I realize that 'how much compression is too much' is difficult to answer. My short block is now assembled (383) but due to decking and the longer stroke the pistons are between .009" and .011" in the hole. I picked up some Edelbrock 60859 60cc aluminum heads last year and was planning on using them to top the shortblock, but those heads with a Felpro 1003 gasket (.041" compressed height plus 4.100 gasket bore) give me a static compression ratio of 11.37 to 1. Pistons are forged flat tops with two valve reliefs (-4.86cc). I have a Comp Cams 08-502-8 that I was planning to install -2 degrees at the crank to lower dynamic compression and to move the power up (TPI doesn't need any help with power down low). Wallace racing dynamic compression calculator tells me 8.93 to 1 dynamic compression, while the Project Pontiac calculator comes in at 8.84 to 1 dynamic compression ratio. With a 160* thermostat, a throttle body bypass, rich tune, conservative timing, and aluminum heads, what are the thoughts on keeping detonation out of this motor? Or will I have to suck it up and go with a bigger chamber head and blow my budget? Any input greatly appreciated, thanks in advance--captainkawasaki

 

Try a thicker head gasket.

 

Kind of high for pump gas. Ok with race gas.

 

You could also resell (trade) for different pistons...and get some inverted domes. Technically, that shape provides the best [concentric] quench.

If have have all the sharp [top piston] edges beveled, run convervative timing, install that cam as suggested...or even at -4-deg, AND run 93 octane, I think you'd accomplish the goal.

Obviously, pulling the heads isn't as bad as pulling the entire motor -- if you're wrong.

I wouldn't do anything thicker than the .041 gasket you purchased. When you're THAT close, decreasing quench can often make the difference between pinging and not.

Of course, even if you run the 160-deg thermostat, I'd bet the big bucks you'd have to increase radiator size (BTUs) and install a higher speed fan. You might have to do that anyway...but it's something to consider when you're weighing options for repurchasing any given part.

FWIW, Fan and radiator (waterpump too?) upgrades would run from $400-$800.

With higher compression, heat/timing management are definitely key.

 

Run E85. I do.

 

Definitely a thought. What's the minimum compression you'd consider for that option? How readily available is it? Overall, what about fueling costs vs gasoline?

 

maybe I'll have to start looking around for some budget priced aluminum heads with a 64cc chamber...I'm worried I'll compete assembly and then I'll end up with knock counts through the roof.

 

If using on line compression sites to calcuate CR you need to refigure.

Any dish in the piston is + (plus) CC's not - (minus) CC's. Minus CC's come into play when using domed pistons.

A difference of 9.72 + CC's from your figures.

Good luck

 

E85 is typically 40-50 cents cheaper per gallon.

The ethanol really needs the higher compression ratio to get the best bang from the fuel. Before my engine guy died, we were planning a 13:1 motor for mine running E85. He had some drag guys running that same combination getting 700 FWHP, and the alcohol keeps it running cooler. In the summer heat, that is a big plus.

 

I've heard good things about e85 and racing. Sure beats $9.00+ race gas.

 

When its 50¢ cheaper than premium, its a wash. Every time I decide to switch back to Premium, they end up jacking with the prices so I dont. I get about 15 mpg on E85, 18 on premium. Depends a lot on what traffic is like. I live semi rural, that helps. It is a problem finding E85 in some areas. Not here though.

 

Just take a die grinder to the combustion chambers of your current heads.

If you unshroud the chamber around the valves (open it up to within .030 of the head gasket bore) and then lay back the quench pad you can easily gain 4cc's.

Will

 

would "cleaning up" the combustion chambers and maybe rounding off the sharp edges around the valve reliefs on the pistons give you an extra c.c. or so ?

 

You can also run a cam with a little more overlap, to bleed off a little compression. A new cam is cheaper than new heads.

 

FWIW...I get 9.05 DCR using Paul Kelley's calculator. The cam I found using the part number provided shows a 108ICL.

 

10.5 and under for the street...without problems.

 

yep...

with 93 oct vanishing around the country as part of the federal clean air act, being phased out and away...gone for good, thats how low compression engines were born...the 8.5:1 engines of the 70s.

Now most places still offer 91, some have 93 and thats only good for 10.5:1 at most IF you want to run the ign advance that is needed for a performance motor. Mine calc'd out to around 96 oct, since thats no longer a possibility mixing trick-gas with pump gas, (used to live near a trick station) I backed out of the ign timing to end the low oct knock that comes with compression and timing advance...and lost an appropioate amount of performance.

I tried water injection. That does work, but its a hassle and it just gets old having to fill the bottle with water/alcohol about 2 times to a tank of gas.
So...
Unless you;ve gopt deep pockets and can afford to blend, or have access to av-gas, I'd try to drop the comp to under 11 and work with the timing to keep the thing in one piece. That knocking from low oct will eventually break something.
The one thing that I would do, is to use gaskets or dish the pistons (machine shop) and avoid cutting the heads much...Heads are harder to build up again. Pistons will get replaced sooner or later. Cutting/grinding heads is often a one way ticket...whatever you do you gotta live with.
Good Luck !

 

Looking into this further....I'll add my 8.3 DCR engine isn't "happy" when it gets up to 220. (If it was an LT engine -- with reverse cooling -- I'd have no problem.) Using this chart, your MAX engine temp would probably need to stay under 200 which is fairly hard to do. And, there's no guarantee the top-end temp wouldn't be lower.

BTW...I have access to 93-oct where I live.

I vote you NEED to drop compression -- because it's an L98.

 

where are you located .. I know some one that can CNC the champer to gain the CCs that you need ..