J Heads

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Daves67Cuda

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My 340 has the "J" heads with it. Do you guys have any suggestions on what to do to them. I think they're the 1.88's, I also hear these flow better than the 2.02's on the J's. Does anyone know about this? Thanks.
 
My questions to you are these.1,what do you intend to do with the car.2,The max RPM you intend to shift at.3,What type of bottom end do you intend to build,listing the parts and specs,C/R,piston type and,stock stroke or stroker.Do you intend to drag the car alot.Will this motor be a daily driver,friday night special,cruising?Mrmopartech
 
Mrmopartech said:
My questions to you are these.1,what do you intend to do with the car.2,The max RPM you intend to shift at.3,What type of bottom end do you intend to build,listing the parts and specs,C/R,piston type and,stock stroke or stroker.Do you intend to drag the car alot.Will this motor be a daily driver,friday night special,cruising?Mrmopartech
Click to expand...

what HE said!!!!! :dontknow:
 
Well. Right now, its a daily driver, but I'm probably gonna get something a lil better on gas milage, but I'm gonna race it pretty much every weekend I can, also drive it to the track and back. For the street probably shift about 3-4 grand, but racing, just go all out, whatever it'll take. I'm not too sure about the bottom end part, what do you mean by that? Right now its a 71 340, j heads with normal stroke. It'll need to be bored, probably .030 over, then I'm probably getting KB243 pistons about 10:1 CR. Comp cams XE268H, MP race flywheel, street/strip pressure plate and clutch. Thats about it on parts, and I'll put different gears in it, its got 3.23s now I think. Maybe go with 3.91s. I'm also looking to get into mid 13s if not faster, faster is better, lol. Hope this info helps and thanks.
 
I'd buld the lower end as you say, add the larger intake valves, and have some bowl cleanup and gasket matching done to them while they are being worked. Replace all the valves with decent stainless, they come swirl polished and back cut already. Make sure your true compression is 10:1..You'll more than likely need to either deck the block, or mill the heads to lower the volume to get that. Witht he stick, the larger intakes will help, and you can keep the rpms where it wil like them, street or strip. If you are running headers and decent pipes, you could upgrade (meaning larger) the cam a little too.
 
The 1.88's are very good for the street. A street machine type ride. Nothing to out of the box. Small cam dual plane type combo.
A 2.02 can be used in the street as well, but the price of the part and work on valve size alone doesn't give a great bang for the buck until you start porting and making use of large (r) cams.
Having done the swap myself on my J heads, it's not a bad upgrade. A minor loss of torque for a minor gain in HP.
 
Dave, the 268 cam is mild for a 10 to 1 340, with good heads. I agree with Mopar on this, Also not ALL stainless valves come back cut. Check them. 340 + .030(344.5 CID), 10 to 1, 1 3/4 hookers, bowl ported, gasket matched 915's(J) heads with stainless, 2.020/1.600, Air Gap, 750 (3310), with spacer, camshaft range, 274-280*, 230-235* @ .050, .520 lift, (to give a true lift of .500 at valve installed (hyd.), you would need .540 lift to achieve true lift of .500 at valve with mech. (solid) cam, true 1.5 rockers. Also, Use file-to-fit rings, .005 oversize (4.075), and gap them yourself. Always good idea to do this even on street rebuilds. Don't build a new engine with 100,000 miles on it. Most pre-gaped sets will measure gaps in the .030 range, I have seen sets as large as .040 before,(extreme cases). All will depend on what finished size your machinist leaves you. If you gap them yourself, you control true ring gap. This combo with 3.91 gears, will be good street package that will behave well on the street and strip, and can get in the 12's with good tuning. Take your time, select good matched components, pay attention to detail, and check EVERYTHING, (clearances), don't assume anything. The machine shop will tell you "it's clean" ... ok, but clean it yourself, clean everything several times, and when you think it's clean...clean it again, no such thing as too clean of an engine. Sorry to writ e a book here...but basics are often over looked. Good luck Dave. E-mail if you have questions. Terry.
 
one thing about ring gaps- on the KBs you need excessive end gap, even the little instruction sheet with the pistons will tell you this, they have a little chart, but what they do not list is that you have to gap both rings, the paper stated just the top- this has been prooved to be very bad on both the dyno and on the street. If you just gap the top one in about five hundred miles or so you'll start pumping. I second the cleanliness statement, At work when I'm showin new guys how to build engines I tell them that an engine has to be clean enough to eat off of, and then cleaned again!
 
