[Autoengineer]

My paperwork from my engine builder states 9.8:1 compression. Going from a .050 head gasket to a .043 a few years ago. I then thought I was at around 10:1.

But....I had a suspicion that my compression was lower based on the 150psi pressure I had measured with a compression tester (back before my broken timing chain fiasco). My old Patriot heads are off the car now because of bent valves so I measured my combustion chambers. I greased 2 of the valves, slid them onto the heads, inserted a spark plug, and filled the chamber with alcohol. I definitely have the 74cc heads as I was able to easily pour 70cc without spilling a drop and there was room to spare. With my dished pistons (+12cc) that puts me at only about 9.3:1 !! ....AND thats assuming 0 deck clearance (Which I don't think is the case, but I haven't gotten a good measurement on that yet.)

For my new heads, I was going to go 65cc. That should put me no more than 10.15:1.

Are my calculations sound? I have a .040 overbore.

Thanks/

 

[tucson67ss]

My paperwork from my engine builder states 9.8:1 compression. Going from a .050 head gasket to a .043 a few years ago. I then thought I was at around 10:1.

But....I had a suspicion that my compression was lower based on the 150psi pressure I had measured with a compression tester (back before my broken timing chain fiasco). My old Patriot heads are off the car now because of bent valves so I measured my combustion chambers. I greased 2 of the valves, slid them onto the heads, inserted a spark plug, and filled the chamber with alcohol. I definitely have the 74cc heads as I was able to easily pour 70cc without spilling a drop and there was room to spare. With my dished pistons (+12cc) that puts me at only about 9.3:1 !! ....AND thats assuming 0 deck clearance (Which I don't think is the case, but I haven't gotten a good measurement on that yet.)

For my new heads, I was going to go 65cc. That should put me no more than 10.15:1.

Are my calculations sound? I have a .040 overbore.

Thanks/

The compression test has nothing to do with compression ratio. There is no calculation from compression test to compression ratio.
The cam duration is what will raise or lower the compression test results.13:1 motor could be 150 depending on the cam. As long as all cylinders are the same within 5 lbs you are good, unless they are under 100 then they all are NFG, but that would be very rare!

You also said "dished pistons" then followed it up with a +12cc. I think you meant -12cc ?

Below is a link to a comp ratio calculator. Fill in the numbers and magic happens when you hit "calculate CR".

Engine Compression Ratio (CR) Calculator

 

[jeff swisher]

some people do not measure correctly.
Some builders are out for money only.
Friend had a buddy that owned a machine shop and raced cars and built race cars.

I built my friend a good 355 and he wanted more power I told him i would do it but his machine shop buddy got the job.
That is fine as he wants to go fast and i care less who gets him there.
Well he was told the heads will be milled as much as possibly and will be decked to .005" and going to put some bad A44 lunati springs in the heads and a lunati cam that will rock.
The guy sold him a hurricane intake and a 800 DP holley.

Well it fizzled at 5500rpm popping and puking. No tune could fix it as I tried a bunch of them

Nothing was milled the pistons were .035" down the hole and dished cast used crap.. the valve springs were from 55-110 seat pressure and many different springs.
In all it was junk ,.. the cam was an old nitrous grind on a 113LSA the thing ran 14.7 with 370 gears and 3500 stall in a 3000 lb truck with slicks. 993 heads.

he told his buddy what it ran and the reply was it went that quick..WTF we were thinking something in the 11's.

Well lesson learned.
You do not sound like you got shafted that bad.
but still lesson learned

 

[ddeennis]

The compression test has nothing to do with compression ratio. There is no calculation from compression test to compression ratio.
The cam duration is what will raise or lower the compression test results.13:1 motor could be 150 depending on the cam. As long as all cylinders are the same within 5 lbs you are good, unless they are under 100 then they all are NFG, but that would be very rare!

You also said "dished pistons" then followed it up with a +12cc. I think you meant -12cc ?

Below is a link to a comp ratio calculator. Fill in the numbers and magic happens when you hit "calculate CR".

Engine Compression Ratio (CR) Calculator

just so it's clear, you may not be able to say 150 cranking compression is equal to 10 to 1 compression, but a cranking compression check is a good indication of where you are at.

