331 v 347 Thoughts


I expect its been run over a few times, pardon the pun, but what are teams views and thoughts on a 331 v 347. Just about to start the build of SGT chassis 54 so am looking into engine options, going to be a beefed up UN1 gearbox, Mick @ SGT mentioned a while back a 331 is a nice engine to use, free revving, etc.
I have been looking at a 347 import a chap close to me has up for sale, its an unused crate engine from the USA built up by Mark Performance Engines and was dyno'd at 400bhp+ which also worries me re the UN1 option as to its ability.
So there we go, into the discussion pot, thoughts, ideas suggestions and banter, all welcome
347 will rev and it has usually more torque. It would be easier to drive on the street with more torque but it also allows you the opportunity to be harder on the box.

Randy V

Staff member
Lifetime Supporter
347 will build more power, but the stress on the block is also greater. Be careful of building much over 400 HP with an OEM 302 block. They can split down the middle.
My 331 with webers built 415 HP 430 TQ... I’m building another one pretty much the same way.
I built myself a 331 stroker for my Cobra many years ago. The main thing (for me) was to get it fully balanced. It was a fabulous engine, torquey, powerful, but oh so smooth. Of course you may like the "lumpy" effect......


Thanks chaps much appreciated, lot to think about but I guess that's part of the fun!!

Howard Jones

Here's my thoughts on a GT40. They are free so they inherently have no value. But for what it's worth, here it goes. My decision list goes like this. They all should be selected for the same goal.

First: Budget then:

1. Power train capability/limit. (1 and 6 should be the same within - = 0, + = 15-20% of power number.)

2. RPM range of engine. (See 4. practical rev limit of Hyd Lifter is 6000 rpms)

3. Octane of fuel. (Street gas will limit CR to about 10.5 to 1 with alum heads.)

4. Lifter type, (use hyd roller cam. need more power? make engine bigger not rev higher)

5. Available in hand usable parts. (see first)

6. Desired power. (a really strong street number is 375hp, 25 more, 400, will all be above 5500 rpms and rarely used.) >500...…………...great bench racing.

So a [email protected] 375HP is 1.24hp per CI that is a good number for 93 octane pump gas. The same heads/cam/CR that make that number on a 302 will make about 400 with a 331 and 425 with a 347. That (425) is very close to the long life street engine number for a stock Ford block.

So now what? If you have a good stock block and crank, reuse them and build a 305 (bored over .30 standard 4 inch bore, standard stroke crank. Use ARP main and rod bolts and have the block line bored. Balance the whole rotating assembly including flywheel, hormonic balancer, clutch pressure plate as close to perfect as possible. Convert to hyd roller cam if you have older block. Then the worries about cam/ZDDP/new oil wear are over.

Select pistons to achieve 10 to 1 CR.

Heads: I have seen several SBF use Edelbrock RPM Performer alum heads and make north of 400hp on 305 CI.

Cam: Comp and others make a good hyd roller in the 280 degree (both in/ex, will have good idle and run up to 6000 RPMS well) and .540 lift range. Keeping lift under .55 will extend spring life and generally will work well on a long life street motor.

IMHO: reusing a good stock block/crank/rods, limiting revs to 6000, real good heads, a hyd cam selected to rev range and street use and a really good balance job with ARP hard wear will make 375-400 HP on pump gas. It will last the lifetime of the car and pull like a friggin monster when you want it to.

As you can see 331 or 347 really isn't necessary.

As an aside the power train will work well if you have a upgraded input shaft R21 and live a long life.

Last but should be first! Use the money you have saved to put a Quaife TBD in the gearbox! The difference between a open diff and a TBD are so profound that I can't
over emphasize how big a deal this is.

Last, really last this time: Some one once said "Decide if you what to talk about it or drive it. Many a great hotrod has been ruined by putting too much power in it"

Good luck.
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I chose to build a 331 stroker for my GT350 clone using a new Ford Boss 302 block and AFR heads. I felt it represented the best compromise between the free-revving nature of an original-displacement engine and the torque of a big stroker.....


RCR GT 40 Gulf Livery 347 Eight Stack injection
here is my experience

In 2000 I build a 302 fuel injected motor with the Holley SysteMax II system for my Cobra replica. It dyno'ed to the rear wheel about 285 hp & tq.
All my dyno's are chassis dyno in my Cobra.

After a couple of years I wanted more. I found a Eagle forged stroker kit cheap. It was a 354 c.i. kit which has .007 more stroke than the 347. So out with the 302 and in with the 354. I bought a pair of Pro Comp heads and a new cam.
Dyno'ed about 320 hp and 375 tq. I made a mistake on the push rods (to short) and trashed the valve guide (quickly). So I bought a pair of Keith Craft Brodix heads and a Victor 90 mm intake to go with all the other 80 & 90 mm stuff I had on the motor. I gain over 80 hp and 45 tq. at the rear wheels. 350 ft lbs of tq at 2200 rpms. I could blow the 11" wide tires off in 1st, 2nd and 3rd. Pulled like a freight train to 7,000 rpms. I tried to shift at 5,800 - 6,000.
Would cruise at 1,400 rpms with no problems.
I put over 20,000 miles on that motor. It was a beast.
With a 20% drive train loss that motor made 500 hp.

My vote would be for the 347. Same cost, more hp if you need it, more fun when you take someone for a ride.:laugh:
Oh yea 22 miles per gallon. And any gear was a pasting gear. Cruised at 2,000 with no jerking. What we call trailer hitch jerk.

My buddy build me a 347 for my GT 40. I called Comp Cam and told them what two cam I had in the 354 but I wanted to move the torque and hp up 1000 rpms in the range. They shipped me #35-522-8 grind #FW XE282HR-12.

I have a 911 tranny in the 40 which has lower 1st, 2nd and 3rd so I can give up some of the low end torque the other motors made. I have different heads and injection system but should be about the same.
I plan on having the car chassis dyno'ed in the next couple of weeks. If it doesn't go as plan then I'll be buying more parts:sad:

I have a back room full of parts. Don't need more.

I will not be driving the 40 like I did my Cobra.

If the engine is running over 400 ft lbs of torque id be worried about using it in a UN1 unless you have the single piece shaft conversion. You would be better off with a higher revving lower torque engine in the 350 ft lb range and even then you need to be careful about how you use the torque when shifting gear.


Thanks all for the info, plenty to digest and Jason good point, it was always going to be more frankly that I was aiming for so will go for a lower output unit, popping over to Franks as well for a chat, chassis is being powder coated as I type, gulp, got to build it now :) Will start a build log soon, cheers all

Kevin Box

My two cents worth

You could do this in 2 stages

Get your self a good set of heads that would flow enough for a 347
Do up your 302 with a good set of pistons, proper block prep, blue print and balance, and cam to suit your requirements and see if this enough grunt.
Buy any other bits for the motor with the 347 combo in mind.

If the above is not enough then buy a 331 or 347 rotating kit from a place with good reputation and have it installed in your already nicely machined block. Retune to suit and go. If the machining etc. was done properly in the first place it will cost you a set of gaskets, a bit of notching around the bottom of the bores, balance and assembly cost.

For preference the 347 is where I'd go but do the upgrade on your UN1 regardless. 347 can then be as tame or as wild as you like.