gt40p combustion chamber porting to 70+cc?

ryan7662 · Aug 30, 2009

Hello again everybody, anyone know a lot about porting. I want to increase my gt40p heads combustion chamber from 58 to 74-80cc if possible. I notice that the combustion chamber isn't a full circle and leans more toward the exhaust side of the head. Can I widen out the chamber towards the intake side of the head and increase the combustion chamber volume? If so is there anyway to tell how much material I should remove. I need to lower the compression for my 81mm ball bearing turbo. I'll have a 4340 3.75"stroke crank, 6.2" rods, and kb step dished pistons from summit. Could I lower the compression by using a shorter rod? I figure I can go as low as 7:5:1 with my turbo. Any help would be greatly appreciated

faststang90 · Aug 30, 2009

i would talk to some eng builder. i dont think u can remove that much to help.
i was doing a bbf and was looking at going to a shorter rod because i found piston that was 6.800 rod but i had rod 6.700. i had a good eng builder tell me no on that. he said that it has to be so far from the cylinder head and that would be to much. i would get another set of heads with the right cc and i would not go down that far in compression

ryan7662 · Aug 30, 2009

Yeah I was shooting for around 8:0:1 actually. Could I instead maybe take some material off the step dished forged keith black pistons. Already got the heads and just drilled them out for 1/2' studs and don't really want to switch them anytime soon. I gotta spend my money on forged internals instead and keep the restrictive gt40ps for now. I haven't bought the pistons yet but I can't seem to find low comp pistons for a 383 stroker anywhere. Thanks for the input man

ryan7662 · Aug 30, 2009

Oh yeah and 6.2 is the shortest rod I can use for a 3.75" stroke so yah your right on that man.

90FoxBdy · Aug 30, 2009

I would search around for different pistons. Check JE and probe they have some pretty big dishes. I will be just under 8:5 with an off the shelf JE and 60cc head. I think you would be asking for trouble, if its even possible to mess with the p's.

faststang90 · Aug 30, 2009

yeah u can do a custom piston and get - dish piston. that way u dont move the piston to far down

ryan7662 · Aug 30, 2009

Couldn't I take off enough metal off these pistons to add 15-20cc? Anyone have a idea cause I think I need to lower my compression for my turbo. Plus I'm not sure 9.5.1 is good for cast iron gt-40ps either.

United Engine and Machine Co.

ryan7662 · Aug 30, 2009

Thanks guys, I think this set of kb pistons would be perfect if I could increase their dish by 15cc or so. Anyone have a idea how much metal I should remove? Maybe I could just shave off that protruding step where thvalve reliefs are and blend it into the rest of the dish?

United Engine and Machine Co.

90FoxBdy · Aug 30, 2009

If it was me I wouldnt, unless you can find a machine shop to do it and take off equal amounts to each piston to keep them the same weight. I think your best bet would be ditch the heads. I know it sucks but there are deals to be had. Dont ruin a $400 set of pistons for a $100 set of heads.

ryan7662 · Aug 30, 2009

AAARRGH.......I know your right man. I've been trying to deny the truth but i can't when its corn-holing me like it is. I'll just push my 84lx 383turbo project back and finish next july-aug instead of march. Oh well....... leaves me plenty of time for my sn95 dash/full interior conversion and diy body kit. Not trying to sound rude or anything I actually wanna thank you for helping me to not make a mistake with my build. I can go a extra 4months without driving her to get the right heads.

Maryland Stang · Aug 31, 2009

ryan7662 said:
Plus I'm not sure 9.5.1 is good for cast iron gt-40ps either

There's nothing wrong with P heads. Lots of people use them at compression ratios of over 9.5:1 including me.

57fairlane · Aug 31, 2009

you know they do make thicker headgaskets right?!?

If it were my motor, I would open up the chambers around the valves slightly and run a .080" cometic or whatever thickness you need to make it work.

gt40p combustion chamber porting to 70+cc?

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