So I will have about $1900 to spend on mods on June 1st. My goal is 400+ rwhp and a 120+ trap speed. Here is the problem though...I love the MPG my car gets right now. I am seeing low to mid 30's on the highway, while my Dad's basically stock c5z is only seeing mid 20's tops.

My plan was to do stock 243's, ls6 intake, medium sized cam, and shorty headers. Will this significantly drop my MPG on the highway? My other plan was to go a little more aggressive and put AI or Tea ls6 heads/ls6 intake on right away, then wait a month or two on the cam.

My GTO, when stock would get 26-27 mpg on the highway...after doing a fast intake, headers, and a healthy sized 230/232 cam, I could only get 22 mpg TOPS out of it. Around town was atrocious in that thing too, below 10. I just don't want to repeat the same mistake, but I just drove my dads C5z and I need that power LOL.

The car is a stock 99 FRC.

 

Messing with gearing/converters/camshafts will diminish gas mileage. I am bolt-ons with 3.90's but I can still get 19-21mpg in the city. 331rwhp/343tq. 11.80@118. A trap speed of 120 can be hit with less hp than 400.

 

Long tube Headers, 3" X pipe, vararam & a tune. My MPG dropped dramatically over stock.
I've had car since it was brand new.
But with these mods, it made the car quite a bit quicker.

 

When I picked up mt vert I got 30-32 at 85 on the road. Headers, 3" dual exhaust, AFR 205 heads and a comp 232-228 cam, 3:42s, and a 3500 stall converter dropped the mileage to 26-28 at the same road speeds. Supercharger dropped it to 24-26. Biggest drop has been the greenies fuel fiasco. Mileage is now 22-24 with the alcohol crap at most all of the stations.

 

That sucks, not sure I want mine to drop that much. I am doing the full AI heads/ls6 intake/shorties first. Not expecting a ridiculous power gain, but it still should be at least 30-40 rwhp I imagine...and I am hoping my MPG doesn't change since the cam will stay stock for now. I will do the cam in another 2-3 months funds permitting.

I just don't want to put stock 243s on with a cam, make 410 rwhp or something, then get bored in 6 months and need to pull the heads again, so I am doing the port work now to save myself from doing the labor twice.

 

Same as above --I have a 98 A4 with approx 400 RWHP --and still get 27-28 on the frwy at 70 MPH--
The biggest culprit that affects fuel milage is LT headers--If you stick with just shortys it should be great for milage--LT's move the 02 bungs further back than stock---shortys have them in the original locations
Having the bung locations further back gives your ECM false data via your 02 sensrs--typically it will make your closed loop fueling (P/T) richer than the commanded stoich of 14.68 to something richer lin the 14.2 range
More fuel !!
No one realy knows how to 100% correct the fuel back to 14.68--But I've seen some good results by:
Intstalling your REAR 02's onto the front locations ( the rear 02's are more sensitive and heat up better)
Lowering your "02 switching point table" try taking out 100-200 points
Trick your ECM in by commanding something leaner-like asking for something like 14.9 in closed loop to end up with 14.68
All these changes need to be verified with a Wideband 02 sensor to get your actual AFR readings

 

My STOCK 02 ZO6 could easily get 30 MPG highway before I started modding it.

I agree that a LOT of the fuel mileage has to do with proper tuning! I have a fairly heavily modded 02 ZO6. Patriot Stage II heads, Home Ported FAST 90/90mmTB, VARARAM, EG Whiplash cam (240ish/ 0.610 lift / 114 +4), Stainless Works 1 3/4" Long Tubes, high flow cats and a 3mid pipe/cat back exhaust system. Its a great high RPM cam. Makes 450 RWHP.

On a LONG trip with luggage and two people, I manage 25-26 if I keep my foot out of it. If I had a cam more suited for midrange, I would probably get 27-28.

Cant ever see getting the old 30 mpg back. Its too much fun hammering it!

 

With my stock 02 Z with headers, and a CAI, I only got 25 MPG on mostly highway driving. ( untuned) After adding TFS heads, milled to 61cc, and a 228-232 cam, I got a consistent 27 MPG driving basically the same way. I credit most of the mileage gain to the raised compression. I haven't checked it in a long time, due to lots or self-induced issues.

