Performance Mods = FE Increase?!?
 

Okay ladies and gents, I'm looking for some feedback on a touchy issue. Performance modifications that lead to fuel efficiency increases. I'm wrestling with my theory on this and could use your input.

Lets examine a "high performance" exhaust system. Tuned equal length header, larger than stock mid-pipe, and performance axle-back muffler. Now generally speaking, 9-times-out-of-10, exhaust systems like this only increase torque and horsepower at max rpm's. Also, 9-times-out-of-10 there is usually a loss of hp and torque at the lower rpms, specifically at normal highway rpm's.

Now, lets say your Yaris, Fit, Prius, or whatever operates at 2500rpm at your highway cruising speed. Lets also say that your new "performance" exhaust lost 7lbs. ft of torque at that rpm. Since you have reduced available power you now have to increase throttle to compensate, thus reducing your gas mileage...correct?

I've been searching and searching for exhaust products that actually demonstrate a dyno-proven GAIN in torque at low rpms and I have found a few. I am hoping you FE scientists will confirm my theory.

If I increase torque at my 2500rpm highway cruising speed by lets say 10lbs-ft I now have MORE available power and should be able to use less throttle and therefor less fuel. Does this sound correct?

Your thoughts? I would essentially be increasing engine efficiency by reducing parasitic power loss.

Also...this is a "if money was no object" discussion. Yes, spending $1000 to gain 1mpg does not make much sense....I'm more concerned about the theory behind it.

Well not only do top end only enhancements dent the cars low end efficiency they usually make it slower overall, so they suck for performance too, dyno queen specials. About the only type of racing where they're any good are short circuit ovals where you're close to bouncing off the rev limiter all the time. There's many street racer types who are ill educated about this, and thats where these products make their money.

Even the reputable manufacturers just roll their eyes and go with it, for most mildly built V8s for example, 1.25 inch primaries will give a lot of mid range grunt, tarmac ripping, out accelerating ricers grunt, but the top end numbers don't look so hot... so joe average always wants the 2 inch header... which only starts being useful above the stock rev limiter... so the supplier rolls his eyes and sells it to him, the customer is always right. So it's a bit of a numbers game, they can always find some sucker to take the big ones, but shift very few smaller ones to people with a brain.

Edit: BTW I think it might be possible to utilise cheap eBay headers for FE and mid-range power by flattening them. It's the cross sectional area that matters, so flattening the tubing in the top and bottom dimension to about half the original diameter seems to come out about right, but you probably need exhaust wrap to make up for the relatively large heat dissipation area of them, vs custom made at the right size.

the way I understand it is that the reason you lose torque is because of the loss of backpressure. if you decrease your backpressure you will see a gain as long as it isn't too much. also remember that your cat gives you some backpressure as well. also think of free-er flowing system in general. most CAIs have a smoother flowing intake tube that is huge (some as loarge as 4") so if you free up the flow in the front end and the back end (assuming you have adequate backpressure) you should get better FE.

if money is no object, you can also get a custom grind on a cam that will give you much better low end torque. that one would have to be custom because it takes away from your top end which noone wants to sacrifice.

there was a thread a while back on a dodge forum about the first 12 sec normally aspired hemi (new 5.7 hemi) the guy ended up using a custom ground cam made for low end torque which took away from his top end. this gave him unbelievable take off power but he died at the end (top end sucked) he then put on some ported heads which gave him the top end back. there are a lot more things done to that truck than that but you see my point. this is just the abridged version.

I think it was metroMPG that actually changed out a camshaft into his metro for FE. he used the XFI cam shaft in his firefly.

you should check out his web site if you really want to get into fuel economy. he has done many experiments and looks at everything with a pretty open mind. www.metrompg.com

Nope.
Anyway, intake and exhaust systems that flow more freely shouldn't affect fuel economy on most vehicles unless you're running at high RPM all the time. The intake and exhaust have to flow reasonably well at redline, and therefore would flow very freely at the low RPM mostly used by members of this forum.

I removed the 1" or so stock dx exhaust, the highly restrictive OEM cast iron one piece cat/exhaust manifold and replaced it with a oem civic ex exhaust system with a 2.5" high flow cat. The ex header is a short primary 4/2/1 with a 2" collector,2.5" cat, 2" resonator and then drops to 1.75" into the muffler. it is designed to make midrange power and does. I picked up quite a bit of torque and high end while getting 1 ish mpg with no extra noise. $100 total, installed. you can get carried away and lose all sorts of low-mid power with giant intakes and exhausts if you pick parts that are designed to do that in order to make power above 5-6k.

