1985 GMC C2500 (New to me)

[wackypez]

Hello,
I just recently purchased a pretty clean 1985 GMC C2500, it runs good and the body is straight with recent paint, except a faded hood it looks great. Its solid but I can't leave well enough alone and come seeking advice on upgrades and modification. I’m not really concerned with the aesthetics yet mostly because it looks good and I want to keep it as period correct, as convenience will allow, I intend to use it. I have briefly researched some upgrades I want to perform and would like your feedback about whether they are worth while and what order I should go about it?

Pertinent info = 1985 Sierra classic, 350, TH-350, 4:10.

As purchased mods = Flamethrower HEI, Relocated intake to draw off grill, Bullet exhaust "cat" back system (I had to have a cat installed as it would not pass smog without it and the high flow model made no noticeable difference in current output and drastically dropped my emissions which gives me alot of room to play with mods and still be legal here in California).

Planned mods
1. K&N filter (what kind do you recommend? keep stock box or open element?
2. Tachometer, any recommendations?
3. Flex-A-Lite 295 Electric fan ( my fan clutch always seems on )
4. Edelbrock CARB legal shorty headers

This is where I get undecided, should I wait till engine shows fatigue (it may already and I just don’t know) and needs to be replaced or wait to upgrade it when/if I need to replace the engine? Either way I want to put the below on either a GM Goodwrench 350ci / 260hp (any other sane recommendations?) or the 22 year old 350ci / 190hp that appears to be solid right now.

Edelbrock Performer 600CFM carb
Edelbrock Performer manifold (Vortec)
Edelbrock Performer-Plus Camshaft Kit
GM Performance Parts Cast Iron Vortec SBC Heads

Any input on the transmission? I would like better gas mileage and highway cruising at a lower RPM but I have the TH-350 is a decent transmission?

Thanks

[Mickey_D]

Welcome to the boards, pez!!

Having much the same truck (except mine's 305), I would have to say that the first thing you should do to increase highway mileage is ditch the insanely high gears in the diff. I think I've got roughly the same gears (if not higher) and mine wraps out at about 78-80mph. And I only get around 9-14mpg (on the highway) because of it.

Other than that, that is one hell of a nice looking truck, and if she runs anywhere as good as she looks, you got a keeper there!!

[bb1979]

Leave the diff gears, put a T700 or better yet a gear vendors behind the tranny which should increase the mileage because of the o/d

[cochino12]

good looking truck, how is the interior?

[Pyrotechnic]

Sweet truck!

If you want the ultimate in gas mileage, I would recommend a QuadraJet carb. Edelbrocks can get decent mileage but require careful tuning to do so. They come from the factory with a very rich setting.

[wackypez]

Thanks for all the compliments and advice. I realize its a big 3/4 ton truck that will probably never be fast or get great gas mileage. It moves itself around respectably and I have no doubt it could competently tow a decent load but I would like a bit more pep and if I can improve the 8 or 9 MPG it currently gets to 10-12+ I would be a happy camper. I think OD or 4 to 5 speed tranny would be a big help as it really has to rev to keep up on these southern California highways where traffic flows at 80. The below are the interior and engine which are both pretty clean. The truck was white but the previous owner painted it the two tone blue and gray it is now, its a rust free, bondo free southern California truck that has 107000 apparently well tended miles on it. The dash has small enough cracks that it doesn’t need to be covered but it came with the dash mat so I left it on there. The drivers side of the seat is the only area that shows a little wear but it’s not bad so I put seat cover on it to protect it until I get it re-upholstered. I don’t really like the vinyl so once I get the engine and transmission sorted out to my liking I will probably have the seat dash and doors covered in an appropriate color durable tweed. Its not going to be my daily driver but I do intend to use it as a truck and honestly its more fun to drive around than my Camry or the wife’s Mini van so I’m sure it wont just sit in the driveway to much. Does anyone have any experience with the mods I listed as intending to do? Any input would be much appreciated.

[Ivan D.]

