Hey guys,
Well i am going ahead and building my own 390 now. I have a set of c8ae-h heads, i am putting the c7ae-b rods in it with a stock 390 2u crank. I am thinking about putting the 1131h pistons that have 1.76" comp. height, flat top with 8cc total of valve reliefs. Will this combination give me a good quench? how do i figure out the quench distance? Is it the distance from the top of piston to the deck of the head? i will have ~69cc in heads and 8cc from pistons and then another ~14cc from gasket, could someone help me with CR?

Thanks guys!! Sorry about the long post

 

The deck clearance is fron the top of the piston to the flat deck of the head. From my working in the 335 forum the general rule there was you wanted to be under .050 deck clearance for quench. Above that and you will have potential for a lot of pinging. http://kb-silvolite.com/calc.php?PHP...8a4c4f13feafcd Thiis is a real good site to figure your cr both static and dynamic.

 

Quench is basically the deck clearance plus the head gasket thickness. Deck clearance (how far in or out of the hole) is calculated as deck height minus crank throw, rod length, and piston compression height. Crank throw is the stroke divided by 2. For a 390 using the 1131 piston, the calculation for deck clearance would be 10.17-1.89-6.488-1.76=.032 (all in inches). Using a standard felpro gasket would give you a quench of .073 (.032 + .041). Using a steel shim would give you .052 (.032 + .02).

There are different opinions about how much quench is too much. You can run an FE with a huge quench if the compression is low enough. My stock 1976 390 with 7.8:1 compression has .153" of quench. That is triple what most folks recommend. However, for a performance build, most folks seem to think anything under .06 will resist detonation pretty good unless you are running super high static compression like race engines (over 11:1). I am not sure what compression you are targeting, but you might be able to get away with the .073. On the other hand, if you can use a steel shim gasket without putting your static compression through the roof, you may want to consider it. BTW, for calculating compression, keep in mind the felpro gasket has a 4.40 bore and .041 thickness, the steel shim gasket has a 4.22 bore and a .02 thickness. The United Engine and Machine site (KB-Silvolite) has a lot of valuable calulators to play around on and they have some very good technical articles regarding quench, dynamic compression, and such. "Magnificent Quench" is a good read.

Bob

 

Thanks for all the great info!!! I looked everything up and figure i will have 9.6:1 CR using the felpro gasket and 10.1:1 CR using the steel shim gasket. Is 9.6 ok with .073in of quench? Is 10.1 too high, will i get ping even if the quench is only .052in? I will be running 94 octane in it all the time so no worries there. Do you think i will be ok with the 10.1:1???

 

First, I must admit I have no real world experience. All of what I posted was based on what I have read as I have started planning my own engine build up. That said, my guess is that you can run either setup without detonation problems as long as you pick a cam that relieves some of the cylinder pressure with a later intake closing event. There are several folks here and on the Network 54 board that are running 10+ static compression ratios with iron heads. Many folks say it can't be done, but these guys are doing it. I think Baddad457 said he is running 10.25:1 with iron heads. I don't know what quench he is running. Maybe he will chime in.

 

What would you call a later cam closing? Would you guess that maybe a cam with 270-290 duration on intake would be ok? I am also looking at a cam with .514 lift and 300 duration, but i am thinking that might be a little heavy. Also do you know what the stock rocker shafts handle? I am looking at putting a set of adjustable roller tip rockers on but was going to put them on my stock shafts, is this a bad idea?

 

Dynamic compression and cylinder pressure, which influence detonation, are not directly related to cam duration. The best thing to look at is the timing when the intake valve closes. The intake closing event is important because it controls the start of the compression cycle and how much cylinder pressure you will get at TDC. If you go to the KB-Silvolite website you can calculate dynamic compression ratios by inserting the intake closing event and the rod length (6.488" on 390 and 428). I am not sure what the guidelines are for picking the right dynamic compression ratio, maybe someone else can help there. I think below 8.5 is generally consider OK.

I think .514 lift with 300 total duration would run with stock rockers and shafts, but you might be happier with more lift and less duration. 300 total means either really slow ramps or really high duration @ .05. For a street machine with a stock convertor, most folks recommend keeping the duration @ .05 below 230 degrees. In regard to lift, I have heard you can run up to .550 with non-adjustable rockers and stock shafts. Over that and you are looking at adjustable rockers and then even higher lift requires beefing up shafts and end stands.

I priced out a complete roller setup in another post in this board and it will cost you at least $800 more (about $1500 total) to run a roller cam than just running a standard hydraulic with adjustable rockers and end stands. If you have the money it will get you more hp, but rollers are much harder on the valve train and cost quite a bit more. Look for a thread entitled "switching to a hydraulic roller or solid lifter cam".

Bob