331 or 347 for a currently 302-converted MGB

[Northernthunder]

My uncle is going to rebuild his 302 MGB, he can't decide, which one, so I said to him lets ask the opinion of some of the people here, 331 or 347??

[cgrey8]

It all depends on what your intentions are for the build. Stroking an engine is good for a number of different purposes. But if it's raw HP you are wanting, a super/turbo, a set of great flowing heads, or a cam can often do that far cheaper and easier than a stroker kit will.

So assuming that the decision has simply been made to stroke, the only option is 331 or 347, then some more info needs to be determined. For instance, what heads, what cam, what RPM range will be used more often, and what kind of "loads" will be typical. For instance, will this a powerplant for a fun truck, a track racer, or a towing vehicle.

A 331 is better for reving. It can come very near the RPMs of a 302, but do so with 10% more CID. The Rod-to-Stroke ratio are also very much like the 302 giving it some durability benefits at high RPMs. Nodular iron are the cheapest cranks but the cast steel are the step up and what you should go with...not because you'll need the strength but because higher quality cranks usually balance easier. Balancing is a bit more challenging if you get a crap kit. Talk to any machinist about the pains getting a cheap Ford stroker kit to balance properly as compared to balancing a quality kit. If balancing cost/time isn't factored in, nodular iron or cast steel is fine for most any application using a stock 302 block. The block's good to ~600hp and so will either of those cranks. If this won't be a high RPM build, standard I-beam rods will be just fine and will avoid you needing to notch the block for rod clearance. However if this is to be a high RPM engine, go with H-beams just to be on the safe side although H-beams will require notching.

A 347 is better for torque production. So if you'll be using this power plant to tow with, I'd default to the 347. While the added CID is not without its merits all by itself, the way it gets this is through a significantly longer stroke than the 302 (3.4" vs 3"). So while that works great for low RPM torque production, it's a LOT more slinging metal moving at higher speeds producing more internal stresses. To get a 347 stroker to safely rev to the RPM range that the 302 and 331 can, you will really want those H-beam rods and a forged crank. Although if this is a torquer motor, I-beams and cast-steel will do just fine. And again, if balancing costs aren't a concern, the low-end nodular iron is just fine too. Either I-beams or H-beams will require block notching with a 347 crank.

In either case, you'll need to be concerned about the higher compression ratio the longer stroke produces as compared to a 302 using the same heads/cam. You don't want to just assume everything will be fine and throw parts at a stroker. You will need to confirm head chamber cc, gasket thickness, piston/deck clearance, and then figure a dynamic compression ratio using whatever cam you select. There's far too much money that goes into a stroker build to fall short on your homework in this area. Otherwise you may find yourself with a build that's got more compression than you can buy fuel for. But the finer points of engine building such as head gasket thickness, deck clearance, cam selection, head selection, target compression ratio, externally vs internally balanced, etc, can be discussed once you've filled in the gaps as to what exactly you plan to do with this engine, expectations for the build, what you are starting with, and what the budget is.

So why did I not go with a 347 and instead a 331 when a 347 fits exactly what I wanted? Because I knew going into the build that I would be reusing my small CC stock GT40p heads and I knew the cam that I'd be going back together with. A 347 with flat top pistons would've given me way too much compression with that combination for premium gas. However 16cc dished pistons would've solved that and in hind sight is exactly what I should've done. While I'm quite content with the outcome of my 331, a 347 would've been even more torquey goodness with no significant increase in cost (just the cost of notching the block). And if I had it to do over again, I'd go the 347 route as many tried to suggest I do...but I didn't listen.

[cgrey8]

Some more random thoughts on various engine build options...

Bigger CIDs generally produce more torque. That means it can produce more HP at lower RPMs. The downside is the the transmission must be stronger than a transmission put behind a less torquey engine that produces it's power via RPMs, not brute force torque.

Bigger CID engines generally burn more fuel than a smaller CID engines doing "light duty" work like cruising and idling. Although larger CID engines are more efficient than smaller CID engines as the load increases.