I have never heard anyone state that the second ring on a KB piston should be gapped anything but stock gaps. The KB instruction sheet that came with my KB107 pistons had a very good explanation as to why the the first ring needed additional gap and why the second and oil rings did not.

Basically because of the position of the ring on the piston and the piston design the top ring runs much hotter than in other pistons which requires the gap to be larger to prevent the ends from butting and breaking the ring or piston. At temperature the gap is the same as what it would be in another piston with stock gaps. The second rings is not exposed to the added heat so it does not require the addtional gap.

I'm running the XE268H cam in my magnum headed 10.6:1 360 and the engine is a screamer in the 1500 to 5500 rpm range with a dual plane manifold. If you want good performance on the street in the 3k-4k range you mention this would be a good cam choice.
 
Alot of the racers today are having good results with gaping the second compression ring larger than the top ring. I know that is counter to everything you have heard on ring end gaps,..so far. The reasons behind this theory is compression gases build up between the top two rings and cause the top ring to 'flutter', (loose seal). I like to use .004 X bore, for top ring, and .0045 X bore for bottom ring. These are for the top two compression rings. For those who do cut thier own ring end gaps, Keep the gap parallel, and ALWAYS deburr the sharp edge that is left after cutting. You made some good points Mr.Speed. Good topic. Terry.
 
yeah, the larger rign on the 2nd is not cutting edge, but wasnt done that much..It's coming back, although I tend to beleive it's a "do it because so-and-so does it" thing more than having reliable dyno testing to back it up. I'm sure in some applications it's got advantages tho.
 
No nothing new. just some racers are having positive results with larger second ring end gaps on certain combos. I use .004 X bore for top ring and .0045 X bore for bottom ring. I round off to next number. This works for me. Terry.
 
actually I talked to the tech guy at Kb and he recomended it, And I have seen dyno results that prove it- not an instant gain but over time if the engine with the bigger 2nd gap is redynoed it will be closer to the same power than the one with the smaller 2nd gap. its more of a endurance issue- I started doing it on stock car motors and now they tell all thier friends to have me build their engines.
 
What would you guys suggest for a cam then for it, doenst have to be Comp, just suggestions cuz I'm still trying to figure out what all I need to order and stuff, then get the head work done and block cleaned up. Thanks for all the advice and I'll make a note of that ring thing.
 
I'll suggest the above mentioned Comp Cam. The XE268H. It is about as far as I would go with on a 340/4spd car with 3.23's.
 
Dave go back to top and read my post (#7), JMHO on camshaft choice with a good street combo that will be turn key, and lend itself well to street duty. And pull down 12's. Some basic rules to build by: Run the best heads you can afford. Build your engine around the heads. Camshaft should be the last component you choose in any combo. Cut your own ring end gaps. Check ALL clearances yourself. Clean everything several times. Prime oil pump, and verify oil pressure, and oil to top end before starting. After camshaft break-in, change oil and filter at this time. Good idea to use straight water when starting for first time. After you verify no coolant leaks, drain water and put in fresh anti-freeze. Good luck, Terry.
 
Rumblefish, I'm gonna be putting either 3.91s or 4.10s in it hopefully, unless I should go with different ones, if anyone has suggestions on that. I'm taking the block to the machine shop tomorrow, and I still have to go pick up the other J head cuz accidently got a 318 one with it, but the guy found the other one. So maybe I'll ask the machinist the cost to do them up real good and whatnot. Well, at least this is the first step and hopefully I could figure everything out, and I appreciate all the help/info I'm getting.
 
Dave. Must have missed the 3.91 or 4.10 part. Sorry. I'd follow headsbikesmopars build. It's a good combo.
 
if you go 4.10s, and if you truely have 10:1 static compression ratio (at least) then I'd go with the EX247H, it's the next step up from the 268. If you go lower then 10:1, or want to run 3.91 or numerically lower, then go 268. You wont be disappointed. The heads should cot aroun $6-800 done properly, with good parts. Make sure the springs match whichever cam you decide on.
 
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