Performance Mods = FE Increase?!? - Fuelly Forums

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Old 07-31-2008, 11:05 AM   #1
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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.
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Old 07-31-2008, 11:17 AM   #2
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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.
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Old 07-31-2008, 11:23 AM   #3
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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
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Old 07-31-2008, 11:39 AM   #4
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Quote:
Originally Posted by BEEF View Post
the way I understand it is that the reason you lose torque is because of the loss of backpressure.
Nope.
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Originally Posted by dkjones96 View Post
Back-pressure is ALWAYS going to hurt performance and mileage.

People used to think back-pressure was what caused the increased torque and fuel economy and to get that they put in smaller pipes. The smaller pipes increased EGV (exhaust gas velocity) which is what really made increased torque. Small pipes with straight through mufflers are optimal.

That's actually a perfect example of why the automotive world is stuck with cars that don't all get 40mpg or more. No offense intended.
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.
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Old 07-31-2008, 11:52 AM   #5
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Originally Posted by Mightymouse13 View Post
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.

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.
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Old 07-31-2008, 11:58 AM   #6
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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?
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Old 07-31-2008, 12:09 PM   #7
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I have obviously been misled on the whole exhaust thing, but the camshaft has been proven to help with FE as per metroMPG.
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Old 07-31-2008, 12:17 PM   #8
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Quote:
Originally Posted by Mightymouse13 View Post
My argument is that if I picked products that improved on low-mid range power, I would actually GAIN FE? Right?
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.
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Old 07-31-2008, 12:19 PM   #9
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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.
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Old 07-31-2008, 12:23 PM   #10
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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.
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