Setting up Gearing for Diesels

[Highway-Star]

OK, I've got a rough idea of how manufactureres determine ideal ratios for petrol engines (for economy), generally trying to keep the engine in its peak economy region, on the peak economy line etc. However I know squat about diesels, just how different are they?

I'm trying to determine the ideal diff ratios for Dad's Hilux he is rebuilding, and what keeps getting me is that these high torque low reving diesel engines all run quite low diffs in their factory vehicles. His vehicle currently has 4.55 diff ratios, but I think (from my limited available information on engine performance), that even the standard 2L motor should have been running 4.11 diffs (nearest ratio anyway) with standard tyres.

I know some commercial vehicles will be setup with lower diffs to make them better load carryers, and pack mules etc, and of course lower gearing is ever usefull offroad, however is fuel economy just neglected altogether? Do Diesels still give the economy when reving above their peak torque region?

I know thats allot of rambling, but any helps appreciated, thanks.

[midi73]

OK, I've got a rough idea of how manufactureres determine ideal ratios for petrol engines (for economy), generally trying to keep the engine in its peak economy region, on the peak economy line etc. However I know squat about diesels, just how different are they?

I'm trying to determine the ideal diff ratios for Dad's Hilux he is rebuilding, and what keeps getting me is that these high torque low reving diesel engines all run quite low diffs in their factory vehicles. His vehicle currently has 4.55 diff ratios, but I think (from my limited available information on engine performance), that even the standard 2L motor should have been running 4.11 diffs (nearest ratio anyway) with standard tyres.

I know some commercial vehicles will be setup with lower diffs to make them better load carryers, and pack mules etc, and of course lower gearing is ever usefull offroad, however is fuel economy just neglected altogether? Do Diesels still give the economy when reving above their peak torque region?

I know thats allot of rambling, but any helps appreciated, thanks.

I think you will find that if you upped the diff ratios, you would find out how torquey these little diesels are not. Eg, you will find yourself forever going back gears when going up hills.

[KiwiBacon]

On most diesels the most efficient rpm is right where max torque is.

But many older little diesels don't have enough torque to pull a gear like that on the open road on any sort of gradient.
If you've got the torque to pull that rev range then set it up like that. If you don't have the torque and don't mind changing gears often then again it works as long as you don't run out of gears on the low side.

[Willy Hilux]

My 2.8L came with 4.3's. After putting a turbo on and swapping diffs to my comp rig. So then it had 4.8's. Alot better for towing.

[Tapage]

Which tire size you gotta run .. ?

It can make the hole diference .. just if you have a little engine as 2L .. ( noon turbo ) I seen a buddy LJ-70 with 4.88 stock R&P pushing hard in the HW 36" tractor tires .. but it's a 2L-T in a good shape capable to push 14 PSI

[Highway-Star]

Tyres will probably be 31" for on road use, towing, dailing driving etc. He Will eventually buy a set for offroad only, which will probably be 32 or 33.

The engine will not be the 2L, thats for sure! I was just using that for an example, sorry bad one it seems. Still looking at a B series diesel as per my thread in the Toyota section, its getting really full on as to how we're going to fit it.

Most of the diesel engines we're looking at reach peak torque at about 2000-2400rpm region, with the 15B being the only real exception. Even the 3B with 4.11 diffs (which puts it at better rev range) and the 31" tyres should whack out 25kg force more in top gear high range, than a standard 2L with 4.55, and 28" tyres. (I'm using kg force at wheels, as its tyre diametre dependent, torque isn't).

So if its putting a reasonable force at the wheels, I guess make the gearing to keep engine at peak torque at most typical speeds. Anyone happen to know the coefficient of drag of an LN65 Hilux trayback? Then I can figure out how much force is needed for the car. Might calculate it with boat in tow too.

On the flipside, anyone got ballpark ideas for how bad a slightly overevving diesel does fuel efficiency wise?

Thanks everyone so far .

[KiwiBacon]

On the flipside, anyone got ballpark ideas for how bad a slightly overevving diesel does fuel efficiency wise?

Thanks everyone so far .

It's a completely different engine. But raising the gearing in my Isuzu 4BD1T powered rangie from 2500rpm to 2000rpm at 100km/h improved my fuel economy by close to 15%. That's just under 9 km/l to just over 11km/l.

[Gwagensteve]

[quote="Highway-Star"]

The engine will not be the 2L, thats for sure! I was just using that for an example, sorry bad one it seems. Still looking at a B series diesel as per my thread in the Toyota section, its getting really full on as to how we're going to fit it.


So if its putting a reasonable force at the wheels, I guess make the gearing to keep engine at peak torque at most typical speeds. Anyone happen to know the coefficient of drag of an LN65 Hilux trayback? Then I can figure out how much force is needed for the car. Might calculate it with boat in tow too.
/quote]

I think your trying to overcomplicate this. I agree that cruise revs should be just above peak torque. You can calculate it out as much as you like, but you only have a few variables to play with, and from what I have seen if you calculate it out, you will end up with an overgeared or underpowered car.

Stock 2L hiluxes spin about 3K at 100 (4.5 gears) from memory. Peak torque is 2400.

