CI aluminum head - had anyone enlarged combustion chambers?

Greetings Ford Six Fans,

I'm building up a 250 pump gas street engine for my '62 Ranchero. My original thought was to have a 2V conversion done on my '80 large log head, but the relative costs moved me towards picking up a CI aluminum head.

Trouble is, with the .123 deck height on the 250 and the 50 CC combustion chambers on the CI head, it's extremely difficult to get a good quench effect. Any rod, piston, and decking combination that moves the piston up towards zero deck height also raises the CR too high for a pump gas street engine.

If the combustion chamber of the CI head were larger, it would be easier to move the piston up towards zero deck height while maintaining a reasonable CR. Have any of you enlarged the combustion chambers on the CI head? IYO, does that compromise the flow and combustion characteristics of the head? What do you think the reasonable limit of enlarging the combustion chambers might be?

Thank you,
Bob the Builder

http://www.falcon6handbook.com/compcalculator.asp

6cyl
3.68 bore
3.91 stroke
50cc combustion
.050 head gasket thickness
3.81 head gasket bore
.123 deck
7cc dished pistons
100% VE (alum is close enough)
4500rpm


Results:
Displacement, [cubic inches] 250
Displacement, [liters] 4.1
Static compression ratio 8.8:1
Cubic Feet per Minute required @ 4500 rpm, [cfm] 325
Estimated Horsepower @ 4500 rpm: 183hp

with flat pistons you would get 9.4 CR, which is still pump gas, ALSO remember you can run 10.5 CR with an EXTREME cam because gas is dependant on dynamic CR, not static...

IIWIYS... 1) dished pistons, 50cc chambers, .040 overbore pistons... and keep it the same... cam I would stay with the 260/260 .480 112* cam... or 2) drop a v8 for the price of the head alone, then upgrade the suspension...

I would rather play and own the alum head, the bent8 is there for perspective... cause we all know it's cheaper to swap in an 8.

When your taking to CI on the aluminum head you might ask if they could CC it for you first so you know which way to adjust the short blocks build unless you wait until you get it. I think they can be varied from 52 to 48 CC if you can get the 52 CC chamber than you could make it work good with your 264 Cam with a dish piston and even a zeroed deck if you want. If you can only get the 50 CC than just leave the blocks deck a little higher so the math comes out the same on CR either way it will be a very good runner compared to a stock short block setup. X2 Dynamic Compression Ratio is more important than Static CR (it's all about when the intake valve closes). IE 8.4 to 9.1 Dyn. C.R. will work with the 91 to 93 premium pump gas

MPGmustang":1wolcgy6 said:

http://www.falcon6handbook.com/compcalculator.asp

6cyl
3.68 bore
3.91 stroke
50cc combustion
.050 head gasket thickness
3.81 head gasket bore
.123 deck
7cc dished pistons
100% VE (alum is close enough)
4500rpm


Results:
Displacement, [cubic inches] 250
Displacement, [liters] 4.1
Static compression ratio 8.8:1
Cubic Feet per Minute required @ 4500 rpm, [cfm] 325
Estimated Horsepower @ 4500 rpm: 183hp

with flat pistons you would get 9.4 CR, which is still pump gas, ALSO remember you can run 10.5 CR with an EXTREME cam because gas is dependant on dynamic CR, not static...

IIWIYS... 1) dished pistons, 50cc chambers, .040 overbore pistons... and keep it the same... cam I would stay with the 260/260 .480 112* cam... or 2) drop a v8 for the price of the head alone, then upgrade the suspension...

I would rather play and own the alum head, the bent8 is there for perspective... cause we all know it's cheaper to swap in an 8.

Click to expand...

MPGMustang,

Thank you for running this calc for me.

Your example setup will certainly work and on pump gas, too - in fact it was my initial approach.

However, the excessive .123 deck height will make for a very lazy quench effect and will not give the most efficient combustion, right? I was thinking that a piston with much more compression height, along the lines of the 255 piston (although not neccessarily that exact one) would reduce the deck height, improve the quench effect and make for more efficient combustion. But to use the piston with the higher compression height, I'd have to have a piston with a large dish (larger than the stock 6.5 CC dish), and/or increase the size of the combustion chamber to maintain a CR that is sane for pump gas. Thus, I just wanted to see if increasing the size of the combustion chamber was even an option.

Bob

I wouldn't, Gene has 255 pistons, and running 10.4 crazy CR... it's tough to tune, sure you have some meat on your block, but should be alright. in the end you would spend MORE $$ on 'little' more chamber... I know they used to 'custom' the head with 56cc.. but no one wanted that, and they have moved production lines... so I don't know if that is still an option... I think you want to talk to mike, he'll know what is best for your route.

if you talk to mike on your order he will have the machinists add a bit to tthe chamber volume. Might be a small charge but to make it driveable then it'd be worth it. I had him take my chambers down a cc or two so I could stay where I needed.

When we made the first proto types, the chambers were around 50cc. However, when they started running the production heads I asked them to leave as much meat on the head as possible, which got us up to 56cc. That seem to work out really well, so that's where we left it.

Unfortunately when we move the production to LA, and started using a new machinist, I forgot all about it. As such, the new machinist did the same thing and they dropped back down to 50cc. At that point, it was to late to do anything about it, at least for that batch. Fortunately I remembered to say something this time, so the next batch should be back up to 56cc. I just need to call them in a couple weeks, when the castings are done, and make sure they remember to change the programming.