Wackypez, seems like you have yer plans straight. On yer gas-mileage question, I had the same problem till not too long ago, my 350-equipped 1/2-ton with 33" tires and 3.08 rear end wouldn't get anything more than 10mpg, and that's with a decent amount of tail wind. It now consistently pulls about 12-14mpg at 1800roms in 3rd (about 50-55mph), and it's got tons of power, too - solution in my case was replacing the power valves in the double-pumper with softer ones (open earlier), and switching the distributor advance to manifold vacuum and turning the idle rpms down. However, to get such decent fuel economy I really found two things useful - my Holley is a spreadbore design (like a Q-jet, small primaries and huge secondaries), and I am very very easy on the throttle.

So yeah, I'd agree with the recommendation about the Q-jet - stay with that, it's not inferior to the Edelbrock in any way, just send it to a good carb-shop to have it rebuilt based on yer truck's parameters. There was actually a thread about that in the Engine & Drivetrain area yesterday, read it here: http://67-72chevytrucks.com/vboard/s...d.php?t=243408

On the gears suggestion - seems to me yer tires are at least 33", which is larger than the stock 29" so yer 4.10 rear end is effectively more like 3.7-3.8 now. Also consider the way of driving - if truck sees mainly freeways then sure, go ahead with a 3.4 and smaller gears, but if you have do lots of stop and go the numerically small gears will kill you - it's harder on the engine to get the truck up to speed, so it burns more gas. So yeah, for a city-driven truck I'd stay with the 4.10 gears, as a matter of fact I'll be installing a set of 4.10s in my 4200+ lbs Lincoln landyacht for the very same reason - it' will haul ass in the city, and on the freeway I'll just drive slower

[1Ton]

Nice truck. I want to find a clean C30 someday.

[wackypez]

Is 8 MPG normal or should I be looking for a culprit? Does anyone have experience with an air fuel meter installation and use? They have several rangeing from $20 to $300+ narrow or wide band being the main difference.

Thanks

[boatpuller]

nice looking truck.

[bigblock79]

welcome nice truck

[magnethead]

wacky, my truck get's 7 or 8 MPG with a 454 and 7000 pounds of curb weight. My friend with a 305 C10 with mudding tires get's about 12 MPG. Looking at your truck, 9.5-10.5 MPG would be about most you can get. Stick a quadrajet on there first and see where that puts you, that's be the cheapest thing probably. As for overdrive, depends how much your truck weighs and if you intend on trailering. The T700R4 isn't the strongest thing in the world with heavy loads, but first gear is lower so you'll get a little better power curve in acceleration. Retrofit wise, your truck is an 85 so you should just have to source one, deal with the TV cable and what not, and move the crossmember to suit. And if the truck is in good condition, i'd say don't go manual, just to keep it in good condition.

[MylilBowTie]

What kind of rims are those? They look like ford rims to me but not sure. They look pretty good thought about getting a set. As we all know there isn't much selection for 8 lugs without running into stuff that gets real expensive.

[TexasJeff]

Pez, I have a 1980 GMC 2500 I feel your pain brother with gas prices so high. When I purchased the truck it had an electronic overdrive 350 tranny. I got about 14 MPG with that tranny, it went out and I replaced it with a standard 350 ($1000 compared to $300) my mileage is now 11 MPG. That 11 is every day, city or highway, hauling a trailer or hauling nothing it is still about 11 mpg.

Here is my baby.

[kevinr1970]

Those wheels look so good on that truck.

Originally Posted by TexasJeff

Pez, I have a 1980 GMC 2500 I feel your pain brother with gas prices so high. When I purchased the truck it had an electronic overdrive 350 tranny. I got about 14 MPG with that tranny, it went out and I replaced it with a standard 350 ($1000 compared to $300) my mileage is now 11 MPG. That 11 is every day, city or highway, hauling a trailer or hauling nothing it is still about 11 mpg.

Here is my baby.

[Jim85IROC]

What's an overdrive 350 tranny? Wasn't the 700R4 the only overdrive trans that came in these trucks?

[MylilBowTie]

Its the lockup torque convertor that makes it feel like it shifts again when it engages. My 84 had one stock it was a 9in tailshaft version. I don't think they where made all that long. The 700r4 pretty much took over.

[wackypez]

The Rims are Eagle series 186 16x8 and can be had for around $400 for the set. The PO owner had just put them and the tires on it before I bought it or I probaly would have done like TexaJeff and gone with the new HD rims, they look great on that truck.

Im going to put an after market Tach in this week anyone have success with one or advice on brand and installation?

Thanks