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Jeff Smith Technical Questions and Answers - Ask Anything

Building a Bigger Clevor

Andrew Grubb; Ontario, Canada: I am planning the build of a 351, Windsor-based, 427 stroker engine with around 600 hp that is as streetable as possible. Do you think it is worth spending the extra $2,000 for a Dart block that allows the capability to run a 4.125-inch bore, so that a shorter (less than 4-inch) stroke can be used? Most stroker kits use an excessively long stroke that can significantly reduce engine life, so the extra initial investment of the Dart block may be worthwhile in the long run. Also, would having a shorter stroke and a larger bore create a more high-end power engine (spinning up to 7,000 rpm or so)? What are the main benefits of using Cleveland-style heads, and would it really be worth the effort over a set of say, AFR 205cc (PN 1450) ported heads? I intend to run on pump gas, and the compression will be around 10.5:1 with forged internals.

The car will be light (around 2,500 to 3,000 pounds) and will see mostly street use but an increasing amount of track time (road course). I am going to be putting this into either a Cobra replica and/or a stripped out '66-ish Mustang coupe. I am hoping to spend less than 10K on the engine, $12K if it is a Dart block. If you would like to take a stab at how much horsepower and torque this thing will make, I definitely won't stop you.

Jeff Smith: Wow, Andrew, there's plenty to cover here. It's my opinion that you are always better off with a new, aftermarket block, especially if you intend to spin this engine beyond 6,000 rpm. An original Ford 351W block that is now 40-plus years old may not be durable enough to justify the cheap initial investment considering the amount of money you have to invest in machine work, particularly if you want to convert from two-bolt to four-bolt mains. To do all the proper machine work on a stock block, including adding four-bolt main caps (which cost roughly $325 for the caps), you're looking at spending $1,300 to $1,600. We priced the Dart SHP 351W block from Summit Racing (PN 31365235) at $1,865.41, plus freight. Want more reasons? Don Barrington at Barrington Machine mentioned that the Dart block has a priority main oiling system as compared with the stock 351W orientation, in which one lifter bank also feeds the main and rod bearings. If you were going to use the stock block, Don suggests bushing the lifter bores to reduce the oil-feed size to the lifters, thereby directing more oil to the mains. Plus, the Dart SHP block uses the smaller 2.749-inch Cleveland main-bearing journal diameter compared with the Windsor's larger 3-inch one. This reduces the bearing speed, which cuts down oil shear—a serious consideration for a track-day engine that will see lots of continuous rpm. Based on all these considerations, plus the Dart's inherent strength, the decision appears to be relatively easy. If that's not enough, the Dart block's much thicker cylinder walls allow punching the bores out to a maximum of 4.185 inches. That combined with a shorter 3.85-inch stroke creates a 424ci engine, which is fairly close to the desired 427 ci. You mention you would like to push this engine up to 7,000 rpm, but keep in mind that with a larger-displacement engine, you will be able to make 600 hp at a lower rpm, so you don't have to spin the engine quite that high. A longer-duration camshaft that is necessary to make the power at higher engine speeds also pushes the torque peak rpm higher, which helps produce more horsepower. For example, if an engine makes 400 lb-ft of torque at 4,000 rpm, that amounts to 304 hp. But make that same 400 lb-ft at 6,000 rpm, and you get a much higher 457 hp. So you can see that moving the torque higher in the rpm band is worth horsepower. But in the best tradition of no free lunch, that higher engine speed is also abusive on parts such as connecting rods and especially hard on the valvetrain. Often, a component like a valvespring that will live forever at 6,000 rpm can die a quick death when subjected to near-continuous use at 6,500 to 6,800 rpm.

