Ok, so I've been poking around here for a while and I'm starting to get a good idea of what I want to rebuild my 390 to be like. The downside is that to do it right it will take me a few years (the nature of my job requires me to move a lot, not much time to work on the truck). So I was doing some thinking...uh-oh...

My truck currently has a crate 390 in it with about 60,000 miles (I pulled it out of a '76 F250 that another guy had put it in). To get me by for the next few years I plan to put a new cam in it (crane 343901). Then I caught a case of the "might as wells", meaning while I'm at it I might as well put a new intake on (Performer RPM). Then I really got to thinking that while I was that far it wouldn't take much more time or money to pull the heads. This would give me a great chance to see what pistons and bore are in the engine now for reference on a future rebuild...and...

My question is this, as long as I'm there and I check the heads and deck to make sure they are true, couldn't I re-install everything with the Mr. Gasket .020 steel shim? By my estimation this would increase my CR by about .5 point (assuming my current gaskets are .041), maybe help a little bit on the power end??

I'm well aware that this isn't the "ideal" setup, but would it work? Let me know if you think this is a dumb idea...

 

No, not a dumb idea but with 60,000 on the motor, it wouldn't be the best idea as the block/heads probably aren't perfectly true as they would be after machining. If your heads are warped at all even though they show no sign of it now, theres a good chance you could have a leak later. When you do your rebuild in the future, choose the pistons of your choice and have the block zero decked according to those pistons and then use the .037 head gasket that will give you good quench. Unless your truck is eating oil like crazy, I would just wait and do all the work at once when you do the entire rebuild. That is unless you just have too much time/money on your hands right now.

 

Does it have a 4bbl on it now?

 

Why not locate a junk yard engine, complete, and have your way with it over time. Meanwhile, you're still driving the daily driver. When the other motor is done to your wishes, just swap them out over a weekend and store the running piece under the bench in case the new and improved version, isn't.............aMP

 

Yes, it has a 4bbl with the iron intake. I already have a couple of junkyard engines that I've been working on. I bought new (Stan's) headers for the truck, and I figured I'd like to put a new cam in while I'm at it. That is when I caught the above mentioned case of the "might as wells" for the aluminum intake, etc. Not to mention the curiosity of seeing what my current bore/piston combo in the engine is for future reference.

I'm already pretty set on the cam and intake, was just looking for someone to talk me out of (or into) the head work. You guys are my sanity check...lol...thanks for the input!

 

Moon, by all means do it. The block and heads are probably straight enough unless you have overheated the snot out of it at some point. Get some copper seal paint and follow the instructions carefully with those shim gaskets and you'll be just fine. The extra .5 compression ain't gonna make you a world beater, but it will help and you'll have the chance to see what pistons you have now. I'd do it. DinosaurFan, on work's old discarded 'puter

 

DF, thanks, the more I was thinking about it the more I was thinking of doing it. As long as the heads are within tolerance (I read it somewhere in Steve Christ's book) there is no reason it shouldn't work. Just make sure everything is good and clean before I put it back together.

I realize this isn't the "ideal" situation, but I'm interested in doing it just to see if it works. Before and after dyno numbers would be interesting, I'll be home in another month, so if I can swing it I'll let you guys all know how it turns out.

Worst case scenario the head gaskets will blow, and I'll have to spend another couple days wrenching on the truck...oh darn

 

From .041 to .020 is good for .5 on compression????? .021 difference in volume on, as an example , a 4.0 bore might be .05 difference. Would you care to share your calculations with the class?..............Overall volume, chamber volume w/ .041 and then the same with .020........I might be off here but, .021 just doesn't seem like that much...........aMP

 

going from a gasket with 4.4" bore and .041 compressed thickness to a gasket with 4.22" bore and .02 compressed thickness.

For calculations I assumed head volume of 70cc, Bore of 4.08, Piston head volume of 9cc, Deck clearance of .11, and stroke of 3.78. I calculated it here http://www.kb-silvolite.com/calc.php?action=comp

This gives me a stock CR of 8.18:1 and an improved CR of 8.56:1. I think the first time I calculated it I used a head volume of 67cc, wich gives me CRs of 8.379 and 8.778.

Ok, ya got me, its actually an improvement of about .4, but I rounded it up to .5 in the process of justifying all the wrenching

Of course these are all assumptions, actual results could go either way.

 

You're right!! I forget how small these FE chambers are and accordingly how a small amount of "quinch" can make alot of difference. Kinda gets scary when you consider that most of the FE heads out there have been surfaced at least once and the chambers are even smaller than estimated........I did the basic math, just cylinder volume with a flat top piston and found that .021 X.180 difference might be worth as much as .7, quite a bit!!.........Now I'm going to go back and actually figure the 10:1 numbers associated with my 435. Of course without actually CCing the chambers I'm kinda in the "ballpark" with any calculations. aMP

 

Yeah, of course it is all assumptions, part of my justification is because I would like to take a look inside the engine and see what I actually have. And once I get it apart and measure how level the deck and heads are I might have to re-install it all with a composite .041 head gasket again anyway. It'll be a couple months before I can dig into it, but I'll be sure to post my results. Thanks for all the input gentlemen,

Nick

 

Moon sounds good to me!! Being how your gonna be in there anyway..I'd change them out for the shim gaskets..I really dont think your gonna blow the shims! LOL..

You get a thumbs up from me, anytime a guy can get a little more hp and Tq.. Its all good!! I just cant see someone putting a motor back tohether stock..when there's more hp and tq to be made with just simple stock parts swap!!

RJ