what needs to be changed in a 72 340 to be like a 70 340?

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340plot

340plot

the bushy bush
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i have 2 72 340 engines, and i am wanting to know what has to be changed in them to make the same horsepower as a like 68-70 340?

one i have has j heads on it, and i have a friend that knows how to port, so he will be doing the port and polish on my heads for me.

i am wanting to get about 400 -475 hp out of it. any insight?
 
Replace the pistons with some good KB hyper pistons spec'ed out for the earlier engine. What mopar did was move the wrist pin up on the '72 340 and that made the pistons sit lower in the bore resulting in lower compression and less power. The factory 340 pistons are EXTEMELY HEAVY! I can not emphasize the enough. Get the good light weight hypers from KB. That will make a huge difference in how it runs. Light pistons rev faster and take a load of stress off the crank and mains.

Some Scat I-beams are very nice, way stronger then stock, and a lot lighter too.

You will need to have the crank balanced for the lighter parts.

If you can swing a 4" stroker kit now would be the time.

The '72 also had the smaller intake valve. Have the intake seat in the head opened up to 2.02" inch like the early heads and do a bowl blend.
 
Just a note: the distance between the center of the wrist pin and top of the piston is called Compression Height. The '72 pistons had a smaller compression height compared to the earlier pistons. And boy were those 340 pistons heavy!!
 
thank you. so is there any other differences? besides the cast crank?
 
70's had bigger intake valves (2.02 vs. 1.92). They also had higher compression. they are rated at 10.5 to 1 (actually they are lower than rated, usually only about 10 to 1) where-as the 72 is 8.5 to 1 (also probably a little lower than rated). The diff. is in the piston height cause the heads have the same chamber size. Cam is possibly different also but your gonna need a much stouter cam than even the stock 68 4 spd. cam to even make 400 hp. Install 2.02 valves in your J heads and do some port clean up and a perf. valve job. Install higher compression pistons (shoot for 10.25 to 1) and balance the assy. Install a bigger cam, Edelbrock Air-gap intake, 700-750 Holley, headers and your in business with at least 400 hp.

A buddy of mine had basically that same build with a comp solid flat tappet cam (20-231-4) and even though he never dyno'ed it I'm sure it had to be in the 450 hp range as it ran low 12's with just an average tune up. I think if he would have ever dyno tuned it it would have been solidly in the 11's.
 
Looks like me and Bill think alike, LOL. I was typing and he beat me to the punch.

Are you sure your crank is cast? Most 72's were still forged I thought. wasn't till 73 they went to cast. But I could be wrong on that.
 
the block is a 2/9/72, but the damper says on it that "use with a cast crank only"

i really have never had any luck with headers, and i will not put out that kind of money for tti's. so will i be ok with the stock 72 340 manifolds?
 
340plot said:
the block is a 2/9/72, but the damper says on it that "use with a cast crank only"

i really have never had any luck with headers, and i will not put out that kind of money for tti's. so will i be ok with the stock 72 340 manifolds?
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I doubt it. You'll need a good header with a 1.75 primary tube.

The 72 and up cylinder head had a 1.88 intake valve, not a 1.92.
 
The best 340 specs, were for late '71, high comp,forged steel crank, "J" heads with 2.02" valve and electronic ignition.
The worst part of factory engines, is as stated earlier, pistons were heavy, and balancing man that was a crap shoot. 'Cuz they sure weren't balanced when they were built, (and that makes a world of difference). I'd always prefer seeing balanced/blueprinted.
Clearances, depends on application, loose for drag racing/high rev, a touch tighter if you want to cruise...

What is your ultimate goal ?
 
didnt the 72 have j heads and the 70 have x heads ? I know theyre almost the same but he did say from 72 to 70
 
Few '70's with "X" heads, "X" were for '68/'69 mostly, then some "O"...The "J" heads (915 casting) came out in late '69, for '70 production year, was designed mostly for the T/A application then but eventually (with a small shift "pushrod") ended up being the most produced casting "915" in the LA series. Those heads were used in 360's and 318's till...
at least '79
 
340plot said:
the block is a 2/9/72, but the damper says on it that "use with a cast crank only"

Bummer. It's still a good crank. Just not as good as forged.
Click to expand...

i really have never had any luck with headers, and i will not put out that kind of money for tti's. so will i be ok with the stock 72 340 manifolds?
Click to expand...

Not for the power level you want. Try exhaling through a straw while running. Same scenario.

Guitar Jones said:
The 72 and up cylinder head had a 1.88 intake valve, not a 1.92.
Click to expand...
Geez. What was I thinking???
 
The cast crank can be made stronger, and hold up for a while........But i wouldn't do it. If your going to stick with a stock exh manifold, you will be, most likely, safe with a Cast Crank.

Stroker Crank(4") or wait for some one, to us a stroker crank and buy there steel crank from them.

There just isn't a Half Way Decent stock cast iron exhaust manifold.

I Payed big money for a "Hooker Super Comps" on my E body, some 10+ years ago, steal leak free...........Well except for and occasional collector gasket.
I will NEVER buy another $99-130 header, EVER again.:read2:
 
The differences? Carb & intake have minor differences. The crank is cast and exernaly balanced so that makes the balancers/damper/flywheel/flexplates for external type.
Pistons as said earlier. Heads have the 1.88 intake, not the 2.02.

To make it perform like an earlier 340, higher compresion pistons like the above said KB would be a good start. You'll need a new cam. Though all 340's have the same cam minus the one mentioned, the 68, 340 4 spd set up. You might as well start fresh on everything.

tti headers are the ones you want to make the HP you seek. Otherwise, the exhaust manifolds will hold you back 70+ HP.
 
