How To Figure Compression Ratio

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SleeperDart505

SleeperDart505

Stock wheels, Stock Paint, 505" Stroker, Full Cage
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Location
Ringgold, GA
I bought this '68 Dart with a 505' stroker motor. It runs good. Doesn't over heat. Runs well on 93 octane. But who of us can ever leave anything alone? :lol:
This motor has stock steel heads on it but the seller says the machine shop shaved the heads and it "has a lot of compression". It was a little hard to crank when I first got it so I turned the timing down a bit and it cranks fine now. Seller says it was the compression not the timing. But he doesn't know any details really. It has been 20 degrees since I got the car so I haven't really spent much time looking at it.

How can I figure out how much compression I have here? I took the part number from the stroker kit and found out the Scat pistons are forged and supposedly have a D shaped dish for valve clearance. The flat top version would have been better I think. The stock heads were shaved but nobody can tell me how much. I figure I can do a cranking pressure check but I wouldn't know how to convert that to compression ratio. Anybody have a rule of thumb? 160 lbs. = 10:1 compression, etc.???

In the future, I'm thinking of going with aluminum heads. For a street/strip car what compression ratio do you guys recommend on 93 octane with an aluminum head?? I know they are not real popular here but I may go with the 440 Source heads since their casting looks like a stock head. I can paint them and it will work real well with my whole Sleeper theme. :p I hear they offer a fully CNC ported set that flows 320 CFM and has an intake port of 255. That's bigger than the TFS 240 head. It will be plenty big enough for what I want to do.

Thoughts?

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Unfortunately the only true way to figure out your static compression ratio is by cc’ing the head combustion chambers and pistons in the hole which would require pulling the heads. Then with your cam specs you can figure out your dynamic compression ratio at wallace racing compression calculator.
 
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With that much stroke, you need the dished pistons for pump gas. A flattop might be as much as 12 to 1.
You could check a compression ratio calculator (I like the one at Wallace racing calculators) and see if you have enough info. I realize MOST of the info you'd have to use would be guess-work. Probably most important spec would be the dish volume, kit info might have that for you.
Comment above is correct. To get an ACCURATE comp ratio, everything has to be actually measured.
(You can possibly get an idea how much the heads have been shaved, maybe. The outer row of head bolt holes are 1" thick stock. If you can measure them now, and get, say, .950, head has probably been shaved about .050.)
And there is NO direct correlation between cranking pressure and compression ratio. WAY too many variables.
 
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SleeperDart505 said:
I bought this '68 Dart with a 505' stroker motor. It runs good. Doesn't over heat. Runs well on 93 octane. But who of us can ever leave anything alone? :lol:
This motor has stock steel heads on it but the seller says the machine shop shaved the heads and it "has a lot of compression". It was a little hard to crank when I first got it so I turned the timing down a bit and it cranks fine now. Seller says it was the compression not the timing. But he doesn't know any details really. It has been 20 degrees since I got the car so I haven't really spent much time looking at it.

How can I figure out how much compression I have here? I took the part number from the stroker kit and found out the Scat pistons are forged and supposedly have a D shaped dish for valve clearance. The flat top version would have been better I think. The stock heads were shaved but nobody can tell me how much. I figure I can do a cranking pressure check but I wouldn't know how to convert that to compression ratio. Anybody have a rule of thumb? 160 lbs. = 10:1 compression, etc.???

In the future, I'm thinking of going with aluminum heads. For a street/strip car what compression ratio do you guys recommend on 93 octane with an aluminum head?? I know they are not real popular here but I may go with the 440 Source heads since their casting looks like a stock head. I can paint them and it will work real well with my whole Sleeper theme. :p I hear they offer a fully CNC ported set that flows 320 CFM and has an intake port of 255. That's bigger than the TFS 240 head. It will be plenty big enough for what I want to do.

Thoughts?

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I run victor knock offs on my 505 , if they would have been in production , back when I bought these , I`d have went with the trick flows ,altho the 270 doesnt flow any more than mine ....
 
Thanks Guys! Like I said, it runs good. Doesn't over heat. Runs well on 93 octane. But I'd love to know the compression ratio so when I buy aluminum heads I could have a starting point for my decision making. This package works great but I want the higher flowing aluminum heads and a bigger cam so I can get the HP up some for the drag strip.
 
I would bet that most aluminum heads will have a smaller chamber/more compression than any but a radically shaved open chamber iron head
If it runs nice now on 93, I'd guess your somewhere around 9.5. Most aluminum heads will boost that, I would guess a half point at least. You could go to a calculator and plug in head chamber volume, 88/89 cc's for 906s 452s etc, 82 for a shaved iron head, and 75 for a typical aluminum head, and see what happens......
 
If you’re going to replace the heads/cam then use the published specs from the stroker kit and NEW heads to come up with a ballpark compression ratio for the new combo. Don’t worry too much about the CR of the old combo because you’re changing it. Who cares what it is now? With 93 octane and aluminum heads you could run up near 11:1 maybe a bit more if you are confident in your tuning skills.
 
Also unfortunate is the variance of 93 pump gas from station to station in any area.

My buddy hauls gas and on occasion will dump regular into the premium tank if after topping off the regular tank and there is gas left in the truck. They drive that truck till every station on their route has gas no matter how long it takes. He is a relief driver on Sundays and its a long day, 12 hours or more.
 
