Mopar 350 small block

Anonymous · Post by **Anonymous** » Tue May 10, 05 10:15 am

Most 70s/80s 318s are less than 9:1 - IIRC they raised it back to 9:1 in 1986 when they went to roller cams. Stock CR is something like 8.5 in a 70s/80s 318, and with manufacturing tollerances being what they were some are lower than that (I presume due to excess material on the block face). The late 60s 318 is reckoned to be 8.9:1 in my Fury, although can be much less due to castings apparently.

IIRC all passenger car 318s have flat top pistons as stock. Rather than dishing them they sit down the bore at TDC. Compression is varied by chamber size.

The 318 was never intended to be a performance engine.

This could work out quite well...

Disclaimer - all this info is from accumulated tech articles on 318s not from ripping them appart myself... but the multiple sources all agree so I guess its true.

I like this idea. Starting to seem like it has wings. Keep us posted.

Blue · Post by **Blue** » Tue May 10, 05 3:33 pm

I seem to remember that the real steel stroker used 327 chevy pistons to get 350 cubes from a 318, or was it 327 rods? there was GM parts in the mix somewhere.....

Dave-R · Post by **Dave-R** » Tue May 10, 05 3:49 pm

I had a feeling you used either Chevy rods or custom pistons for this sort of thing which is why I wondered how it was done.

paolo_mangusta · Post by **paolo_mangusta** » Tue May 10, 05 4:14 pm

The article I've got doesn't mention Ch*vy bits, though another one in the same book talks about ch*vy rods for big block strokers. They use 318 block, 360 crank, 360 rods and pistons for a early ( i think 67-70) or late (post 77 - again I'll have to check this!) that are made by Speed -pro and have valve cut-outs as standard - thoug these disappear with machining.

I've emailed Real Steel so will let you know what they say (too many bosses around for private phone calls!)

I won't be able to check how far down the bore the piston is till the w/e, but I think special pistons will be necessary, and this is the sticking point - you can't machine a piston too much until the crown gets too thin

I'll let you know how I get on

Blue · Post by **Blue** » Tue May 10, 05 4:35 pm

The more I think about it I remember that the whole point of the exercise was to build a stroker useing only off the shelf components. This sounds like a cheap way to do it but if it involves a lot of machine work and changing to internal balance you might be surprised at how expensive it can become. I wonder what happened to that engine, it ended up in the for sale section in the back of the mag, and I remember thinking he'll be lucky to sell it at that price!

Blue · Post by **Blue** » Tue May 10, 05 4:45 pm

By the way,the old way to build a 451 stroker was to use an offset ground 440 crank in a 400 block with big block chevy rods and custom pistons. you can now buy all the parts aftermarket for that combo and it's a far cheaper/easier job than it used to be. I'm sure the aftermarket will be coming out with similar parts to stroke the 318 before long, be a good seller I reckon!

Dave-R · Post by **Dave-R** » Tue May 10, 05 5:03 pm

You missed my earlier post then Blue?

http://www.hughesengines.com/crank_kits/390crankkit.asp

DaveBishop · Post by **DaveBishop** » Tue May 10, 05 5:22 pm

John they did use small block chevy rods and mopar pistons the pistons had 100 th ground off. If I find the mag i will try to scan it and try to attach it

Blue · Post by **Blue** » Tue May 10, 05 5:43 pm

Aha! I missed that one Dave!

Rich · Post by **Rich** » Tue May 10, 05 9:15 pm

I`m sure their plan was to get 360 horses from a 360.Seem to remember they used 327 Chavy pistons as they are 4"dia.I too have the mag but sure as hell could not put my digit on it..

paolo_mangusta · Post by **paolo_mangusta** » Wed May 11, 05 10:13 am

Ok, still waiting to hear from REal steel, but the hot rod build is definitely all Mopar.

Maybe they use chevy rods because if the big ends are smaller then you could offset grind to adjust the piston height and stroke.

Or maybe it's just because chevy bits are cheaper

One thing I'd forgotten

was that the stroke increase need to be halved to see how much higher the normal piston would sit in the block! This page

http://victorylibrary.com/mopar/a-rod.htm

would seem to confirm that you would need to take 0.135 (half the stroke increase) off a 318 piston to end up with it sitting in the same place, so I'll have to measure how far down the piston sits to and comapre it to the measurement for zero deck height in the calculations.

I think my heads are the 72cc smog heads, so will want the CR upping anyway, so taking .100 off sounds about right if it doesn't hit the cylinder head, if you factor a nice thick gasket in there as well, hopefully taking Steve's figure of 10.5 down a little bit. Though that wouldn't be too bad on 95 octane would it? Maybe with some octane-boost unleaded additive?

Dave-R · Post by **Dave-R** » Wed May 11, 05 10:44 am

The compression ratio has only a bit to do with cylinder pressure. Cam timing/duration effects actual pressure.

With a bigger duration you need bigger comp ratios. With a stock or near stock cam you will want 9 to 9.5:1 at most. Bigger cams and you can go up to 10:1 and higher but you need Optimax fuel.

This is with Iron heads of course.

Compression ratio does not have a big effect on power in real life. Keeping it mild enough to stop it pinking with decent ignition advance is more effective at making power.

paolo_mangusta · Post by **paolo_mangusta** » Wed May 11, 05 12:33 pm

Well, the cam I'll be putting in it is quite pokey - I *think* it's .465 lift and 282 duration (if that sounds like nonsense please ignore!)

I've skimmed .150 off a four cylinder head on an old Citroen without problems

and with a lightened flywheel it really improved the engine response - also with a manual it slows the engine down quicker and speeds up the changes. Even if it doesn't give much more power, it does seem to improve the feel of the motor IMHO

I appreciate the compromise between advance and CR, but a lot of these engines were designed to run on 87 octane, weren't they? Should be room for some tweaking - Are these engines prone to pinking?

I agree that more than 10.1 would be asking for trouble though

, and normal unleaded is expensive enough, let alone Optimax, I want something that will run on 95 unleaded with actane booster additive since it's not much more expensive to buy the octane plus additive as just the additive, if that makes sense...

Dave-R · Post by **Dave-R** » Wed May 11, 05 12:48 pm

With a cam profile like that you want 9.5:1 or a bit over. It will run on normal unleaded. A higher compression ratio will not gain you any real power. It is not worth the hassle unless lowering the compression ratio is more hassle. But keep below 10:1 with that cam and it will be easier to run.

paolo_mangusta · Post by **paolo_mangusta** » Wed May 11, 05 1:46 pm

Cheers Dave - what we'll probably be looking at is machining the top of the piston to get the CR down, and obviously the more you take off the crown, the weaker the piston will be, so we'll need to see if we can get the CR down to a reasonable amount and find a piston taht will still have reasonable amount of material left on top.

Might try and get over to the lock-up one night this week to measure the piston crown thickness and how far down the bore it sits - teh suspense is killing me