Engine and Transmission Tech

Mallory metal should only be needed if you are going for an internal balance. Otherwise metal will have to be removed unless you use stock rods and heavy pistons. For internal balance a combination of the late crank and the lightest rods and pistons may avoid the need for mallory metal or may not. With an early 350 crank, about 600 grams of rod and about the same for pistons, a pair of 1" mallory slugs were needed. That setup uses the modified SBC flywheel and a zero balance damper.

Jim

Thanks again to both of you for the advice--you're a wealth of knowledge.

Jim B.: Are you saying that IF I use an early crank and don't go for internal balance, I'll need to remove some weight from the crankshaft? "Otherwise metal will have to be removed unless you use stock rods and heavy pistons". My rods are about 680 grams and I assume the pistons will be lighter than stock. Just want to make sure I understand what you're saying.

Eric, Your rod weight is close to the stock later 350 rods. The early crank is lighter. The pistons will be lighter. They may add a little weight to crank or flywheel and damper. Chris used the later crank and rods with much lighter pistons and added no weight but crank was drilled and lightened. He did internal balancing though.



Edited 2 time(s). Last edit at 02/25/2022 01:37PM by mgb260.

The heavier rods offset the lighter pistons somewhat so you may be close. If those rods are the same width as stock SBC rods they have .050 milled off each side of the big end and use early small journal SBC rod bearings.



Edited 2 time(s). Last edit at 02/25/2022 01:51PM by mgb260.

There's a formula, half the rod weight is rotating and half is reciprocating but without getting into that, lighter rods and pistons mean less is needed in the crankshaft bobweights. With light rods and pistons considerable weight has to be drilled out of the stock early crank with external balance and even more with the late crank. This is why it's feasible to internally balance the assy provided the rods and pistons are light enough. (~100g. can also be lost at the wrist pins.)

This internal balance allows the use of less expensive SBC flywheels and a variety of dampers including aftermarket fluid dampers. However, as Jim mentioned one hole has to be elongated for the flywheels to bolt up, and it can affect the stack up on the throwout bearing, so those areas need attention.

OTOH, external balance leaves the damper and flywheel stock which means all parts can come from Buick sources. Also an advantage.

I'm building one engine each way. Both are equally valid approaches. I'd suggest you ask your engine balancer for his recommendations given the parts you have in hand.

Jim



Edited 2 time(s). Last edit at 02/25/2022 03:15PM by BlownMGB-V8.

Great advice, thanks. Nice that I can go either way. Hadn't thought about using a SBC flywheel, but that gives me more to think about.

My question about using a 340/350 stroke ? Lightweight LBC actually need that kind of torque ?
Art.

Yep.

Jim

No more so than does a Ford 347 stroker. Probably not to both.

Hey Art,
It's a fair question to ask. Even the best of us need a good technical devil's advocate to pull us up when our perspective gets out of hand and I'm no different.
I had considered doing this only because:
-I was dealing with a core requiring a full overhaul and the additional cost to do this work with a viable late 350 crank was much less than it would normally be;
-I was looking strictly at improving the rod ratio to reduce thrust wear and improve the service life and efficiency of the engine while flattening the torque curve rather than a max effort build with the most torque possible.
Now that I have a good running marine engine it no longer makes sense to undertake, and I imagine I'll be happy with the engine more or less the way it is, but I'll still hang on to my core and pursue this on that engine for some rainy day years from now IF I get tired of the engine I have. I'm not too proud to course-correct if it gets the project finished sooner. I feel like the first rule of filmmaking should be the first rule of hot-rodding: FINISH THE HOTROD. So many great ideas don't get finished and Coppola was a great filmmaker but this car isn't the hill I want to die on. I wouldn't advise someone to take this work on unless the rotating assembly already needs attention - but as Jim B. stated, there's a lot more versatility to an internally balanced engine if you're looking at modifications that aren't compatible with the stock vibration damper and flywheel and that can save money and time that pays for it. One can only answer this question meaningfully if we look at all the goals of the builder and the car they're building.
One could argue the stock 18G/V engine alone is enough for this car - I'm going to be brutally honest when I say I never gave the MGB a second look until I realised you could stuff a V8 into it with no major metalwork. Now I feel like I won the lottery. I don't think any AC Cobra or Sunbeam Tiger owner is complaining about the extra torque, but I'll never afford one so I'll build this one and find out for myself, I guess... It sure looks like fun to me! And let's not forget that a RBB is a malaise era car and isn't getting on the lawn at Pebble Beach anytime soon so if it's a bit overpowered, a bit disorganized, panel fit is off, you're really not going to gain much by obsessing over getting it perfect unless it makes YOU happy. One man's opinion.
Cheers.

If it's torque you are talking about, many of us have by now driven the MGB-Roadmaster and the unanimous consensus is that it's an easy car to drive.

Jim

Should you look on the Stan Weiss, flow reports, website. Much is revealed about the 300 alum. vs 300/340 iron head flow
capacities. Iron head int. dia. was increased by reducing exh. to 1.37". Ported it flows 104-130 cfm., partly because it's
.76 diameter of intake.
Onward, Art.

This?

[www.users.interport.net]

I think most of us know that the Buick 300 aluminum head doesn't flow as well as the iron 300/340 head.

My late Rover 4.9/4.6 head flows as well as my Buick 300 head.

Yes, the marine engine comes with iron heads so that problem is solved, but thanks very much for mentioning Stan Weiss (I had to look him up before Carl posted the link) and the data is fun to run through. As I said I was looking at the stroker for different reasons that the head won't solve, but it's a fair point.