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Topic Review (Newest First)
11-22-2012 05:37 AM
cobalt327
Quote:
Originally Posted by cobalt327 View Post
...piston deck to the block deck is where the quench (quench/squish, or "squench") measurement is taken.
The above should read "...piston deck at TDC to the head deck is where the quench (quench/squish, or "squench") measurement is taken."
11-21-2012 10:47 PM
cobalt327
Quote:
Originally Posted by mouse77 View Post
Your gonna confuse the poor kid with this talk of the quench myth.
You cant calculate "quench" effectively when you are using a full dish piston, the "quench" is essentially the entire combustion chamber with those pistons.
Perhaps the raised ring on the piston, (Probably almost .030 in the hole with an unmachined deck) Might be closer to .070 "quench" (.030 in the hole, stock style .038 gasket), but still will be very low compression.
The 330/350 horse GM crates with a very similar piston, and vortec heads are 9.0 to 1.
I believe the "poor kid" can- if he wishes to- grasp these concepts just fine.

In order to give any meaningful CR estimate there has to be solid numbers to work from. The "raised ring on the piston" as you call it (actually this is the piston deck) to the block deck is where the quench (quench/squish, or "squench") measurement is taken. Without having a point of reference there can be no actual CR measurement; it would be a range, like between 9:1 and 9.2:1.

Now, I do not pretend to think that in this case a variance from 0.040" quench of + 0.010-0.015" will matter all that much. But beyond that it sure DOES matter as to how the engine will tolerate lower octane fuels and/or other less-than-ideal operating parameters when it comes to detonation- even w/a round dish piston. Actually it probably matters more w/a round dish piston because it has such a poor design for giving good quench action compared to an inverted dome type piston.

Take a look here for more on the concept, as well as a disambiguation of the word "quench", quench and squish being two entirely different things).

That the round dish piston has a less than ideal configuration for producing good quench action is well known- that's why a D-shaped or inverted dish piston is preferred when a dished piston is required.
11-21-2012 09:41 PM
smith9300 Thanks mouse but I just bought a double roller timming set I have an accel distributer but the heads I have now are casting #996 I think I dont know if the seats are cracked but iv thought about getting vortec heads but then ihave to but a vortec intake and right now I have a Weiand
11-21-2012 09:18 PM
smith9300 Thanks mouse but I just bought a double roller timming set I have an accel distributer but the heads I have now are casting #996 I think I dont know if the seats are cracked but iv thought about getting vortec heads but then ihave to but a vortec intake and right now I have a Weiand
11-21-2012 08:29 PM
mouse77
Quote:
Originally Posted by cobalt327 View Post
If the stock pistons are going to be reused, AND if the dish is 24cc like the 400 often used stock, the 64cc chamber size will put the compression ratio (CR) at about 9.6:1 using a 0.040" quench.
Your gonna confuse the poor kid with this talk of the quench myth.
You cant calculate "quench" effectively when you are using a full dish piston, the "quench" is essentially the entire combustion chamber with those pistons.
Perhaps the raised ring on the piston, (Probably almost .030 in the hole with an unmachined deck) Might be closer to .070 "quench" (.030 in the hole, stock style .038 gasket), but still will be very low compression.
The 330/350 horse GM crates with a very similar piston, and vortec heads are 9.0 to 1.

I agree that the vortec is probably the way to go in your boat. BUT, i would advise you have a machine shop drill the steam holes (If the vortecs even have a water passage there) I know the lower coolant area in the vortec has a very large opening, and i would be concered about drilling a hole that close to it.

If it runs/ran well, and the valve job doesnt show any cracks around the exhaust seat, you are pretty much good to go. The 400 were bad because of the siamese bore (airlock), and people not bleeding them right. The heads were not weak, per se. They were the same casting minus the steam holes as most other HD chev engines of the time. Lots and lots of people never cracked a head on them.

Roller rockers are a benefit, yes. At your stage, there are probably better things to spend your money on. Roller timing chain for durabilty, rebuilt/new/better distributor, machined valve locks, machine for positive seals on the valves, decking the block etc

Go to whoever in your area has the best reputation, and talk to them, and tell them your budget. They will work with you to get you what you need.
Oh, and you are for sure going to have to put valves and guides in them. They were sloppy in 1976 when they were brand new.