3B's hate to rev. A 2L will spin well past 4K, but a 3B is all out by 3600 from memory. Run the numbers on a calucator such as Novaks to work out your rev rise when you downshift at peak torque with the engines you are looking at. of you run 5th down to, say, 85 Km/h, you need 4th to bring the motor well up into the sweet spot between peak torque and peak HP to have a hope of holding speed in the lower gear. (hope you follow that) Again, your gearbox ratios are effectively set, so you only have diff ratio to play with.

I think with a B motor will want to run 4.3's with around a 32-33. They run 4.11's in cruisers with a 32 (7.50) as a reference. in a lighter car (hilux) it should move along fine, and the slightly shorter gearing will crispen it up a bit.

if you go 15B, who cares how it's geared.... in a hilux it will just fly.

Steve.

[Highway-Star]

Ok Thanks people. From what Kiwibacon said, getting this gearing right should pay off.

Yep, I follow what you said Steve, thank you very much. We'll probably be using the R151 gearbox, and then basically we can shift the rev range to road speed up and down by playing with diff ratios/tyre sizes.

If we do go with a 15B, the gearing will most likely be higher, as the torque that thing pumps out doesn't really need low gearing (for this application anyway). Dad's got lined up options of 1B (I hope not), 3B, 13BT, 14B, 15FBT at the moment, and is running around with a Dyna bellhousing and we're going to see how it goes up to a R151 we've got lined up. Now the axle parts are on order so the choice of C&P has gotta be done soon.

I'll make something up in excel so I can fiddle around easier, but this has clarified quite a few things, cheers.

[Highway-Star]

OK

I whacked this up in excel, I've made others for different combos, but It shows what I'm understanding. I've had to assume the rev range for peak torque a bit, all I know is 1800rpm for peak.

Feel free to comment on this, I know a bit higher gearing may suit this for highway driving, however in this config in only takes just under 2400rpm to attain 100km/h, not too bad.

[Tapage]

Lets figures this combo ...

14B + H151 + 4.11 + 33"

This combo would give to you a nice towing capacity with 120km/h cruising speed ..

[Highway-Star]

Lets figures this combo ...

14B + H151 + 4.11 + 33"

This combo would give to you a nice towing capacity with 120km/h cruising speed ..

H series gearbox proved too much stuffing around thats why where going with the R151. Wish it could have 33" tyres, but Mr Plod would not be too impressed.

I just read a thread in the Yoto forum about an extra high overdrive option for the R151, that could help cruising. 120km/h is not needed, 110km/h is the max speed limit anywhere in QLD I believe, with 100 the norm. plus at 120 the fuel economy would start becoming bad anyway because the wind resistance on the lux would start to become quite significant.

Still trying to decide on the engine. He can get a 14BT or 15BT for the same price. But I have some reservations about the 15BT damaging the drivetrain with its phenominal torque. Though some people put 1UZFE in Hiluxs and get away with it.

[Tapage]

I love my H55F that coul be an option ( more easy to match here ) with your 14B ( never seen a 15BT ) ..

the H55F khave much more reduction at 1st than any H150 or H151 .. and the 5th it's .84 in H55F.

[Highway-Star]

I love my H55F that coul be an option ( more easy to match here ) with your 14B ( never seen a 15BT ) ..

the H55F khave much more reduction at 1st than any H150 or H151 .. and the 5th it's .84 in H55F.

Spent a good week trying to figure out how to mate a H55f to a Hilux transfer, worked out it would cost at least $800! (different splines, etc etc) Otherwise would have been the ideal box.

Got an R151f from a late model surf to bolt straight to a BU85 Dyna bellhousing we borrowed from the wreckers, which will bolt straight up to a B series engine, now we're getting somewhere .

[Tapage]

I love my H55F that coul be an option ( more easy to match here ) with your 14B ( never seen a 15BT ) ..

the H55F khave much more reduction at 1st than any H150 or H151 .. and the 5th it's .84 in H55F.

Spent a good week trying to figure out how to mate a H55f to a Hilux transfer, worked out it would cost at least $800! (different splines, etc etc) Otherwise would have been the ideal box.

Got an R151f from a late model surf to bolt straight to a BU85 Dyna bellhousing we borrowed from the wreckers, which will bolt straight up to a B series engine, now we're getting somewhere .

Holly S**t .. now I figure you was talking about R151 not the H151 !

[Highway-Star]

Holly S**t .. now I figure you was talking about R151 not the H151 !

Don't it suck when you fail to pick up on an obvious little thing, happens to me at least 4 times a day.


I think for the 15B, 3.7 diffs might have been even better, however due to there slight more rarity I've decided that 4.1's should work well, and that slightly lower gearing won't hurt when towing. The engine will still only be doing 2400rpm at 100km/h which should be livable, bit better than 2550rpm if we stick the stock 4.5's back in.

Also slightly off topic, with tyre sizes, 30" (7.00R16 i think) is standard on a live axled Lux correct? If so 31" would be a tad illegal here in QLD, but should be able to pass them off. If we have to put 30" (or even worse) on for road use, then we will want higher then 4.1's, thats all. (No, the car is lacking a tyre placard).

[tanshi]

you can go a given percentage larger than standard tyres in QLD. it usually equates to around one tyre size. IE as far as i am aware, you can legally run 28"s on a sierra as they had 26" standard. so if 30"s are standard on a lux you should be able to do 32s