 

I understand your conflict with wanting more power but yet maintaining good economy. The good news is that with the C5 it is possible to be a "cake eater", although you will have to compromise economy a little depending on how much power you go after. The good thing is some mods actually increase efficiency (i.e. less restrictive exhaust, cold air intake) which can increase fuel mileage. When you start adding larger port heads and a cam with valve overlap is when your mileage will drop, but it's not that bad if properly tuned.
For the 113k miles that I have driven my '99 FRC, I have always logged my mileage at every fill-up, so my mileage isn't based off the DIC. I saw a best of 33 mpg on the interstate @ 70 mph with long tube headers and a Vararam intake. After adding a FAST manifold, 224/230 cam, AFR 205 heads, YT rockers, underdrive pulley, LS7 clutch and putting down 460 whp I recently logged 28 mpg on a 600 mile interstate run to Orlando - and that was with 220k miles on the stock short block. I routinely get 23-24 mpg on my daily commute to work (mixed city/hwy). So yeah, it dropped 4-5 mpg when I added the ~130 whp, but I'm OK with that... By the way, I have also spent quite a bit of time with HPTuners to optimize the tune and get back as much economy as possible. Recent changes to injector timing look promising to increase mileage slightly.

 

No such thing as a free lunch. Inevitably, there's a tradeoff between power and fuel economy. My F1C supercharged 427 is only good for around 13 mpg around town, but the carbureted Ford 427 in my old (real) Cobra got under 10, and with no more than half the power.

 

Thats great...so I guess it is all in the tune. I have noticed a trend in this thread that everyone complaining about MPG have LT headers. I wonder if it does have something to do with the 02's being moved.

What about raised compression? I will probably have my 799 heads milled .030. Also, will stock injectors support these heads on the stock cam? They are 26 pounders I believe in the 99's.

 

No idea on my HP, but I did vararam, power duct, magnaflow magnapack (w x-pipe), diablo sport tune, and I can get 34-36hwy!

 

Ya my car came with a vararam and vortex powerduct. With whatever oil was in there after I bought it, I was seeing mid 30's on the highway. I switched to german castrol, which is a little thicker, so I am only seeing lower 30's...but I like the protection of the german castrol. What is odd is that my dads car has the same vararam, with everything else being stock, and his MPG sucks. Maybe it is his tune.

 

Stock 26# injectors will be max'd out by 380 whp.
I'm not totally buying into long tube headers hurting mileage. Long tube headers were my second mod after the Vararam. Like tblu92 says, there are tuning tricks to help maintain proper AFR. I have my engine running in open loop up to 1200 rpm, with a commanded AFR around 16:1. Once engine speed goes above 1200 it returns to closed loop and the ECM maintains 14.7.

 

What do you recommend? I don't want to spend too much on injectors just yet..

Can I just step up to the 28 pounders? Again, car will just have ai ported 799s and an ls6 intake to start..MAYBE shorty headers, but from what I am reading, they seem to be a very small gain. I estimate the heads and intake will give me 35 hp.

 

Not all long tube headers move the o2 sensors further back, I have Kooks and they are in the stock locations.
I also agree a lot of the milage is in the tune and the way you manage the right foot.
Last fall the wife and I went to Sandestin for the Corvette show and on the way there I set the cruise on 70 mph and got 31 mpg.
On the way back I played a bit and it dropped to 25 mpg and this is with 600 rwhp & 3.42 gear through a stalled A4.
Oh and H/C also.

 

Yeah true, depends on right foot for sure. I was just assuming everyone was cruise controlling and watching the DIC.

 

I like this thread. Hope that others continue to chime in with their mods and mileage results. I'm in the same boat as the OP, I'd love to get a few more HP, but, only if I don't have to give up any of my MPG. Given a choice between HP and MPG I'll take the MPG. The C5s have plenty of HP for most of us right out of the box.
Drive safe!!

 

I will post my results when my heads are back June 1 . I will be the guinea pig for doing it ***-backwards (heads before cam).

 

The physics/book logic would say that anything you do to increase the efficiency of the airflow in and out (intake, heads, headers, reduce cat restriction, etc.) should improve fuel economy at the same speeds and throttle inputs on the same fuel/spark map. Changes to gearing, cam, injectors, and fuel map (adding fuel) will tend to decrease fuel economy.

The major issue that makes the book logic problemmatic is that when it's got more power, you tend to play with that power more.

Having a more conservative tune for street driving and a more aggressive tune for HPDE/drag/autox/etc. is how some guys get around this trade-off. I've always been amazed at how many guys sit trackside with their laptops installing and playing with their tunes before a run.