On The Edge 355
Truth be told, I went into this project with a bad attitude. Not that I had a problem with building an engine, mind you. I mean that I wanted to get a bit nasty with this 355ci build and do something out on the edge. Past endeavors have always been built with two ideas in mind: attain nice, fat peak numbers, but get them while maintaining a broad, street-friendly powerband. When compared to these past street/strip efforts, this little beast would be of the strip/street variety; we intended to make as much high-rpm power as possible, something good for ripping off fast e.t.'s at the local digs, and as long as we could at least pretend the thing was streetable, that would be good enough for me.
When we conferred with JMS Racing Engines' Mike Johnson, we quickly decided that we would push the limits for a streetable, pump-gas compression ratio, running somewhere in the neighborhood of 11.5:1. Johnson also thought a bit outside the box, suggesting that we run a long, 6- or 6.125-inch connecting rod. Although the idea was to produce high-rpm horsepower, a long rod usually makes more torque, and we didn't want to be deficient in this area. (In other words, we didn't want a total low-rpm pig.) Dwell time at TDC would also be increased, promoting a more complete burn. We'd also get a favorable rod angle in the bargain, producing less piston rock. Mahle had just the small-dome piston we needed to work with a 6.125-inch rod while producing our desired compression ratio.
Our cylinder heads of choice were Trick Flow's Super 23 Street/Strip lungs. These heads flow well and are a great bargain, especially considering that they come with CNC's combustion chambers. They're available with 64- or 72cc chambers, 195cc intake runners, and several spring packages that can handle up to 0.600-inch lift. We intended to far surpass that figure, however, so we ordered ours bare and outfitted them with Isky springs and hardware to handle up to 0.730-inch lift. That should give you and idea of just how crazy we intended to get with this thing, but our rapidly approaching deadline dictated that we go with a shelf grind instead: 0.640/0.640-inch lift, 272/280 degrees duration, and a 106-degree lobe separation angle.
It's a good-sized cam for a 355, to say the least, but a funny thing happened on our way to the dyno: We came up with the big, high-rpm horsepower number we wanted-539 ponies at 6,500 rpm, to be specific-but we also came up with a pretty broad torque band. Check out the chart, and you'll see that our creation is making more than 400 lb-ft from 3,800 rpm all the way to 6,900. Our backup cam choice is indeed radical, but damned if we didn't end with a usable powerband in the bargain, thanks to all that duration and the tight LSA. Reading the cylinder pressure numbers from our runs, our dyno guru Steve Brul concluded that we'd hit on an efficient combo, leading to all the torque that came along with our horsepower.
This thing is rumpity as a Harley, to be sure, and idle vacuum (at 1,200 rpm) is a miniscule 4.5 inches. Is it streetable? With a converter and gears, sure it is. We used 100-octane for our testing, but with a little less timing it'll even run on 91. In the end, we got more than we bargained for: a nasty but stout little mouse that can run hard up top but still get out of its own way down low. What did we learn? Every once in a while, things actually go better than planned. -JN