The Dougs have the fit and quality of a TTI without the price:

http://www.summitracing.com/parts/DOU-D453-R/

This article has some good info on how to build a mild 340:

http://www.compcams.com/Community/Articles/Details.asp?ID=-738676943

If you have the money I would go with a 4" stroke crank. Don't buy a balanced kit, have a good local shop balance it. If budget is tight just go with the good KB pistons and have it balanced.

And the X heads aren't any better then the J heads other then the bigger valve which can be put in the Js (some Js had the big valve but that is just a machining difference the cast is the same).
 
340butterduster said:
and balancing man that was a crap shoot. 'Cuz they sure weren't balanced when they were built, (and that makes a world of difference).
Click to expand...

I never knew how bad the balance was in my original standard bore 340 until I dropped in the stroker engine. So much smoother going down the road. It is incredible how smooth a correctly balanced engine is. And the stroker kits that come balanced, yeah, that is like a factory balance, total crap. You have to have a good shop balance it correctly if you want it balanced.

Balancing also takes a lot of stress off that bottom end too. And less rattles in your car so that is nice.

Did I mention those standard 340 pistons are HEAVY.
 
rumblefish360 said:
tti headers are the ones you want to make the HP you seek. Otherwise, the exhaust manifolds will hold you back 70+ HP.
Click to expand...


I thought it be more like 20hp. I know those 1970 hp cast manifolds can allow for some decent flow. I was running them on my 318 with 360 heads and 3.90 gears. Switched from cheap headers, notice a slight drop in power.
 
dodge freak said:
I thought it be more like 20hp. I know those 1970 hp cast manifolds can allow for some decent flow. I was running them on my 318 with 360 heads and 3.90 gears. Switched from cheap headers, notice a slight drop in power.
Click to expand...

Go back and click on the 340 link provided by "340sFastback"
Here let me repost it.

http://www.compcams.com/Community/Ar...?ID=-738676943


In there they did a dyno HP comparison with just the header replace.

Most performance parts are a compromise.... It will make more Power but loose torque, or have lots of toque/power down low but won't rev
THE HEADER IS THE EXCEPTION TO THE RULE!!!!
More bottom end power, more mid range cruise power and more top end power to boot(every thing in you power range is enhanced)

Go read that article and don't pass up the best performance you can add to a stock, or full race eng.
 
Headers will always be better but the HP loss can change depending on build.

For example…… My build. High compression, high RPM. I went form 11.78 @ 118mph to a 13.2 @ 105mph most (but not all) of that is related to exhaust changes.

My engine is built for a shift point up around 8,000rpm which was fine with Hooker super comps and 3" exhaust.

Basically I lost 1.42 seconds and 13mph when I went to 340 manifolds and 2.5" exhaust.
 
1968FormulaS340 said:
Headers will always be better but the HP loss can change depending on build.

For example…… My build. High compression, high RPM. I went form 11.78 @ 118mph to a 13.2 @ 105mph most (but not all) of that is related to exhaust changes.

My engine is built for a shift point up around 8,000rpm which was fine with Hooker super comps and 3" exhaust.

Basically I lost 1.42 seconds and 13mph when I went to 340 manifolds and 2.5" exhaust.
Click to expand...

Wow! Now that's a huge difference.
 
Sounds too much--what else was done besides the header swap--I am running the same exhaust pipes and mufflers as before. My shift was at 6,500 (aiming for 7,000 this time and only that "low" cause its got kb hypers-not forged.

Sometimes the carb is jetted too rich and headers help lean it out.

Sometimes its not til the headers are on before somebody fines tunes everything-jetting, timing-ignition up grade, I run msd 7al and hvc coil and high end wires with heat-shield to fully stop the wires from shorting to ground..my old msd 6 was ok but the 7 system was a noticeable improvement. Helps to burn more of the fuel.
 
One of the reason's I think my change was so huge is because all my power is in the area where the manifolds simply don't flow. They are not doing the job from 5,500-8,000.


The other changes were…..

Electric fan and electric water pump drive to stock belt driven fan and standard water pump.

Slicks to stock tires.

SS springs to XHD springs.
 
Hmm, well I never timed it after the switch and had not so long before the rod went out. Just a few months.

They were cheap 1 5/8 Headman headers. Hooker Super Comps likely would have flowed better.

I have hear of the BB boys running even in to the 10's still using some type of factory cast manifold. But they make stroker kits for those motors too.
 
dodge freak said:
I have hear of the BB boys running even in to the 10's still using some type of factory cast manifold. But they make stroker kits for those motors too.
Click to expand...

This is true but those guys have the manifolds extrude honed which costs as much as TTIs and they have mega mega dollars in those engines. If cost big money to look stock and go as fast as they do. Only the deep pocket guys can play that game.

The high hp engines with the cams designed to work with headers will really suffer when manifolds are used. Here are some dyno tests of some small block strokers with stock manifolds:

http://www.hughesengines.com/TechArticles/6408416strokerdynotests.php
 
"In Test 2 we switched to a pair of TTI, stepped headers, 1 5/8"/ 1 ". To say the difference is amazing is a gross understatement!

The power was up 42 hp and torque up 85 ft-lb at only 2600 RPM. And the improvements just kept coming with a maximum improvement for the horsepower at 4900 RPM with 66HP more and the torque was up 72 ft-lb at 4800 RPM"


I knew that stock manifolds would limit your hp/torque, but that's amazing!! I realize that those numbers are from a stroker motor, but I imagine the results would be similar for a high reving 340.
 
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