Heads question

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Captainkirk

Old School Mopar Warrior
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Here's the question at hand....
I've got 2 sets of heads; 1 set are the J heads with 1.88 intakes. These have been ported, polished, and cc'd to run about an 11.5:1 CR with the TRW 13:1 pistons I've got. The other set are high mileage stock X heads with 2.02 intakes. The X heads would require quite a bit more work, and my builder tells me the J heads will give me better off the line torque with the smaller valves and won't affect top end that much. True or false? Would I be better off investing the money in the X heads, having 2.02s installed in the J heads, or leaving well enough alone?
 
I'd run the J's myself. I've heard 2.02/1.88 swaps that didn't show a change in E/T. Sounds like the J's are pretty well setup with 1.88s, I'd run 'em!
 
i'd run the J heads too (I do in fact ) but it isnt that much$ $ to drop a set of 2.02 valves into them either. Infact you don't even need to replace the seats - a good machinist can back cut the 1.88 seats to open them up for the 2.02 valves. My j heads happened to be the factory 2.02 version of heads and when I dropped a seat once the replacement part were the same for either head ..

and if your running ported /polished with 11.5 compression you probable need as much air as you can get or it would be like running a 2bbl on a funny car (large exaggeration but you get my point )
 
Thanks. Kinda what I thought, too, but some folks on the sight act as if the X heads are the Holy Grail. I wasn't sure if there were any casting differences besides the valves.
Plus, if I put the X heads on, I'd be back to 13:1 compression, which is over the top for pump gas and composite head gaskets (I'd need o-rings for sure). I was just curious about the castings themselves, and whether my engine guy was just assuming the valve size would make no difference.
 
Captain, the 'X's are more expensive and harder to find but they don't flow anymore than the 'J' casting with 2.020 valves. I agree with everyone on this one...use the 'J' heads. Terry.
 
From everything I've read the J heads do flow as well as the X heads. The only restriction is the smaller valves and your high compression. That will restrict high end power quite a bit. I'd still agree with everyone also on using the J heads but if your looking for maximum HP install large valves in them.

The one major thing I can't agree with is the difference in compression ratio when switching heads because X and J heads both have basically the same size combustion chambers when stock, give or take a couple of CC's. Unless your J heads have been ground out to a higher CC or something and it would take a BUNCH of grinding to drop 1.5 points of compression.

BTW: If you decide to not use those X heads shoot me a PM. A buddy of mine is looking for a set.
 
As I mentioned in the original post, they have been cc'd to give me a more "streetable" 11.5:1 CR. The pistons are TRW 13:1 when used with stock heads. Funny thing was, when I bought the car the motor had these pistons installed with STOCK J heads; I often wonder how the car managed to run on pump gas (the guy I got it from had no idea these pistons were installed and probably ran leaded regular!) and managed not to blow head gaskets!
 
Kirk,you have not mention at all the motor size or your intend rpm of shifting or use for the car.Both sets of heads can be great if used in the right applacation forum.A 360 street motor will benifit more with the "J"s on it,where as a strickly race motor for the weekend will be great to use a bigger valve head if the rpm,s are going to be well over 6500rpm,Mrmopartech
 
My Bad!
The motor is a 340, std. bore, forged crank w/stock rods. Crank and rods are being balanced. It will almost certainly use a hydraulic cam; haven't decided whether or not to reuse the same cam (.450/.475-298/308 duration) but will most likely go for a new one. I'm interested in some of the new hydraulic roller lifter grinds. Ignition will most likely be stock electronic or MSD-my engine guy likes and recommends the MSD so I'll probably try it this time around. Induction, not sure. I'd love to try a six pack or dual quad setup, but may settle for a Victor or RPM Air Gap with my 650 DP. Intended use is street/strip, max RPM probably limited to the cam (6500, unless I go with solids). Right now I have the stock valve train; if useage dictates I could upgrade but I'm not trying to get too flamboyant as money IS an issue. This will be bolted to an A833 4 speed with Competition Plus, 8-3/4 rear end bolted to a '72 Duster. By the way, if you have any cam or induction recommendations for this combo, I'd love to hear them. I'm shooting for low 12 e.t.'s....without nitrous.
 
