[Brant]

hello, being my first time building an engine - it's important I have second opinions, as they help me make proper decisions and learn.

I have a 340 fit with KB243 Hypereutectic pistons. I've machined my J-heads down to 67cc. My deck height is 0.02 above the deck. With the head volume and a 0.04 head gasket I will reach my 10.2.1 compression (rounded up).

Does this mean that my quench area is 0.04 minus 0.02? Isn't the optimum quench 0.035 – 0.04? Will I be OK with this set up, what will the effect of having a lower quench area have? I need to be schooled

Also, I understand quench is directly realated to how the engine performs in relation to pinging etc, so am I going to be OK on pump gas, as this is a street driven car. Thanks soooo much guys, I'm a bit worried.

[dave-r]

Measure the quench clearence directly.

You already know the piston height above the deck right? 20thou if I understand your post correctly?

Now measure the distance from the head surface to the quench area in the chamber (these are open chamber heads right?) and add the gasket compressed thickness to this figure.

Now take away your 20thou deck height from that figure. What is left should be the gap between the top of the piston and the quench area in the head.

The quench distance should be as close to 40thou as you can get it. Anything less than 35though and you run a real risk of the pistons hiting the heads under some conditions. So you will need to either take some off the piston tops or take some out of the chamber. I am not sure which would be the best route to go on that.

With over 10:1 compression you are going to need at least 95RON fuel I would guess. It all depends on how wild your cam is because that effects cylinder pressure.

[Christer]

The quench distance should be as close to 40thou as you can get it. Anything less than 35though and you run a real risk of the pistons hiting the heads under some conditions. So you will need to either take some off the piston tops or take some out of the chamber. I am not sure which would be the best route to go on that.

How about adding a thicker head gasket? Is that a possibility? Are there any thick head gaskets on the market?
(I really don´t know much about this....but if you don´t ask, you don´t get any answers...)

[spitfire9137]

How about adding a thicker head gasket? Is that a possibility? Are there any thick head gaskets on the market?
(I really don´t know much about this....but if you don´t ask, you don´t get any answers...)

I think head spacers are made that will add a bit of thickness. I know I have seen them for chevys dont know about mopars though.

[dave-r]

Most composite head gaskets have a compressed thickness of 35 thou.

Steel shim gaskets have a compressed thickness of 20 thou.

Those are about your only choices.

If he was planning on using a 20 thou gasket and going to a 35 thou version would give him the quench distance he needs it would work like that but it would also lower the compression more than just giving yourself a bit more clearence in the area where it is needed.

Also going to much more than 45 thou in the quench takes away any advantage to having it at all.

[Brant]

Thanks for the response. Yes, I do have options, I just don't want to decrease my compression ratio. I'll do my measuring - Dave, thanks.

To acheive the 10.1:1 compression ratio, we were going to use a fel-pro compressed 40 thous head gasket. I'm so confused right now. I've talked to Kieth Black and they seemed to clear things up - but now I'm confused again.

In relation to the image below of my pistons, the highest flat area sits .04 above deck, and the lower ridge around the circumfrence sits about .02 above deck. The piston has a 6cc piston compression height. Keith Black said this is how they sit, and I use the .02 above deck to determine compression ratio keeping in mind the 6cc PCH. So, If I use a 40 thous compressed gasket - subtact the .02 negative deck height - it leaves me with .02 do I need to add the height in my cylinder head to get the quench.


I'm really sorry I don't understand - but I want to.

thanks

[Brant]

OK Dave, I read your post agiain slowly, and I understand now. Silly me, it was written very clear. I'll be good now, thanks again.

[dave-r]

My small block experience is too small to be able to give you better advice. Having only ever owned a stock 318 in the LA series.

If you look at the 440 piston below (where i have more experience) you can see that there is a definate "D" shaped quench area that exactly fills the open chamber part of the 440 head.
This is how I imagined your piston would look but obviously there is more to it than that.

[Brant]

Just thought I'd post a couple pics of where all my money has gone over the last 6 months. This is the engine a friend and I built – I was the superviser, as this was my first engine experience.

340
4.08 bore, line bored, re-surfaced
Keith black Hypereutectic pistons
Stock rods, resized, bronze bushed, ARP bolts
Flywheel resurfaced, rotating essembly balanced
Stock crank, turned 10 thous, clevite bearings
Ported/polished J heads, gasket matched, new guides, seats, decked for 10.2 compression, fel-pro head gaskets
Edelbrock air gap intake, gasket matched, machined
Hydraulic Direct Connection .475 lift, 238/238 @ 0.50
750 DP classic Holley
Center force clutch
running with 3:23's, will change to 3:55 next year.
(this car will be driven lots, will also see highway) hence the cam and gears.

[Follicly Challenged]

The old J heads were an open chamber head, 68-72 cc ? with a nominal quench area depth of around .100" if memory serves me correctly, I don't have a book. Which means, as long as no part of you're piston goes higher than the .038" gasket thickness, you've still got approx .100" quench depending upon how much the heads have been cut.

You're pistons look surprisingly like the old KB 190 piston for a 360, are these the 340 version of that design ? Sorry, I haven't used KB in awhile.