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?? :eek: 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!! :D

[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.

[DmaxMaverick]

About that fan thing.......

The "27 HP" it takes to turn the fan is at full rated RPM. It is substantually less at lower RPM's. The marketing of the alternative kinda' left that part out. Also, power requirements increase exponentially with increasing RPM, so at 50% rotational speed, it is much less than 50% of that 27 HP.

The engine driven fan is many times more efficient than any electric setup.

Both fan types use power from the acc. drive belt. The electrics lose efficiency through the alternator, wiring, battery, and motor(s). The engine driven fan loses efficiency only due to its mass and operation period. It is "clutched", but is never really "off".

Sure, it takes more HP to turn it. And it turns when it doesn't need to. There is a better alternative than electric, if you goal is efficiency.

If you were to reduce the mass and capacity of the engine driven fan to near the capacity of an electric, it would be much more efficient, except........when it doesn't need to turn. A thermostatic controlled on/off clutch would also have to be used. Electric, like the A/C clutch, or air operated, like OTR applications would work well. The viscous coupling (fan clutch) will have to go. A remaining drawback is the low engine RPM operation.

Now.......
If an electric fan is capable of keeping a 6.5 cool, a similarly capable engine driven fan would be more efficient. Electrics have generally about a 50-60% capacity of the OEM fan. So, if you were to reduce the capacity of the engine driven fan to near that of the electric, it should be more efficient. The capacity can be reduced by downsizing the mass, and rotational speed. If it doesn't work, then the electric would not have worked under the same conditions.

There are still other factors to consider, like water pump speed, climate, and load, but I think the maximum efficiency can be had w/o an electric fan. Perhaps a combination of both. An electric for lower, and engine driven fan for higher RPM's. The OEM fan was designed to work (or supposed to) while operating at the maximum capacity of the truck, so it has to be overdesigned for less than those conditions. An electric fan alone would be no match under the same conditions.

[65TD]

27 HP is a ridiculous number to assign to this.

If you examine the power required to replace this 27 HP with alternator output you can convert to amps.

1 hp = 745.699872 watts

27 * 745.69 = 20133.63

watts / volts = current

20133.63 / 12 = 1677.8025

1677 amps of current to provide 27 HP.

So lets say the electric fan is drawing 25 amps or less than 1/2 hp.

Cruising down the highway at 45 mph your truck may produce 50 hp. 27 hp for the fan and 23 to get down the road. So our electric miracle fan is going to save millions.


Loss of power in electric components considered it could take roughly 100 amps of electricity to produce 1 hp of electric fan power.

[baker2acre]

DmaxMaverick...

I see your point, to a degree. However in this condition I must respectfully disagree. I think your comparison isn't apples to apples... or well it may be but in an orange basket. :o In the setups that use the electric powered fans they are thermostatically controled!! Meaning if not needed they can be turned OFF. In this situation where mileage is the goal the ability to turn a power requirement OFF would be invaluable!! This is why our oems have clutches... but they still draw power when off. Apples to apples I agree that an electric unit is less efficient when operating, but in a 500 mile cruise where an electric fan may come on for... let's say 50miles of that 500 while the oem unit has been operating in full or partial mode for the entire time. Also, I'm sure that the OEM setup could be reduced in size and or speed to lessen it's draw but you will in turn reduce it's effectiveness by the same measure. The electric would still be there when you do need it.

[DmaxMaverick]

DmaxMaverick...

I see your point, to a degree. However in this condition I must respectfully disagree. I think your comparison isn't apples to apples... or well it may be but in an orange basket. :o In the setups that use the electric powered fans they are thermostatically controled!! Meaning if not needed they can be turned OFF. In this situation where mileage is the goal the ability to turn a power requirement OFF would be invaluable!! This is why our oems have clutches... but they still draw power when off. Apples to apples I agree that an electric unit is less efficient when operating, but in a 500 mile cruise where an electric fan may come on for... let's say 50miles of that 500 while the oem unit has been operating in full or partial mode for the entire time. Also, I'm sure that the OEM setup could be reduced in size and or speed to lessen it's draw but you will in turn reduce it's effectiveness by the same measure. The electric would still be there when you do need it.
I agree, and I stated that as well. The viscous coupler will have to go. A different method of engagement will have to be used to gain maximum efficiency. The OTR applications use an air operated clutch, and it can be seen as being many times more efficient. The engine can be running, and the fan is barely moving, if at all. Engagement is obvious....squeel (clutch engagement), and roar (fan moving air). Unfortunately, the OEM fan would very likely be inappropriate for this application. It is designed to operate with a viscous coupler, and be under load all the time, just more or less depending on the clutch engagement.

I was comparing the apple to apples. The OEM fan against the aftermarket's offering. The goal is the same, and they do the same thing....move air. My reason for posting was to compare the raw efficiency between the two, all else being equal.

