Reading torque/power curves

[rickybobby23]

Out of curiosity, what should I be looking for with power and torque curves when looking for a good driveable engine on dirt? I have some curves on a couple engines I'm looking to buy but not sure what type of curve is smooth enough and in the correct rpm range for dirt racing. Anybody with any suggestions?

[perfconn]

Flatter the curves the better off you are.

[rickybobby23]

Is there an rpm range that the most power/torque should be found? Also it looks like both gain around 300-350hp between 5 and 8000 rpms. One gains 100ft/lbs, other gains 150 ft/lbs within the same rpm range. Is this ok, decent, bad or what? Both are steel 358s.

[perfconn]

No one particular number at a given rpm is important.What is important is that you don't see a big jump in hp or torque within a short span.

[let-r-eat]

Perfconn is spot on.You want flat torque curve Post up the sheets?

[rickybobby23]

I don't have the resources to scan them and get them on here. There are no spikes in the curves, the biggest gains in torque and hp in a certain span were what I described above.

[dualdj1]

Those guys hit it pretty well. The only other thing I'd say is keep in mind where you're racing. stop and go tracks benefit from more torque, long rounder tracks are more wide open and benefit from more HP. also, note where the power starts to fall off. if it begins to drop at say 8k, then you can probably push it to 85,8600 nicely. if it doesn't peak until 82,83, then you're looking at readlining higher, meaning more expensive valvetrain components to keep it together/rebuild. Not a dealbreaker by any means, just something else to consider.

[andy16]

there are alot of hidden little details that the normal person not even me could tell you about if you contacted a dyno operator they could. there are things like the fuel curve and i believe where the hp and torque numbers meet are also important although im not the guy who knows. theres alot more to it than two lines. ambient air temp watergrains timing jetting carb size fuel etc. usually the one that pulls the most fuel with the least amount of timing will be faster no matter what dyno says. drag racing guys are real good at seeing what matters on the dyno sheets they are able to compare what actually makes the motor more powerful to what the dyno says is more powerful and they dont always agree. its tricky to compare dynosheets from two differnt dynos about two different motors. A well tuned carb to a out of the box carb could be the difference in that 100 to 150 hp. are you buying total packages? if your not getting the exact parts that were dynoed its pretty much a guess. to me seeing a dyno sheet on a motor for sale says the motor is part matched and runs well and was taken care of. its like your race car scales at home every week my car weighs 2350lbs at home and every track i go to there scales my car is 20-50lb different.hard to compare unless weighed on my scales?? but if both motors were dynoed they both were prob well taken care of and fine pieces. hard to say?? i suggest contacting frank good he has done some dynoing for me and is very knowledgeable can find him on facebook. good luck!!

[50j]

You should find that hp and tq meet or cross at 5252 rpm. "Horsepower" is a function of torque x rpm/5252.

[Matt49]

You should find that hp and tq meet or cross at 5252 rpm. "Horsepower" is a function of torque x rpm/5252.

EXACTLY
HP is derived from torque and RPM as HP is a function of torque over time and RPM contains the "time" element. Where the 5252 comes from is complicated but HP ALWAYS ALWAYS ALWAYS equals torque times RPM divided by 5252.
I'm always surprised at how many people believe that torque and HP are somehow uncorrelated numbers.

With all that being said, a small spike in torque at the wrong time can result in a huge spike in HP which is what makes some motors hard to drive. The higher the RPM a torque spike occurs the more affect it has on HP.
I made a little spreadsheet with a graph that lets you plug in torque numbers at various RPMs and then plots the torque and HP curves. It's pretty interesting to play around with.

[andy16]

checked that against my sheet and yep thats true. hmm... didn know that. still alot of info in there i wish i knew more about.

[Matt49]

checked that against my sheet and yep thats true. hmm... didn know that. still alot of info in there i wish i knew more about.

This does about as good of a job explaining it as I could...
http://www.howstuffworks.com/question622.htm