After reading thru, I'd say if you have the $$, build the shortblock, and then build the X heads to run on it. What is the part number of those pistons? I'm guessing they are rated at 13:1 with certain heads. With the high static ratio, and small cam, you could run either set. The valve size will be more than made up for with 3 points of compression. You probably will have just as much trouble with detonation with the polished Js, than the Xs, because they arent designed for pump fuel at that level. The cam will just make things worse. That's why I say get the shortblock together first, at least to set the deck heights, and then chose the heads that will do the better job. I would truely re-think using pistons that will yeild anything over 10.5:1 with any iron head. And at that level, cam choice will be critical to stay out of detonation. It's not static ratio, so much as dynamic compression that really gets you in trouble. Dynamic takes into account the closign event of the intake valve in relation to crank position, and static ratio. If your dynamic ratio is higher than 9.5:1, you will detonate on any pump fuel I can get.
As an example, I plug in the specs for 74cc head (stock is around 71-2cc or larger regardless of factory specs, then polishing), .030 over bore, an 18cc dome (guess on my part for piston), a typical FelPro gasket, and a Crane with 308° duration and it spit out 11.8:1 static, 10.94:1 dynamic. That's 110 octane C-12 for fuel.
 
moper said:
After reading thru, I'd say if you have the $$, build the shortblock, and then build the X heads to run on it. What is the part number of those pistons? I'm guessing they are rated at 13:1 with certain heads. With the high static ratio, and small cam, you could run either set. The valve size will be more than made up for with 3 points of compression. You probably will have just as much trouble with detonation with the polished Js, than the Xs, because they arent designed for pump fuel at that level. The cam will just make things worse. That's why I say get the shortblock together first, at least to set the deck heights, and then chose the heads that will do the better job. I would truely re-think using pistons that will yeild anything over 10.5:1 with any iron head. And at that level, cam choice will be critical to stay out of detonation. It's not static ratio, so much as dynamic compression that really gets you in trouble. Dynamic takes into account the closign event of the intake valve in relation to crank position, and static ratio. If your dynamic ratio is higher than 9.5:1, you will detonate on any pump fuel I can get.
As an example, I plug in the specs for 74cc head (stock is around 71-2cc or larger regardless of factory specs, then polishing), .030 over bore, an 18cc dome (guess on my part for piston), a typical FelPro gasket, and a Crane with 308° duration and it spit out 11.8:1 static, 10.94:1 dynamic. That's 110 octane C-12 for fuel.

i agree about the static VS dynamic compresion the cam makes alot of diference, i get away with over 11:1 static compresion on a J headed 340 on pump gas but thats with a pretty big cam that kills of alot of cylinderpreasure!
 
And you're in Sweden...The fuels I get around here (from March to October) are 92-93 octane, and all octanes and blends contain 10% ethanol. From Nov to Feb is the really bad stuff..lol.
 
hi, the 1.88 valve head will provide more than enough air flow to run low 12 secs. i have seen, putting 2.02 valves in j heads, loose port velocity. in the mid lift area, the 1.88 will out flow the 2.02, this is where the valve spends 90% of it's time, opening or closing. .100 to .400 lift. a 340 or a 360 stocker can run low 11 secs with this head and valve size.
 
Thank You perfacar,
For stating what I've been saying all along, and with the right port work being done the 1.88 valve can perform as well or better than a 2.02 valve below 7000 rpm's.
I'm glad that there's someone else out there that has found what I/we have found and that it does work despite the contrary. Kudo's perfacar!
 
hi, should let you know, that most stockers are in the 7000 to 7500 range at end of 1/4 track.
 
To add to what Perfacar and BJ said. Adding the flow numbers @ the different lift values and averaging them will give you a better look at total flow from seat to full open and back to seat. Big numbers up top in the lift cycle doesn't usually make the best power. The head with the most average flow will just about always make better numbers on the street and the strip, ..usually. Unless your building an 'rpm' motor like a pro-stock, that thrives on rpm and will not see anything less than 5000. The most flow with the smallest port at the lower lifts...equals BIG torque and power, equals great street manners. And lots of tire smoke. I said all that to say...I agree Perfacar and BJ. Terry.
 
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