Good luck!
11-21-2012 11:26 AM
smith9300 Thanks for the help guys but does anybody Know a rough estimate on getting heads magna fluxed and the motor on has like 45k miles on it but I wanted to redo the heads BC of the new cam and I needed push rods and lifters but another ? Would it be beneficial to run roller rockers over stock rockers
11-21-2012 04:47 AM
cobalt327
Quote:
Originally Posted by smith9300 View Post
If I put the 350 heads on it would it have to run premium gas and its a bone stock 400 when I got it it didn't have a cam the bottom end is good but I just want it to have some good torque and I dont want the heads to be the thing that holds it back but its going in a 85 Chevy c20 lifted bout 6 inches it deffently shows its age but I got it as a gift and its my project
If the stock pistons are going to be reused, AND if the dish is 24cc like the 400 often used stock, the 64cc chamber size will put the compression ratio (CR) at about 9.6:1 using a 0.040" quench.

But if you want to use 64cc heads, at least go w/Vortecs. The old small chamber SBC heads are ~45-plus y/o and will usually need guides, seats, springs possibly valves, milling, etc. Not worth all that. Or just go w/aftermarket if the budget allows.
11-21-2012 03:16 AM
LJM97Z
Quote:
Originally Posted by smith9300 View Post
Hello im 19 and im pretty new with engine work and i have a stock 400 sbc i have a comp cam xe256 that i bought but i wanted to redo all of the top end and dont really know where to start. I dont have a lot of money but just want to run some stock chevy heads on it. just lookin for ideas on what heads would be good to run on it. Any Feedback would be apperciated
IMO, if the stock heads are in decent shape use them, If you don't have alot of money to work with, don't get caught up in all the "YOU NEED" this & that, A simple stock 400 will have enough torque to keep you happy, I'm talking stock right down to the rotating assemble, That little xe256 you have will work just fine in a stock 400, If you want, just add bolt-ons like a decent set of headers (1.5/8) & Perf. intake, I got caught up in the "your compressions too low", The stock heads are junk, your leaving too much HP on the table Yada Yada Yada......, I'm "somewhat happy" with my 406, its got Iron eagle heads, decent cam, 10.4 compression, 750dp, RPM intake, headers, but i had a stock "re-ringed" 400 in a 78 2wd blazer back about 12 yrs. ago, all it had was headers & RV cam, That Blazer was a blast to drive, loads of torque, & yes, it had the crappy 882 heads & 8.1 compression, that was it, nothing fancy.
11-20-2012 09:45 PM
idontdrivericeieatit You would only need premium gas if you compression was high but with stock 400 you probly have dish pistons and with 64cc head you would not need premium gas you would he able run 87 the head itself either a 350 or a 400 head that you use wont determine what octane gas you need the cc of the head and. Type of piston and the cam you use will
11-20-2012 09:31 PM
techinspector1 Here are the instructions for drilling the steam holes in the heads....
Technical Articles at Greg's Engine & Machine
11-20-2012 06:23 PM
smith9300 If I put the 350 heads on it would it have to run premium gas and its a bone stock 400 when I got it it didn't have a cam the bottom end is good but I just want it to have some good torque and I dont want the heads to be the thing that holds it back but its going in a 85 Chevy c20 lifted bout 6 inches it deffently shows its age but I got it as a gift and its my project
11-20-2012 04:21 AM
6426yy Vortecs if it has sock type dished pistons
11-18-2012 08:57 PM
idontdrivericeieatit I agree 400 heads stock are bad about being cracked and most not good and most are 76cc chamber head with open chambers not good for power at all I would check around for a good 350 head. 64cc closed chamber head and. Just drill steam holes you can follow the head gasket as to where the holes need be i m sure a quick search on Google and gasket in hand most can handle the job I use do mine with hand drill but best use drill press if u have one
11-18-2012 08:52 PM
vinniekq2 any quality head will work.you dont need a big flowing head for a near stock engine.Just get a machinist to show you or drill the 2 holes for you.
Tell us a little about your project and goals?
11-18-2012 06:59 PM
632Mantis Stock 400 heads are notorious for cracking.

You can run these, but you will need to drill some steam port holes in them. ( Or let your local machine shop do it. )

Summit Racing® Cast Iron Cylinder Heads for Small Chevy SUM-152123 - SummitRacing.com
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