The Dirty Bird!

Here's what I'm running right now:

1989 Firebird Trans Am GTA

Engine: Unmodified L98 (5.7L 350 TPI) with N10 option
Transmission: 4-Speed Automatic (700R4)
Suspension: WS6 option package

Aftermarket Applications:
[list=1][*]K&N Performance Air Filter[*]Dynomax "Super Turbo" Muffler: single inlet, dual outlet[*]Twin Dynomax "Super Converters": 3 in (7.62 cm) width per converter[*]3 in (7.62 cm) exhaust manifold[*]3.5 in (8.89 cm) Y-Pipe[*]MSD 6A ignition box[/list=1]

Gas mileage:

City: 17.8 MPG
Hwy: 22.5 MPG

Here's what my project is:

Basically, I want to maximize the fuel's effectiveness. I want to get more power than what I have without losing (and preferably gaining) MPG, and keeping it street legal all at the same time, plus I want to stay with a fuel-injected engine. I'm saving up money to do this, so in couple of years or so, cost shouldn't be that big of a deal. I’d also like a speed shop or other specialist to do this, since I’m one of those people that has more money than time nowadays.

I'm wondering, if I can increase the amount of air that flows through the car, from the intake all the way to the muffler, will that make for better use of the fuel, since larger (and cooler) amounts of air will come into play?

Here's what I’m looking at in terms of L98 upgrades:

Larger headers - Obviously this will improve air flow at the exhaust level, but are aluminum headers better than iron heads, despite their light weight, but inability to transfer heat as well as iron? Or, is heat transfer a moot point due to the fact that it's at the exhaust level?

Siamese intake runners – Manufacturer (Arizona Speed and Marine) claims that this can increase air intake as much as 45%. How true is this? Or is it a gross exaggeration?

Upgrade from 48 mm throttle body to 62 mm throttle body – Manufacturer claims that this upgrade can increase air intake as much as 25%. Is this true?

Single or Dual Cold Air Intake – This is kind of a duh thing. :P

8.5 mm Spark Plug Wires – Will a hotter spark from performance wires mean better combustion of fuel?

With all this in mind, I’ve also been reading about people that are doing things like LS1 swaps and so forth. I’ve also read that you can get ~28-30 MPG HWY from using the LS1 engine, and they apparently have 310 BHP right out of the box. I’m not sure if these kind of numbers would justify the cost of purchasing and having someone replace the L98 with the LS1 (or perhaps a more powerful fuel-injected GM engine?). Also, what accommodations would I have to make if I dropped an LS1 (or similar) engine? Would I be able to keep the bolt-on mods that I have already? Or would I have to lose all the mods I’ve done in order to accommodate the new engine? Can I keep my existing transmission, or would I have to purchase a new one? Or should I just keep the L98 and make modifications to it from there?

If anyone can correct (or add) any info or assumptions I might have made wrong, please do. I’m just researching to see what I can do for my car. I’d love to have more power, but I don’t want to sacrifice fuel mileage.

Thank you so very much for your advice.

DB

five7kid

iTrader: (14)

Not to flame, but why is gas mileage a concern? Money? Braggin' rights?

It is an important question to answer, though. You can buy a lot of gas for $6000 (or whatever an LS1 swap would really cost you). Probably the best bang-for-buck is good tuning and synthetic lubes in all of the running gear. Above that, the up-front costs will take a long time to recoupe.

I think your exhaust is overkill. You need big flow areas for high power outputs at high RPMs - not what you want to be doing for low fuel consumption. Sure, improving flow will help some, but head work and a cam change would do more for you than an over-sized exhaust. Same for the TB - for economy, the 48mm is the way to stay. AFPR to get the mix right. 19# injectors to better vaporize the spray (coupled w/higher pressure). Ignition to make sure it ignites cleanly. Stuff like that.

The Dirty Bird!

I'm just interested in how I can get best performance *and* maximize my gas mileage.

My reasoning? I just find it an interesting concept, that's all.

Dragula

Don't fall into the "bigger is better trap"! Not always do you want the largest size available. As for the throttle body, Max size maybe 52mm. You can make lots of ponies with that size. No need to go bigger. Siamese runners are good, but make sure to get a high flow manifold as well. Your system is only as good as the weakest link (stock TPI manifold!). Cold air intake good. High flow exhaust good. As for headers, 1 5/8" is good unless you want to run over 400hp (my opinion). Besides, you will have to pay $600 dollars for larger emissions legal headers. The only ones who make larger headers are SLP (see my previous post on this topic......I could find no one else). The 1 5/8" are sold by many...Hooker, Hedman, Edelbrock, and SLP just to name some. And get some better heads for the airflow. As for plug wires, not gonna see much there unless you get an aftermarket ignition as well. Might see a little, but not much.

T o quote five7kid "Not to flame, but why is gas mileage a concern? Money? Braggin' rights? "

By increasing airflow you will increase horsepower. Once you do that you will NEVER get better gas mileage........because the pedal is ALWAYS on the floor

five7kid

iTrader: (14)

Man, I can back up that statement! My mileage went from 18-19 commuting to 14-15 after my mods. Not because it was so much less efficient, just that those on-ramps are a great "test strip". If I stay out of it, I can get 17-18 (if mostly freeway), but that is sooooooo hard.....

PONTIMANIAC 90

I'm just interested in how I can get best performance *and* maximize my gas mileage.

My reasoning? I just find it an interesting concept, that's all


Any High Performance mod will give you a more efficient engine. Meaning your "foot" will have a greater range to "travel"
As far as your MPG concern, it all depends on your driving style.
If you got a "heavy foot" than you can make an economy car a gas gusler.
I have 4.10 w/a 5 speed. With a stick you can get really nice MPG, even with my set up. I can spend .5 of a second in 1st, than swich to 3rd and to 5th, during street driving. On my highway trips, of over 2 hours, I didn't notice any difference in fuel consumption as long as I kept it under 85mph. But since my Firebird is not an everyday car, whenever I do drive it, I go through every gear and floor the pedal every chance I get, especially on short highway trips, so my MPG is Very poor.
So in summary your MPG are directly effected by the way you drive.
Good luck
Mike

The Dirty Bird!

So it sounds like, if I can control the temptation to floor it, fuel usage shouldn't really be a big issue after the mods.

PONTIMANIAC 90

You got it.

jmd

iTrader: (4)

To increase mileage you want velocity, not more flow. One way to get velocity is to run small ports. Another way is to come up with a technically superior port (vortecs) that does both well.

Exhaust that is not restricitive up to 3500 will be better for mpg than exhaust that's not restrictive up to 6500. But it won't make your mpg that much better to be worth running crap exhaust.

Roller cams are great. Quick ramps mean short duration and good lift. This makes torque and mileage.

You should look into PROM tuning to increase mileage. And maybe under-car paneling to make the car more aerodynamic. And adjust shift points to put the motor in it's healthy torque curve as much as possible with a balance of pre-shift and after-shift.