Semi-hypothetical situation...

You're building a 427 and you have a large amount of money spent/wasted/invested in a set of heads and intake you don't want to part with, and the combustion chambers are only 64cc.

With a 4.125" bore, 4.000" stroke, 4.155" gasket compressed to 0.038" and 0.025" deck, a piston with a 20cc dish gives you roughly 10.0:1 CR. A piston with a 32cc dish, on the other hand, drops CR to roughly 9.0:1 which would be more boost friendly. To get down to 8.5:1, you'd need a 39cc dish.

Assuming a rod length of 5.85" and a deck height of 9.025", compression height for the piston would be 1.15". A 5.7" rod would give you 1.3". Rod ratio would be 1.463:1 or 1.425:1, so not much difference. Either way, you'd get enough "meat" in the piston for a big dish.

How big a dish is too much dish for forced induction? For a naturally aspirated engine, you'd want to preserve your quench as much as possible. Does the same hold true with forced induction? If the dish isn't uniform, but is concentrated in one area like a large valve relief, is that more desirable than a uniformly round (and shallower) dish?

I bought the block bare because I wanted to leave my options open, even though one of them was a 20cc dish. I figured 10.0:1 was fine for naturally aspirated operation (I'm losing reverse coolant flow, more than likely), especially with a mid-sized solid roller, but for forced induction, I'd either need to go with a bigger dish or reconsider my top end.

Is there really any benefit in trying for 8.5:1 vs. 9.0:1? The amount of boost you could run (supercharger with an intercooler) wouldn't differ that much, or would it?

Any input is greatly appreciated. Thanks!

 

Properly designed dish pistons have a "D" shaped dish that does not compromise squish - there is still a flat part of the crown that coressponds to the squich area of the head.

http://www.thermaltechcoatings.com/images/JE_piston.JPG

Yes, 8.5:1 is a lot better than 9:1 for a pump gas, high boost setup. But you are going to be limited by the available piston choice anyway.

Rich

 

Thanks Rich. I know off-the-shelf choices would be limited, but I already figured I'd need custom pistons.

 

Just for the record, squish is ALWAYS important.

We didn't always know this. Well, at least I didn't always know that.

If you ever want to see a perfect head (IMO), look at a VW VR6 head. I won't spoil the surprise, but you'll see what I mean.

 

JE has a 4.125" piston in their Extreme Duty inverted dome 400 series with a 1.145" compression height and 28cc dish (PN 232516). 9.025" - 1.145" - 5.85" - 2" = 0.030" deck. I could have the block decked, of course.

They've also got a 4.125" piston with a 36cc dish (PN 232514) which would give me 8.7:1 CR, but with a 1.445" compression height. I'd like to be able to run 5.85" rods, if possible. Even a 5.7" rod would be too long for that piston.

 

Jim,

Don't have the block decked... and do a reverse dome piston modled off the chamber (since you pretty much have to with the odd valve spacing of the 215RR) but do a custom and it will get you exactly what you need..... JE and CP are two good choices for this.

With a 5.85" rod you will be ok.

Bret

 

Thanks Bret. What should I be looking for as far as piston deck height? My 396 block was decked at 9.0" and piston deck height was 0.003", IIRC.

 

Since you want to minimize compression, ask your machinist to take off as little as possible to make the decks true, and be sure he takes the same amount off each bank. Unless I'm mistaken, small block deck heights are usually around 9.025 inches from the factory, but most aftermarket pistons are made for zero deck at less than 9.010 inches.

 

We are talking about a aftermarket machined block so you can pretty much be sure that the decks are flat and parallel with the mains at 9.025"...

4" stroke crank... 5.85" rod... makes a 1.175" Compression height piston.

Make sure the crank can run a rod that short in the motor, the counterweights have to be cut down enough to run that setup.

Bret

 

Yep, that's why I didn't buy the short block already assembled.

 

Not according to the testing/research done by SRP. But then, they were using in excess of 25# boost.

 

Jim,

If it was me I would talk to Carl Hinkson at CNC Blocks Northeast about a block...

http://www.cncblocksnortheast.com/

He can pretty much set you up with a block ready to go and knows the ins and outs of about every aftermarket and production block out there.

Bret

 

After my Motown Lite block shows up or before?

Looks like they just modify existing blocks. I have no concerns about the block I chose, and I don't have the room to run a taller deck, not to mention that it would sort of make my Hogan's intake worthless. I'm happy with a 9.025" deck and a 4" stroke. The only question now is whether I stay naturally aspirated, or do what I've been wanting to do since about a year after I was committed to my NA 396 and supercharge it to get the same (or more) power with better street manners.

At this point, if I spent all this money to end up with a car that's a pain in the *** to drive on the street (where it will live 99.9% of its life) and "only" makes ~550 RWHP, isn't that sort of like buying an aircraft hanger to work on one car in?

 

http://drr.infopop.cc/eve/ubb.x/a/tp...6541077031/p/1

Good thread there on blocks... don't know if I would count World out because of it, but something to look at none the less..

The bad drivabilty part would be a big issue if it was me.... 550rwhp is great but i'd rather have 500rwhp and have fun driving the car around.

Bret

 

Link? I'd like to read that.

Or, if you have the time...?