So I'm building the heads/cam on my Corvette.
We're going to try and get to 500whp with a stock 346... So I'm trying to get every last bit out of each part with the car. I'm going to be buying PRC 227cc heads. I'm wondering what to get the combustion chamber done at and the heads milled to as well. (I know I'll have to flycut in any case because I'll be running a very aggressive cam)...

So basically what compression can you run using 93 octane? Assuming a tune is dead on. After reading Pat G's "recipe for 500whp" thread, I see sometimes higher compression is ok with tighter quench... So ultimately, what am I looking at here for compression ration and quench (which might bring us in to chamber size in cc, mill amount, etc - feel free to add in info)...

Also - tell me if this would go over better in advanced tech... if so I'll re-post or request to have it moved. Thanks

 

I believe 12.5:1 is probably as high as you'll be able to go on pump gas, and that might be a bit high.

I'm no expert though and I'm curious to know also.

 

There really is'nt 1 answer to this question and You'll see alot of different answers . There are things that change safe compression ratio (static) ,,,,,,,,,1 thing You can do to help is either buy a larger radiator and fab/modify the car a little , or a good sized tranny cooler if it's an automatic car ,,,,,,,You can buy another 4ft. of each size of the heater hose and couple them in and just make 1 curl and hold them in place with wire ties . With a larger cooling capacity even when the gauge reads the same as before , actually most of the engine is running cooler and thus less pre-ignition allowing more advance . Of course fuel additives to raise the ignition temperature or the "volitility"of You're fuel will do the same thing ,,,,,,,maybe a water inj kit or a meth inj kit ,,,,,,,,coat the tops of the pistons and the exhaust runners with the ceramic coating ,,,,,,,,,just don't do 2 or more of the previiusly mentioned or it will never warm up . Google Wallace racing and use some of their calculators ,,,,They have a bunch of different 1's ,,,,,,that should help .

 

im running 12.1 static in my 347. It has diamond dome pistons with no coatings. I have no problems with detonation on 93 pump. You mainly want to worry about cranking/dynamic compression. You can change your cranking compression with cam timing events.

 

12 to 1 is about the limit to be SAFE.Depending on who tunes the combo after its done.

 

I'm running 12.5:1 in my 408, but my huge cam drops drops drc down to 8.22:1....I've heard of others going higher, but "safe zone" with proper quench is about 8.5:1 max dcr

 

it all depends on the cam,head gasket,piston design and the tune.If its a very large cam then shoot to get your dynamic compression in the mid 8's and static at like 12.Use the calculators to figure it out,there are a few out there.

 

anyone know where the find the calculator? google wasn't much help

 

Just do a search on this site for DCR calculator,i believe it was by pianoprodigy screen name.

 

found this one...
https://ls1tech.com/forums/generatio...alculator.html

using the heads I'd like with a .041 hg, and 57cc comb chamber, I get 12.12:1 compression and 8.92 dynamic with 0.034 quench with flat top pistons... however I think with any big cam I'll probably need to cut reliefs

1cc relief is 11.95 / 8.79
2cc relief is 11.79 / 8.68

 

So I'm thinking, I'd like to be close to the number for the 1cc relief but with a 2cc relief... So I assume I'd need to mill the heads then, right?

 

That must be a big cam, what are the specs if you don't mind me asking? With a can that size honestly you will probably be looking at 4-6cc valve reliefs to get good ptv clearance....is worry about ptv first, plus add to the depth of the valve reliefs for the extra milling.

 

DCR is what's going to limit you.

Pat Kelly has about the best one I've used.
DCR FAQ's, the answer to your question could be here.

http://cochise.uia.net/pkelley2/dcrvb6.zip 1.55 MB

http://cochise.uia.net/pkelley2/dcr.zip 423 KB

 

I was probably going to goto Pat G or EPS for a custom cam but I was using a T-Rex for the calculations, I figured if I don't go custom I'll buy that. I know its pretty big.
Will check this one out... and yeah I'd like SCR to be like 11.8-12.0 and DCR to be like 8.4-8.75

 

Others have touched on the items that you should consider when determining what compression ratio you have or should have.

Take a look at the following site for calculating your compression ratio - http://www.kb-silvolite.com/calc.php?action=comp

A gentleman who business does development work for GM and Pratt and Miller told me that 12.0:1 is the most he would recommend for 93 octance pump gas.

Also, you should realize that your block is going to grow 0.012 inches in height from 70 degrees F to 190 degrees F. So when you use the United Engine and Machine Company compression ratio calculation page, be sure that you enter both your hot and cold deck heights so you can see the difference.

Also, you will need to know exactly where your closing point at 0.050" is for your intake valve. Add 15 degrees to that and enter the number in last box of the UEM calculation page.

Good luck.

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I'll probably just use the hot deck height number - as that'll keep it safe. I appreciate you bringing that up tho. I'll factor it in.

So how do you add in the milling to the calculator? Also is there a guide anywhere to how much milling = how much CR?

 

The subject of milling can be very complex. Some do straight milling, while others do angle milling.

The important thing to remember is that your milling is going to bring the cylinder head closer to the piston. Your aftermarket camshaft is probably going to have more valve lift and a longer duration than your OEM camshaft. All of these factors need to be MEASURED so you can accurately determine your compression ratio and your piston to valve clearance.

Oh yes, the cold static compression ratio is safer than the hot static compression ratio.

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