4 bolt mains on a 360/408

[gfriedline]

At what level of power do 4 bolt mains NEED to be installed on a 360 block stroked to 408. I have read about 2.0 hp/cu.in.
Does that mean 2.0 for the block which is 360, or for the 408 cubes? Reason being I may want to run a 200 or so shot of gas on occasion.

Also, the cast cranks are pretty beefy, but when do forged cranks need to be considered? My plan is to make around 500hp, and 500lb-ft into a 408 stroker. I plan on 2 bolt mains, cast crank, eagle H-beams, and forged pistons. I have been told that this is a mild buildup, and shouldnt take a lot to complete.

I also want to run nitrous for better times on the strip. Would a 200-250 shot be too much, or would a lot of considerations need to be made in the bottom end?

[451Mopar]

I haven't looked at the total costs, but I think if you plan on running 600+ HP you will be ahead to invest in the Mopar race block than retrofitting 4-bolt mains onto a 360. The stock later model 360 engines have thinner webbing above the main caps.
The very early 360's ('71?) had a full thickness webbing.
The race block is even beefier with filled pan rails, and a thicker deck (better gasket sealing and less cylinder wall distortion) and can be bought with the 4-bolt mains.

Same with the crank and connecting rods. The stock peices are probbably OK at 500 HP, but at 600+ you should go with the forged crank and H-beam rods.

[6 packin]

Running 500 ponies and 200-250 shot. You may just consider the heavy Main caps like the ones from Pro-gram. I would forget the 4 bolts all togather. That kinda power I would run a good steel crank before I would spend money on a 4 bolt setup. I think with a early block and main studs in with the steel crank you should be alright. But the Heavy main caps and arp studs will help more than adding 4 bolt main caps on a block that was made for 2 bolts. I belive the heavy maincaps and studs would be much stronger than 4 bolt caps added to a 2 bolt old block.

[sanborn]

You may not be interested in all this explanation-but what the heck!

The suitability of a cylinder block for performance/racing is determined by it's rigidity. You want a block that is as rigid as possible. The more rigid a block is, the more HP/torque the block can absorb before failure.

Deck thickness, cylinder wall thickness and the main cap design all affect block flex(or lack of it) as well as block material, etc.

The basic 360 block is OK up to a point and 500HP is about it! The earlier blocks may be a little better. You can add four bolt caps but you can't change the deck or cylinder wall thickness-although "Hard Block" helps. In my humble opinion(and it is humble) you would be better off to buy a "race" block for anything over 500HP. The two bolt "race" block will absorb about 600-650HP quite well and of course the four bolt version will take quite a bit more. Why can the "race" block take so much more than a stock block? Because the extra cast iron makes the block much beefier and the block doesn't depend on the mains for rigidity.

The purpose of four bolt mains is to add rigidity to the block by tieing the side pan rails together. That's the reason for the cross bolt design of Hemis, SB aluminum race blocks, and the Nascar block with (gasp) six bolt main caps.

The billet two bolt main caps don't help block rigidity but do help to reduce "cap walk"-that's a whole different issue.

[6 packin]

Drilling 1/2 inch holes in the main webing and addind studs or bolts is not my idea of making somthing rigid. I agree with all that, but the hemii blocks and race blocks were cast with the extra meat and extra bolts around the main area. If that were true every one racing a big block would be drilling there main area and adding cross bolts to there main are. I am just looking at the point of drilling or modifying stock blocks.

[gfriedline]

These all seem like excellent points to me, but why then did Steve Dulcich and those Car Craft guys make that 408 on 4 bolt miloden caps, and a cast crank on a 360 block? They were dynoing at 600 hp, isnt this a case where the R block should have been used?

Or does this add up to the fact that they get parts for free, and use em'?

[6 packin]

I stand corrected!

If they done it in Mopar magazine then I stand corrected. I have seen failures in brand X engines were the 2 bolt mains with studs was stronger than the factory 4 bolts setup in the same block. Because of the much more material that was moved for the extra bolts was. The use of the extra bolts was offset by the material removed. Having that said with all the machine work and extra hardware provided it would be cheaper to go with a aftermarket block raedy to go!

[sanborn]

All very good comments and questions. I personally agree with the comment about adding 4 bolt caps to an already weak block.

As to why people do it, I don't know.

I've been at this crazy business a lot of years-tried to skimp on cost too many times-and got caught by it too many times.

To me a good race block is just like building a house- you can't make the foundation too strong!

Everything in the race business has limits of strength. A stock block has a limit of about 500HP(my opinion) and a two bolt race block about 650HP(my opinion) and a four bolt race block has a limit of about 800HP(my opinion). BTW, we have busted them all! A lot of it has to do with the type of racing done, smooth tracks(drag and pavement oval) are much easier on blocks. Off road and rough oval(dirt which is what we do) is much harder due to the constant load/unload(also called hammering) on the engine.

[6 packin]

Amen brother!

[451Mopar]

Now I'm going to have to re-look at my ols mopar magazines
What issue was that build up?

I know the Indy built small block in the engine masters challange uses a race block with the extra inner/outter head bolts.

If the other Mopar buildup is what you say I would hope they started with one of the early thick webbed blocks or the 340 "x" block (I think that was the old performance block?)

Magazines do alot of stuff to push advitisers products, and they have built many engines that failed on the dyno, remember the stock 318 with a 300 HP shot of Nitrous?