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Old 6th May 2006, 11:33 AM  
United States
Join Date: Jul 2004
Location: USA
Posts: 4
331 Alternative

Originally Posted by 1074R/001
I was pretty sure that many of the modern heads would outperform the original GW heads. Curiosity getting the better of me, I was wondering if anyone had some specifications so that I could compare them.

I'm also curious about the original cam grinds.

My goal is to equal the power of the '69 Gulf cars with a 331. The slight increase in displacement will allow for a slightly milder setup for the same peak power and with EFI and a good computer, the engine would be pretty friendly on the road while still being able to headbutt the horizon with enthusiasm.

Hi, Stew.

If you haven’t already purchased your reciprocating assembly and had your block work done, please seriously consider the M-6010-BOSS302 block, punched-out to a 4.125-inch bore. With your 3.250-inch stroke, this will give you 347.47 cubic inches (5.694 liters), which is almost identical to the extremely popular “347” combination (like the M-6013-B347/C347). The latter uses a 4.030-inch bore and a 3.400-inch stroke for 346.95 cubic inches (5.685 liters).

The bigger bore provides more of a power increase than the simple increase in displacement suggests. First, it unshrouds the valves, making your heads more efficient: the better your engine breathes, the more power it makes.

And just so we’re on the same page: in the 1970s, NHRA Pro Stock racers like Bob Glidden campaigned the 351C -- using blocks that had been sleeved (rather elaborately: the process involved furnace-brazing and stress-relief) and bored to 4.080 inch. If they could have gone larger, they would have – not so much for the displacement as for the unshrouding of the valves.

The other benefit of a larger bore is a piston having greater surface area. Assuming a perfect seal between the piston and the cylinder, and a perfectly round cylinder:

Bore (inches) -- Surface Area (square inches)
4.000 -- 12.5664
4.030 -- 12.7556
4.080 -- 13.0741
4.125 -- 13.3640
4.155 -- 13.5591

If you apply 1000 psi to each (for instance, during the combustion event), the force translated by the 4.030-inch diameter piston is 12,566 pounds; the force translated by the 4.125-inch diameter piston is 13,364 pounds (nearly 8 percent more). Of course, if you’ve got the bucks, the M-6010-F302 block will shave about 70 pounds and let you go to a 4.155-inch bore!

Alternatively, the M-6010-S302 block weighs a few pounds more, but gives you an extra half inch of deck height to play with – room for a slightly longer rod and a much more “normal-looking” (and robust) piston!

There are a variety of ways to do EFI with the slightly-taller block, but the easiest is probably to use spacers between the heads and the intake; everybody seems to have his or her own idea about how to space an intake, so I’ll skip that – except I must mention this: don’t forget to add the thickness of the spacer (and gaskets) to the length of your intake runners when you’re getting a custom camshaft ground. Later in this message, I’ll get to what I mean by “custom camshaft.”

You want to use a 3-ring stack (compression, wiper, oil) on the street. Ford Racing Parts claims their M-6013-B347/C347 uses a set of 1.090-inch tall pistons, each featuring a 1/16-inch, 1/16-inch, 3/16-inch ring stack. The set of pistons is part number M-6108-C347. The distances between the rings must be incredibly thin! Even at that, it seems the top ring is going to take a severe beating from the heat.

The “assembled height” (cold) of a 3.400-inch stroke, a 5.400-inch connecting rod, and a 1.090-inch piston is 8.190 inches. Working backwards, if you limit yourself to an “assembled height” (cold) of 8.190 inches and you use a 5.400-inch connecting rod with a 3.250-inch stroke, your piston is 1.165 inches tall.

For a 3-ring stack, that’s still an awfully short piston -- imho -- however, it offers a lot more room for the rings than what Ford is selling. If you shorten the rod to 5.315 inches (a common aftermarket length for the SBF; the same length as the D0ZX-6200-A “302 Boss, 9000 rpm, competition-only” rod from Ford’s Off Highway Parts program), it gets even better: your piston is 1.250 inches tall -- which will allow the use of canted valves.

With a 3.250-inch stroke, the 5.400-inch rod will give you a rod:stroke ratio of 1.6615:1; the 5.315-inch rod will give you a rod:stroke ratio of 1.6354:1. For comparison, the 351C (including the Boss 351) has a rod:stroke ratio of 1.6514; the 350 SBC has a rod:stroke ratio of 1.6379:1. The 410 and 428 FE big blocks from Ford each has a rod:stroke ratio of 1.6304:1, and the 400 SBC has a rod:stroke ratio of 1.4840:1.

Any one of the foregoing engines can be configured to provide both a reasonably smooth idle and a broad powerband that is easily accessible.

To give you an idea of the potential of a small-bore 347 that has the wrong camshaft, check out but remember that’s an engine purpose-built to make good power numbers on pump gas during only a few pulls on a dynamometer. One look at the lift indicates the cam isn’t intended for street use -- and the insane valve spring pressures simply confirm this.

On the other hand, it says you can make 93.2 horsepower per liter using racing gasoline or E85, with a 10.6:1 compression ratio.

For something a lot more streetable -- and featuring EFI -- check out The power curve looks a little strange, but you’re looking at a pretty mild, generic small-bore 347 that will probably run all day on 87PON petroleum gasoline (though you might have to retune for crappy gas), and crank out 70.9 horsepower per liter.

Holley gets almost the exact same power density from a 5.0 EFI engine while using a smaller cam; see the details at

But of you want to see something REALLY impressive, you’ll have to go to, where a small-bore 347 having 10:1 compression produces 421 RWHP (rear wheel horsepower) at only 6000 rpm in a 1991 Mustang GT.

For what it’s worth, that translates to well over 500 horsepower at the flywheel. That engine uses the 5.315-inch connecting rod with a 3.400-inch stroke, giving it a rod:stroke ratio of 1.5623:1 -- and though the camshaft will produce a significant lope, it is streetable.

Oh, here’s another link: The engine is another small-bore 347, and it makes “only” 380 horsepower -- at only 4600 rpm! It’s carbureted, but still: pretty freaking impressive for pump gas and natural aspiration. Oh, and none of the above figures included laughing gas: “Ride too slow? Add a little N20 -- and away you go!” offers carbureted and EFI versions of the small-bore 347, in 9.5:1 compression with horsepowers to 435, and torques to 425 lbf.

But if you want to get the best output, I think the 4.125 x 3.250 combination is the way to go -- especially with the 5.315-inch connecting rod. And if you need even more punch, boost the stroke to 3.400 inches for 363.5 cubic inches (5.957 liters)! Vrooom!

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