Thread: 500 hp/550 lb-ft from a 390?

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It would be a shame to waste all the time and work cuz a rod let go and window'd the block!!! Never have ran a Hawk's rod, can't say that I've heard much good or bad about them....but then I'm one of them fools who spend the extra and go with Carillo's, Crower, or similar US made rod.... Lots of folks on here use the low buck imported rods with great results, I know Pat has used them in some big horse engines with no problems.

Jeff, you mentioned earlier that you had a 750 you wanted to use. I guess I missed that when I did the DynoSim. Go ahead and plan on using it, horsepower drops only 3 to 547 @6000 and torque drops 3 to 535 @5000, so no big deal and it will save several hundred in cash outlay.

I didn't go into detail on the block deck height/squish/piston deck height/gasket, but that's the critical area of the motor in my opinion. That's where you either have a motor or you don't in terms of preventing detonation on pump gas.

What you want to do is add up your "stack", the total of the crank radius, rod length and piston compression height and compare that figure to the block deck height. Of course you want to measure everything closely before final assembly, but you can get an idea of where you are before you ever buy parts. I believe the block deck height of a 390 is 10.170". That assumes that some knothead previous owner hasn't decked the block. The crank radius is 1.890", the rod is 6.490" and the L2291F piston has a compression height of 1.776". If you add up this "stack" parts, you get a stack height of 10.156". Deducting that from the block deck height of 10.170" reveals a piston deck height of 0.014" (piston in the hole 0.014" with the piston at TDC). Now, to get the squish, you add the thickness of the compressed head gasket to that. Most composition gaskets that are available are in the 0.040" thickness range, so using one of these will put the squish at 0.054", a little too generous in my opinion. FE gurus may have a different take on it. What I might be thinking is to take a light skim on the block decks (0.009") to just clean them up and set the piston deck height at 0.005". A 0.040" gasket such as the Ford M-6051-A427 added to the 0.005" piston deck height would produce a squish of 0.045". Or a 0.014" cut on the decks and putting the piston at zero deck with the 0.040" gasket would put the squish at 0.040".

I might be telling you stuff that you already know, but I'd rather repeat it than to have you make a mistake on figuring the squish for the motor. Of course you don't want to cut anything until you have done some careful measuring.

Now, for those of you who are just tuning in and haven't had experience with "squish", I'll expand on this for you. It is desireable to generate turbulence in the chamber just as the plug fires to prevent any "dead spaces" where the mixture wouldn't fire. This feature will serve to allow the motor to operate on a lesser quality of fuel or to allow it to operate on the available fuel at a higher static compresssion ratio, so long as the intake closing point on the intake lobe of the cam is matched to the static compression ratio to close the valve at just the right time to trap the mixture in the cylinder as the piston is ascending on the compression stroke. For a given static compression ratio, closing the intake valve too early can result in excessive cylinder pressure and the inability of the motor to run on pump gas without detonation. Closing the intake valve too late can allow too much of the fuel/air charge that has just been pushed into the cylinder by atmospheric pressure to be pushed back through the still-open intake valve as the piston ascends in the bore, pushing the mixture up the intake tract to disrupt the signal at the carburetor venturi. The venturi sees flow going both ways and doesn't know whether to sh** or go blind. This is what makes the rump-rump in an over-cammed motor at low rpm's. The carburetor venturi is incapable of metering fuel properly. If you get the chance to see a cammed-up motor run at night with the air cleaner off and a strong light shone on the top of the carb, you'll see the phenomenon of "stand-off". This is a ball of mixture that is being blown out the top of the carburetor by the piston coming up in the bore while the intake valve is still open. Once the motor gets "up on the cam", everything smooths out and you have a runner, but at low r's, you get the rump-rump.

Anyway, I'm getting off-subject. The point is to tighten up the squish so that the flat crown of the piston matches up with the flat underside of the cylinder head and results in the "jetting" of the mixture toward the spark plug, generating turbulence and optimizing the combination of parts. Here is a photo showing the underside of the Edelbrock head (photo showing the valves). On the right side of the photo, you can see that the head is flat at the area where the piston crown would mate with it. Think of it as dropping a book down on your desk and visualize the air that escapes as the book closes in on the desktop. That's what squish is. The other photo is of the piston. You can see that there is a nice flat area on the crown to mate up with the head at top dead center.
http://store.summitracing.com/largei...dl-60069_w.jpg
http://store.summitracing.com/largei...l2291f20_w.jpg

The dimension of 0.035" to 0.045" has been arrived at by professional engine builders who have experimented with this stuff and determined that this is the optimal dimension. Any wider and you have too much clearance for the mixture to be "jetted" across the chamber. Any closer and you run the risk of a piston/head collision. As the motor runs, things change. The crank flexes a little, the rods stretch a little, the piston heats up and gets a little taller and you end up with a very close clearance between the piston and head at operating temps and rpm's. David Vizard says he has run a small block Chevy at 0.026" with no collision, but that is not for the faint of heart to try.