Superchargers and turbochargers can improve the performance of smaller CID engines by SUBSTANTIAL amounts giving them the ability to produce way more power than larger CID engines without a supercharger. The amount more power they produce is relative to the boost level. The catch is you have to "pay" for the compression of the boosted air which is HP the engine is producing that does not go to the transmission. So if you have an engine that, by itself, can produce 200hp naturally asiprated (NA). You add a supercharger and now it might produce 300hp with 8-10PSI of boost. The engine maybe actually producing ~350hp, which means you are feeding it 350hp worth of fuel to only get 300hp to the transmission. That's a parasitic load of 50hp. So a supercharged 300hp engine is going to burn more fuel than a naturally aspirated 300hp engine. That sounds bad, but what it gets you is the POTENTIAL to idle and cruise around with the smaller engine, thus burning less gas at the running conditions far more common for street engines...idling and cruising down the road. I say potential because purpose-built high HP boosted engines run lower compression ratios to allow for the higher boost pressures. But lower compression reduces idle and cruising efficiency. So if this is taken to an extreme, it is possible you wind up with a smaller CID engine that gets worse fuel economy than a larger CID high compression NA engine. But if the boost level is kept in a reasonable range of say 5-8 PSI, then you can build a smaller CID engine with enough compression to idle and cruise like a normal engine and really put out some impressive WOT power numbers that an NA 347 would have difficulty attaining.

So what's different between superchargers and turbos? Well the obvious difference is the supercharger uses a belt drive so the engine is directly producing the power the supercharger uses. Turbos work on a different principle. They "reclaim" cylinder blow-down pressure that would normally be lost out the exhaust pipe. So turbos don't put up the same parasitic load that superchargers do. But they do have some amount of parasitic load associated with them. They do reclaim a lot, but they also restrict flow of exhaust thus causing the pistons to work harder to push exhaust out which is a small parasitic load...but FAR less than a supercharger.

My point in all this? If this is a small little MGB, you may not need a stroker to make more power. Although if space constraints are an issue, then a super/turbo may not be in order either. What you may want to consider is a simple head/cam/intake/exhaust upgrade unless it already has all these things. Even after a stroker kit is paid for, there's a good amount of machining costs associated with preparing a block for a stroker kit that a lot of people fail to consider or fail to actually do. Again, it just all depends on what your intentions are for the build and what it is the engine isn't doing for you now that you want it to do after you spend some money.

[Northernthunder]

cgrey8
thanks for all the information, this is the second MGB 302 build, first one was a boss 302, toploader 4 speed, jag rearend, frame connectors, twin roll bars, it look mini cobra, I just talked to my uncle, he chose to go with the 331 set up, this being my first ford, and i like a challange, i'll stick with my plan, it's 347 for my 2000 ranger,
thanks again

[cgrey8]

331 makes sense for an MGB although I'd think an H/C/I 302 would work just as well for likely a lot cheaper.

But a 347 in a Ranger has got to be cool. Like I said above, knowing what I know now if I had to do it over again, I'd have gone the 347 route.

[Northernthunder]

I hope so, i'm driving the truck now (3.0 l), but i have another 2000 ranger frame which i'm using for the build, with a beat up cab and front end, i'm going to use for my mock up.

[Dave]

Like Chris said, all depends on end use. As a DD I'd go with the 302-347 plan, just so many good options. If I was into serious road type racing I'd sure step back and go with a 289 spec build. On a tight road course they will out run a big block. Where you at anyway, I see NorthernThunder and think up Nord.
Dave

[cgrey8]

Are you getting a machine shop to build the whole thing for you or are you designing it out, choosing the heads, choosing the head gasket thickness, piston deck clearance, cam, etc? If you aren't thinking about all these things, then you need to be. Most machinists are good with their tools, but it's hit-n-miss whether they are good engine builders. Many of the old schoolers just know a combination that they've used before that ended up doing alright so that's what they build over and over for people that don't know what they want. You can either trust yourself to someone that claims they know what they are doing or you can educate yourself and go in there prepared to ask for exactly what you want. But to speak intelligently about engine building is a learning curve. It's not something you'll just get from reading a single article or post. It's going to require you really think about what it is you are doing and learn about concepts you've probably never thought about before...one of the biggest being the difference between static and dynamic compression ratio.

[Northernthunder]

Hi Dave
Thanks for your input, Nord, yes, northern is for Canada, Thunder is for the storm that happens when the wife's on the warpath, usually because I've spent to much time in the shop.

Hi Chris
the truck is to be streetable, but run at the strip, mid to low 12's, that is the plan, 347, 5 speed, 9", CR 9 OR 10-1, AFR heads, T- ram 2x4 450 cfm, mallory ignition system, scat rotating assembly, i have a solid group of guys who help build and design my engines and setups, everything is match to work together.