performance and fuel economy

[crazycuda]

I had a good debate with a friend last night. I was hoping for some outside input, The conversation was about the LT1 350 chevy in stock form.
If you were to improve the flow on the heads (port) and make no other changes from stock aside from small tube headers I said you would also increase fuel economy too. If you were to drive the car sainly I think you would see an improvement in gas mileage. He said yes he agrees you would have a power increase but he is positive there would be a large loss of fuel economy.
Maby Im looking at things the wrong way but I always thought if you could improve the VE of any given motor you would also see an improvement in fuel economy. Just like if you reduce internal friction you would get a power improvement in return.

[Ron E]

I think during part throttle there would be no real difference as the VE (or lack of) is restricted by the throttle blades. The throttle position required to run, say 55 mph may be a little less due to the increased flow, but the net airflow will be so close as to not matter on the intake side. Maybe some increased mpg due to a free-ing up the exhaust side.
The improved VE shows up at WFO.

[67RS502]

Yes you will see a gain with the headers, 1-3mpg.
Porting may show a bit, because it will take a bit less throttle opening to stay (cruise) at the same rpm.

[Robert Kane]

If you improve the wet flow of a port/chamber you will see a definite improvement. If the flow numbers only increase and the wet flow characteristics remain the same, fuel economy will most likely not improve, since the A/F ratio would be the same and the more air induced would carry more fuel. The engine would not be more fuel efficient. If you raised compression alone, you would see an improvement. You need to improve the burn quality inside the chamber i.e. better charge motion, or higher compression to get the BSFC's down.
Robert.

[crazycuda]

thanks for the input. Heres a far fetched question, what would you guys do to that same motor to improve it's fuel economy. Lets use a stock Impala ss as a base line, and assume its been converted to alum heads , stock L460e transmission and 3.73 gears.
I think better exhaust would help. Larger mass air meter and any other intake restriction improvements. I don't know if a greater rocker ratio would help economy but it would help on the hp end so Im ify if that would be a worthwile improvement in the intake side rockers. Of course headders, and low restriction cats.

[OldSStroker]

thanks for the input. Heres a far fetched question, what would you guys do to that same motor to improve it's fuel economy. Lets use a stock Impala ss as a base line, and assume its been converted to alum heads, stock L460e transmission and 3.73 gears.
I think better exhaust would help. Larger mass air meter and any other intake restriction improvements. I don't know if a greater rocker ratio would help economy but it would help on the hp end so Im ify if that would be a worthwile improvement in the intake side rockers. Of course headders, and low restriction cats.

Put the 3.08s back in for highway mileage. It's like another .83 OD, and worth about 400 rpm @70.

Hopefully you raised the static compression ratio when you changed heads. That helps mpg as well as power.

Unless I was doing a cam and probably a converter, I'd keep the exhaust manifolds and use a 2-1/2 catback like the one from Dynomax with Ultraflow SS mufflers. They outflow the B**** brand. Those manifolds are fairly good for the power you are making, and Imp headers really heat things up under the hood. Some of that gets to the incoming air which drops power and mpg.

I don't think rockers would be much help, but it you are thinking a cam (not for mpg, but for power) in the future, you might check with your cam designer about what rockers to get. My cam guy likes ProMags.

I like the K&N FIPK. Larger MAF isn't helpful until you are making more power, and I doubt if it helps cruise fuel economy.

Approaching 100K on my Imp. A little long of tooth now, but it still does fairly well on mpg. I kept the 3.08s, and have the other things mentioned. Haven't gone to aluminum heads yet, but for it's 10th birthday, I want to give it some Lloyd Elliott LE1 or 2 heads and a very special cam. It still doesn't use a drop of oil. Mobil 1 has been in it since the first oil change.

My $.02

[panic]

Porting = reduced pumping loss = more mileage at all throttle openings.

[OldSStroker]

Porting = reduced pumping loss = more mileage at all throttle openings.

Perhaps, if the porting was done to optimize low flow, throttled conditons found during 30-50 hp cruising. As you sure that the porting that works for max power also maximizes the low-flow condition? How many head porters even test at those flow conditions?

Aren't pumping losses for a 355 V8 at 2000 cruise around 5 hp? It seems to me that even a huge decrease in pumping losses at cruise rpm would hardly be noticed in fuel economy. What am I missing here?

[Ron E]

Not to argue the earlier wetflow comments. There could be some help with that. But, on porting (talking intake, here) The motor is throttled by restricting the inlet flow. Assuming the same engine, same load, and the same RPM/MPH, the better heads will have to be restricted a little more (with the throttle blades) or you're going to get a ticket, right? So, intake side pumping losses at a given MPH, and all else equal, need to be the same to achieve the same speed. Or, so it seems to one who has never spent 30 seconds thinking about part-throttle performance.
Now, improved exhaust would reduce a pumping loss that would allow the engine to do a bit less work (at any RPM)and offer up some potential MPG, it seems to me.
Make it happy at lower RPM and pull a taller gear. I went from 3.08's to 3.55"s a few years back and lost an instant 1.5 MPG. Even though a bunch of "experts" told me it wouldn't make a measurable difference. Thats like an extra $1200 a week these days.

[Joe Mendelis]

If you have a stock head and you foo foo the short turn, valvejob it with something the makes it flow 30 cfm better, I bet it would get better gas milage and make more power. (regardless of wetflow)

I used to have a 1990 mustang GT. Stock with 2.73's and only a cat-back system it got 27-28 MPG. I built a stock bottom end 302 with Windsor Jr. heads (bigger and better flowing than stock), a 73 mm mass air meter, a 224 HR cam and a GT40 manifold. I put 3.55's in it and a Tremec TKO with the stock OD ratio. It Still got 28 MPG!

