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Old 05-03-2019, 09:24 PM
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Detonation vs compression

Posting this to try and learn. I have been doing a lot of on line research trying to figure out how much compression can be run on pump gas. I see a lot of posts with people saying you cant do over 10 to 1 and still run on pump gas and I also see posts of people running 11 to 1 and higher on pump gas. I know camshaft selection correlates with compression to set your DCR. My question is how to know how much camshaft you need depending on compression ratio. I have also been told you cant only look at DCR as the deciding factor. Another thing is cylinder pressure not exceeding 200 as a standard for some. I know aluminum heads ups the compression ratio that can be run. I have what if questions and just curious if it can contribute to running more or if it doesnt change a thing. Say you're quinch is .040 in the optimal range. Say DCR IS 9 TO 1. Do higher flowing heads or having chambers cleaned up by heads being professionally ported contribute to being able to run higher compression? Does having the heart shaped combustion chamber of newer heads contribute also. Or is it just to be prevented you need optimal cam timing. Just looking for thoughts from some of the experts. It's all interesting to me and I like to be as educated as possible.

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Old 05-03-2019, 10:07 PM
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Originally Posted by Tylerburn0882 View Post
Posting this to try and learn. I have been doing a lot of on line research trying to figure out how much compression can be run on pump gas. I see a lot of posts with people saying you cant do over 10 to 1 and still run on pump gas and I also see posts of people running 11 to 1 and higher on pump gas. I know camshaft selection correlates with compression to set your DCR. My question is how to know how much camshaft you need depending on compression ratio. I have also been told you cant only look at DCR as the deciding factor. Another thing is cylinder pressure not exceeding 200 as a standard for some. I know aluminum heads ups the compression ratio that can be run. I have what if questions and just curious if it can contribute to running more or if it doesnt change a thing. Say you're quinch is .040 in the optimal range. Say DCR IS 9 TO 1. Do higher flowing heads or having chambers cleaned up by heads being professionally ported contribute to being able to run higher compression? Does having the heart shaped combustion chamber of newer heads contribute also. Or is it just to be prevented you need optimal cam timing. Just looking for thoughts from some of the experts. It's all interesting to me and I like to be as educated as possible.
You're camshaft has absolutely nothing or dog in the fight from a mathematical standpoint in figuring SCR or DCR. Your cam is the brains of the engine, that's its job. General rule on being able to use pump gas, 9:5 max w/ iron heads, 10:5 max w/alum heads. YOUR DCR will always be a set figure. if you want to figure SCR/DCR, you need to know rod length, intake closing at BDC .050, stroke, combustion chamber, piston volume, bore, deck, head gasket, EDIT: Yes, i think proper quench can help you out just a bit on the gas.
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Old 05-03-2019, 10:11 PM
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Then why do you need the intake closing point off of the cam card for all of the dynamic completion ratio calculators?
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Old 05-03-2019, 10:14 PM
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By changing camshafts doesnt it change DCR? Bigger the camshaft the lower the DCR?
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Old 05-03-2019, 11:01 PM
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Originally Posted by rdobbs77 View Post
You're camshaft has absolutely nothing or dog in the fight from a mathematical standpoint in figuring SCR or DCR. Your cam is the brains of the engine, that's its job. General rule on being able to use pump gas, 9:5 max w/ iron heads, 10:5 max w/alum heads. YOUR DCR will always be a set figure. if you want to figure SCR/DCR, you need to know rod length, intake closing at BDC .050, stroke, combustion chamber, piston volume, bore, deck, head gasket, EDIT: Yes, i think proper quench can help you out just a bit on the gas.
↑↑↑ This is all ***-backwards. SCR (STATIC) is a fixed value based upon chamber volume and full stroke. DCR (DYNAMIC) is a variable number that is based upon the effective stroke calculated from when the intake valve closes. The cam dictates when the intake valve closes, and the later the valve closes the shorter the effective stroke the engine has when calculating the DCR. Remember, I said this is a variable number as the DCR will actually change with RPM.
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Old 05-04-2019, 02:21 AM
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Some guys don't feel the term Dynamic Compression Ratio correctly describes the situation as it really is a single point of stroke utilization and OEMs not take into account throttle position. If you talk with motorcycle engine builders they call this Compensated Compression Ratio. But a rose be any other name,,,, yada, yada, yada.

