high rpm vs low rpm

[Execut1ve]

Hello all, I'm trying to figure out my strategy for modding my '94 F150. A lot of mods seem to enhance high end power at the expense of low end and vise versa. Unless I am mistaken, some items that fall into this category are exhaust (long tubes vs shorties), timing, and cam profile. Now my question: Is it better to enhance high end or low end if one is trying to get better fuel econ? It seems like the low end would be more beneficial for getting going from a stop and high end helps more at maintaining highway speeds. I imagine it'll come down to whether I drive more city or highway. I can't decide which is more important though, high end or low end, because the truck burns plenty of gas on both! advice plz

[George Tyler]

lower RPM is much like having a smaller engine, it's how much power is available to how much you are using, more so with a spark ignition engine than a compression ignition (deisel). Small Cylinders suffer from high heat loss to the head at low RPM so peak efficiency is at higher (but not high) RPM. I know the Chev 4.1L in my landrover peak torque was at 1600rpm, and peak efficiency would have been around there as well. For efficiency more cylinders is generally bad, all else being equal, but it seldom is. a V6 Estima gets around 10L/100km, and my 4 cyl one gets 15L/100km. that V6 is a lexus one, and it is a really good engine. Maybe an engine swap for a start? I don't think it lends it self to more mods though, it is too complicated.

[Frank Lee]

Low end for sure, less air and fuel going through the engine, less internal friction, less load caused by fast spinning accessories. You are correct in that most performance stuff is geared towards high rpm output.

[Execut1ve]

I'm not asking whether to drive in such a way as to keep RPM's high or low, if anyone's confused... I realize in general the engine operates more efficiently at lower RPM. But there's a difference between cruising through town at 35mph and ~1500 rpm and driving down the highway at 65mph and ~2000 rpm. I'm asking which provides more fuel econ benefit: enhancing low rpm performance or high rpm performance.

Frank I have definitely noticed that most performance mods for these trucks seem designed to give more high rpm power. Are there any to enhance low end torque? and which would help my fuel econ more?

As an example, I am under the impression that as far as exhaust, long tube headers enhance high rpm power at the expense of low end torque while shorties do the opposite. Is that correct?

[Frank Lee]

Originally Posted by Execut1ve

I'm not asking whether to drive in such a way as to keep RPM's high or low, if anyone's confused... I realize in general the engine operates more efficiently at lower RPM. But there's a difference between cruising through town at 35mph and ~1500 rpm and driving down the highway at 65mph and ~2000 rpm. I'm asking which provides more fuel econ benefit: enhancing low rpm performance or high rpm performance.

Did I not just spell it out for you?

Frank I have definitely noticed that most performance mods for these trucks seem designed to give more high rpm power. Are there any to enhance low end torque? and which would help my fuel econ more?

Offset cam keys and RV cam grinds come to mind first.

As an example, I am under the impression that as far as exhaust, long tube headers enhance high rpm power at the expense of low end torque while shorties do the opposite. Is that correct?

Bass-ackwards!

[Execut1ve]

Originally Posted by Frank Lee

Did I not just spell it out for you?

you did, but you aren't the only one who might read (or who has read) the thread. my remark wasn't directed at you

so long tube headers are more what I want? and it seems like I'd be going after gear meant more for towing/hauling since those guys want the low end torque, yes?

[Frank Lee]

Here's a more in-depth discussion:

http://ecomodder.com/forum/showthrea...peed-1477.html

[Execut1ve]

a very interesting thread. It reinforces in my mind that the engine has a certain RPM and load where it operates most efficiently. I had thought that in terms of what that RPM actually is, the lower the better. But that thread inclines me to think the RPM is more in the middle of the range.

However it does nothing to address my initial question: If my %highway and %city driving remains unchanged, which will provide the greatest increase in fuel econ: enhancing low end torque or high end power? In short, long tubes vs shorties?

If you've already answered this and I'm just being dense I apologize

[pounsfos]

so evan when your cruising on the highway your revs are only 500rpm more than they would be in town.

long tube headers going to a collecter
cam designed for towning (power from 1200 to 2300 or something like i'l let you sort it out)
GET SOME LONGER GEARS TO DROP THOSE REVS MORE
experiment with air intakes
dont forget those aero mods
oh and a ecu tune will do you good

or you could buy a smaller car bacause ive never seen the point of owning a big car like you americans but thats just me (not taking a swing at ya, just putting my 2c in)

[Execut1ve]

If I could afford a vehicle I would buy one. my other car is a civic I don't think regearing to higher gears is an option since it already struggles a bit maintaining highway speed, likes to downshift on all but the slightest hills.

so at what RPM does "low end" end and "high end" begin? If my "around town" and my highway RPM's both fall into the low end range then my question is answered. Is high end only of concern for folks running the RPM's up way high in racing or what not?