I have a hypothetical question for all you highly trained individuals. If you take two identical trucks with same rear end and trans, and equip one with a diesel engine and the other with a gas engine of equivalent horse power which would get the better miles per hour?

miles per hour or miles per gallon?

You have a very complicated (simple) question.

Diesel fuel has more energy gallon for gallon than gasoline.
1 gallon of gasoline = 124,000 Btu
1 gallon of diesel fuel = 139,000 Btu
http://www.eia.doe.gov/kids/energyfacts/science/energy_calculator.html

So, one would expect a diesel engine to get AT LEAST 1.12 times the fuel mileage.

Diesel engines are also more efficient than gasoline engines.
Modern gasoline engines have an average efficiency of about 25 to 30% when used to power an automobile.
The most efficient type, direct injection Diesels, are able to reach an efficiency of about 40% in the engine speed range of idle to about 1,800 RPM.
http://en.wikipedia.org/wiki/Engine_efficiency
So, Diesels are up to 1.33 times as efficient (or so).
So, multiply the two together, and one should get somewhere around 1.12*1.33 = 1.6 times the fuel mileage with Diesel.

However, in your "ideal" vehicle, you are stipulating the same drive train, and HP.

A Diesel tends to run more efficiently at lower RPM's than a Gasoline Engine.

So, you could cripple a Diesel engine if it was coupled to a gasoline transmission, or visa-versa.

I'll happily add more complication: the diesel engine will have greater displacement than the gas engine, for equal HP. In every case I know of, engines offered in gas and diesel versions (same block, different heads, pistons and so-forth), the diesel versions only make about 75% the power of the gas version, and have less peak torque (which always shocks people). However, diesels are usually tuned for peak torque at lower rpm than their gas cousins, and so run at peak efficiency at lower rpm too. This means that two identical vehicles, with identical rear end and trans, and (unlike your example) identical engines except one gas and one diesel, the diesel will be slower, but get better fuel mileage. I own just such one (64 Rover). VW's probably have similar results.
A diesel tuned for peak torque at high rpm might be faster than an equivelant gas vehicle of lower rpm torque curve, both having equal HP. Turning HP into speed is not so simple, but I think you knew that.
Cheers,
JohnO

Compression ratio plays a part.

As the compression ratio goes up so does the thermal efficiency. There is a trade off, the pay back decreases as the compression ratio increases. Wear and tear on bearings and decreasing improvement in efficiency make harder to gain any more improvement.

Andrew, I failed that part of the proof reading. Yes I did mean miles per gallon.

Thank you keelec, and johno for your explanation of the torque, rpm, and horse power. I do understand the performace difference between the gas and diesel since I am truck driver.
Now that I've got you started, lets complicate (or maybe simplify) things a little bit.
If we were to take a 6.2 chevy diesel and put it into a chevy with a 3 speed with granny low, what sort of rear end ratio would you think I might need to get good pulling and desent mileage?
My feelings is that the hang up for good mileage is going to be the 3 speed trans.
Alex

Ideally you need to have the performance curve showing BSFC (Brake Specific Fuel Consumption, or fuel burned per hp). Most engines have best BSFC at a point just past peak torque. That's where you want the engine rpm at cruise for best MPG. For best pulling power at cruise, you'll want a point closer to peak HP, which means higher rpm and worse BSFC. TANSTAAFL.

A 3-speed tranny will need very large steps between gears to include a granny low and a best cruise. You're problem then becomes driveability - you'll need to rev the engine to screaming point before shifting, then it may bog down in the next gear, if the torque curve is narrow. How broad is the 6.2 torque curve?
If using an automatic transmission 3-speed, the torque convertor will compensate for most of the bogging problem described, but I get the impression you're talking about a stick shift. Auto-boxes consume power, robbing gas mileage. 3-speeds rob less power than 4-speeds, thanks to fewer gears splashing in oil, but it's a small difference.

The 6.2 may not be the best engine for your use, although they're cheap and easy to get. BSFC will be better in most any other diesel of equal hp.

Tell us more about your project. It sounds interesting.

An example (warning, Rover content). My 1964 Rover came from the factory with a 2.25 petrol engine, 4-speed transmission, 2-speed transfer case and 4.7 differentials. After lots of fiddling I was able to get between 17 and 19 mpg on the highway.
The Rover 2.25 engine was designed as a diesel, but also offered in the petrol version commonly found in the NADA (North American Dollar Area) trucks. By converting a diesel to a gas, Rover got a very robust engine, contributing to Rover's reputation. I tracked down an original 2.25 diesel and installed it in the vehicle, with no modification to the gearing (not that there's any options). Mileage went to 26-28mpg, but top speed dropped from 70 to 60 (remember that torque curve thing?).
The most dramatic difference was evident off-road. A particular nearby course that takes 2-days to drive commonly produced petrol fuel consumption of 4-6mpg. The same course, same vehicle, but with the diesel engine, say 20mpg. Suddenly I didn't need to carry spare fuel cans anymore, unlike my fellow Rover owners, many of whom have now converted to diesel, and a few (Ed Beggs is a good example) to biodiesel/svo.

