Compression Ratio Decision
Thread Starter
Melting Slicks
Joined: Apr 1999
Posts: 2,010
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From: Moon Twp. PA USA
Compression Ratio Decision
I have to make a choice soon on what pistons I want to run in my 383. Choices are 7cc flat top @ 10.78 CR - 8.8 Dynamic CR or 19cc dish @ 9.5 CR - 7.8 Dynamic. Both Dynamic CR numbers are with a cam IVC @ 66 degrees ABDC. Engine will have aluminum heads. I want to run it on 89 - 91 octane pump gas. Quench (squish) will be .039. What is the lower limit on quench using steel con rods? If necessary, I can use a different cam to bleed off pressure to reduce Dynamic CR. I don't want to put in pistons and later find I have pinging with the flat tops or I could use the flat tops after I put in the dished pistons. I could get up to 9.8 CR with the dished pistons by reducing quench to .030 and using a different head gasket.
What do you think the best way to go to have a nice street cruiser?
What do you think the best way to go to have a nice street cruiser?
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I run 91 octane with 11:1 CR and aluminum heads, now I have a big cam and run no vacuum advance but I was actually running 45 degrees advance when I first started it up with no pinging. To me quench is over rated I have never worried about it or should I say you can run into trouble with piston to valve clearance and other clearance problems that make it a headache and not worthwhile to put alot of emphasis on when I am building a motor. I am in the .060" range. 
Race Director
Joined: Jun 2000
Posts: 10,046
Likes: 675
From: FL
This is from the readme from my DCR program:
Dynamic Compression Ratio Calculator (DCR).
This is a useful little VBasic program I wrote to calculate Dynamic Compression Ratios (DCR). Knowing the DCR is very useful for cam selection, since compression in the cylinder cannot start until the intake valve is closed. Knowing the remaining amount of crank stroke left after the intake valve is closed is needed to calculate the DCR. According To David Vizard, a street engine running on pump gas should have around 7.5:1 DCR, a race engine should shoot for 9:1. These ratios will produce the best torque output. All these calculations must be done with seat timings. Since compression doesn't start until the intake valve closes, using .050" tappet lift gives an incorrect result. At .050" the valve is still open .050" times the rocker ratio. In the case of a SBC, the valve is still open .075" (that's around twice what a plug is gapped at) and a BBC's intake valve is open .085". That's a big leak as far as compression is concerned.
The DCR Calculator provides three separate calculators. One for calculating the valve closing points, another for calculating the remaining stroke length after the intake valve closing, and a third for calculating compression ratios. There are various way the DCR Calculator can be used.
If you don't know the valve event times, use the "Cam Timing and Overlap Calculator." Enter the advertised duration for both the intake and exhaust, the Lobe Separation Angle, and the installed Intake Lobe Centerline Angle. The calculator will return the valve events for that cam.
Next, enter the results along with the rod length and crank stroke for your engine in the "Dynamic Stroke Length Calculator." This calculator returns the amount of the crank stroke remaining after the intake closes. This is the Dynamic Stroke Length (DSL). The distance the piston is from BDC is also shown.
After getting the DSL, go to the "Compression Ratio Calculator." Enter the required information using the DSL for the "Stroke" input. The calculator returns the Compression Ratio for that combo, in this case the Dynamic CR. You can also use the crank stroke to get the Static CR.
Where this info is most helpful is in selecting a cam and setting the proper CR. Since a pump gas engine wants around 7.5:1 CR, you can work with the chamber size and piston dome/dish to get this for a chosen cam. Going the other way, if you already have the heads and pistons, it is easy to try different cams to find one that give the desired results. Of course, this works for race engines, too. Just shoot for 9:1 DCR.
After getting the DCR where you want it, enter the crank stroke to find out what the static CR needs to be. Order the parts accordingly. As for the cam, the intake lobe center, and LSA don't really matter as far as the DCR goes. The only thing that affects the DCR is the intake closing time. Get a cam with the correct intake closing point and you will have the desired dynamic CR. The rest of the specs are up to you.
David Vizard never specifies if these numbers, 8.5 and 9:1, are for iron or aluminum heads. Since almost all he talks about in his books are iron heads, I'm assuming these numbers are for iron. I know of a fellow running at 8.12 DCR with aluminum heads and 92 octane without any problems. It appears that aluminum heads can tolerate (and probably need, due to thermal loss) another 1/2 point of DCR.
