View Full Version : 350 or 400 mains for my Motown 406 build ??

DRJDVM's '69
03-26-2005, 04:32 PM
I'm making plans to build a D1SC procharged 406, and I'm starting with a Motown block. I have the option of the 350 or 400 mains.

So whats the real advantage of each ???

One guy I talked to, said go with the 350 mains since you will have a ton more crankshaft options etc....

If I go 350, do I have to do all the "extra" work as far as balancing etc ( I know 400's are usually external balanced, but was going to do the extra stuff to go internal balance etc)

Any tips about advantages/disadvantages of each would be great....

03-26-2005, 05:08 PM
I say 350 main's....... bearing speed and loading.

03-26-2005, 05:55 PM
with 350 mains you will have less bearing contact and speed - some "free" hp. with the 400 mains there is more overlap between the rod/main bearings - stronger crank.

how much boost?
how much hp?
what crank do you want to use - stock, forged, cast, billet?
what strke do you want?

yes the 350 mains do have more stroke options, but if you look hard enough every crank manufaturer makes 400 main cranks too. if it is a stock stroke, chk out doug herbert performance, they probably have the largest selection of cranks, rods, and pistons in one catalog


03-26-2005, 06:41 PM
If you don't plan on reving the motor to the moon, I would suggest using the 400 mains for the extra strength. The 400 mains help crank strength a lot, especially with longer strokes.
The extra weight will be in the centre of the crank and won't have much of an effect on balance. Some crank manufacturers offer gun drilling on the mains (in addition to the rod journels) to reduce the toal weight of the crank.

DRJDVM's '69
03-27-2005, 12:01 PM
My hope is to put down around 600 RWHP, with around 12-14 lbs boost.

I'm modeling my motor after Don Estels car right now.... he put down 691 RWHP with 14 lbs boost on a 406, so I'm shooting for that right now.

Everything will be forged.... billet is too much $$$

Probably going 4.155 bore and 3.750 stroke..... I may try and get a few more cubes (i.e 415-427) if I can do it and still keep the CR down and not put additional stress on the motor etc by compromising the L/R ratio etc.

With a big inch small block, I dont think I will need or want to rev over the 6-6500 RPM range at the absolute most. Probably less..

I'm trying to build the motor so I can get the maximum benefit of being supercharged.

03-27-2005, 03:19 PM
i would definately get the 400 mains then

03-27-2005, 04:29 PM
A couple quick notes.

400 mains, yes.

It requires high dollar pistons to keep compression down as stroke gets longer. As it is now you should be able to get by with shelf forged pistons.

Since you are buying a new block, consider a standard bore, or minumum overbore to save the block for future overbores. The extra couple cubes are not worth it, and unless you are building a class racer, you do not really need the benifit of the larger bore. Besides, your supercharger will take care of that.

Internal balance stuff is readily avaiable in Eagle or other budget 4340 cranks. Should be plenty for what you are doing.

Eagle H Beams are heavy, but plenty strong. Using an internal balance crank and H beams will make a heavy rotating assembly, but will help torque anyway.

03-27-2005, 05:22 PM
Uh oh, I see the boost bug did indeed bite you.

If the price difference is minimal I'd go with the larger 400 mains just for more bearing surface area. This isn't going to be an all-out race engine where you're looking for every possible frictional advantage to have the engine rev faster, so I would sacrifice a bit of performance in the name of durability and go with the larger 400 mains. Even with the 400 mains it will make a lot of power, trust me on that, and my guess is that in the "real world" the actual power difference will be difficult for a dyno to see, probably single digit differences at the RPM levels a street engine usually sees. Start spinning it to 9500-10000 and it will be a more significant difference.

I'd also push you towards a Dart Little M block. The MoTown is a good block, but Dart's casting and machining quality is a bit better and it's not that much more money.

Prodigy's advice on going smaller on the bore is good too. The few extra cubes won't help that much, and you'll want thicker cylinder walls to help with the increased combustion pressures. Thicker cylinder walls won't flex as much, and more importantly the smaller bores will leave more "meat" for a headgasket inbetween the cylinders, where most problems usually occur. Go too big on the bores and you may find yourself burning / distorting the headgaskets between the cylinders since the gasket will be about at wide as a piece of dental floss!

The best piece of advice I can offer is buy the very best flowing heads you can afford and focus on the I/E ratio and shoot for as much exhaust flow as possible. I would also strongly suggest inconel exhaust valves just for a bit of added insurance. They're a bit heavier than stainless, but you can make all of that weight gain back and then some by using titanium keepers and retainers.


03-27-2005, 05:40 PM
400 with squirrels....

DRJDVM's '69
03-29-2005, 09:01 AM
More opinions..

Dart says go 350..... World says go 400..... Scat says 350 would be better for weight and thats what most people would do, but 400 would be slightly stronger, so ?????

It sounds like the 350 has the slight edge..... lighter with a "stock wt." crank (i.e not a lightweight version), more selections, keeps more material in the block, bearing speed and load... downside is not as strong as 400....

The 400..... strength......downside is extra weigh and removing more material from block at the mains.

Whats the likelyhood that I'm going to break a forged crank (either a 350 or 400 main)putting out 800 HP under 6500 RPM on a street driven car ???

03-29-2005, 10:08 AM
Whats the likelyhood that I'm going to break a forged crank (either a 350 or 400 main)putting out 800 HP under 6500 RPM on a street driven car ???

Highly unlikely if it's a good crank, things are assembled properly, and you don't detonate the crap out of the engine.

BTW, I should have my new injectors within a week, and with local carshows starting up I may be seeing you soon. My offer for a ride still stands. :geek:

What is planned for shows in Manteca now that the Perko's closed? Last I heard there were rumors of a show across the street at Wienerschnitzel, but I haven't got confirmation on that.


DRJDVM's '69
03-29-2005, 10:35 AM
Hey Troy
No idea on the informal car shows like Perko's..... since Perko's really didnt do anything, I dont see why we cant still meet in the same parking lot....Perko's used to actually participate in the shows, but that stopped a long time ago, so essentially all we were doing was using the parking lot.......

I'm sure people will still meet there and figure out what to do.....

May 14th, East Union HS has a car show.....

As far as cranks... thats my point...... isn't it going to be pretty hard to break a forged 350 crank with what I'm planning?? so whats the real benefit of going with a 400 ??? Added insurance is always nice, but with a "day to day/real world" view, the extra weight will have more of an impact on the performance of the motor than the extra insurance.

When's the last time you heard or saw someone break a crank, period ???? And if they did, would it really have made a difference if it had been a 400 rather than a 350??? In other words, when something goes wrong enough to break a crank, then some serious sh*t must have happened and the difference between a 350 and 400 wouldnt have saved it.... a lot of other components probably would have given way too...

In reality, I dont have very much experince with this kind of debate, so thats why I'm asking the people on here for opinions.

03-29-2005, 10:54 AM
I think your spending too much time thinking about this little thing..

Just go either way... FLIP A COIN...

A good forged crank should handle 1000hp easy..in any size..

DRJDVM's '69
03-29-2005, 12:10 PM
I do have a tendency to over-think and over-analyze stuff :)

But on the other hand, its the details that make the difference......

03-29-2005, 07:53 PM
I sent you a pm. :)