Run a compression test and record the numbers. Post them here along with the specific camshaft specs and we cen make an educated guess as to what the static compression is. Static compression ratio really by itself does not affect what octane fuel an engine runs on. Cylinder pressure, cylinder head material and combustion chamber shape all have much more of an effect, so when we get those numbers, we can give better answers.
 
ch1ll said:
Also unfortunate is the variance of 93 pump gas from station to station in any area.

My buddy hauls gas and on occasion will dump regular into the premium tank if after topping off the regular tank and there is gas left in the truck. They drive that truck till every station on their route has gas no matter how long it takes. He is a relief driver on Sundays and its a long day, 12 hours or more.
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I have a friend who's driven a gas truck here in Georgia for 30 plus years and he's told me before you can get put in JAIL and companies fined huge fines for doing what you describe.
 
If it's running well, I would pull all of your plugs and see if you're getting any detonation speckles under a magnifying glass with a light. If there's no detonation, let her eat!

Like mentioned previously, if you have your cam specs and you know the exact PSI for each cylinder, you can use those figures to calculate a dynamic compression.
 
SleeperDart505 said:
I bought this '68 Dart with a 505' stroker motor. It runs good. Doesn't over heat. Runs well on 93 octane. But who of us can ever leave anything alone? :lol:
This motor has stock steel heads on it but the seller says the machine shop shaved the heads and it "has a lot of compression". It was a little hard to crank when I first got it so I turned the timing down a bit and it cranks fine now. Seller says it was the compression not the timing. But he doesn't know any details really. It has been 20 degrees since I got the car so I haven't really spent much time looking at it.

How can I figure out how much compression I have here? I took the part number from the stroker kit and found out the Scat pistons are forged and supposedly have a D shaped dish for valve clearance. The flat top version would have been better I think. The stock heads were shaved but nobody can tell me how much. I figure I can do a cranking pressure check but I wouldn't know how to convert that to compression ratio. Anybody have a rule of thumb? 160 lbs. = 10:1 compression, etc.???

In the future, I'm thinking of going with aluminum heads. For a street/strip car what compression ratio do you guys recommend on 93 octane with an aluminum head?? I know they are not real popular here but I may go with the 440 Source heads since their casting looks like a stock head. I can paint them and it will work real well with my whole Sleeper theme. :p I hear they offer a fully CNC ported set that flows 320 CFM and has an intake port of 255. That's bigger than the TFS 240 head. It will be plenty big enough for what I want to do.

Thoughts?

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Love the car, especially the license plate frame. I remember Hines.
 
No easy way to check the CR once the engine is in the car. When you pull the heads off to switch to aluminum you'll be able take some measurements. Once you know a few numbers you'll be able to calculate the CR.
 
Bodyperson said:
Love the car, especially the license plate frame. I remember Hines.
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Thats a pretty cool machine. Wish I could find one to borrow! This car was built by a guy in California that moved to Vegas. I've never heard of Hines but apperantly he had and wanted it on there for nostalgia sake. :thumbsup:
TT5.9mag said:
Who cares what it is now? With 93 octane and aluminum heads you could run up near 11:1 maybe a bit more if you are confident in your tuning skills.
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I only care about the current compression ratio because it runs so well on 93 octane and never gets hot. I'd like to know where I'm at because I don't want to stray too far from here with the new set up.
 
ch1ll said:
Also unfortunate is the variance of 93 pump gas from station to station in any area.

My buddy hauls gas and on occasion will dump regular into the premium tank if after topping off the regular tank and there is gas left in the truck. They drive that truck till every station on their route has gas no matter how long it takes. He is a relief driver on Sundays and its a long day, 12 hours or more.
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:wtf:
 
RustyRatRod said:
Run a compression test and record the numbers. Post them here along with the specific camshaft specs and we cen make an educated guess as to what the static compression is. Static compression ratio really by itself does not affect what octane fuel an engine runs on. Cylinder pressure, cylinder head material and combustion chamber shape all have much more of an effect, so when we get those numbers, we can give better answers.
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As soon as it warms up a tad and we don't have ice hanging off of everything, I'll get out there and check it out. Us Georgia boys ain't used to this kinda weather!!!
 
SleeperDart505 said:
As soon as it warms up a tad and we don't have ice hanging off of everything, I'll get out there and check it out. Us Georgia boys ain't used to this kinda weather!!!
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No, it's been pretty cold. Worse way up there where you are though.
 
SleeperDart505 said:
I only care about the current compression ratio because it runs so well on 93 octane and never gets hot. I'd like to know where I'm at because I don't want to stray too far from here with the new set up.
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I guess I understand that but I’d still not worry too much about it. The aluminum heads and cam change (if going bigger) will change the detonation tolerance of the engine.
 
I agree with the ''if it runs good, don't stress over the current set up''. That's a great looking sleeper Dart and i feel for those Radial T/A's, especially if you go to more head/horsepower. :)
 
SleeperDart505 said:
Thats a pretty cool machine. Wish I could find one to borrow! This car was built by a guy in California that moved to Vegas. I've never heard of Hines but apperantly he had and wanted it on there for nostalgia sake. :thumbsup:

I only care about the current compression ratio because it runs so well on 93 octane and never gets hot. I'd like to know where I'm at because I don't want to stray too far from here with the new set up.
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I have a katech whistler an use it alot at the dyno. Some circle tracks will have rules that mandate a maximum compression ratio. If you have one near you maybe you can get the tech guy to check your motor for you.
 
92b said:
I have a katech whistler an use it alot at the dyno. Some circle tracks will have rules that mandate a maximum compression ratio. If you have one near you maybe you can get the tech guy to check your motor for you.
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We have a circle track close by. Maybe I'll get lucky?? Thanks!
 
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