All right, here's what I've got.

It's my 390, thing's bored 0.030" over. I picked up the TRW/SpeedPro forged pistons. Now, assuming my rods are the right length, trying to figure out how much I need to have trimmed (since they have to be, 428 crank, which is btw, .02 under, rods and mains) so I can get 9.5:1 or less. Pretty sure around here I'll have problems with 10:1.

I spoke to the shop doing my heads. The combustion chambers are 69cc's.

Been playing with Dyno2000, and per their compression ratio math, if I have the piston at .1" from TDC with 43cc's of volume above the piston (not counting the space of the chambers or the gasket, assuming felpro .041" displacing 10.1cc's) says it'll get me to 9.47:1 compression. If I tell it I have flat top pistons, it tells me that I'll have 9.48:1.

Now, I have been looking, and with a stock 390 crank it looks like the TRW's would give me around 10:1 with 72cc heads. But with the 428 crank we have to mill.

Is .1" gonna be enough off the pistons? I want to get as close to 0-deck height as I can. Rough guess as to what we'll need to carve out of the eyelets after the mill?

Sorry if I've had a bunch of questions but I really want to build this motor up right, and have it last me for a good long while. And I don't want to have to deal with putting in octane boosters every time I fill up.

 

Getting 9.5:1 from your setup is going to be very difficult. I don't know how much the TRWs can be shaved but you want to maintain a zero deck. I would recommend enlarging the combustion chamber. Otherwise you will end up with thin pistons and still a lot of compression. Desktop dyno's compression calculator sucks. It doesn't take into account everything and is difficult to use. I know of a good one that uses microsoft access and it is great.

 

[updated:LAST EDITED ON 28-Jun-02 AT 05:53 PM (EST)]err, restate that.

Pistons are here. Care to hazard a guess at how much to enlarge the chambers? And would a thicker gasket help? (Or would it be worth the extra trouble, since it could theoretically be weaker and more likely to break)

 

You are right about a thicker gasket. I wouldn't know where to find one anyway. I'll do a quick calculation and see ho much you would need to go.

 

With regular felpro gaskets and milling your pistons down to acheive zero deck you will have 11.84:1 compression. Remember you are increasing the stroke so bringing it up to zero deck with a longer stroke seriously increases compression. To get the compression down to 9.65:1 you would need 89.1cc combustion chambers. Are you going to run nitrous? I don't know if you can get your chambers that big and still be safe. I would recommend running 1976 stock 390 pistons which are really 410 pistons. These puppys already have a dish to them

 

No, no nitrous. But 89 cc's seems a bit large. Wonder if the trw's will have enough space... hrm. Suppose I could go ahead and get a set of the sterling cast pistons, and see if I can return the trw's.

 

I was in the same boat that your in. I bought some 390 60 over L2291F forged pistons with a 390 kit. I then found a 428 crank and wanted to use the pistons.

I found some C7AE-A heads from a 67 TBird (74 cc). I enlarged the chambers to 80.5 cc by removing all the casting and getting to a smooth surface, then unshrouding the valves.

The actual difference in the pistons would be .1 I had mine zero decked for best squish effect and ended having about .08 taken off. I measured the pistons at the thickest point and it was over .5" so .08 wouldn't be a problem.

I used a standard sized gasket and didn't carve any more from the eyebrows.

I THINK I got it down to 10.25 but it could be a touch higher. It cranks at 205~210 with < 100 miles on it and hasn't broken in the rings yet.

I knew this was going to be a problem from the start and I did several things to help:

I got the Edelbrock alum water pump and 160 high flow T stat and ground out the water passage in the intake and heads for better flow (you'd be surprised how much they're off). I blocked off the exhaust cross over in the heads and run the 4 row jumbo rad on an F250. I had a pro curve the dist given all the engine specs and now I'm looking for a cooler plug and cold air intake.

I've been able to run reg gas so far, but I haven't put any real miles on it yet.

By the way the C7AE-A large chamber heads are cheap and have the same basic port size and the common C8AE-H heads.

Hope this helps.