Edelbrock heads on a 340 with KB243 pistons

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hemichuck

hemichuck

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I've got a 1973 340 + 030 short block that I built with a cast crank, Eagle rods, and Keith Black 243 pistons which are a flat top with valve notches but they extend .018 out of the hole. I picked up a set of closed chamber Edelbrock heads and was wondering what is the minimum piston to head clearance you can run? With an .040 gasket won't that leave me with .022 clearance? Anybody ever run this combo? Do I need to run a thicker gasket?
 
.035 is generally the accepted minimum, but I have heard of people running .020 with steel rods.
 
I wouldnt run the clearance that close. Im not sure if you could get much thicker of a gasket set, Fel Pro are .040. I ran those pistons in my old 340 with open chamber Eddy heads and the thinner .028 Mopar gaskets. Isnt that the quench dome of the pistons you are refering too?
hemichuck said:
Wouldn't optimum quench be .040
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.028 sounds better, but you can get .025 with the 519SD felpro and .018 out of the hole.
Rpm's?
 
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The KB243 is flat but sticks .018 above the deck so if you had a .058 compressed gasket that would give me about .040 between the piston and the head.
 
This motors going in a 1939 Plymouth coupe with an automatic and 3.23 gear. I was going to run a small cam, about the same as a 68 340 cam, 441 lift and 260 something duration. I might be better off just using my iron heads. This car is for my 83 year old Dad to drive so it don't have to be real bad ***.
 
use the felpro 519sd and tune it clean as you can get it, high initial 20-24* and if can't tune a carb, take it to a chassis dyno.
 
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What do they recommend? And I missed a question earlier,yes they are RPM's. I got the heads new at the Mopar Nats last year for $1250, I got the guy to throw in a Victor intake(which I'll never use) I bought a performer RPM intake at another swap meet for $120, brand new. I have several cams laying around, I might go a little bigger on the cam but I don't want it too radical, maybe a 484/284
 
MOPAROFFICIAL said:
.040 is huge and any bigger and there is no quench to benefit.
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I have read anything above .050.
I liked the "Cometic" post! I have them do me up a custom set.
I have run a quench as close as .028. There were signed that there was rod stretch and the slug was getting to close.
Current quench is .038.
 
hemichuck said:
What do they recommend? And I missed a question earlier,yes they are RPM's. I got the heads new at the Mopar Nats last year for $1250, I got the guy to throw in a Victor intake(which I'll never use) I bought a performer RPM intake at another swap meet for $120, brand new. I have several cams laying around, I might go a little bigger on the cam but I don't want it too radical, maybe a 484/284
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I already told you their recommendation. .035". But as stated, lots of people have run less and had no trouble.
 
Edelbrock sells a modified head for the early 340's with the pistons that are .018" proud of the deck, they're machined for clearance with those pistons. They're basically an open chamber head. Not sure what their thinking is, but they actually make a head with a modified chamber. Maybe their reasoning is debatable, but they must have their reasons for doing that instead of just telling folks to run a fat head gasket, which would be a lot easier for them.

I have a set of KB243's in the '68 340 in my Duster, and they really are .018" out on my engine anyway. I ended up using a set of the later 308 heads (open chambers) and having them ported. Chambers ended up at 65cc's, compression is 9.8:1 and runs 91 octane ok. I pulled the timing back a couple degrees to 34* all in mechanical, no vacuum advance.

The problem with the closed chambers is that you have to run a thick head gasket for piston clearance so you end up dropping the compression anyway, and then you're running an aluminum head with less than 10:1 compression. Use the modified chamber eddy's and they're 65cc chambers, same compression. Obviously you can play with the clearance value a little. My expertise is not as an engine builder and I wanted an engine I could daily drive for a long time so I didn't want to get crazy with the clearance and the lunati 60404 I've got has .513"/.533" for lift so I ended up going open chamber iron heads and spending my money on porting. Which worked ok, the 308's flowed 264cfm at .500".

Like I said, I'm not some crazy experienced engine builder, I had my reasons for doing what I did and it's all a compromise. It's a heck of a lot easier to figure with zero deck pistons and closed chambers.
 
It's not like quench is the end all be all. It's not going to add 50 horse power. I wouldn't give it another though if you're skiddish of the closed chambers. Run the open chamber and don't sweat it. The air flow is more important, IMO anyway.
 
RustyRatRod said:
It's not like quench is the end all be all. It's not going to add 50 horse power. I wouldn't give it another though if you're skiddish of the closed chambers. Run the open chamber and don't sweat it. The air flow is more important, IMO anyway.
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That's exactly the conclusion I came to. I decided I didn't want to risk hard parts coming together for a better quench number.
 
72bluNblu said:
That's exactly the conclusion I came to. I decided I didn't want to risk hard parts coming together for a better quench number.
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You did the right thing.
Nice flowing head that 308. 260 cfm @ .500 is nice. That can make some real good power for sure.
 
A local shop here experimented with quench gap and when the gap got down to .025" the pistons would juuust kiss the heads under circle track racing conditions, revving to around 7-8k RPM. Pistons rock and expansion rates vary between engine parts, so I take that as an absolute racing minimum. For most folks, I'd recommend the .035" minimum.

I agree with Crackedback..... .050" quench gap still gives some benefit from all I have read on the subject for 40+ years.

As for the benefit of quench: I like to run CR as high as possible to extend the torque band down as low as I can; that is all due to my street use and rally experience. Quench just gives some margin against detonation, and thus some margin against things going wrong as one drives around with variable gasoline, etc. It also contributes to a small amount of combustion efficiency, and thus power and mileage.

But, I can see others deciding to forego it.
 
Don't forget that unless the block has been properly square decked, not all 8 pistons will be out of the block exactly the same.
 
Yeah, I'll get on Summits site and order some at .053, that way I'll be able to use these heads. I really want a good quench because my Dads going to be driving it and he won't put the greatest gas in it. Just trying to make it as seamless and bulletproof as possible. I got a great deal on the heads so I want to put them on something. I might dig through my pile of cams and see what I have, there about 10 new ones laying out there so hopefully I can find something that makes a little bump without vacuum problems. I might buy a Fitech unit to put on this engine also. I just want it to run out pretty good and not give him any problems. At 83 years old, if he has a problem, it will surely be my problem.
 
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