Other things to consider....Electric fans are expensive. Repair/replacement of electric fans is expensive. Would there be enough economy gain by using electric fans to offset the initial cost and maintenance? How many miles would it take to recover the loss? And.....how much of the truck's capacity is lost do to a lesser ability to control engine heat with heavier loads, grades or summer heat? I realize maximum economy is the scope of this argument, but it is still a truck. At what point is it no longer useable as a truck? What price for that?

[gmctd]

The 27hp is probably rated in static conditions, such as full engagement at full throttle while sitting still, as in traffic, where actual dynamic also involves forward vehicular movement at 70mph, in 4rth overdrive with TCC locked up, and a great volume of 70mph air forced thru the radiator by that forward movement.

27hp is simple scare tactics, imo

[More Power]

Most things in a diesel engine are a compromise. Engineering manuals that discuss the Ricardo Comet precombustion chambers say swirl velocity is key to efficiency (best use of fuel & air). I suspect the size of the port in these 6.2/6.5 precups are a compromise as well. The smaller port precups allow for designed swirl velocity when running NA and the TD precups provide similar swirl velocity when under boost pressure. Unfortunately, most 6.5TD's aren't generating a lot of boost pressure when driven for fuel economy. This likely produces a swirl velocity below optimum.

Conversely, running an engine with NA precups at 10+psi boost pressure and higher RPM could result in less than complete evacuation of spent gases in the NA precups. Just a theory.

Most 6.5's are run somewhere in the middle, which I feel would benefit from the more efficient (for economy) NA precups. If I were building another power engine, I'd use the biggest TD precups available, and would run up to 20-psi max boost pressure using one of the new emerging turbochargers that are way more efficient than the GM-x series at the higher power levels.

My hunch is that the difference in power production capability between NA & TD precups is just a few % on an otherwise identically equipped engine running up to 12-psi boost pressure. On the other hand, the fuel economy advantage could be as much as 25%. :)

Jim

[baker2acre]

Thanks Dmax...

I think I see where you're going now. I must have jumped the line where you ommitted the viscous coupling for a more efficient connection. I mad some assumptions and generalizations and I appologize. :o

I was unaware that the OTR rigs used air!! I think that's rather clever. By the way, you brought up the issue of expense and maintanance/repair with the full electric setup... out of curiosity how would the full-independant clutch system either air or electromagnetic compare? How durable are the clutches and contact surfaces?

[DmaxMaverick]

With an air operated clutch, like on the OTR's, I am not aware of any significant repair issues. Another TDP member more exposed with this will have to chime in. I've been around them since they were first used, but have never seen one in need of repair. Perhaps that's my unique experience, or a testement to their reliability. For the application we are discussing, an electromagnetic may be a better option. The use of an air operated clutch would require an air source. The A/C compressor clutches are generally reliable, and will see similar power transfer with either a fan or A/C compressor. They are common parts and not very expensive. Some enginuity will be needed, but a clutch could be fabricated that will work. I've thought about this for many years, but never put it to practice. Maybe it's time.

[baker2acre]

Jim,


How would compression ratio play into this... I.E. Higher CR + Smaller Cups+ Low Boost or Low CR + Big Cups + High Boost or any combination of... ? Would any combination offer the best fuel economy?

[TurboDiverArt]

Money wise I'd think a lighter fan might be a better option. Building an electric clutch (like an A/C compressor clutch) might be too expensive to sell commercially, what would the pay off be? I think when I replaced my A/C clutch the price was somewhere between $200 and $300 for the part from NAPA. A fan would be cheaper I

[arveetek]

I know electric fan clutches are available for the 5.9L Cummins. I remember an episode of Two Guys Garage when they installed one on their Dodge truck. I don't know if there are any available for the other light duty engines or not.

I think the electric clutch is the perfect answer. You get the best advantages between the electric fans and the oem fans. Another benefit is the ability to manually engage the fan and let it run when you want. If you're approaching a large hill and you'll know the engine will get hot before reaching the top, you can flip a switch and engage the fan to get the maximum cooling before you get into trouble.

Casey

[ronniejoe]

The clutches used by motorhome chassis manufacturers should work. I saw some electrically actuated fan clutches on GM 8.1 L gas engines during an interview with the Workhorse Chassis folks. Should probably look up what they are using.

[kevin77]

The 27hp is probably rated in static conditions, such as full engagement at full throttle while sitting still, as in traffic, where actual dynamic also involves forward vehicular movement at 70mph, in 4rth overdrive with TCC locked up, and a great volume of 70mph air forced thru the radiator by that forward movement.

27hp is simple scare tactics, imo

I have a little 24hp tractor that I hooked up to a silage blower (4' diameter fan) When I get the fan cranked up, I can blow leaves 30 to 40' - it sounds like an airplane taking off - and the neighborhood knows I blowing leaves - I cannot see a fan that is about 2' in diameter taking that much HP to run.

