340 to 372 c.i.

-
ir3333

ir3333

Well-Known Member
Joined
Jun 1, 2008
Messages
5,836
Reaction score
1,578
Location
ontario,canada
help me with this math...
3.58 stroke crank (360) turned down to 340 journals
What compression height (pistons) is required for zero deck?
..who have built a 372....pros/cons?
 
Way back in the day a local engine builder built one of these motors and used chevy pistons to get the right deck height.
 
o.k..... 3.58 - 3.31 x 1/2 or = .110 increase in stroke
340 @ 1.820 = zero deck
1.820 - .110 = 1.710 comp height = zero deck w/3.58 stroke?
 
They ran the motor in a dragster, but I do not know what et it ran. The motor is still sitting in the engine builders shop. I thought about buying it, but it is bored out 60 over so I am not sure if it would make a good street motor. I worry that it would not cool properly.
 
With a 3.58 stroke, you would basically need common flattop 360 pistons, which tend to be around 1.66"-1.67" compression height. Just gotta find the right bore starting at 4.04". But if you don't want to deck the block a bit, it may involve custom pistons and more expense. I believe KB 107's go to 4.08" bore.

In the '70's, Tom Hoover (father of the Hemi) built a budget 360 + .060" which ended up at 371 or 372. He used low compression 340 pistons but had to mill the tops down and clearance the bottoms under the wrist pin bosses for the counterweights. That was before there was a variety of 360 pistons to choose from.
 
Been looking at this myself with the Scat crank. Most have said by the time you grind and balance the factory 360 crank to internal you could buy the Scat crank and save some $$.

Anyway, like Loco said, any 360 piston (for your desired compression, heads, stock length rods, etc) with the matched bore to your 340. So if your 340 is stock bore of 4.04", you need a .040 over 360 piston.
 
using a .358 stroke crank with 340 journals this could be built with off the shelf parts.
 
I have one with the Scat 340 main 3.58 stroke crank. I use stock length rods and the KB 107 pistons as Locomotion said.
I have not installed it in a car yet, but assembly was uneventful.
 
I'd like to know how this turns out if you go through with it. I've thought about this particular combo myself. I believe they can be built with off the shelf parts unless you have an oddball situation.

I like the 340 in my car, goes to 7,000+ rpm but unless I have steep gears, it leaves soft cause there is not much torque at low rpm. I would think a little extra stroke might help with that.
 
this is always controversial but the square bore / stroke 318/390,408 and 416 make a lot
of torque but do not like to rev quickly.
the 372 - 377 could produce good torque,be quick revving and more efficient.
...oh oh, now i've started it!
 
if a 3.58 crank is turned to 340 journals and the correct KB 107's are used how is the balance affected?
 
Once you start mixing parts, you have to balance the rotating assembly. KB pistons are lighter than OEM 360 pistons. Guesstimating without actually weighing what you will use is a waste of time. Even the OEM balance job with OEM parts can be improved upon.
Not sure if you can get light enough using forged pistons, like SRP or Probe, and lighter rods, to get rid of the regular 360 counterweights on the balancer and converter. But it would help towards internally balancing and minimizing the use of expensive Mallory Metal.
 
I've been running my 372 (340) for 10 years. It is without a doubt the best Mopar motor I have ever owned. So here's the build if your interested. Wisco piston, Eagle H Beam rods, ported Edlebrock heads, Edlebrock Victor intake, Harland Sharp rockers, Smith Brothers pushrods, Crower roller cam, MSD disty locked at 34 degrees timing, MSD Retard box, 750 Holley DP worked over, Mopar Perfromance 360 cast crank, block filler half filled, Milidon pan. The motor makes 540hp and 500tq. It gets shifted at 7000 all day long and I have only had one hiccup with it, a broken pushrod adjusting screw. The motor is in a 3100 # car and runs 11.50's with 4:30 gears and a 904. Its a little to much gear as the trap speed is 111. This is my street car and it raps so fast in first you better be on your toes or you'll be on top of the limiter. Pulls hard all the way to red line and is a blast to drive. There's a lot left in the ET if I were to put in the effort but being a street car it gets the job done.
 
ir3333 said:
...awesome.
how did you manage the balancing?
Click to expand...

you take your parts to the machine shop and have him balance them based on the weight of the pistons and rods you use....

lighter the pistons and rods...easier it is to balance..

and lighter the bob weight...faster it will rev..
 
so this particular stroker can likely be balanced without issues?
no mallory weight req'd?
 
...that is the plan, but 360 pistons are very light and turning the 360 crankshaft will also change the balance
 
I'm building a 372. Using a Scat rotating assy, ie forged crank, I beam connecting rods and Icon Flat top pistons. It's also "internal balance", so a stock 340 flywheel and harmonic balancer could be use.

Zoro deck height with my engine block (1970) and that kit

I had the combustion chamber enlarged to 70cc to lower the compression ratio to 10:1
 
here is the specs on my new motor:

MRL - 378 Stroker Motor
340 Block 4.100"
3.58 eagle Crank
2.100 eagle sbc H Beam rods
Diamond Piston 9.7 comp
Edelbrock 63CC Heads CNC ported my MCH
Houghs 1.6 roller rockers
Solid 243/251 duration @ .050" .550/.570 lift
Edelbrock RPM Intake
Kevco pan with baffles
750 quickfuel carb

520HP 6150rpm
491TQ 4400rpm

378ci 1.375 HP per CI
 
I have a .030 over 340 block with the 360 crankshaft in my car. I used a cast 360 crank and had the mains turned down. The balancing cost me a lot as I had the motor internally balanced. The pistons were KB's off the shelf that I had the dome milled flat for a zero deck, but they have been discontinued.
It's a smooth running engine and it's strong considering it's a really mild build, around 335hp.
 
Always wondered why you would 3.58 stroke a desireable 340, when a 40 over 360 is what what you are going to end up with? I have a 360 crank turned down to 340 mains and might use it in a crazy 273 build. Figured the 273 low compression piston would be about +.150 out of the hole....but both motors love to rev..., seems a larger stroker 4.00 crank would be going the opposite direction. 3.58 is still somewhat rev-able. Balancing this 273 would exceed the value of the block though, but using light 273 rods would help.
 
ir3333 said:
so this particular stroker can likely be balanced without issues?
no mallory weight req'd?
Click to expand...

If you are using a stock 360 cast crank turned down....probably going to need mallory metal even with light weight scat i beam rods and kb 107 pistons...

last 360 cast crank I assembled...light weight i beam rods 585 grams....and light weight Ross pistons 484 grams...still would need mallory metal to internally balance....or you can externally balance it with a 360 damper and a B&M weighted flexplate....
 
So what's the HP and torque difference going to be if the parts are all the same 372 inch to your run of the mill 360 .030 over 365" . Maybe 5hp and what 10' pounds of touque ? Doesn't seem worth the effort unless you have all the parts, and you can have it all machined and balanced for much cheaper than the going rate!
 
You must log in or register to reply here.
-

Back
Top