Horsepower

[L B0MB]

Power is the answer Tom W

Torque is not the driving factor as it needs to be provided at a given road speed which if you work back through the gearing will give you a crankshaft speed. Crank torque time rotational velocity equals power.

Talking in power units removes any confusion of gearing in the drivetrain.

A lower power engine will use less fuel but will be slower... IF the track is open enough that a high percentage of time is spent at max power and the lower weight does not provide sufficient gains in braking and cornering

[flavorPacket]

Why don't you guys just run a 90 bhp 4-banger on two cylinders and find out by testing?

[VFR750R]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by L B0MB:
Power is the answer Tom W

Torque is not the driving factor as it needs to be provided at a given road speed which if you work back through the gearing will give you a crankshaft speed. Crank torque time rotational velocity equals power.

</div></BLOCKQUOTE>

Agreed.

Power is most important. Torque by itself is almost meaningless. A flat torque curve is often misrepresented as powerband width which is not the same thing. If you have a wide gap between gears, poor driver, etc., you need a wide powerband, not wide torque band.

For the doubters, create a rear wheel force vs speed graph for your engine in each gear plotted against a curve of constant HP equal to your max HP. The graph will tell you where to shift

[J. Vinella]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by VFR750R:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by L B0MB:
Power is the answer Tom W

Torque is not the driving factor as it needs to be provided at a given road speed which if you work back through the gearing will give you a crankshaft speed. Crank torque time rotational velocity equals power.

</div></BLOCKQUOTE>

Agreed.

Power is most important. Torque by itself is almost meaningless. A flat torque curve is often misrepresented as powerband width which is not the same thing. If you have a wide gap between gears, poor driver, etc., you need a wide powerband, not wide torque band.

For the doubters, create a rear wheel force vs speed graph for your engine in each gear plotted against a curve of constant HP equal to your max HP. The graph will tell you where to shift </div></BLOCKQUOTE>

Now take that plot and add the max tire force you can produce. Then see how much HP you can get away with and at what gearing.

[Kirk Feldkamp]

Power shows up as a 1/X function on a vehicle speed vs. thrust force graph. Your gearing shifts and scales the gear cascades along the "iso-power" curves. The power output at any given RPM does not change with changes in gearing.... but the forces do. The difference between the vehicle drag curve (make some estimates and plot it) and the thrust curves is the sum of your forces that accelerate the vehicle (duh). It doesn't take a whole lot of time to figure out that either a 'heavy' 4 cylinder car or a 'light' single car can be traction limited for most of the FSAE competition speed range by playing with final drive ratios. Within the traction limited portion of the speed range, you will not realize any more acceleration if you add torque or power through changes to your engine or gearing... you just don't have the ability to put down any more force than the tires can handle!

The trade off you make with your gearing is, of course, vehicle speed for a given engine RPM. Given that FSAE courses are *generally* pretty tight and therefore slow, the speed range you care about is relatively small (compared to even a street car, and especially a "real" track/race car). In other words, if you can find an engine that cranks out some RPM but doesn't make a lot of juice, you can probably end up being traction limited (or close to it) for a significant portion of the speed range you need at comp just by playing with the gearing. The offroad 250 singles make about 34hp and 19ft*lbs and rev out to ~13000rpm [stock], right? Now how about modified a bit to add some top end? How about a UTA-style turbo 4-cylinder 250 (low torque but high power) revving to 19,500rpm? Some quick calculations can tell you what the realities and possibilities are for these, or any other, engine package. Make sure you are taking into account the weight differences between the different setups.

What are some maximum top speeds you top-team guys have ever seen at a FSAE competition in autocross or endurance?

This is all a very non-standard line of thinking, and for very good reason. As soon as you step outside of the realm where the tracks are super tight and low speed, the amount of time you spend outside of the traction limited speed range increases significantly (hence my earlier question about throttle position histograms). In that case, I'll take more power and speed range nearly every time. Within the FSAE realm, I see an opportunity to decrease engine displacement (to a point) without suffering too much with regard to performance. Furthermore, because the economy score has such a direct impact on the points earned equation (10%, right?), the perceived performance losses you get by reducing power and torque are likely more than made up in the economy calculation. The indirect effects of going to a lighter package (improvements in tire wear, lower inertias, etc.) permeate into other areas, but most of the points improvement will likely be seen in autocross and endurance where having an ultra nimble car (transitionally) that doesn't fall off can be a huge benefit.

In the end, this is all an optimization-based argument. Can you get the same amount done with less? This is "engineering" in a nutshell... figuring out ways to meet or exceed goals with the least amount of resources as possible.

-Kirk

[VFR750R]

http://www.sportrider.com/news..._revealed/index.html

For those without budgets, the best FSAE engine ever may have just arrived.

[Homemade WRX]

Oh an engine debate is always fun, especially when it starts with so many variables with different cars (and DRIVERS) but only power and weight are only a small part of the picture.

I'm with Kurt as a big fan of the 250 4-cylinder engines. Hence I had purchased 3 of them for my senior project. I'm actually going to pick up 3 more MC22 engines to turn into my own fun project and build on my existing 92 hp platform. Looking to build a personal FSAE car as I miss driving them

There will always be people to argue for a larger displacement engine being better because of the torque. I'm a fan of usable RPM, gearing and weight. Complexity does play a role, especially with limit funds and knowledge/ability. Those last details can be a real issue for a young or novice team.

[nowhere fast]

I'm not sure if that MV engine will actually end up being 600cc, if it was many potential customers would just buy the Daytona 675 instead.

[Mikey Antonakakis]

Yeah didn't that link say their sources tell them it's going to be 675?

EDIT:
Okay so if you don't have a budget I guess decreasing that displacement a bit is a possibility.

[RANeff]

MV Agusta... *drool*. I was hoping that engine would be 600cc for a while, but looks to be untrue. I suppose if you have enough m oney to buy the enigne, you should have enough to reduce displacement...