Kevin

[65TD]

Take an air boat for instance.

In 4" of water over marsh grass she glides along with ease.

Put it in sand it's not going to move well and it takes a lot more throttle.

Surprisingly it acts about the same in very tall wet grass and reeds.

In the case of sand it's just the friction on the bottom. Requires more volume of air. In the tall grass it's more the obstacles preventing the free flow of air that requires a lot of power.

In both it seems it's not as much the weight of the prop as it is the airs resistance to movement.

[JohnC]

One benefit to having a fan clutch that does not disengage completely is that it provides airflow for the intercooler and AC condenser even when the engine temperature is under control...

[baker2acre]

:confused: Shouldn't the exposed frontal area of our trucks handle that heat load at speed?? The only time that would not be so would be at low speed manuverings?

[TurboDiverArt]

One benefit to having a fan clutch that does not disengage completely is that it provides airflow for the intercooler and AC condenser even when the engine temperature is under control...
That's probably a good point. With the A/C on it would probably have to lock up the fan or command it to be on very frequently to generate airflow in around town driving. With the A/C off the clutch might do great for mileage but might deliver poorer mileage with the AC on. Maybe if you wired the fan to the compressor or something so that when the compressor turned on, so would the fan. I don't think you would want the fan locked up the whole time the A/C is blowing.

Art.

[65TD]

Electrics you see used in production all come on with A/C. Some have two fans. One of the two fans may be dedicated to the A/C and the other fan could still be off.

By the way almost all electric fans I have seen were on vehicles that did not have an option. Either due to motor alignment or positioning in the vehicle, or lack of room.

Also I pretty much run my A/C all the time if needed. If it's hot, or if the windows are fogging up. In humid areas it's needed a lot.

[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

[Chevrolet4x4s]

which pre cups would be good for a mild to medium twin turbo intercooled 6.5 built for economy? Also what years did the 6.5s get roller cams?
Shane

[MaxPF]

I heard from a VERY reputable source that the problem with using small port precups on a turbo motor is that the cup will get excessively hot and you will get rapid erosion of the port. I wonder what material the precups are made of. They are simply called "stainless steel", but that covers a wide range of alloys, none of which are the best for a precup IMO. HD IDI's usually use a Nimonic alloy (similar to Inconel)... is that what the cups in the 6.x are that are reffered to "stainless steel", or is it some cheaper alloy...?

[DmaxMaverick]

which pre cups would be good for a mild to medium twin turbo intercooled 6.5 built for economy? Also what years did the 6.5s get roller cams?
Shane
I'd suggest using 6.5 TD cups. The gain/risk with N/A cups won't wash.

6.2/6.5 engines have always had rollers.

[Chevrolet4x4s]

I'd suggest using 6.5 TD cups. The gain/risk with N/A cups won't wash.

6.2/6.5 engines have always had rollers.

Well I shouldnt have to pull the heads then.Glad to hear that its a roller cam.

Also which years of 6.5s had the 18 to 1 pistons and is there a casting number on the engine that will tell you if it is the 18 to one engine....Its a late 95 or a 96 engine
Thanks
Shane

[MaxPF]

No factory 6.5 ever had 18:1 compression. 6.2's and early 6.5's had 21.3:1 compression. Later they dropped the compression to 20.3:1. I don't remember when the change was made though...

[More Power]

I heard from a VERY reputable source that the problem with using small port precups on a turbo motor is that the cup will get excessively hot and you will get rapid erosion of the port. I wonder what material the precups are made of. They are simply called "stainless steel", but that covers a wide range of alloys, none of which are the best for a precup IMO. HD IDI's usually use a Nimonic alloy (similar to Inconel)... is that what the cups in the 6.x are that are reffered to "stainless steel", or is it some cheaper alloy...?

The cups are, according to GM, stainless-steel.

I have a disassembled 6.2L engine in the shop that ran for 200,000 miles with a Banks turbo. The cups are no more worse for wear... Peninsular produced turbocharged marine 6.2L diesels from 1984 till 1991. There is no more reputable source than Pen, for info concerning turbocharged 6.2 precup life. ;)

Jim

[Chevrolet4x4s]

No factory 6.5 ever had 18:1 compression. 6.2's and early 6.5's had 21.3:1 compression. Later they dropped the compression to 20.3:1. I don't remember when the change was made though...

Aaaw shucks further proof not to take everything in a wiki to the bank
http://en.wikipedia.org/wiki/Detroit_Diesel_V8_engine :( on the 20.3 to 1 would 10-14 max psi intercooled with close watch on egts be ok?
Shane

[More Power]

Actually....... AM General/GEP currently builds 18:1 6.5's on an infrequent basis. They all go to the marine vendors.... The 18:1 6.5's that Peninsular, Marine Diesel, and other marine vendors offer are all factory built as 18:1 engines. GM never offered them in their trucks/subs though. ;)

Jim