish, and be more detonation-resistant.

But,

It will work.

 

[1low4x4]

Thanks for the responses. The engine has been together and running (hard) now for about 8 years lol. I just finally got curious as to the CR

 

[HotRodPC]

Man, If Id have known buying 4 relief flat top pistons was basically dished pistons, I wouldn't have got them. lol

They're not "Dished". Point he's making is, the 4 valve reliefs are Negative in the calculation like dish pistons would be. Those are still considered flat tops. The negative space is very minimal considering an OEM dish piston.

Just call Comp Cams back and tell them you have a Static compression ratio of 9.5 then give them your cam part # or spec and see what they'd say should be your dynamic ratio taking valve overlap and valve timing into consideration.

 

[1low4x4]

Gotcha. Thanks yall. I appreciate the quick answers. Now, while I have yall here, My engine is a roller cam capable block. One piece rear main seal.....should I convert it to roller cam? Is it worth the price?

 

[74 Shortbed]

Dished piston.

You must be registered for see images attach

4 valve relief piston, there is a difference.

You must be registered for see images attach

 

[SkinnyG]

Go roller cam.

I cheaped out and went flat tappet, and for every oil change I have to find a zinc-rich oil or a ZDDP additive. Break-in was stressful, because my lumpy cam has large spring rates, and there is always the risk that the cam won't last break-in.

Further, roller cams can open the valve MUCH faster and MUCH further than a flat tappet, and (as long as the heads can work with the higher lift) that results in MORE power everywhere, with likely better driveability.

But, staying flat tappet saved me HUNDREDS over going roller. Not sure I was being wise at the time though....

 

[HotRodPC]

Gotcha. Thanks yall. I appreciate the quick answers. Now, while I have yall here, My engine is a roller cam capable block. One piece rear main seal.....should I convert it to roller cam? Is it worth the price?

Go roller cam.

I cheaped out and went flat tappet, and for every oil change I have to find a zinc-rich oil or a ZDDP additive. Break-in was stressful, because my lumpy cam has large spring rates, and there is always the risk that the cam won't last break-in.

Further, roller cams can open the valve MUCH faster and MUCH further than a flat tappet, and (as long as the heads can work with the higher lift) that results in MORE power everywhere, with likely better driveability.

But, staying flat tappet saved me HUNDREDS over going roller. Not sure I was being wise at the time though....

My opinion will differ a bit. If it's good running engine right now, then leave it alone. No need in reinventing the wheel so to speak. If it's a good running engine, it's NOT WORTH the upgrade.

Now, if it's a bad running engine and you are rebuilding it NOW, then by all means, it's worth the upgrade for longer life, more hp to the rear wheels with less parasitic hp loss in friction, and a little added mpg.

 

[SkinnyG]

Ah, yes. I agree with that.

I don't often start with a good engine, so my mind immediately went down the rebuild/significant-upgrade path.