I have taken the advice frome Blu93GT and currently am looking for some "reverse dome" 351C Pistons. Anyone know where I can find some? KB doesn't make any so I am pretty much stumped. I would like Hypereutectic if I could get some but "Reverse dome" is the important part. Thanx for all the help

 

[updated:LAST EDITED ON 01-Aug-02 AT 05:15 PM (EST)]here ya go! look at KB-148

http://www.kb-silvolite.com/page45.htm#SECT1

they're calling it a "D-cup" (nice name! i love d-cups!) and you'll notice the dish is the same shape as the dome on the 351C domed pistons. also, they say it's 9.8:1 with a 62cc chamber, which is the size of the aussies, according to what i just now found on a search).

this should be healthy enough for good power, with very good resistance to detonation, as long as the piston/head clearance comes out right. seems like 351C pistons in a 400 come out very good for quench, but you should do the math yourself just to make sure...

ALSO - i found a listing of STOCK GASKET THICKNESS (compressed) last night. i think the 351C was listed at .048 or .052, and it showed .032 or something for the 400... Profanity Removed!!! i thought they were pretty much all .040, except for "special" ones...

not only could this possibly affect the compression ratios that the piston makers list, but it also means that if you only have a .032 gasket, your pistons shouldn't come ANY above deck, and .01 BELOW would be better. and with a .048 or .052 gasket, you need AT LEAST zero deck, and preferably about .01 ABOVE.

sheesh! that's one more thing to consider...

 

Blu93GT,

I just want to thank you so much. Everybody has been real helpful here but you have been the most helpful to me. It just seems like you have a genuine interest in what I am doing and that says a lot to me. A lot of people don't know this but I am only 17 years old. I am building a hot 78 F250 that is going to waste any chevy in town. I know a lot about engines (I am a machinist at a racer's machine shop) but apparently you know more. Again, I just want to thank you for helping me so much. I really appreciate it!

 

Ok Blu93GT, I just ran the numbers and the results came out as follows:

62cc Heads, 13cc D-Cup pistons, .041 gasket thickness, 4.100 gasket bore, 4.030 cylinder bore, 4.000 stroke, .010 deck clearance


With those specs, I get a 10.73:1 compression ratio. Isn't that going to be too high for 91 octane premium and a low end cam like I want? And on top of that, I didn't think I was going to have to deck the block that much. I was thinking my deck clearance would be .025-.030, which would give me 10.387:1. But you keep talking about how the deck clearance needs to be within .010 of zero deck, or zero if you can get it. I am just worried that 10.7 is going to be way too much compression. Please advise

 

[updated:LAST EDITED ON 01-Aug-02 AT 06:45 PM (EST)]don't thank me yet - it looks like i screwed up!

i just did some math, and it looks like there is a problem with the 351 pistons - the compression height is too short. it looks like you'd need to take .050 off the block deck surface to get zero deck with those pistons - even if you don't mind reworking the intake, i don't think the 400 block can handle that much being cut off.

also, the 9.8 compression is for a 351 - the extra cubes of a 400 will give a higher compression ratio. but with the piston top only coming up to .05 below deck, those numbers are all wrong anyway.

i couldn't find any pistons with 1.72 compression height (which looks about right) so maybe i'm still making a mistake somewhere, because i thought 351C pistons were supposed to work in a 400.

i'll keep looking into this, and the gasket thickness question. we both need to know...

ANYONE?


update - i checked using:

http://www.projectbronco.com/Technical_Articles/335CR.xls

din't know if those formulas are right, but it came to 9.97:1 - still to high, since you'd have like .088 piston/head clearance, meaning no quench at all. 9.8 with quench sure looked a LOT better! there have GOT to be some 1.72 pistons - i'm still looking..

 

http://www.kb-silvolite.com/calc.phtml

another calculator - this one says it's like 9.6 or something but i think it's wrong. even if it is right, that's a little high with open chambers and a short-duration cam. and even if it wasn't, i'm still on a tear about not finding the compression height i want!

no luck finding 1.72 pistons yet - obviously. i e-mailed KB tech to find out what the deal is. i'm sure they'll tell me something like they don't make them because the 400 needs 8:1 compression because of it's open-chamber heads, and it's not a performance motor anyway, and why don't i leave them alone and go build a nice 351W like everybody else...

but maybe they'll say they can cut the 351C relief or d-cup on some other piston for the same price - that would be sweet!