I agree. By installing mods that only increase power in the high rpm region and sacrifice low-mid range power will definetely KILL FE.

My argument is that if I picked products that improved on low-mid range power, I would actually GAIN FE? Right?

I have obviously been misled on the whole exhaust thing, but the camshaft has been proven to help with FE as per metroMPG.

If you're still talking about intake and exhaust, I seriously doubt it. Like I said, if it's going to flow acceptably at high RPM/WOT, it should flow completely free at lower RPM.

If it's another modification, then it just depends on exactly what it is.

When you put it that way, it makes sense HolyCow. Unless I'm at full-throttle, it probably won't make a difference.

But I bet it would sound pretty cool. :eek:

Hmm...thinking about it a little more, I suppose if you went to something that increased EGV, or an intake with a different length between the throttle and the valves, you could conceivably affect FE.

not the THROTTLE BODY SPACER!!!!!!!!!!

well htink about this: if it frees up/creates torque then you can use less gas to reach X speed in the same time than it took without the mods.

yes intake and exhaust only really help at higher rpm (also sounds cooler/looks better under the hood :P ) it may help some at low rpm but don't expect a miracle/it to pay for itself...

It's about velocity and momentum, just because there's a wide open space doesn't mean it flows optimally at a given speed.

Think of regular sized cans of coke and mixer/mini sized cans of coke, and line up a table top full end to end, the mini sized are obviously easier to push lengthwise than the full size, they have less mass. Air has mass, we easily forget that, but at the speeds exhaust gases would like to escape the cylinders it matters. Put on pipes that are too big and you are having to push full sized cans of coke, they will get moving slowly and resist piston pressure, and the maximum speed you can move them is reduced because the pressure is spread over twice the area, whereas the mini cans will get moving quickly, and velocity will develop quick, quickly relieving pressure on the piston and evacuating the cylinder more quickly. Of course if you go down to too small a size then the wall friction is the limiting factor as for how fast you can push, that's when you "need" bigger pipes.

However it can be a touchy balance, most cars do benefit from opening up the stock exhaust size a little, like one size, 2-2-1/4 not 2-3" however on the header/manifold, you're usually better off going smaller. The greater volume after the collector is beneficial because the gases will slow down from losing heat energy and from friction, so having space for them to move slower yet still move the same volume per minute is the idea. But at the head, velocity is crucial.

The cost of the performance parts outweigh any money you will save via better mileage. I have a 2005 mustang with a high performance tune, cold air intake, high flow exhaust and 4.10 gears. My mileage in the city has not changed. I have yet to test it on the highway. I know others have gotten increased mileage with under drive pullies, and an alum driveshaft but both of those items cost is over 1000.00.

Well not necessarily if you pick them right... I was paying $65 for a generic muffler, they tended to last 12-18 months before rotting out. So I thought, screw that, I'll go for a semi performance one, looks a bit better quality, so I bought a Walker Sound-FX for $70.... wow, talk about a lot of difference for $5 bucks, uncorked the the motor nicely, began seeing 23mpg high tanks instead of 21 high mpg tanks, and it's lasted 5 years so far. So it's outlasted 3 generic ones at $65 a pop, and I've put about 60K on it, so it's saved around 113 gallons, at only $3 a gallon average, that's still $339... so $70 bucks pays $539, that's a deal right there.

Yes I totally agree. If you can find high performance parts cheap especially ones that reduce rotational mass or friction they will increase mileage. There a number of 500hp supercharged fox bodied mustangs getting 20mpg highway. In a mustang forum I frequent one guy went with a 4.6 to 5.1 stroker kit and boasts 30mpg highway.

That's the reason I got a lightweight flywheel for my car. It was only $165 from eBay. It takes less power to accellerate in lower gears, so I can shift at lower rpm and still accellerate at an acceptable rate in city traffic.

Hmmm, I maybe didn't explain quite so adequately why bigger exhausts from the collector back are more okay than having bigger header primaries. The situation is that in the header, the mass of gas in each tube has to be started from a standstill every second revolution of the engine, whereas after the collector where the flows combine, the flow of gas is more constant, I know you can feel the pulses at idle, but at higher RPM it's closer to a steady stream. Therefore it doesn't matter so much about the volume and mass of gas that has to be stopped and started. To go up the rev range quickly though, there is some limitation imposed by the mass of gas that has to be accelerated, but it's not as bad as starting it from a standstill every time as in the header pipes.