With that in mind, the answer to the question of bore size is easy: Bigger is always better. Add a cylinder head that flows some air, and you have the basis for an engine that will make outstanding power. In my admittedly conservative approach, it seems wise to choose a shorter-duration camshaft that will maximize torque between 4,500 and 5,000 rpm. You will probably discover that a typical performance engine will spend much of its time in this rpm range. If you are really serious (and it sounds like you are), do some additional research and find a car that races your road course that has a similar engine and drivetrain combination. You could just ask the owner, but I would suggest asking specifically if he has a data logger (Racepak makes a good one, but there are many others) that would allow viewing a log of the engine's rpm curve on a quick lap. What you're looking for is how much time the engine spends in the rpm bands of 4,000 to 5,000, 5,000 to 6,000, and then 6,000 and above. More than likely, the engine will spend a clear majority of its time between 4,500 and 6,000 rpm. If that's the case, I would concentrate on emphasizing power production in that rpm range. For example, if your engine makes more torque it will accelerate the car hard coming off the corner and will carry more speed all the way down the straight as opposed to just making power right before you have to lift to enter the next corner. It's the same concept in drag racing, when torque accelerates the car all the way down the track.

To address your cylinder head question, I think you are on the right track with the AFR 205cc (PN 1458 w/72cc chamber) Windsor heads, since these heads not only flow really well but also offer excellent velocity, which will help make gobs of torque. If you subscribe to the torque concept, these heads are an excellent choice, even for a 427ci engine. AFR makes a larger, 225cc head (PN 1456 w/ 72cc chamber) with some significant flow increases over the 205 head, especially in the midlift flow areas where it might help to make even more power. Valve sizes are the same between the two heads (2.08/1.60-inch dimensions). Another thing to consider is that the exhaust ports are raised on both of these heads, which means bolt-on headers might not fit as cleanly on the Mustang.

If you think you'd still like to spin the engine to the higher speeds to take advantage of the increased horsepower, you might consider the Cleveland-style heads offered by Trick Flow Specialties (TFS). On paper, these canted valve heads produce similar specs to the 225cc AFR inline heads with 2.08/1.60-inch valves, although the TFS combustion chamber is much smaller at 60 cc (PN TFS-5160T005-CO1; $2,749.95 Summit Racing). The basic as-cast heads offer some decent flow numbers. The TFS catalog lists intake flow numbers for the 190cc as-cast Cleveland head as such: 248 cfm at 0.400-inch lift, 286 cfm at 0.500-inch, and 299 cfm at 0.600-inch lift. These are excellent numbers, matched with good exhaust flow numbers that average 76 percent exhaust-to-intake flow. This is really important because the original factory Cleveland castings are notorious for their extremely poor exhaust-port flow that killed any power potential above about 5,000 rpm. Clearly, if you wanted to spend a little more money on the CNC-ported heads, the flow would increase significantly. TFS also shows in its catalog a dyno test of a 383ci small-block Ford using the 190cc as-cast Cleveland heads making 524 hp at 6,500 rpm using a Crane 236/240 at 0.050 hydraulic roller camshaft with 0.621/0.631-inch valve lift. If we take the horsepower per cubic inch of that engine and multiply it by a 427ci engine, we get 595 hp. A larger engine will make the above cam smaller, which means it would probably reach horsepower peak at closer to 6,000 rpm than 6,500. TFS says the 383 made 480 lb-ft of torque at around 4,500, so we can assume the torque would also jump up to around 540 to 550 lb-ft with the larger 427. On deciding between the two heads, keep in mind that you will need to consider which head you will use before ordering pistons, since the canted Cleveland valve angles are completely different from the inline Windsor head. This means the valve reliefs in the pistons will be different depending on the cylinder head. You will probably need a dished piston to run pump gas, since these heads have a 60cc chamber. A 4.185-inch bore and a 3.85-inch stroke, a 20cc dished piston, a deck height of 0.005 inch in the hole, and a gasket thickness of 0.041 inch will produce a 10.6:1 compression ratio.

Finally, you might want to consider how the Cleveland head will physically fit in either of the cars you are considering compared with the Windsor head. The Cleveland package will basically require a custom set of headers, while you might be able to find a set of aftermarket headers that are designed for what would essentially be a 351 Windsor if you went with the AFR heads. It's these little things that can make a big difference in terms of how much the project will cost.

More Info

Airflow Research (AFR)
Valencia, CA
877/892-8844

www.airflowresearch.com

Dart Machinery
Troy, MI
248/362-1188
www.dartheads.com

Trick Flow Specialties (TFS)
Tallmadge, OH
330/630-1555
www.trickflow.com


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