I would shoot for no more than 8.0 DCR for a street motor even with aluminum heads. I. My Crane HR276 has an IVC of 69 ABDC vice the Comp XE274 that was at 63 ABDC. This despite th fact that the Crane cam is rated at only 214@.050 vice ~230@.50 for the Comp cam. All the Comp XE series have very quick valve closing events to build DCR but as a result, they prefer a lower static compression ratio.
Motorhead is right that a big cam will reduce DCR by having a later ICV. Is definitely helping bleed off some pressure. BTW, most engines can take up to 50 degrees timing under little to no load so the 45 on startup does not surprise me.
Dynamic Compression Ratio Calculator (DCR).
This is a useful little VBasic program I wrote to calculate Dynamic Compression Ratios (DCR). Knowing the DCR is very useful for cam selection, since compression in the cylinder cannot start until the intake valve is closed. Knowing the remaining amount of crank stroke left after the intake valve is closed is needed to calculate the DCR. According To David Vizard, a street engine running on pump gas should have around 7.5:1 DCR, a race engine should shoot for 9:1. These ratios will produce the best torque output. All these calculations must be done with seat timings. Since compression doesn't start until the intake valve closes, using .050" tappet lift gives an incorrect result. At .050" the valve is still open .050" times the rocker ratio. In the case of a SBC, the valve is still open .075" (that's around twice what a plug is gapped at) and a BBC's intake valve is open .085". That's a big leak as far as compression is concerned.
The DCR Calculator provides three separate calculators. One for calculating the valve closing points, another for calculating the remaining stroke length after the intake valve closing, and a third for calculating compression ratios. There are various way the DCR Calculator can be used.
If you don't know the valve event times, use the "Cam Timing and Overlap Calculator." Enter the advertised duration for both the intake and exhaust, the Lobe Separation Angle, and the installed Intake Lobe Centerline Angle. The calculator will return the valve events for that cam.
Next, enter the results along with the rod length and crank stroke for your engine in the "Dynamic Stroke Length Calculator." This calculator returns the amount of the crank stroke remaining after the intake closes. This is the Dynamic Stroke Length (DSL). The distance the piston is from BDC is also shown.
After getting the DSL, go to the "Compression Ratio Calculator." Enter the required information using the DSL for the "Stroke" input. The calculator returns the Compression Ratio for that combo, in this case the Dynamic CR. You can also use the crank stroke to get the Static CR.
Where this info is most helpful is in selecting a cam and setting the proper CR. Since a pump gas engine wants around 7.5:1 CR, you can work with the chamber size and piston dome/dish to get this for a chosen cam. Going the other way, if you already have the heads and pistons, it is easy to try different cams to find one that give the desired results. Of course, this works for race engines, too. Just shoot for 9:1 DCR.
After getting the DCR where you want it, enter the crank stroke to find out what the static CR needs to be. Order the parts accordingly. As for the cam, the intake lobe center, and LSA don't really matter as far as the DCR goes. The only thing that affects the DCR is the intake closing time. Get a cam with the correct intake closing point and you will have the desired dynamic CR. The rest of the specs are up to you.
David Vizard never specifies if these numbers, 8.5 and 9:1, are for iron or aluminum heads. Since almost all he talks about in his books are iron heads, I'm assuming these numbers are for iron. I know of a fellow running at 8.12 DCR with aluminum heads and 92 octane without any problems. It appears that aluminum heads can tolerate (and probably need, due to thermal loss) another 1/2 point of DCR.
I would shoot for no more than 8.0 DCR for a street motor even with aluminum heads. I. My Crane HR276 has an IVC of 69 ABDC vice the Comp XE274 that was at 63 ABDC. This despite th fact that the Crane cam is rated at only 214@.050 vice ~230@.50 for the Comp cam. All the Comp XE series have very quick valve closing events to build DCR but as a result, they prefer a lower static compression ratio.
Motorhead is right that a big cam will reduce DCR by having a later ICV. Is definitely helping bleed off some pressure. BTW, most engines can take up to 50 degrees timing under little to no load so the 45 on startup does not surprise me.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
my 383 with 11.2 static and .041 quench runs fine with a Dynamic compression ratio of something like 8.42 it's very near the edge of 91 octane though.
With your aluminum heads I would try for at least 8.1 - 8.25 to get the best power.
With your aluminum heads I would try for at least 8.1 - 8.25 to get the best power.