 

okay - i tried both C/R calculators. it looks like you'd need about a 24cc reverse-dome to get 9.8:1 with the aussie heads, with a 1.72 piston.

i'd need the same thing, but only 12cc, to get 11:1 with 4v heads.

now where can we GET these pistons...

 

Ross can make them, unfortunately the cost upwards of $800. Yeah I wish I could find these damn things because it looks like I am gonna just have to go with flat tops and stock 400 heads. I know you can get performance like that because my bro built a 425hp/500# 400 using stock heads. I was really hoping the aussie's would work out for me. BTW, do you know where I can find some flat tops for the 400? Thanx dude

 

[updated:LAST EDITED ON 02-Aug-02 AT 00:19 AM (EST)]the KB-108 on that page should give about 9.4 - probably ok even with the U.S. 2v heads, but i wouldn't mill anything. i guess one nice thing about open chamber is that you can just use a thicker head gasket to reduce compression if you need to.

i thought any 351C flat-tops should be about the same, but here: http://www.kb-silvolite.com/slvpg48.htm is one with a lower compression height

here is another good place to compare specs: http://www.flatlanderracing.com/pistons.html


it looks like the KB-148's d-cups would actually be just right for what i'm doing - IF i could cut the block down .045" or so...

oddly enough, there is a flat top 351W piston shown on the silv-o-lite page that has the 1.72 compression height needed for a 400 with quench heads, AND it has some strange valve reliefs at odd angles that kind of look like 351C valve reliefs, but it's regular cast instead of hyperU - and without the reverse dome, compression would still be too high.

as for the ROSS pistons - their regular off-the-shelf 5.0 pistons are like $500, so if they add 60% for custom pistons, then maybe KB will do customs for 50%, which i might JUST be able to swing...

 

My machine shop had J.E. custom build dished pistons that have the outside edge comming close to 0 deck height. Sorry I don't have the specs on em.About $450 but they are forged. 165 to 170 PSI static compression. and no knock on 91 octane at 5,500 above sea level. Also using total seal rings.

 

thanks! that's a good price for custom-made forged pistons, but i was hoping to find something even cheaper - i think the KB's i was looking at are like $200 or $240. i see others in these forums that are looking at either aussie or 4v heads on a 400, so i thought maybe if enough people contact KB, they might consider adding some 400 quench pistons to their lineup.

details here:

https://www.ford-trucks.com/dcforum/DCForumID12/1058.html

https://www.ford-trucks.com/dcforum/DCForumID12/1056.html

if this is something you're interested in, please contact them and let then know - who knows, it might work!

 

has anyone considered ofset grinding (stroking) the crank to get zero deck height? how much can a crank be ofset ground? If I'm not mistaken this will add to the cr but you would get more quench.

 

that would be sweet! but then the stock rods wouldn't work.

240 I6 rods might do it - but the machine work for the crank would still cost more than the extra i'd have to pay for custom pistons.

the KB pistons in my jegs catalog range from $200 to $320 a set, so i guess $500+ wouldn't be TOO bad, considering they'd be forged... still, it'd be nice if i could get some good hypers for a couple hundred less. it'll be N/A, so i'm not convinced i need to spend the extra for forged.

 

Sorry it has taken me so long to write back. My connection gave out on me friday night and the tech office didn't open till this morning. Anyways...

I had an idea the other day and I was wondering if it would work. Ok, if I squared up the block and heads and then took all the measurements, couldn’t I have a set of custom pistons made with a custom compression height? For example, if, after block cleanup, my deck clearance was .040 (10.290), couldn’t I have JE make some custom 400 pistons that have a compression height that is .040 taller than stock? That would bring it to zero deck, wouldn’t it? I could then have them make a reverse dome in the piston large enough to bring my compression down to 10-10.5:1. I know it would have to be quite a large reverse dome but I really want these Aussie heads. If I made the reverse dome that big, would I still be getting effective quench? I’ve ran out of ideas and so if this one doesn’t fall through, I am just going to port and polish some stock 400 heads and use flat tops on 9.5:1 compression. Sure it’s not the ultimate setup, but if my brother can make 425hp/500# with it and not use headers, I think I can make 350 with headers. Plus my motor will last forever. Once again, thanks for all the help, Blu93GT, and tell me what you think about this idea. I realize it would be expensive but assume money is no object.

P.S. Is the combustion chamber shape of an Aussie 2V the same as a 351C 4V?