DCR and Octane
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06-01-2008, 10:24 PM
#22
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ok, what would the VE have to be to have a DCR that much higher than the SCR? From what I understand the DCR can never be higher than the SCR, and definately not by that much.
06-01-2008, 11:09 PM
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Last I knew Pro stock was 125ish, but that has been years, I am sure it is still in that range. Why do you think they don't run anymore compression then they do?
That is why I do not believe the DRC calcs, because it can be!
If you believe the simple DCR formula, A prostock car would need MORE compression to run as good as they do.
From what I understand the DCR can never be higher than the SCR, and definately not by that much.
If you believe the simple DCR formula, A prostock car would need MORE compression to run as good as they do.
06-02-2008, 12:31 AM
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damn, a 125% VE is higher than I thought. That would mean they pull in 25% more volume of air, than the actual cylinder volume... which means the intake tuning has to be "Ramming" the extra air into the cylinders so I guess perhaps in that application that would be the case... just treat it as a minor forced induction motor. I doubt these street motors are seeing VEs much over 100% if that. DCR calculators are just using DCR as a simple mechanical calculation, just like SCR. THrowing VE into the mix is not what the intention is, it is just to give a good guideline... which was the point of this thread in the first place, I just want to know why is 8.8 the magic number for 93 octane? It just happens to work that way? people get lucky? There has to be a good explination, and perhaps it involves VE, I just want to understand this crap more because I do agree that DCR is too simple and is not telling the whole story.
06-02-2008, 01:34 AM
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damn, a 125% VE is higher than I thought. That would mean they pull in 25% more volume of air, than the actual cylinder volume... which means the intake tuning has to be "Ramming" the extra air into the cylinders so I guess perhaps in that application that would be the case... just treat it as a minor forced induction motor. I doubt these street motors are seeing VEs much over 100% if that. DCR calculators are just using DCR as a simple mechanical calculation, just like SCR. THrowing VE into the mix is not what the intention is, it is just to give a good guideline... which was the point of this thread in the first place, I just want to know why is 8.8 the magic number for 93 octane? It just happens to work that way? people get lucky? There has to be a good explination, and perhaps it involves VE, I just want to understand this crap more because I do agree that DCR is too simple and is not telling the whole story.
06-02-2008, 07:27 AM
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DC does not account for VE, chamber design, etc, etc. To many varibles and not enough data. Every engine builder who is worth much has always told me DC is not accurate enough to be of much use. I know people would like easy numbers to build off of, but the best thing you can do is experiment, or talk to people who have done it, and then experiment for yourself. I use to beleive in the DC crap years ago to. Funny thing is one of the people who got me thinking about it correctly (Bret) has been kicked off this crap site! Maybe you guys should look for other places to research this stuff if you really want the truth!
Maybe the person saying 8.8 is the magic number, has data that points to that direction, but I wouldn't trust it 100%
Maybe the person saying 8.8 is the magic number, has data that points to that direction, but I wouldn't trust it 100%
06-02-2008, 07:59 AM
#28
DC does not account for VE, chamber design, etc, etc. To many varibles and not enough data. Every engine builder who is worth much has always told me DC is not accurate enough to be of much use. I know people would like easy numbers to build off of, but the best thing you can do is experiment, or talk to people who have done it, and then experiment for yourself. I use to beleive in the DC crap years ago to. Funny thing is one of the people who got me thinking about it correctly (Bret) has been kicked off this crap site! Maybe you guys should look for other places to research this stuff if you really want the truth!
Maybe the person saying 8.8 is the magic number, has data that points to that direction, but I wouldn't trust it 100%
Maybe the person saying 8.8 is the magic number, has data that points to that direction, but I wouldn't trust it 100%
I like this post very much. It makes me happy.
My honest opinion is that there are way too many variables involved to say that a simple formula will tell us what octane fuel an engine will run on. The chamber design is one of them. If you took an average ls style head, flow numbers be damned, and changed nothing but the chamber design the octane requirements or tolerances would change. The question was brought up about boosted motors. DCR does not take into account any added air to the cylinder. Same with a high VE NA motor.
I don't believe there is a quick and easy universal formula to determine this.
I based my answer above mostly on his SCR rather than his DCR. He will be on the edge, but it should work. I would run at least a tr6 heat range of plug in it, but he should be OK.
