Thread: pump gas

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whats the highest dynamic compression ratio you can have and still run 93 octane gas? I have two choices of pistons one with 30cc dish with a 10.2:1 static CR and a 9.1:1 dynamic CR. the other is a 18cc dish with a static CR of 9.30:1 and a dynamic compression of 8.34:1. I would like to run a bit over 10:1 so my cam has enough compression. the heads are vortecs on a 406sbc the cam is 230/230 camshaft. any thoughts and ideas would be helpful!

Very funny.... I was thinking of the exact same question last night but with the variation of 91 octane at high altitude (which should be roughly the same as 93 at low altitude). I think techinspector1 has some crazy mind-control that makes us think of these things

BTW: I've heard you should run DCR no higher than 8.5:1 on "pump gas" but I was wondering the same if you can run higher with "premium" and maybe alum heads....

Everything I've read says 8.0 to 8.25 dynamic for pump gas. In reading threads on other forums, I've come to believe that the calculator I've been using (KB pistons) is a little conservative. I still believe the 8.0 to 8.25 is valid, depending on the calculator used, but I tend to favor 8.25 to 8.50 on the KB calculator. Of course any engine build that is optimizing the DCR should also include de-burring of the chamber to prevent hot spots and proper squish, which on a SBC would ideally be set at 0.035" to 0.040". Big blocks should use a little more because the bore is bigger and the piston has a better chance to present the edge of the piston above the deck as it rocks on the pin. Same with forged pistons in a small block. You'll use maybe 0.005" to 0.007" piston to wall clearance, so the piston is looser in the bore and has a better chance to present the edge of the piston above the deck. Here's the KB calculator if you want to play. Determine the intake closing point of the cam you're going to use and add 15 degrees to that figure for entry into the calculator. (ex: a cam with a closing point of 41 degrees after bottom dead center would be entered into the calculator as 56 degrees).

http://kb-silvolite.com/calc.php?action=comp

www.summitracing.com has a very complete list of head gaskets for you to find the gasket bore and compressed thickness. A common gasket used on the 350 with a zero deck would be one such as the Fel-Pro 1010, 4.166" bore, 0.039" compressed. If you don't want to cut the deck to zero and the piston in a little ways down in the bore, use a thinner gasket to reach your correct squish figure. For instance, Chevy makes a 4.100" by 0.026" shim gasket under part number 3830711 and Fel-Pro makes a steel shim gasket at 4.100" by 0.016" under part number 7733SH1. Cometic makes a custom gasket in any thickness starting at 0.027" on up and any bore size.

Originally Posted by skids72

Very funny.... I was thinking of the exact same question last night but with the variation of 91 octane at high altitude (which should be roughly the same as 93 at low altitude). I think techinspector1 has some crazy mind-control that makes us think of these things

BTW: I've heard you should run DCR no higher than 8.5:1 on "pump gas" but I was wondering the same if you can run higher with "premium" and maybe alum heads....

Yes, a higher DCR can be used with higher quality fuels such as race gas or alcohol. I'm not sure about the figures, but I think somewhere between 9.0 and 9.5. Again, not sure but I think it was Erik who said he has run motors with cranking pressure in the area of 220 psi on pump fuel with all things being optimized, no PCV or EGR to pollute the incoming mixture in other words, everything polished, squish optimized, exact intake closing point for the SCR, etc., etc.

If you read the tech tips on Crane's site, they suggest limiting cranking pressure to about 160 on pump gas. Now, of course, they are gonna be conservative because the rodder in East Treestump probably isn't going to go to the trouble or even have the facilities to set the squish properly and do the other things that will produce a valid motor with 220 cranking pressure.

techinspector1: Thanks for the explanation... as for pump gas we get up here, 91 octane premium is as high as it goes. Thinking about cranking pressure, I decided to play with some math:

At sea level typical atmospheric pressure is 14.7psi or 29.9"Hg (at 32*F). Typical pressure I see here in the summer is < 24"Hg which amounts to 11.8psi(?) but at higher temperature ~80*F

At 14.7psi ambient air pressure a 160psi cylinder pressure would come from 10.88:1 compression (I think this would be dynamic compression). For 11.8psi air pressure (high altitude) you get 160psi cylinder pressure from 13.56:1 compression (!)

I need to find out how to account for pressure conversion at more realistic temperatures... there are flaws in the math from the different temperatures but I think the general point is it takes more compression to get 160psi at lower air pressure.

At any rate I guess this begs the question is it the DCR that matters or the actual cylinder pressure that matters? If cylinder pressure is the key, it seems higher DCR can be used at lower atm pressure (i.e. high altitude).

Am I thinking right on this?

Of course, if I want to drive to Kansas I might run into trouble but I'm looking to get the most out of a bracket car that always runs at high altitude on 91 octane.... thinking out loud.

I'll try some more research on this and let y'all know if I learn anything else...

-Chris

Thanks, Denny.... I'll have to spend some time with this stuff.

-Chris

Wow, Denny... that's a gold-mine. Where did you get this?

Question on the ratios listed under different fuel types... these look like static CR's? Although the first part of the article was talking mostly DCR's?

If I'm reading this right, with good squish, alum head and high altitude 10.5:1 is okay (SCR or DCR?) maybe a tad higher on "premium"?

vk: sorry I hijacked your thread! I gotta stop doing that!

Thanks again,

-Chris