A few years later I built a 347. Same gears and tranny. Heads ported from 206 @ 500 to 262 @ .500 and 223 @ .700 to 280. Volume only increased from 179cc stock to 186 (most air was in the valvejob and chamber). I used an XER282 cam and same manifold (temporarily). The car then got only 23/24 MPG.
The heads were obviously way more efficient at moving air through the given space. The cam was way better than the junk FMS cam, but 8 degrees bigger at 50 with 2 degrees less seat timing. Where did that milage go? the fuel economy dropped a lot more % than the piston speed increased, and I had thinner rings with less tension on the 347.
My best guess would be that the engine was throttled a lot more with the 347. The engine rpm on the highway would be the same. Now it is trying to draw in more air, so the throttle position is closed more to sustain the same rpm. The increase in piston speed made the 2nd wave tuning point lower as well, so maybe I was out of that sweetspot.

Bigger engines are usually less efficient in a MPG case because the BSFC just goes up the more you have to throttle it. You also have more friction in the bottom end.

[lil289]

Joe,

How do you calculate your gas mileage?

[Ron E]

Joe, mine was a '90 Mustang also. Dead stock, w/ cat-backs it was good for hi-22/lo-23 MPG w/ 3.08's. The 3.55's hurt that about 1.5 MPG. Adding adding a little (9lb boost) centrifugal blower /I-cooler (depending on your foot) made no measurable difference in MPG.
Next car was a '93 T-bird 5.0 w-2.73's. Even with the AT, and an extra 400-500 lbs the avearge hovered around 24. All this through what has to be about the most pathetic excuse for an exhuast system ever offered to the general public.

I don't think anything will stretch your gas better than setting up cam,converter,etc to make it more comfortable at a lower RPM, and gearing accordingly. But, that's not the point of the thread (just throwing in due to current gas prices)

Is there a gain in intake-side porting (increased airflow, only)?, maybe. It still seems as if it would require more throttle induced restriction at a given load, so as to maintain the same net flow (I.E. RPM). Unless the "quality" of the mixture is improved greatly, chances are you'd have to pay close attention to notice the change, while there are substantial increases to be had on the exhaust side.

(I still don't know how the T-Bird even cranked, much less got good numbers with that exhaust)

[SupStk]

I've watched this tread with some interest hoping to learn something I've missed over the years of building engines. I've seen cases where the focus was on street performance and the end result was engines that made better than expected MPG. I've also had combinations that made good mileage for myself but when trying to duplicate it for someone else the end result wasn't up to expectations. I've pretty much given up on promising anything for fuel economy, if I do it seems to bite me every time. I know there are loads of variables such as fuel quality, weight, engine tuning parameters, driving habits, drive train losses, aerodynamics, ect that all effect MPG's. It makes sense that it takes a certain amount of fuel to do so much work. The economy is made by minimising losses.

[automotive breath]

I have been following this thread fo the same reason hoping to learn something. This is a subject that interests people very much with the price of fuel on the rise. I believe it is possible to achieve performance and economy but it's a very delicate balance, get one part of the equation off and mileage drops.

Here's some things I've learned:

BSFC increases at high engine speed due to increased friction
BSFC increases at low engine speed due to increased time for heat losses to the piston and cylinder wall
About 9% of the fuel supplied to the engine is not burned during the combustion phase
Improve combustion efficiency by reducing heat loss, increase flame velocity and turbulence intensity
BSFC decreases with higher compression due to higher thermal efficiency
Reducing frictional losses improves mechanical efficiency
Throttling increases pumping resistance thus mechanical efficiency decreases, at idle mechanical efficiency approaches zero


Some very good points were made on this thread.

Improving volumetric efficiency should show small improvements in mileage. Porting on the intake side should be limited to reducing turbulence in the flow cone and maintaining port velocity. Most of the improvements will be found at the valve seat and 1/2" before and after. The goal being increasing cylinder filling at part throttle conditions. If the volumetric efficiency is very low at cruse RPM exhaust gasses present in the compressed charge will impact combustion quality.

I believe we all agree a good flowing exhaust is essential. One thing that hasn't been discussed much is camshaft design. With a good flowing exhaust and increases overlap raw fuel will be pulled out of the exhaust during overlap. Continued improvements in fuel efficient performance camshafts are essential.

If you raised compression alone, you would see an improvement. You need to improve the burn quality inside the chamber i.e. better charge motion, or higher compression to get the BSFC's down.
Robert.

As Robert suggest for continued improvements in fuel efficiency we will need to focus on burn quality. Increasing compression ratio, eliminating detonation. Once the overall combination is set up for optimal fuel efficiency additional gains will be realized by improved combustion and thermal efficiency.

[crazycuda]

I've noticed no one talked about fuel injector size or fuel pressures. I am guessing a smaller injector run at higher pressure would help the fuel atimization, which I would guess would help limit the amount of unburned fuel being exhausted.
Why I say this is the wifes car has a tpi 350 in it. I had to change the injectors and I used the 305 21lb injectors instead of the replacement 24lb (no one had the 24's in stock at the time). The car had a bad lean area until i raised the fuel pressure to 50psi instead of the factory 45. What was interesting is the car ran better and also got better mpg (approx 3 miles per gal). Maby it is just a concindence or her old injectors were just that screwed