Getting The Right Compression Ratio...

-
K

krondo.wg

Well-Known Member
Joined
Oct 17, 2008
Messages
53
Reaction score
0
Location
Dallas
I'm about to get my engine back from the machine shop, with the rotating assy. already together, and torqued properly. I plan on getting some edelbrock performer heads, but I'm not sure what I need to get 10:1 cr or better.

I have a '73 340 bored .30 over, decked just enough to clean it up, and I ordered the kb243 (6cc flat top) hyper. pistons. Again, I plan on using the eddy heads, so am I getting close to 10? Or is there a couple of things I still need to do?

Also, what eddy head should I get? The standard, or the 340? I only see a difference in 2cc.
 
Hi you can go to the KB web site and use there CR calculators and play around with head CC and gasket thickness. just plug in the things you know for sure
deck height, stroke, bore, etc. if you go to Summitracing.com you can get gasket spec's on a whole bunch of different options.
Richard
 
The standard head. The "340" Edelbrock head is designed to work with the OE pistons of the earlier 340's, which were a positive piston. Er. positive deck height. They pop up above the deck.

You have no use for them.
 
Try this calculator on the Keith Black website

http://www.kb-silvolite.com/calc.php?action=comp

I have the KB 2308, with the Edel Performer Heads, and a 0.039 Head Gasket, and my compression is too low, static at 9.6, and dynamic at 8.7. Should have gone with the 2310's more compression! As it is, I can run 87 octane with no knocking or pinging. According to all that I've read, with AL heads you can run an extra point of compression, so mine should be closer to 11.5:1, to maximize the engines power. Dave Hughes confirms this, but his shop (Tim) is the one who sold me the rotating assembly in the first place, and knew my cam & heads. Go figure...
 
You had the decks cleaned up... You should have had it square decked... That way the deck height is a known quantity. You need to see where they ended up in your block. Every one is different. I always square deck to get the piston at 0 deck. For that piston, running a closed chamber head, the correct deck would have been 9.617. Running the .040ish Felpro gasket, you'd get a decent quench. If you use the recommended deck of 9.599, you have .018" of piston sticking out of the bore. So if you use a closed chamber head, you have a piston to head clearance(quench distance) of .022"... WAY too tight. If you use teh open chamber RPMS (pn 60179) you have the .060 deep open chamber, and the 65cc chamber you end up with .080" quench, too far, plus the compression ratio goes down. If it's all together, measure where the pistons stop at TDC. If they do pop up, get the 60179 heads and have them milled so the chamber is more shallow and you end up with .040" quench distance... That might be a bit of milling, so you'll need the intake sides milled too.
 
I think they took .10 off the deck. It sucks cause I was just there today checking out the weld job they did on the mounting ear, and I forgot to ask.

As for the gaskets, I havent picked out a brand yet. Most likely I'll go with fel-pro, since they are more commonplace. Once I get the gaskets picked out, I'll enter in their specs.


Here's where I'm confused. Those kb243 I ordered said:

NOTES (HYPER) Designated as O.E. replacement for engines manufactured in years 68-7l. All other years not legal for sale or use on pollution controlled vehicles. High compression flat top. Factory compression ratio 10:2:1. PISTON COMES OUT OF THE DECK .018".


So...doesn't that mean I need the 340 performer rpm heads instead of the stock 779's?

Sorry for the newbie questions, but that's what I am.
 
The 60779 are a closed chamber design, 63ccs. They are meant for use with pistons that do not come out of the deck. Like the 340s from '73, or any 318/360. The 60179s are machined to have a large recess to mimick open chamber heads for the early 340s that had higher compression. but they cut is .060 deep, so you lose any possibility of quench. In other words, you give up a huge potential reason for using them in the first place. If you block was tall, like most factory blocks are, your pistons may NOT come out that .018". The only way to check is to measure the height of the piston when it's at the top of it's bore. Square decking allows the machinist to accurately machie the block to the hieght you specify. Standard decking or milling does not. The KBs are a flat top, but its called a shallow dome. Not a true total flat top design. It's modeled after the TRWs and Speed Pros that have been around for years. Great with open chambers, but not usable with closed 60779s. Keep asking questions. Newbiw isnt a lifetime monicker...lol
 
Correct me if I'm wrong, but I'm thinking I need the 340 RPM head, with 65cc #60179, because I have those kb243 (OEM) replacements? I'm trying to achieve the earlier 340 compression ration, instead of my dinky 8.5:1

I had the machine shop put the rotating assy together cause I didn't want to mess it up. I'm just afraid I'm going to screw things up with the selection of heads, and gaskets. The reason I say that is because of the measurements, and clearances.

I would have the machine shop put the valvetrain together, but I want a little part in the engine building. Airfilters and valvecovers ain't enuf :p
 
I got it! Heh, after 30 minutes on hold I finally talked with an Eddy rep. He said I need to go with the 60179's, since those pistons are comin out of the deck.

Looks like I'm cookin with gas now!
 
I'm installing the the closed chamber 60779's on my 340 right now. The pistons are TRW 2316's. I bought the closed chamber because I plan to build a 408. Well i decided to try them on my 340 with a cam change also. After talking BJR racing and Don at FBO I purchased a pair of Cometic head gaskets that thick enough to give me proper quench and clearace.
 
You must log in or register to reply here.
-

Back
Top