06-02-2008, 09:48 PM
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FYI, here's an engine running 9.2 DCR on 87 octane: http://airflowresearch.com/articles/...e003/A3-P1.htm
That's on a dyno, of course, but it oughta run fine on the street on 89, and certainly on 91.
That's on a dyno, of course, but it oughta run fine on the street on 89, and certainly on 91.
06-02-2008, 10:31 PM
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I ran a 9.2 DCR engine with great results and no predenotation in 110 F weather in Phoenix.
Currently in the break in stage of another with a DCR of 9.4.
Granted, I know how to tune and that does make a difference.
Currently in the break in stage of another with a DCR of 9.4.
Granted, I know how to tune and that does make a difference.
06-02-2008, 10:51 PM
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06-03-2008, 01:18 PM
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It would be nice if it was all really simple to calculate wouldn't it? 
I understand the VE argument, but I also think that SCR in reality is more important than DCR the more I read and the more I talk to people, and that overall VE of the motor as well as head design (even material), temperature, squish, piston design, DA, yadda yadda will untimately determine wether or not the motor is pump gas friendly. I guess I just had foolish high hopes that there was some benchmark guideline as to what is safe and what isnt. I guess it just comes down to running what you know is safe and then if you get greedy, try and push the envelope some and experiment and see how high you can go in compression with a certain fuel.
With a site like this, I can see why DCR would be hyped up because it is the easy "comparo number". It is the "If so and so has these heads, and this cam and has this DCR and he has no problems running 93 then you should be safe" mentality, and so far it has been working fine but I'm sure it has bitten some people. I'm just gonna use DCR as just another number to see if I'm at least in the ballpark of what is acceptable to run 93, and just hope for the best. If that doesn't work, I'll just have to back up and punt.
I understand the VE argument, but I also think that SCR in reality is more important than DCR the more I read and the more I talk to people, and that overall VE of the motor as well as head design (even material), temperature, squish, piston design, DA, yadda yadda will untimately determine wether or not the motor is pump gas friendly. I guess I just had foolish high hopes that there was some benchmark guideline as to what is safe and what isnt. I guess it just comes down to running what you know is safe and then if you get greedy, try and push the envelope some and experiment and see how high you can go in compression with a certain fuel.
With a site like this, I can see why DCR would be hyped up because it is the easy "comparo number". It is the "If so and so has these heads, and this cam and has this DCR and he has no problems running 93 then you should be safe" mentality, and so far it has been working fine but I'm sure it has bitten some people. I'm just gonna use DCR as just another number to see if I'm at least in the ballpark of what is acceptable to run 93, and just hope for the best. If that doesn't work, I'll just have to back up and punt.
06-03-2008, 03:32 PM
#37
DCR has alot more factors that people dont consider.
You could have a 9.0 to 1 static engine with more DCR than a 11 to 1 engine.
All depends on the cams intake valve closing point, head/intake/exhaust efficiency.
You could have a 9 to 1 static compression engine with 8 to1 dynamic detonate on 93 octane and a 12 to 1 static with 8.8 to 1 dynamic NOT detonate on 93 octane. Why? Because the 9 to 1 motor has the piston .025 in the hole with a .039 head gasket. The quench, which is THE most important item when on the edge running pumpgas, is way too big.
DCR can be stated as the ACTUAL cylinder pressure/load that is acutally SEEN by the engine after the intake valve closes and the piston reaches tdc on the compression stroke.
David
You could have a 9.0 to 1 static engine with more DCR than a 11 to 1 engine.
All depends on the cams intake valve closing point, head/intake/exhaust efficiency.
You could have a 9 to 1 static compression engine with 8 to1 dynamic detonate on 93 octane and a 12 to 1 static with 8.8 to 1 dynamic NOT detonate on 93 octane. Why? Because the 9 to 1 motor has the piston .025 in the hole with a .039 head gasket. The quench, which is THE most important item when on the edge running pumpgas, is way too big.
DCR can be stated as the ACTUAL cylinder pressure/load that is acutally SEEN by the engine after the intake valve closes and the piston reaches tdc on the compression stroke.
David
06-04-2008, 06:50 AM
#39
At the least you will need to know your
1) Bore
2)Stroke
3)Deck Height
4)Chamber volume (combustion chamber on your head + piston dome or dish)
That should be enough to get you close.
Static compression is what the factory uses not Dynamic compression.
For a good reason I might add....
