So the diesel bronco is going to be getting some fuel and a turbo soon (after I dyno N/A) and I know I am going to be upgrading the rear axle. I originally was going to drop a 9" in it, but in my research, stock for stock, the 9" isn't really much stronger than an 8.8. When I started pricing out the cost of building a 9" to take the torque plus the upfront cost of the axle, upgrading the 8.8 is looking like a better option. The weak links of the 8.8 are the carrier, axle tubes, and (Some say) the C-clips.


My plan is to get a better aftermarket LS carrier to take the carrier issues out of the picture, do a smaller 3pt truss and weld the tubes, solid pinion preload spacer, and do a girdle. I fully assume the bronco will be doing 600-700ft/lbs at the wheels with stiff 33x11.5 tires. It wont be doing any crawling or anything like that, its my DD and will see 99.9% street action, with the occasional desert/logging road blast...


My question is, have any of you guys ran into issues with the 8.8 that aren't related to the carrier or tubes spinning? How many of you are running torquey setups?


Before anybody asks, the reason Im not swapping in a 10.25" is for weight reasons. I want to keep the bronco light, and it will be staying with 1/2T running gear.

 

Never had any problems out of any of my 8.8 my dd tow rig has 300,000 and my sledpuller has built 351 and 33s been sled pulling for last 8 years but im not puttin down ur numbers so it may not hold up for you

 

Are you thinking your engine will only put out in the range of 60 lb-ft? To get torque at the driveshaft multiply engine torque by the first gear ratio. To get torque at the wheels you multiply that by the rear axle ratio.

So even only 400 lb-ft at the engine will be around 1200 - 1600 lb-ft at the driveshaft and 3,600 - 5,200 lb-ft at the wheels.

I'm not saying an 8.8 can't be made to work (I don't know enough about building them to have an opinion there). But make sure you're working with realistic numbers.

 

The power numbers I listed are numbers measured at the rear wheels on a chassis dyno, so figuring a 20% parasitic would be around 850ft/lbs at the flywheel. Chassis dyno numbers aren't a direct measurement of torque, it factors in engine RPM to calculate the amount of Engine power/torque but after all the drivetrain losses.


The weight of the vehicle and tire size is huge, but im going to be on the lower end of both of those as far as a bronco is concerned. At the end of the day, its going to be how easy it is to break the tires loose more than anything.

 

Not true. If you measure POWER at the rear wheels you'll get the same power as at the flywheel (minus 20% or so for parasitic losses). But the same is not true for TORQUE, and torque is what an axle cares about.

Power is proportional to torque times speed. When you spin your drive shaft at 1/3 the speed of the engine (by using a 3:1 gear in your trans) you get 3 times the torque. 3x the torque times 1/3 the speed = the same power. But you are still getting 3x the torque. Same holds true at the ring and pinion.

So with 850 lb-ft at the flywheel times about 5:1 for your ZF5s 1st gear (more if it's a wide ratio ZF5), times about 3.5 for your axle ratio (more if you've got 4.10s), that works out to 14,875 lb-ft at the wheels. OK, take into account the 20% parasitic losses and you're still at 11,900 lb-ft.

Yes, you'll spin the tires before that. But don't be making decisions based on bad math that says you only have 1/20th of the rear wheel torque that you actually have.

 

You didn't even read the what I said in the quote....


The numbers a Dyno spits out (which was what I was referring to when I said 600-700ft/lbs measured at the wheels) are calculated in relation to engine speed...They are numbers the engine is effectively powering the vehicle with after all the parasitic losses are accounted for.... Don't believe me? Point me out a dyno that shows any street vehicle putting out 17,000ft/lbs of torque when measured at the wheels... You sir are just being obnoxious.

Are the net torque numbers higher at the rear wheels?? Absolutely, as they are in every vehicle. When I was stating the numbers it would be making, it was a reference to all other vehicles in the same platform, as measured on a chassis dyno.

 

No, I'm not being obnoxious, I'm being an engineer (although my wife would argue that there's very little difference sometimes...).

If you measured 600 - 700 lb-ft of torque at the rear wheels you would need to divide that by the gear reduction to figure out the torque at the flywheel. Let's say you measured 700 lb-ft at the rear wheels, you have 3.55 gears and you had your ZF5 trans in 5th gear (0.77:1). Then the torque reflected at your flywheel is 700 divided by 3.55 divided by 0.76 = 259 lb-ft. Divide by 0.8 to back out the 20% parasitic loss and you're at 324 lb-ft of torque at the flywheel. Maybe not too bad with an N/A diesel (it'd put you somewhere around 200 - 250 horsepower depending on the engine speed), but I'm betting you're aiming higher than that when you turbo it.

And even if that's really what you mean, your axle is still going to see more than 700 lb-ft because you're not going to be in 5th gear all the time. When you downshift to 1st the rear axle will see that 324 lb-ft times the first gear ratio (5.08:1) times the rear axle ratio (the 3.55:1 I assumed) times 0.8 for the parasitic loss and you're at 4,680 lb-ft.

