View Full Version : Engine mods for lower torque
05-17-2008, 01:22 PM
Im just tossing ideas around right now, not really planing anything in the near future. What engine modifications can be made to get the motor to get better low end efficiency? I have heard of motors being "de-tuned" for fuel efficiency, but I am not sure what that is supposed to mean. What is "de-tuning" a motor?
Big things for me are:
a) How can I get the idle to be lower and more efficient?I have a manual transmission, so I don't need it to bear load at idle. If I could get a smoothish 500rpm idle I would be thrilled, assuming lower rpm=less fuel.
b) My Accord is factory tuned for max power/torque in the 4000-5500rpm range. I am quite willing to sacrifice that lovely top-end power for some low end efficiency. How? Can you narrow the throttle body? Wrap the exhaust headers to reduce heat lost and keep the gas flow high? Could I possibly find or order a cam that would focus my power/efficiency in the 1000-3500 range where I actually want to use the motor? Retard ignition timing?
c) Ignition timing, knock sensor, etc: I strongly suspect that when I am using heavy throttle in the sub 1600rpm range I am causing the knock sensor to engage and the ECU is retarding my timing (reduced efficiency?), because it feels like it bogs down a little bit. This 2.2L engine has plenty of power in that range for my purposes, but I suspect that this is causing inefficiency. Anyone know how badly efficiency suffers when the knock sensor kicks in, or what it feels like when it does?
05-17-2008, 03:31 PM
The problem isn't that motors don't make enough torque at low RPMs,they make plenty.There are two main problems.
1)The gearing is too tall for max FE in highway driving.Most V-8's are geared to do about 1800 RPMs at 60 mph in top gear.This is strictly top gear stuff-not a problem for your MT in the city.You can select your gearing to turn whatever RPMs you want in the up to 40 mph range-city driving.
2)The other problem-your's also-is that the motors have too little flywheel effect to turn low RPMs with high loads.The motors could make the power, but they would lug if you geared tham that tall. 5 liter V-8s could and do probably make the 40- 50 hp they would need to push the vehicle along at 60 mpg at 1300 rpms, but most would lug if they tried.
The engineers went for quicker acceleration and less flywheel effect,so they just can't gear too tall.
Bottom line it wouldn't matter much if you moved peak torque a bit lower on the RPMs band.The problem is gearing,and lugging. You probably can make the gearing about 10% taller without lugging-if you could fine the gears.It would drop your RPMs at 60 mph about 250 rpms.
A crude guess is that a 10% drop in rpms might improve mpg by maybe 2-3%.Engine speed kills mpg.This is probably one reason GM is planning to turbo charge the little ICE generator on the VOLT-keep the RPMs down(and size).A 1000 cc motor 3 cyl can easily make 140 hp without a turbo,and all GM needs is maybe 70 hp.70 hp-without a turbo-would probably require maybe 8000 rpms-with a Turbo maybe 5000 rpms-lot less friction.
05-17-2008, 04:54 PM
Thanks for the info Phoebeisis. Can you explain flywheel effect a little? Are you referring to the rotational inertia in the engine? Is it possible, then, that more rotational mass in an engine could allow better fuel economy by making lower RPM operation viable?
Also I was not trying to say that the engine doesnt make enough torque at low RPM. You are correct in saying it makes plenty, and then some. I accelerate just as fast as everyone else while keeping my rpm under 2100. I was just hypothesizing that power(hp=torque X rpm, so power is gained by favoring mid to high end torque) could be sacrificed for efficiency by modifying the engine in favor of low-end torque. For instance, I know that narrower throttle bodies generate better low-end torque but don't breathe well enough for high rpm, high displacement usage.
05-18-2008, 02:26 PM
I just joined this forum, but have lurked for awhile. I am more familiar with older V8's, but these principles apply to any engine. Generally speaking anytime you install an aftermarket camshaft it has to be degreed in. This means the valve opening, and closing cycle has to be in the correct relationship with the crank as dictated by the cam maker. This is not written in stone though. You can advance the cam (relative to the crank) for more low end power (torque goes up, peak hp rpm comes down), or you can retard the cam for more high end power. You can observe this effect by checking cranking pressure (doing a compression check) , as you advance or retard the cam. Cranking comp. will rise with the cam advanced. This is usually done with offset keys.
I am convinced that most street driven vehicles have there cams retarded (favoring high rpm power) just so the manufacturers can advertise higher hp numbers, than the competition.
In a street driven vehicle the more miles you have, the more a cam chain stretches, this "slack" has the same effect as retarding the cam thus favoring high rpm power, and in my opinion giving worse fuel economy. I don't know how much a cam "belt" actually stretches as it wears though.
Increasing ignition timing will usually result in more power, right up to the point that the motor detonates (pings) at part throttle. this can be observed by increasing the timing while the engine is idling.
I'm not real familiar with Accords, but for most popular engines there are many aftermarket "towing", or mileage cams available. They usually come with a disclaimer that the emmisions will be affected.
I've had many sets of headers wrapped with fiberglass cloth over the years, and think that it increases power, and efficiency, but ends up destroying the metal. Uncoated pipes turn into piles of rust. Cast iron will crack like crazy. Modern stainless exhausts holds up better, but are not indistructable.
A cheap, and easy fix is to install better wires, and or coil pack, and then increase the spark plug gap. MSD ignitions are popular with racers, and typically run .060 - .080 gaps.
05-18-2008, 05:23 PM
Thank you blokhead! So if I understand correctly, it is possible that an adjusted cam gear could improve fuel economy by making the engine operate more efficiently in low rpms. Yes I would agree that most motors are tuned for peak hp numbers instead of good operating power. I really wish cars came with sample dyno charts so you could see what kind of engine power you are getting into.
For ignition systems, I have always heard that manufacturers over-engineer their ignition systems so that they will perform sufficiently even when they are in poor condition, and that upgrading ignition is a waste of money unless you are making significant other engine modifications that make the bigger spark necessary. I don't know - MSD ignitions probably draw more current from the alternator too, which means that you would need significant power increases to justify the extra power usage.
05-18-2008, 06:44 PM
Yes, I believe that adjusting the cam timing would help FE. I'm kind of an old school hot rodder, but my muscle cars days are over now. I just picked up an 03 Focus ZX3, and want to see what kind of mileage I can get out of it.
There's a lot of people on the net who are modding these cars for more hp, but a lot of there tricks also get you more mpg. You just have to keep your foot out of it.
Along with the wire, and gap mods, and tire pressure increase, etc., I plan on picking up a set of underdrive pulleys for it. These lower the rpm that the accesorries spin at, resulting in more hp, and less fuel consumption. I might even try a long tube header set up, with a low restriction cat. conv.
I'm going to look into how much trouble it will be to advance the cam, but I don't even have a manual for it yet.
As far as the MSD goes, it require a LOT more juice to run, in fact if the battery voltage is low it will misfire.
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