All we're doing here is adjusting the remaining stroke after the point in crank degrees where the intake valve closes
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Old 05-04-2019, 05:15 AM
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So that being said, how does one know how much risk to take with it. Is it just trial and error? From all the reading I've done it seems like every situation could be a little different depending on vehicle weight and gearing and other factors. What about the heads? Is it just the combustion chamber of the heads that effects it or does flow also effect it? Dont really know how flow could effect it with just better cylinder filling but I'm definitely no expert and just want to know.
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Old 05-04-2019, 07:15 AM
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I went thru all this last fall before I built my sbc.
I have 9.9 compression with iron heads and have no problem running 91 octane.
I have the heart shaped chambers and .039 quench.
My cam is a Howard's. [email protected] with .470-.470 lift
I can't remember of the top of my head what the intake closing was but my dcr was 8.2ish.
Your cam has everything to do with how much compression you can run.
The more compression you try to run, the bigger your cam needs to be to bleed some of that off. Playing with the dcr calculated helped me chose the right cam.
I'm no expert engine builder but I did my homework because I couldn't afford to do it twice.
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Old 05-04-2019, 10:21 AM
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Quote:
Originally Posted by Greg T View Post
↑↑↑ This is all ***-backwards. SCR (STATIC) is a fixed value based upon chamber volume and full stroke. DCR (DYNAMIC) is a variable number that is based upon the effective stroke calculated from when the intake valve closes. The cam dictates when the intake valve closes, and the later the valve closes the shorter the effective stroke the engine has when calculating the DCR. Remember, I said this is a variable number as the DCR will actually change with RPM.
Don't build my motor.

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Old 05-04-2019, 10:29 AM
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Originally Posted by Tylerburn0882 View Post
Then why do you need the intake closing point off of the cam card for all of the dynamic completion ratio calculators?
you do need it which i quoted in my response.
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Old 05-04-2019, 11:31 AM
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You said earlier that camshaft has nothing to do with mathematically changing SCR Or DCR. I understand it wont change your SCR but it does change your DCR with all of the different intake closing points of different cams. But my question I guess is more or less what people have run and got away with on pump gas without detonation.
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Old 05-04-2019, 12:06 PM
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I use Performance Trends engine analyzer to crunch the numbers on my computer. You have to be very careful to measure everything accurately or the numbers will be skewed. But, it will actually calculate what your cranking compression will be, and your idle vacuum, as well as providing a power and torque graph.

I don't like to go over 185 on pump gas. I grind off or sand any sharp edges in the combustion chamber, and on the top of the pistons.

Sent from my moto x4 using Tapatalk
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Old 05-04-2019, 12:40 PM
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Is that something that has to be purchased online?
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Old 05-04-2019, 01:08 PM
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Originally Posted by Tylerburn0882 View Post
Is that something that has to be purchased online?
Yes, Performance Trends Engine Analyzer is a computer program you would have to purchase if you wanted to use a simulator program. There are many others out there.

If you look you may be able to find a free version of Desktop Dyno somewhere online, I've got the 2000 version that way but all the download links I've got to it are dead now.
Comp Cams now has version 5.0, along with DynoSim 5 w/ProTools which is a more in-depth version.
There are others out there too, I just can't think of them at the moment. Max Race Software(Larry Meaux) I believe has some of the best, PipeMax comes to mind but that is just for header and intake sizing.

You're not going to find one simple answer for the question you are asking, "How much compression ratio can I get away with on pump gas??" due to the fact there are literally 4-5 dozen factors and issues that affect it, of which you've already learned the first easy dozen.

Best advice I can give is don't try to push the limit unless you feel you have a good handle on the info.
I've done 10.9-11.0:1 on pump 93 with iron heads...but this was a street/strip deal, not a daily driver...fairly light 3200 lbs, 3400 stall converter, 4.56 rear gear, coated headers, large radiator, big solid lifter cam, Vic Jr intake, 950 carb, Dart heads with polished chambers and good spark plug location, hand smoothed piston tops, very careful tune on the carb and ignition
Said engine made in the area of 500 HP, ran to low -mid 11's in the 1/4 mile driven to and from the track.... but would quickly kill itself if I stuck it in a 3800 lb car or a truck, with 2200 stall and 3.42 gear...it would detonate itself to death unless so much was tuned out of it that it wouldn't be any fun to own.
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Old 05-04-2019, 01:56 PM
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When I build it will be going in a truck that does weigh approximately 3800 lbs but I dont mind using a 3000 stall convertor or 3:73 gears. I also need to talk to Holley and see if the newer EFI units that take the place of the carbs can run with a 110 LSA to further protect it. I want it to be as streetable as possible without ruining the engine but not have such a big cam that all the power is in an rpm range that I dont use. The truck will only have a couple hundred miles per month on it and might go to the track occasionally. Only want to build it once without leaving anything on the table. I have already purchased most of the parts and they are all high quality so I dont want to just throw it all away from not having all my ducks in a row. I bought a cam but second guessing putting it in.
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