But more likely is that your dyno shop measured the horsepower at the rear wheels and then calculated the torque based on the engine speed rather than the wheel speed. This gives you a number that isn't real, but it reflects the amount of torque the engine would have been putting out if the driveline had no losses. So you back out the assumed 20% loss and estimate the 850 lb-ft of torque at the flywheel. If that's the case, then the numbers I ran earlier are the correct ones.

 

The problem here is that you are thinking too much like an engineer, and not reading the actual words. The obnoxious comment may have been just giving you too much credit, I don't think you actually understand how a chassis dyno works.


So lets review:


I stated my bronco would be making 600-700ft/lbs at the wheels...Which is simply saying, when it is on the dyno, the engine power, instead of being measured at the flywheel on an engine dyno, engine power will be calculated through the dynamometer computer based off of load (load=force=torque) and engine RPM transmitted through the rubber to the rollers on the dyno. The calculated engine power is exactly the same as the flywheel number, minus parasitic loss of the drivetrain. I simply said 600-700ft/lbs at the wheels (which you took in a rigidly literal sense) because that is a common way to refer to the engine power numbers when they are measured at the wheels on a chassis dynamometer.


You were correct in stating the large numbers in your reply, this was what I referred to "net torque" in my second reply to you... However in terms of the thread, "net torque" doesn't apply, it means essentially nothing, every vehicle has more net torque at the wheels than measured engine power on a chassis dyno... The 600-700 I stated was in relation to whatever other "chassis dyno'd engine power" people who have had issues with 8.8's were making. Unless you can tell me what net torque an 8.8 grenades at (which is my point, its a ridiculous number with too many variables when you factor in tires and gearing and such), you are simply being obnoxious with numbers. "Chassis dyno'd engine power" numbers are a good reference to go from, because they can be applied across the board as an input to the axle.


The Dyno I test on is a Load Cell Dyno, which means it measures force at the rollers, applies engine RPM, and calculates engine torque minus parasitic losses of the drivetrain. It then calculates horsepower off of torque (force) pulled from the rollers over engine RPM. This is measuring engine power via the wheels. The other kind of dyno is an inertia dyno, where you have a constant mass (The drum) and the computer measures how fast your tires spin the drum over time, and directly calculating horsepower from that, and if you want a torque number, you have to have an RPM input to the computer.


So just so we are clear, we aren't talking Net-Torque-at-the-wheels (As if you bolted a load cell onto the hub and took a direct measurement), We are measuring Engine-Power-via-the Wheels (As if you bolted a load cell onto the hub, took a direct measurement, compared hub RPM to engine RPM, and formulated torque based off that) which there is nothing inaccurate about.

 

OK, I guess we're on the same page. I just didn't understand that when you specify torque "at the wheels" you "aren't talking Net-Torque-at-the-wheels " but instead "are measuring Engine-Power-via-the Wheels ". Most people talk about power when they want to talk about power so that just confused me for a minute.

 

Awesome!

 

Interesting argument guys,I enjoyed reading.

RacinNdrummin, after the upgrades you mentioned,I think the weak spot will be the axles. Don't think they'll handle diesel power and 33" tires.

 

no one measures torque applied on the wheel....


I know this is a reiteration but its simple, rpm is measured and the data supplied to the dyno computer, as well as hp at the wheels, and the torque in relation to hp/rpm is calculated on the fly.




R&D: you're going to be asking a lot from the 8.8, even with 33's . I suspect, despite its street plans, youre going to be subjecting the 8.8 to that available torque rather frequently. the tubes spinning is typically more prevalent with later housings, not so much the earlier ones such as yours. welding the tubes to the diff housing solves that either way. the way your turbo hits (I think its safe to say harder than most) would play a part in the survivability of the 8.8 as well. slow gradual spool is going to be gentle on the rear vs hard hitting spikes of power.


I think if you wanted to run the 8.8, the only thing you need to address other than what youre already aware of is the u-joint size. the stockers aren't likely going to live very long. then you're into driveshaft work, misewell get the front u-joint upsized if youre going to upsize the rear.


I wouldn't try to shy you away from an 8.8 build, but a dana 60 should be a consideration IMO. full floater, "1/2 ton-esque" size and weight characteristics, disc brakes, larger u-joint, cheap. 150 dollar axle, convert to 5 lug, driveshaft work, call it a build.

 

Sterling 10.25" 5-lug conversion - FSB Forums


This for the semi-float 10.25, but with the larger ring gear it would be the best option.

 

I'd call these guys.
http://www.strangeengineering.net/

I have c clip eliminators and 33 spline axles on my 90 Mustang. I think if you look around you find guys running some pretty serious power thru the 8.8 on the drag strip. Not your application I know but there are lots of upgrade parts out there. My bet is you would still need to be diligent with your accelerator pedal in certain circumstances.

 

Interesting...I wonder what the weight difference would be? Getting rid of the drum brakes and maybe (At best) modding a set of explorer discs to work, or (at worst) doing what the OP in the link did (Id like to retain the E-brake). Budget wise, I bet it would be similar to building the 8.8, and I know the factory trac-lok isn't going to grenade like the 8.8 will.