150 cranking compression is a little low, it's needs to be around 175-185 in a nice little street engine and will be fine on pump gas easy, so you can raise the real compression ratio (keep the same camshaft) and move the cranking compression up. It's just not all about the cam.

I too was told when I was younger my first bbc 396 was 10.25 to 1 compression, years later I found out it was 7.7 to 1 and had 135 cranking compression, I swapped heads to closed chambered raised my compression up to 8.8 to 1 and my cranking compression came up to 150. it was better, worth a full second in the 1/4 doing that. cranking compression was still low I would have needed to change pistons to get it near 175-185.

Later on I changed pistons and got it to 10.2 to 1 and changed cam slightly bigger to a 233/239 and I had cranking compression of 185 and ran it just fine on 87 octane and that was good for 107 mph trap speeds.

So cranking compression can help in getting your combo together, just not as a tool "as long all is with in 5 psi" deal.

In fact I used the cranking compression as a tool to help pick a cam, my sons 350 had 165 psi cranking compression and he wanted a bigger cam , but I knew he needed more torque, so that requires more cranking compression, so I picked a larger cam by about 15 degrees in duration but made sure the cam gave more cranking compression because then I knew it would make more torque, so the new bigger came was making 173 psi in cranking compression, that was good enough to drop his 60 ft time and make his overall 1/4 mile about 3 tenths quicker.

So use it as a guide as well.

The most cranking compression engine I ever built was a 14.5 to 1 bbc 468 roller cam, it had 275 psi cranking compression.

 

[Rokker]

The compression test has nothing to do with compression ratio.

You also said "dished pistons" then followed it up with a +12cc. I think you meant -12cc ?

Dish adds cc´s to the equation, so dish is + and dome -cc.

Also as stated, a compression test does not tell you your compression, but it does tell you if you´ve got a high or low cranking pressure, and if you know what cam is in the motor a compression test will give you a indicator if the cr is where it should be.

 

[Dave Birdwell]

With a .005" deck height, you're pretty close to 9.2 Going to a 64cc head would put you right about 10.15:1 yes.

You need to verify you're close to "0" deck height because if you bump the compression up, your quench will become more important.

 

[Autoengineer]

Let me clarify...the 150psi cranking compression test didn't have any input into my calculations. I only mentioned it to explain why I bothered to look into my compression into the first place. It was the reason I was suspicous about my supposed 10:1. Thanks

 

[tucson67ss]

Dish adds cc´s to the equation, so dish is + and dome -cc.

Also as stated, a compression test does not tell you your compression, but it does tell you if you´ve got a high or low cranking pressure, and if you know what cam is in the motor a compression test will give you a indicator if the cr is where it should be.

"With my dished pistons (+12cc)"

Wrong, that is not how pistons are sold. dished pistons have a - cc not a +.

Correct, All I said was "The compression test has nothing to do with compression ratio. There is no calculation from compression test to compression ratio." 100% true!

 

[bracketchev1221]

It depends on the calculator you use. Some input dish as + some input it as -.

 

[65chevelle300post]

It should be - for pop-ups, and + for flat top or dish. Compression distance can sometimes change this but it's not the usual. You only need to go to Summit to see this.

 

[tucson67ss]

It should be - for pop-ups, and + for flat top or dish. Compression distance can sometimes change this but it's not the usual. You only need to go to Summit to see this.

Still WRONG!
"you only need to go to summit to see this"
Summit has great prices and shipping, but I would not turn to them for tech.
I would go to the piston manufacturers and you will see what I said is true.

Every piston manufacture I've seen measures a dome as + and a dish as -.
Check out this link:

13730 - Chevy 454 +39.0cc Dome Top SRS Pistons
As well as Mahle, Wiseco ETC.

Summit is stating how much volume is lost and is not the way the piston industry refers to piston domes or dish.


Try calling up a piston manufacturer and tell him you would like a -20cc piston.
I assure you they will give you a dished piston #.

 

[65chevelle300post]

Just about every piston there at Summit shows the opposite of what you are saying.

The number used to calculate compression using a standard compression calculator is the one I'm referring to.

Since we are talking about calculating compression that is the number to use which is why they list the values this way. This would be a negative value to subtract from the chamber volume for pop ups and a positive value to add chamber volume with a flat top or dished piston.

