390 Aluminum intake?

[wyngnut55]

I will call when I find out bore size. Bet they will line me up with pistons I want. Yes, I don't think I want low compression. This is a good heads up for me. Thanks

[DuckRyder]

CastHyperForgeW.jpg

I will have to recreate the post with the pictures later, and the pictures are not great, but here are the 3 pistons all lined up, the pins are through pin bosses, so the pistons are lined up at the piston pins.

You’ll see how much shorter the cast “truck: piston is, the middle one is Hyper (H304P) and the last one is Forged (L2291F)

[wyngnut55]

Head casting # C8AEH, Shop thinks its 69cc. The old pistons were flat top & thats what I'm going to replace with. I'll talk with Summit when I know bore. Shop said keep compression below 10:1, 9.5:1 preferably.

[DuckRyder]

Yep C8AE-H are what I have, with new valves and a shave for straightness they were more like 66.

The flat tops have any markings, usually on top or around the pin boss?

[wyngnut55]

Pistons have #30 on top. .030 over? also have #2 by the wrist pin boss.

[DuckRyder]

Id say the #30 is almost certainly the oversize, the 2 I’m not sure of. Most all Federal Mogul (TRW, Speed Pro, Sealed Power) i have seen have the piston number on top. KB I believe have “KB” near the pin boss.

I ran some DCR #’s on that cam, personally I would definitely try to stay under 9.5 static.

See: http://cochise.uia.net/pkelley2/DynamicCR.html

[wyngnut55]

Ooo Kay...Probably going to have to read that quite a few times! Looks like its not as simple as I was thinking. I guess the questions are? Will the compression ratio be kept below 10:1 with that cam and flat top pistons? Can that be determined after all machining on heads is done before pistons are ordered? Should I just go with dish pistons and be safe? Would the engine still produce more power with low compression dish pistons? Or is the whole thing being over thought?
I want a reliable engine, really don't want to be pulling this thing out over and over.
Thanks

[DuckRyder]

To answer your questions:

You can calculate the compression given numbers you have, the more real numbers (versus specs) you have the better. The heads in particular are an important piece as well as the compressed thickness of the head gasket the bore diameter of the head gasket and piston height compared to deck.

I don’t think the 2291F pistons are going to drop below 10.0:1 with your current heads, I make them at closer to 10.25:1 but I’m guessing with several numbers. Summit has a PDF that gives you approximate CR on the L2291 for various head CC. They also have a compression calculator. Those may help, though unfortunately I’ve been unable to find a similar PDF for other choices.

More compression will make more power right up until you start having to pull timing out.

A gross oversimplification of DCR is “effective CR” or what the engine actually “sees” vs Satic, the big variable once static compression is set is the camshaft, most particular the intake close event. This is why some big cams say things like “needs 10.5:1” on the cam cards and also what people are really talking about when they say things like “big overlap cam bleeds of compression”...

You could also call Comp, been a while since I talked to them but they were helpful, friendly and knowledgeable last time I did.

Out of time at lunch, but I have L2291F in my engine and the same heads, so I’ll be back with real world story time tonight...

[DuckRyder]

OK, so when I did my engine, I calculated things pretty carefully using everything I knew some of it published specs. I have L2291F-30 pistons, C8AE-H heads, Victor gaskets (about the same as FelPro) and I put CJ sized stainless valves in it. I had figured the compression would be slightly over 10:1 (between 10.1:1 - 10.2:1). I changed my cam choice a couple of times, but I ran DCR on each one and the numbers said I would be OK to run on pump premium.

I left very detailed instructions for the machine shop (they actually appreciated that) and they followed them very well, however they ended up having to deck both the block and heads more than expected which changed the deck clearance and the head CC, the pistons ended up at zero deck and the heads at 66cc this put the compression closer to 10.6:1-10.7:1.

My cam is a fairly large cam, though it is a street oriented solid roller and the actual seat to seat events are not nearly as radical as the .050 events would lead you to believe. So it builds cylinder pressure (higher DCR) like a smaller cam. It would have been perfect with the originally planned static compression and it does work, even where I am at now, but I have to be conservative with both total timing and how fast I bring it in or it will ping on premium.

If I had aluminum heads or a few degrees on the intake close It would be fine.

You’re at least 10 degrees more conservative on the intake close than my cam, so my engine with your cam would be a hot mess, probably have to run very retarded timing.

So that’s my cautionary tale.

If i was in your shoes I would pick the small dish pistons or similar. (not truck pistons)

HyperTopW1.jpg

The flat top forged are prettier, though.

VariationTop.jpg

Truck/410 piston

410PistonW.jpg

[wyngnut55]

Wow, thanks for all the input. I'm kind of leaning towards just staying with the dish pistons that Summit originally quoted me. I also visited the machine shop today and talked to them about pistons. He is sure he could do the calc's, find me some flat top pistons and keep the CR below 9.5:1 using his source, not Summit. This would probably mean more money and ordering a different rebuild kit from Summit w/out pistons. Might be worth it. He hasn't seen the cam specs from Summit yet and hasn't got to my engine yet.
Thanks

[DuckRyder]

I’ll run some more numbers today using the H304P, I think this piston would do what you want.

There are a number of companies that do custom pistons and KB Silv-O-lite also offers several pistons I’m not familiar with.

Definitely worth the effort to get what you want. The truck/410 pistons aren’t a good idea, well unless you want to put a 428 crank in it and make a 410.

[wyngnut55]

Decisions decisions! I had this truck torn apart for 3 1/2 years in my garage & yard to my wifes dismay. Got it going about 6 months ago. I used to do a lot of fleet maintenance, including paint jobs (school bus yellow) for a heavy equipment dirt moving contractor that had about 50 of 1970s vintage Ford trucks. They were beat pretty bad. Always liked the 72 and earlier the best. Never did engine rebuilds just bought long blocks from National Engine Exchange. This was back in the early 80s. I uploaded some pics of my project. Painted it in the driveway.
Thanks

[DuckRyder]

Cool pictures, you should start a thread in projects and bring us up to speed.

I didn’t have a chance to run numbers today (the program is on an old windows computer) I promise I’ll get to it ASAP.

[DuckRyder]

Anyone sees any mistakes In my math let me know.

390 deck height 10.17, stroke 3.78, Rod 6.488.

Fel Pro 1020 head gasket .041 compressed, 4.400 bore diameter.

Formula: Deck clearance = 10.17 - (.5 stroke {1.89} + rod + piston compression height )

OK, so lets look at 3 pistons:

L2291F, H304P and KB 150

L2291F has 7cc volume, 1.776 ch = 10.2311:1 static and 8.744:1 dynamic @ .016 deck clearance with 67.5cc head.

H304P has 12.1cc volume, 1.759 ch = 9.396:1 static and 8.072:1 dynamic @ .033 deck clearance with 67.5cc head.

KB150 has 20 cc volume (step dish), 1.78 ch = 9.1139:1 static and 7.83:1 dynamic @ .012 deck clearance with 67.5cc head.

I think the L2291F is unworkable with that cam and head combination on pump gas.

Either of the other two should work fine.

[wyngnut55]

Thanks again for all the input. Those 2 choices sound workable. I'll confirm it with machine shop when they get to my block.