Matching shift points to get the most power from the 5sfe, a discussion about the 5s Options

[enderswift]

alot of people (myself included) go on and on about power modding the 5sfe when in reality theres really not that much to do except throw cams in or turbocharge. Because that sucks I figured I would try to squeeze out the most power from the 5s by shifting at the perfect rpm's. After mulling over stock 5s dyno charts and testing at the local warehouse lots, I figure shifts at 5200 get you the most power.

heres my reasoning. prepare for your eyes to bleed:

The power produced by an engine is defined by two units of measure, HP and TQ. Alot of people have a hard time telling the difference between the two. I like to think of TQ as the ability to overcome the inertia of mass, and HP as the rate of change of torque. So a high torque low horsepower engine wont accelerate very fast because the rate of change of its massive amounts of torque will be low. The benefit to this setup however, is either you can tow ALOT, or dont have to worry about having a heavy ass car. The way people make cars with this setup go fast is to use steep gear ratios where the torque muscles them to speed, like my friends mustang. The flip side is high horse power low torque cars READ: honda. With these setups the engine cant overcome alot of mass but the rate of change of its torque is impressive. Weight is the plague to these setups as the torque cant fully utilize the horsepower. The way people make civics (non turbo) fast is strip down as much weight as possible and multiply torque through gearing. The PERFECT kind of setup has HP and TQ distributed equally throughout the whole rpm range.
good luck finding/making one. all this being said the ideal TQ/HP balance on the 5sfe sort of occurs between 4600 and 5100. At this point the horsepower and torque are able to utilize each other better than at any other spot on the rpm range. To make up for our transmission gearing, shifting at 5200 brings the revs right into the meat of the 46-51 range. this = most power. observe:



This chart was borrowed from celicatech and cleaned up a bit for our purposes. I'm sure theres a member here who will recognize it

anyway our engines have a good amount of torque at low rpms, which makes the heavy ass celica feel peppy but the horsepower isnt there to really make ideal use of it. further up the HP starts to match the TQ and thats when the greatest acceleration occurs. At one point the HP and TQ are equal but because of our ****ty flowing head thats actually a bad spot to shift for. After 5200 rpm the 5sfe turns starts to wheeze out as the head wont let it flow any more air.

Im no engineer (yet) so theres probably a flaw in my reasoning somewhere so free to correct me politely or chip in some wisdom.

please tell me if I am completely wrong or not. this is as best as I could figure

This post has been edited by enderswift: Aug 14, 2008 - 12:06 AM

[enderswift]

what i meant by finding a car with equal tq and hp throughout the curve was just that. your bmw chart still shows two separate lines. im talking about one line representing both measures... THAT would be sick. I think the tesla roadster with its electric motors is able to come close....

either way, I agree with the idea that shift point heavily rely on gear ratios, but I still think 5200-5300 is the rpm to shift at simply because its the best balance reguardless of the gear your in.

why would I shift at 5400 rpm when at that moment the engine is producing 109 hp and 109 tq.Granted its the most efficient, but If you look a few hundred rpm back to 5200, the hp is like 108 while the tq is 120! in a heavy car like this the extra tq matters and since the hp is the same as at 5400, why rev higher and rob yourself of time? If this car weighed less I would say sure go ahead shoot for that extra horse, but the trade off of 10 ft/lb for one hp doesnt make sense. its better to have 108 hp working with 120 ftlb than 109 hp working with 109ft/lb. Again if this car weighed less horsepower would matter more, but in this case you want the torque.

This post has been edited by enderswift: Aug 14, 2008 - 12:37 PM

[Rayme]

what i meant by finding a car with equal tq and hp throughout the curve was just that. your bmw chart still shows two separate lines. im talking about one line representing both measures... THAT would be sick. I think the tesla roadster with its electric motors is able to come close....

either way, I agree with the idea that shift point heavily rely on gear ratios, but I still think 5200-5300 is the rpm to shift at simply because its the best balance reguardless of the gear your in.

why would I shift at 5400 rpm when at that moment the engine is producing 109 hp and 109 tq.Granted its the most efficient, but If you look a few hundred rpm back to 5200, the hp is like 108 while the tq is 120! in a heavy car like this the extra tq matters and since the hp is the same as at 5400, why rev higher and rob yourself of time? If this car weighed less I would say sure go ahead shoot for that extra horse, but the trade off of 10 ft/lb for one hp doesnt make sense. its better to have 108 hp working with 120 ftlb than 109 hp working with 109ft/lb. Again if this car weighed less horsepower would matter more, but in this case you want the torque.

You idea is flawed, its impossible to have a horsepower and torque curve following themselves. The Horsepower curves is calculated from the torque curve.

I know what you mean by "shifting to get back in the torque"...but you are shifting ! Even though the engine only makes 109 tq @ 5400 RPM, shifting to land @ 4200(You typed 5200 but I see you meant 4200) with 120 tq will results in LESS torque to the wheel because you just changed the mutliplication ratio by changing gear.

torque without counting final drive:
1st gear with 3.2 ratio @ 5200 rpm @ 109 tq = 348 @ wheel
2nd gear with 1.9 ratio @ 4200 rpm @ 120 tq = 228 @ wheel

that's why you should look at the HP curve to check your shift point.

This post has been edited by Rayme: Aug 14, 2008 - 1:09 PM