Thanks again johno, I've always thought that the 3 speed was going to be the real problem to good pull and mph. I've entertained the idea of manufacturing a split shift setup to go between the trans and the engine and then change the knuckle to a highway speed, but there might be an easier way. Bottom line, it sounds like to get what I want I'm going to have to change something in the drive line too.

Now did I follow you correctly in that the automatic trans robs horse power?
And how can I find out what the BSFC is an engine? Would the manufacturer have that stored in a file somewhere?

As far as the driveablility, that's the problem I have with this 235 straight 6.
(You're problem then becomes driveability - you'll need to rev the engine to screaming point before shifting,)
and diesel engines don't normally like to be reved up that high.

john galt, where did you find your 3.4 turbo? I also have a 89 toyota truck that like to drop a diesel into but have not been able to locate an engine.

Thanks to everyone that has contibuted so far for all your knowledge and input.
Alex

Yes, automatic transmissions reduce fuel mileage, compared to a manual-shift tranny. However, some properly designed torque convertors behave rather like variable ratio transmissions, multiplying torque at low rpm (like a lower gear), yet approaching 1:1 direct drive at higher rpm (referred to as "lockup"). Thus it can allow a 3-speed transmission to have good "driveability".
BSFC curves should be available from the manufacturer, though they're sometimes hard to get. These forums might be a potential source.
What vehicle do you want to put this in? Consider overdrives, split-ratio differentials, and 4-speed transmissions. Hmmmm, Mckenzie Tenn. Let me guess that you've got a 1960's 3/4 4wd pickup?
Cheers,
JohnO

Your close johno, the truck is a 61 chevy, one ton dually, flat bed.

Good grief, I had one myself - 12-foot bed with a stock rack, but I thought it had a 4-speed. The 235 was marginally underpowered even when empty, and eventually swallowed a piston. It got replaced with a 350 and seemed pretty happy except for the embarrassing oil consumption (2-1/2 gallons oil to drive 600 miles). The 350 wasn't rebuildable, so the truck got scrapped.
Swapping a 6.2 for a 350 sounds almost trivial, and the 350 fit without modification in place of the 235.
Need a set of 19.5 tires and rims?

Well I quess you call it a 4 speed but that first gear is so low it's more like a granny gear. I use this truck to pull my trailer with logs and other wood equipment on and with the price of gas I'd like to convert it to diesel and then run bio in it. The 235 has pulled a lot in the last 14 years and it's getting tired.
As for the rims, right now my tires are good. I ordered a set through the coop that are 8 ply if I remember right.

[QUOTE]
Swapping a 6.2 for a 350 sounds almost trivial, and the 350 fit without modification in place of the 235.
QUOTE]
Johno, if I follow this right, a 6.2 would replace a 235 just as easily as a 350 would. Now we're back to the 3 speed trany causeing problems with driveablilty. I really don't want to go with an auto tran. Anyone have any other ideas on transmissions?

quess we don't have many tranny techs on the forum. Any suggestions on where to take my inquiry?

quote:

Well I quess you call it a 4 speed but that first gear is so low it's more like a granny gear.

Since you've confirmed it's a 4-speed, I think the 6.2 would work. If it had been a 3-speed, as I originally thought, it wouldn't have worked. Do you have any specs on the tranny (gear ratios, or designation) at all? That might help if someone out there has the same numbers in a modern truck with 6.2. I'm not sure what year the 6.2 was first offered, but you could start by looking it up in a Motor's Manual for Trucks and comparing the ratios listed there (sorry, I don't have one).

growyoung:

The final drive on this tranny is 1 to 1 or direct drive. I remember this tranny from my youth (learned all about double clutching behind one). We used to call them growlers due to the square cut gears and no synchro in first. If memory serves they were made by both saginaw and warner gear.

The specific fuel conpumption on these engines is .43 lbs/shp/hour at 1,800 RPMs. The trick to getting good mileage with these engines is keeping them at 1,800 to 2,000 RPMs at highway speeds. As they are governed for 3,400 RPMs, this restricts your choice of rear end ratios available.

Depending on the conditions, you might look at the 700R4 automatics. They have a very deep first for an automatic, a .75 overdrive in forth plus they have a lock-up torgue convertor. You wil have to get a diesel governor for the tranny plus you should use the diesel torgue convertor (1,200 RPM stall).

The 6.2 is not exactly a work horse but if you are careful, they will certainly do the trick. Heat is their biggest threat and if you have to spend alot of time with your foot into it, spend the money on lots of cooling capacity.

I have an RPM calculater but it is in an Excel spreadsheet and this board doesn't want to accept it as an attachment. If you will give me the current rear end gear ratio plus the intended tire size (diameter) I will be more then happy to calculate the need gears for max MPG.

Hope this helps.

Bill