View Full Version : Horsepower limit for stock 318 block?

03-23-2008, 07:25 PM
I have a 390ci 318 using a Hughes stroker kit and Hughes stud girdle,Diamond pistons, H-beam rods etc. My concert is the block, my motor should be making around 500hp and with 150 or 200 shot of nitrous is this block strong enough to hold together? I am very familiar with nitrous system and have tunes 3 stages on my buddys drag radial car, so I know my tune-up is safe. Just wondering if I should go ahead and try to find a Mopar race block. The block is a LA out of a 91 Dodge Dakota.

03-23-2008, 08:38 PM
Some may dissagree, but I doubt it will last for long if it is going to be raced regularly.

I've seen a similar build on a 360 with a big shot, take the main and cam webs right out the bottom.

03-23-2008, 08:55 PM
That's kinda what I thought. The block already has about 50 1/8th miles on it with a 100 shot. I guess I better stick with what has been working and start looking for a race block to build.

03-23-2008, 09:35 PM
here ya go..http://www.moparchat.com/forums/showthread.php?t=110263

03-24-2008, 12:02 AM
Thats a lot of power to be pushing out of that block, I'd play it safe and pick up a racing block.

lil red fred
03-24-2008, 09:13 AM
i used a 318 in a circle track car for about 3 years the block handled it no problem but i broke #1 main cap. the motor hit 8000 rpms every race. but i wasnt at that power level. i was running in the 450 range. i agree with 7duster0 the race block would be better for that kind of power. but the race blocks ive used came with the same material caps as 318 so check that out if you go that route

03-24-2008, 12:26 PM
Block is the same as a 340 so close to the same i would guess,,wayne

03-24-2008, 02:17 PM
500 would be the limit. I'd go with a race block if I was you. : up:

03-24-2008, 06:13 PM
Thanks for all the input. That is pretty much right on with how I was thinking. Now my question is will my top end(heads and intake) Fit an "R3" block. Heads are Edelbrock performer RPM(Hughes stage 2 porting) and Edelbrock victor intake.
Thanks again guys,

03-24-2008, 07:48 PM
I looked into the R-3 when i did my stroker,and there are sure a lot of versions out there! There is a tall and short deck version,along with siamese and full water jacket versions!. also there is a 59 and 48 degree tappet bore model!. You will be looking at the 59 degree unit as this is what a std. LA/Magnum uses!. That way you can use,the same heads,cams,timing sets,heads,etc. Everything will bolt up,including both styles of motor mounts!. This block should be good for 7-800 hp!. But the 48 degree block is more for high sustained rpms,as this has the favourable 48 degree tappet bores and other features!. The only problem i seen on these blocks were there were no oil drainback holes in the valley were the oil drains back to the sump,as it was intended for a different oiling system!. HOWEVER,you can drill 5/8" drainback holes yourself,and offset them so the oil does not drain straight down on the camshaft!. I think Hughes can set you up with a modified block,fully machined. If you are going this route to an R-3,....I would really give it some thought of REALLY increasing the displacement as yo can go right up to 476 cu. in. WOW,just think,.....476 inches weighing in at about 375- 400 lbs!!!!. Just remember,....you can ventalate this one pretty fast also,only this one would make me cry a little louder!!!:crying:


03-24-2008, 08:07 PM
Yeah I planned on going with more cubic inches. I have my short block sold so I was planning on building another. Thanks for the input.


B body Benny
03-25-2008, 09:22 AM
In my opinion, if your running forged everything, and have ARP studs in the top and the bottom end, AND your running a stud girdle (man they work good, i shoulda used one on my 390), I wouldnt be too scared, as long as you havent bored her out to far. My buddy runs 150 gas on a 060 thou over LA 360 block, stroked out to 410, and has three seasons on it, with regular (read about 20 - 30 passes per year) outings to our local strip.
He runs RPM heads and all the same as you, exept no girdle, and he has had no problems as yet. One concern people overlook, is the cylinder pressure in an overbored engine. If the cylinder walls are thin (bored too far) then they will tend to flex under high cylinder pressures and run the risk of splitting a bore. Best insurance here is use a good block, with minimal over bore. (Avoid the magnums, there crap after .020") use a reasonable compression ratio, on or below 10.5:1, and dont overrev it. If the piston speed gets too high, componants dont work properly, then fail, and sh*t happens.
In short, 'ya built it, so go lean on it and see what she got!':pile:
Thats my plan anyway!

03-25-2008, 12:16 PM
I believe 318 main caps are weaker than 340 caps.
If any thing were to happen with the 318 block @High RPM max power levels it would be main cap failure or cylinder wall splitting or valley cracking. [rare]

03-25-2008, 01:54 PM
I believe 318 main caps are weaker than 340 caps.
Why would that be? Would you have a picure of the 2 caps side by side by chance?

If any thing were to happen with the 318 block @High RPM max power levels it would be main cap failure or cylinder wall splitting or valley cracking. [rare]

We had a fella here a while back that turbo'ed his create 360. I forget the amount of pressure applied, but, he had some great shots of the block splitting in half at the center, right down the cam journals.

Dakotamike, I beileve that if you keep the NOS mild, you'll not have a block problem. The hit NOS gives is a hard one and you'll loosen up the clearances in the crank and rods long beofre you split the block. Also, to much NOS can very well melt piston crowns. Which is much more a common problem.

I'm with DW, a 500 hp limt on the gas is where I would yeild at just for longevity and saftey concerns.

03-25-2008, 03:16 PM
Heres the picture you asked for.
340 no.1 main cap on right, 318 no.1 main cap on left.
Notice more bolt length required for the beefier 340 main cap.
Im not sure how many people know this, good info to have though.:study:

03-25-2008, 04:40 PM
Hey, lookie there. Aint that some chit. Never knew that. I'm surprised theres such a diffence in a way. Considering the 318 was out for a bit before the 340 and not needing anyting to beefy for such a basic people mover engine, I guess they thought that was fine until they upped the displacement for a peormance engine.

Nice domes in the backroubf there.