Class 7 race engine advice

[sigmatero]

If you could draw an ideal torque/hp vs RPM curve for a desert race engine what would it look like? Assume you're after about 300hp and it's going in a Class 7 open truck. You can pick any drive train gearing you like. Assume you don't break down (gotta finish to win of course) and that whatever power production approach you choose is reliable enough to get you through the race.

As an example... I just rebuilt a 22RE for my '87 4Runner "logging-road" truck (and I'll hopefully mate it to the tranny tonight, yea!). It will be used on really rough N.ID, Canadian Border roads at "fun" speeds. Oh, before you ask, I'm only hauling me and my family, not "cargo"- you do that in the winter with snowmobiles (from what I hear of course) :). The only thing I changed on the engine was to add a RV (high torque) cam because I figured that I would be mostly interested in torque and not needing oversized valves, headers, etc which typically are most beneficial at high RPMs. I'll have 33" tires for clearance and will probably run 5.29 gears. Should be a real stump puller (well, as much stump as I can pull with about 115 hp and a relatively light truck).

But what about a desert race truck- does this type of approach still apply? If you look at the average speeds of the Class 7 trucks they're around 30 mph for a race (well, a lot of the races anyway). That's pretty close to what I'll average with my torquey truck when I'm running around the backcountry. How often do the trucks really need full horsepower at the high RPMs? Is is mostly about how quick you can accelerate out of corners, around obstacles, etc or is it the speed you can make on the fast sections that wins races?

Thanks for your advice and expertise folks.

[Eric Tadeja]

oh you need horse power at high RPMs

you want to be able to keep your HP up in high RPMs so you will be able to stay on top of the whoops if you start bogging down you are going to screw yourself up and also when you hit sand and silt

[jesusgatos]

Well, if you're still planning on building a Toyota (http://www.race-dezert.com/forum/showthread.php?t=20545), I don't think you'll find a better engine to build than the new 4.0L V6. I'm sure there are a lot of engine builders that could get more power out of it.

[sigmatero]

Oh, I ran across this article a while back. Very interesting and relevant to this discussion...

http://www.cannell.co.uk/Technical%20Articles/pwr_tq_explain.htm

[GeoffMilke]

If you got 300 HP, and can get 275 lbs of torque or more, and get the torque curve to stay up until abover 5000 RPM before it starts dropping you will have a great setup.

We have 250 HP from a 4.0 pushrod motor, and about 220 lbs of torque, which drops off drastically at 4500 rpm, leaving us with "No More Go" when we really need it. For instance, top speed is about 85mph on the drylake (Laguna Salada) at about 4500 rpms, and it just wont go faster. But on the flat highway, at 4500 we are doing 97 mph. dont really need torque to 'get up and go" on the highway

Now, mind you, I don't know what the heck I'm talking about, but I've seen our truck on the DYNO and talked to the engine builder, and what I'm telling you about the truck is exactly what he told me was going to happen.

[ntsqd]

The only thing I can add to whats already been said is this: the flatter the torque curve the easier the truck will be to drive. Those surprise corners where you're too busy to downshift, but need the engine pull you out of trouble? Yea, that's when a flat torque curve comes in really handy. With a peaky curve you're likely going to be in the wrong gear and the engine will NOT be happy about it. At the extremes think 2 stroke MXer vs big block Chevy.

Since HP is a function of torque & RPM, pay attention to what torque where and the HP will follow.

[Byrdman]

It also depends on if your running an automatic or a manual. Automatics require more ponies to spin. They are in some cases more reliable and easier to drive than manuals, but do help in getting the power to the ground. It also depends on how heavy your truck is going to be: If its a tank, pushing all that weight around requires more horses and eats more parts. My suggestion- stay away from the smaller motors, start with a large stock motor and build it up down the road once you've got the truck working and finishing races. Make the truck light- it's really easy to start bolting a bunch of junk on it to look cool and keep up with the Jones- You'll end up spending a bunch of $ and chasing your tail!

[sigmatero]

Makes sense, that's sort of the direction I'm leaning but it's a long ways down the road. I like to plan early :) What's the old saying, the fun of the trip is 90% dreaming and 10% doing.

[Kritter]

300 hp in a 7 open sounds small... in score and bitd the busch motors are pushing some ponies.

[punkassslacker]

4.5L V6 pumping 500-600 ponies is pretty much the standard for class 7. When we bought Rick Taylors truck it came with two 4.5L engines the odd fire was rated at 550Hp and the even was rated at 500Hp. Unless you can get you 7 to be really light I would plan on going some the 4.5L other wise going to have problems keeping up.

Not to say that the fasted truck always wins. Dan Chamlee is a perfect example. He has a 3.8 punched out to 4.2. Dan is getting roughly 250 Hp no more than 300 (these are estimates, no dino info) He never won a race two seasons ago and managed to win the championship for SCORE.

Basically think about what you really want to do. Do you want to race and push it hard or are you going to drive and try to save the truck? Then think about your budget. Do you have 10K to throw at a motor just to rebuild?

Back to your question about rpm's and stuff. It seems like everytime I look at the tach Scott is has her reving about 8K, she is just screaming. At San Felipe we did 114 MPH and could have gotten a couple more, but we backed off. With the right gear and a Busch car I can see how it is that they can get 200 MPH. Those 4.5L engines aren't a motor you want to take lightly. They teach you respect real quick.