6.5 Precups - Power -OR- Economy?

[More Power]

I received this email a while ago asking whether there would be a big sacrifice in power when using NA precups in a turbo'ed engine.

I am building up a Turbo 6.5 corresponding to your fuel economy buildup article. Are we going to see a follow up on that soon? I am held up at the pre cup stage to see how that works out. I am getting so many nay sayers that tell me I will sacrifice too much power if I go with the early style pre cup.

Thanks in advance, -TMiller

Back in late 1991, Motor Trend did a comparison test between a Banks turbocharged 6.2L diesel and a brand new 1992 model-year 6.5L turbodiesel. The Banks TD outperformed the new 6.5 by a little.

If maximum power is more important to you, go with the TD precups. If better fuel economy and nearly as much power sound like a good compromise, go with the NA precups.

Jim

I would also add that NA 6.2's have routinely produced low 20's for fuel economy. A 6.2L GMC that I drove for over 200,000 miles would routinely deliver 22-24 mpg, with an all-time high of 27. High teens are more common in carefully driven and high geared 6.5TD's. If precups don't explain the difference, let me know what you think does.

The fuel economy project is still on. Member Gordon Marks recently donated a complete serpentine set. Still debating whether to run an electric rad fan or go with an engine-driven fan. I'm leaning more toward electric, since this is about ultimate 6.5 fuel economy.

[baker2acre]

Jim,

I'd lean towards the electric fan as well. Like you said being that the ultimate goal is fuel economy you'd waste energy at the crank and I doubt you'll be stick'n it's nose into the Rockies w/a 34' 5'r in tow!!! Or will you?? For it's intension, hwy cruising will suffice to cool the rad in light duty use and the fan would only be needed to suppliment on a long climb or in Wally's parking lot in August and most of that would come from energy already stored in the batteries. It's a setup used by all the "economy" minded compact cars... evan VW's diesels.

Plus, it wouldn't be hard to switch back to crank driven. That territories already been traveled!!

[kevin77]

I'm gonna see what the difference is on my 6.5 - I had a broken valve spring on #7 and a cracked head on #8 - I replaced the heads and had 6.2 precups installed. I also replaced the exhaust with a 4", replaced the oil cooler (stock) and oil cooler lines and dropped a gear drive in for the timing chain. I have not yet gone through one full tank of fuel but right now - it looks like I am getting the same mileage but It has been around 0F around here and I have been keeping it in 4 wheel drive.

She actually feels like she has more power now - I would say the exhaust helps. I figure the next thing I might have done is a new chip in the ECM.

Kevin

[chickenhunterbob]

Also interested in reading about how the fan bit transpires.

Fuel economy is my one and only gauge of "performance" on my old truck.

I saw a show on TV a while back (Trucks, or some such) where the guy held up an electric fan assembly with two fans (like my wife's old windstar, only bigger) for a Duramax, and claimed it would save something like 27 HP.

Perhaps the mfgr's claim, maybe a bit high, doesn't matter, point is I myself can easily see that not having to spin that fan even with the clutch disengaged most of the time, would result in a little extra HP to generate to the wheels.

If'n of course it cost almost nothing, and cooled the engine sufficiently.

[Robyn]

We had an 82 GMC Jimmy that would deliver 24-26 on the highway.
Later we bought an 86 Burb 2500 and that thing would run 20 regularly on the highway.

I believe all that is written about the 6.2's and mileage.

I have never owned a 6.5 that made 20.
My 93 extended cab would do 18-19 Hwy if you were real careful
The 94 dually crewcab was about the same but not near the power as the 93
The 95 Burb we had was in the 17-18 range
The 94 we have now that got the rebuild last summer is running High teens around town.
The Clearwater heads have smaller precups than the factory ones that came off.
Butt dyno can't tell any difference in the power. The thing has 4.10 gears too.

Now my 91 Burb is getting 15 in mixed service. I am not sure of the condition of anything on that beast. Its got 3.42's with 35 inch tires too.

Definately something to this stuff for sure.

[Robyn]

The fan is a big boat anchor for sure. Unfortunately the little electric units wont even come close to pulling the wind the direct drive units do.

I am faced with a serious fan dilema in my Hummer Kit in that the room up front for the radaitor is not enough to allow for the factory unit. Its gonna get a small aluminum unit in front as well as a small aluminum unit under the truck in the rear with a set of electric fans. Gawd I hope it works.
Total cooling area will be more than the one factory unit BUTTTTTTTTTTT.

[WRB Diesel]

I am curious to know if anyone has some solid data on MPG numbers when running the 6.2 pre-cups in the 6.5TD. I too have spoken with several people saying that I should avoid using those pre-cups in a 6.5 due to power lost. However, most of these people are power junkies and think I should be shooting for 500+ hp. I would like when I rebuild the motor in my '94 suburban down the road... to install the earlier pre-cups and get that increase in mpg. the burb is used as a commuter, and for long trips to various whitewater rivers around the northwest and I have no need for tons of hp.
Thanks in advance.

[More Power]

Most 6.5 owners would not likely experience a difference in power, but they would see a difference in fuel economy. On the other hand if "Power" is the mission, the turbo cups would be the obvious choice.

In 1991, Motor Trend magazine compared a brand new 6.5TD powered truck to a prior year 6.2L equipped with a Banks turbocharger. The Banks 6.2 outperformed the new 6.5 by a slight margin.

Jim

[TurboDiverArt]

Any idea what kind of MPG we are talking about here? Is it .5 MPG or 2MPG? Just curious.

Art.

[Chevrolet4x4s]

Any idea what kind of MPG we are talking about here? Is it .5 MPG or 2MPG? Just curious.

Art.

I too would like to know how much the precups will improve it. And a set of theromastiacally controlled daul electric fans should pick up what 3mpg over the stock fly swatter?
Shane

[More Power]

The 6.2L and the 6.5TD longblocks are basically the same, with the exception of bore diameter and precups.

Assuming 3.73 gears, a 6.2 powered truck can deliver low 20's for fuel economy. With the same gearing, a 6.5 equipped truck can produce high teens. There could be 5-mpg difference. Some or most of which is, I believe, attributable to the precups.

GM once sold a 6.5L diesel engine named the "Fuel Miser". The only difference between that engine and a typical 6.5 is that the Miser used smaller port precups and just the rear cylinders received oil spray.

Jim