Anyone out there with hands-on experience with the Dart block or World products Mowtown II block? I'm planning on building a 427 or 434 TPI motor and am looking for any recommendations or experience with these products...

 

I built my SBC427 using a tall-deck Dart Iron Eagle block. Dart actually makes the "Rocket" block for GM - they cast and machine it. The Iron Eagle is identical to the Rocket block except that the Iron Eagle is capable of being either a wet or dry sump, depending on how you plug it, whereas the rocket is offered under two different part numbers depending on if you want a wet or dry sump.

I used part number 31122222, which is a tall 9.325" deck block, with splayed steel mains, 400 main size, and Big Block cam bearings. All of the blocks feature a .391" raised cam, and .800" spread pan rails, .400" each side.

Contrary to what World Products/Bill Mitchell Hardcore implies in their advertisements, the use of a raised cam and spread pan rails does not require expensive race only hardware. Summit offers a pretty decent Moroso oil pan for this block in their catalog for $250, and Stef's can make you a custom sheetmetal aluminum pan for it for around $300. These are both good pans too, with kicked out saumps, teflon screens, trap doors, and include the pump and pickup I think. Felpro offers pan gaskets for it for about $25.00. As for the raised cam, Cloyes offers a Tru-Roller timing chain for it for about $95, and a stock timing cover and gasket fits it just fine.

Even the tall deck, if you chose does not add alot to the cost. Dart sells the necessary spacers for $40, or you can use one of their single plane intake manifolds that are designed for the tall deck blocks - and convert it to EFI if you chose. You don't have to go with a custom Hogan's sheetmetal intake manifold like I did if you don't want.

I have also helped two guys build SBC 427's based off of the HArdcore Motown SBC427 shortblocks. They are good blocks and the shortblocks are a rpetty good deal if you're satisfied with the quality of the components he uses. My only reservation is that there have been reports of the main webs cracking around the 1/2" main bolts/studs. There isn't alot of material left on the sides, they didn't widen the mains to accomodate the larger bolts/studs. I heard they were going to go back to 7/16" mains bolts/studs, so maybe they did if there is a Motown II block out now. This has only been a problem on very high output motors (like 800hp+), so it porbably isn't a concern if you're just gonna built a typical 600hp/7000rpm street or street/strip motor.

I don't know all the specific details of the Motown blocks, but the Dart block is what I'd recommend based on having used a couple of Dart blocks myself, and having helped build some Motown blocks - plus what I've seen in the NMCA/NSCA scene.

If you want all the specs on the Dart block, they are detailed on my webpage

http://www.montygwilliams.com . go to Engine, then Block.


[Modified by Monty, 1:09 PM 1/17/2002]

 

Monty,how does the GM boxtie blocks compare to the Dart and Motown?
Do you recommend 1 or 2 piece rear mains?Also,how would a 3.875" stroke compare to the 4.00" in a 427inch motor.
Thanks....Phil

 

Phil,

The Bowtie blocks are very comparible to the Motown blocks in that they maintain the same overall production dimensions but feature better main caps, thicker decks, cylinder walls etc. They are good to around the 700hp levels or so, although people have certainly taken them higher - alot higher in some cases. The GM Rocket and Dart blocks are a step up in that they feature raised cam locations, spread pan rails, and even thicker cylinder walls, decks, bulkheads etc. You also get the option of taller decks, main bearings sizes and cam bearing size and type. All of these things mean you can safely build the engine bigger, with more reliability, and with better ratios/geometry/angles, etc. What I mean is that you don't have to run small base circle cams, you don't have to clearance or grind into the block as much, no cam to rod interference, and you end up with better rod/stroke ratios. With the tall deck block, I am running 6.125" rods with a 4.00" stroke, that yields a 1.53 rod/stroke ratio, which is slightly better than the common 5.7" rodded 383 with a 1.52 rod/stroke ratio. With a 6.00 rod, or even a 5.7" rod, the rod stroke ratio would be worse, and cylinder side thrust loading would be higher, increasing cylinder wall, piston, and ring wear.

The 1 or 2 piece main is just a matter of personal preference I think or a matter of being able to use any existing parts you may already have. The 1 piece rear main is supposed to be less prone to leaking, but the 2 piece rear is alot easier to change if it does leak. Obviously, you crank selection needs to be based in part by the rear main design, but almost all aftermarket cranks are offered either way.

I would be very surprised if there was a significant difference in power or torque between the 3.875" and the 4.00" SBC 427 motors. The 3.875" stroke 427 requires the larger bore, which would benefit from unshouding the valves a slight bit more, and may show a small gain at high rpms, but the longer 4.00" stroke version would probably have a slight low to mir range torque advantage. I don't think there would be a significant benefit of one vs. the other in terms of performance, and I doubt it would show up on the dyno or at the track.

The one advantage the 4.00" stroke version has is that it doesn't come as close to maxing out the bore of the block. A Bowtie block would not have any room for anything more than a slight clean up if the engine needed rebuilding, while the Dart and Rocket Blocks can be bored out further sicne their cylinder walls are thicker. Dart/GM recommends a max of a 4.200" bore on their blocks, but even then, the cylinder walls are at least .300" thick. With a 4.125" bore to start out, you still have plenty of room for reboring, refreshening if necessary - the block has more life in it left. There are alot of huys out there running .090" cylinder wall thicknesses on the thrust side in their production SBC 400 based blocks though. While they may get aways with it now, there is not life left in the block, short of resleeving it, if it needs to be rebored or cleaned up. Also, cylinder walls that thin have a negative effect on piston ring sealing and cost some power due to the cylinder wall movement.




[Modified by Monty, 2:11 PM 1/17/2002]

 

Monty,thanks so much for your thorough explaination.
Phil