That is the same number listed for nearly every piston that Summit sells and I can show you a hundred of those numbers right off the web site just as easily. There are only a few manufacturers who list the values as you say but those values would need to be reversed in order to be accurate with the vast majority of compression calculators.

 

[tucson67ss]

Just about every piston there at Summit shows the opposite of what you are saying.

The number used to calculate compression using a standard compression calculator is the one I'm referring to.

Since we are talking about calculating compression that is the number to use which is why they list the values this way. This would be a negative value to subtract from the chamber volume for pop ups and a positive value to add chamber volume with a flat top or dished piston.

That is the same number listed for nearly every piston that Summit sells and I can show you a hundred of those numbers right off the web site just as easily. There are only a few manufacturers who list the values as you say but those values would need to be reversed in order to be accurate with the vast majority of compression calculators.

Jeez - All of my post's are refering to the piston not volume of the cylinder.
When you say "my dished pistons (+12cc) that is referring to the piston and is WRONG if you are referring to a dished piston.
You keep referring to Summit as if they are the God of pistons. They only sell the stuff. I think I would go with the manufacturers and it makes total sence
So, EVERY piston manufacturer is wrong ?
Please, show me another supplier who list's like Summit.
Yes, Summit is telling you the volume you loose, but they are not referring to the piston directly and it could be misleading if you didn't follow the wording.
How can you argue with facts ? pistons are sold as - for dished and + for dome. period, end of story. Go check for yourself please!
Summit is doing the math for you.
I've presented you facts now it is your turn and Summit does not count.
At the end of the day we are all on the same team and I certainly don't mean to insult you in any way. That"s the problem with computers as they have no emotion.
I'm sure if we had this argument face to face we would be laughing and saying no to each other or at least I would. as this is really a trivial matter.

 

[Dave Birdwell]

Either way, the two calculators, one being summit's, came up with the same answer.

 

[Tom Mobley]

jeeze you guys, cabin fever much? There is no "Right Way", it's however the guy writing the calculator program wants it.

anybody who writes a CR calculator can specify the dish cc's input as positive or negative. The one I wrote back in the olden days expected the dish cc's as positive because the dish added to the chamber volume at TDC. When I ported it to the TI-85 calculator it was reversed because it was easier that way.

If you're not sure just run a typical stock flattop 350. 4.030, 3.48, 0 deck, .039 gasket, 6cc valve notches and 76cc chamber. Should come out around 9:1. change to 64cc chambers it should come out about 10.25.

 

[jeff swisher]

Years ago I would do the math all by hand with pencil and paper no calculator at all not even a hand held.
It takes a lot of paper. I figured in the space above the top piston ring and the top of the piston.
It is a small area but all those small area's add up.

You figure in all the math to convert to cc and you have a lot of chicken scratching to do.
Most of those online calculators aren't that accurate.

 

[Tom Mobley]

I've been cheerfully ignoring that small volume between the piston head and cylinder wall above the top ring for 40 or so years now. I'll calc one out one of these days so I can see how horribly wrong I've always been.

 

[Autoengineer]

OK, so using a dial indicator, I measured how far the edge of piston 1 is below the deck at TDC. I got .032"

Plugging that into the calculator, this knocks my compression even lower. About 8.7:1 w/the old heads and 9.5:1 with the new. I definitely made the right choice with the 65cc heads. I think the 9.5:1 is a better # for my cam.

I know why the builder probably did it. I originally had a very mild GM ZZ4 cam and he probably felt that the lower compression fit that cam, its just irritating that he didn't fix the paperwork to match that. Also would have helped if Patriot had marked their heads with the size!

 

[Tom Mobley]

>>> I got .032"

huh? is that including the gasket? that's outta whack. did you measure at the center, like over the end of the wrist pin?

if you measured at the top or bottom you need to center the piston or measure top and bottom and split the difference. If it's really .032 the guy must have used the cheapie rebuilder pistons or something. They can be .020 short, not suitable for a performance build at all.

 

[Tom Mobley]

The compression height of a 350 piston is supposed to be 1.56". that's from the center of the wrist pin bore to the flat surface of the top of the piston. I've had the cheap rebuilder pistons that were 1.54, .020 short. I've heard there's some even shorter than that, but haven't seen any.

BTW, 9.5 is a good number. Pretty much what I always shoot for on normal street engines.