Pump gas and dynamic compression ratio

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NukeBass

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I've done a few different searches and went through a few posts (there are lots of them depending on search criteria) that discuss dynamic compression ratio and pump gas. Most of them seem to agree that 8.5 is the max DCR to target for regular pump gas compatibility, I think, for 93 octane, but that's generally as much info as I can find. I also found on the web this website that has a table for DCR vs. pump gas, but it is the only one I found. http://classicinlines.com/CompressionRatio.asp

Since it is on the web, I generally like to find a couple of sources with information before really trusting it and this chart shows that you can go as high as 9:1 on 93 octane.

Is the classic inlines chart correct (which would be nice since it breaks down different octanes) or should I use something else, like the general 8.5? Thanks!
 
I don't think that you're going to get a straight cut answer on this since there will be variables. Iron or aluminum heads? Some combustion chambers are more efficient than others which will help reduce detonation so that may need considered. As far as my narrow range of knowledge is informed, 8.0:1 is about the max you should probably go with 91. With 93 it is probably a little higher, maybe 8.5:1 as you've read. You can run 91 with 8.0:1 DCR though. I know that this works. Any higher DCR than that is beyond my range of knowledge.
 
I don't know anything from anything but your numbers seem low to me... maybe for 89 reg fuel but like I said I don't know.

My iron headed 360 is 10:1 and I regularly run 91 fuel. As a back up I will run 94 if I take it to the track. I've never had an issue on the street with 91 even if I play around a bit. I know timing makes a big difference as well as other things.

I am currently putting together a 505ci motor. I've been lead to believe that with alum heads I should be OK at 11.5:1 with 94...

Like I said I don't know anything. In passing I was listening to a seminar at MMW and they put the emphasis on cylinder pressure not ratio.
 
It is more important to look at cylinder pressure over static compression. But in reality, Dynamic compression is more like cylinder pressure as it takes into account the intake valve closing, rod length and stroke. You actual cylinder pressure is going to follow the DCR so it is a direct relationship. But in general, the more optimized your combustion is, the more DCR you can run with a given octane.
 
I agree with MRL.. I am running a 410 with 10.15 Static compression, but the dynamic is 8.8ish...I run 91 Octane....Keep in mind I also have closed chambered heads. I am about 20-50ft above sea level also. 33 degrees timing.
 
Fst73_360 said:
I agree with MRL.. I am running a 410 with 10.15 Static compression, but the dynamic is 8.8ish...I run 91 Octane....Keep in mind I also have closed chambered heads. I am about 20-50ft above sea level also. 33 degrees timing.
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I'm surprised you can run 91 with that high of DCR and that much timing.
 
clinteg said:
I'm surprised you can run 91 with that high of DCR and that much timing.
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Yup. Engine built by Brian at IMM.. I mean the camshaft is 251-260 @50 so that has a little to do with it and the heads.
 
mderoy340 said:
I built my j head 340 to 8.3 dynamic. Runs great on 93. All piston top edges smoothed and combustion chambers were polished and cc'd so all cylinder pressures would be equal.
Here's a link to a good read by Pat Kelley.
http://cochise.uia.net/pkelley2/DynamicCR.html
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That's the website I used to get the dynamic stroke calculation. It is a good read. His DCR numbers are a little lower than that of the classic inlines chart I linked to, but he does say between 7.5-8.5 is good for pump gas (91 or better octane).
 
I run 200 psi cranking presure, 11.25 cr 360 with j heads, no heat crossover, hi stall, 30 total, electric fuel pump, very cold plug with MSD. I run 93 from the better stations. If I was to race it I would 50/50 mix with some race fuel.
 
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