Which Rear Axle Ratio? (31ft MH, 7.3idi turbo, Allison 1000)

[Garbage_Mechan]

It is a 31 ft motor home so it will be loaded and then more loaded. Being an airstream it isn’t quite as bad of an aerodynamic brick as most. Use the overdrive in the trans to get your road speed. F Superduties with the 7.3 IDI have 4.33, 4.56 and 5.13 gears for a reason.

 

[Rusgo]

Honestly, since that turbo doesn't do much until 2000 rpm, it's just acting as dead weight your engine has to pump past until then. The hypermax system is meant for winding rpms up, so you only have a good usable power increase for the upper 1000rpm. Not a typical diesel workhorse type powerband. You're going to want that initial boost between 1500 and 2000 rpm. It's just so damn useful and helpful in so many situations. Makes its feel like a different (better) engine.

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[Rusgo]

And if you put the engine under heavy load before 2000, you'll call for more fuel, have the turbo restriction, won't get much extra air, and just turn that extra diesel fuel into heat and smoke.

I like the hypermax system's plumbing, but that turbo...man. Might as well be pumping lard with it.

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[Oledirtypearl86]

I jus st read through this pretty quick so I agree with Wes and a lot of the others I run a hypermax on my 86 F-350 and use it to pull my 34 foot camp trailer and I can say I max my boost at 14 psi and don't see anything before 1850 rpm but when it does boost it's there also I run 4:10 gears and a 4 speed I think keeping your gears lower will help a bunch keeping your trans from always shifting and holding a steady rpm range an that will help with fuel mileage and wear and tear that's just my two cents worth

 

[Macrobb]

I ran my Banks sidewinder as well as Factory turbo setups on the dyno; peak torque(with sufficient fuel, this was with a RD2-110 IP) was at 2750 RPMs. Basically right where the 'crossover' point between the turbo pushing more air and the IP running out of it.

The 1400/1600 torque peak is N/A, where you are just starving for air beyond that. You still get all that torque with the turbo setup, it's just that you get so much *more* torque higher in the revs.

Now, if you can get a turbo that'll spool down low, you might be able to do better.

 

[sjwelds]

@genscripter

 

[genscripter]

@genscripter

hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

Lots going on in this conversation. For the most part, I pretty much agree with everyone, especially Wes's comments.

Goal: don't create a traffic jam, doesn't matter where I drive but be able to cruise with good fuel economy and low noise.

With that huge rig, there is no way you can get both "keep up with traffic" and "get good fuel economy." Aim for one. You can't get both.

Here are my mods on my IDI van for fuel economy: http://www.nickpisca.com/diesel/turbo/the-great-big-list-of-eco-mods-for-6-9-and-7-3-idi-vehicles/ Maybe some of these will help you with eeking out 1 or 2 more MPG than what you are currently getting. I'd be very surprised if you get better than 15mpg on that beast with a tailwind and going down a grade.

Frankly, even with a decent turbo, you aren't going to be keeping up with traffic. The turbo will help, but

Even with a wicked wheel 2?

I put a Wicked Wheel 2 on my van's hypermax kit, and it did help a lot on the lower RPM's. Check it out here: http://www.nickpisca.com/diesel/tur...lation-on-a-hypermax-idi-van-turbo-kit-video/

The problem with the hypermax van kit (amongst other things) is the low RPM issue. The WW2 does help with that so it's a moderate improvement. Also, thanks to the doghouse access, the WW2 install is easy peasy. I'd recommend it if you are putting in that hypermax kit in the RV. You might even want to install the stock hypermax kit, tune it, and drive it around first, just so you can get a baseline. Then install the WW2 and record the difference.

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I'd like to comment on my people's responses, but i gotta run.

Overall, I'd be concerned with that kind of load with a 6-speed and a 3.55 axle. The EGT's will be a lot with a 90cc IP and hypermax. I have that exact setup, but instead of the 6-speed, I have the C6 + GVOD. Even with the WW2, if I go up a decent grade with a fully loaded van, I have to feather the pedal to keep the EGT's under 1200. The boost is fine, but it's just too hot. I gear down when I'm above 6000 ft like in Colorado and whatnot. When I was driving across Wyoming last summer, I had no problems with the C6 gears, but once i got the GV engaged and tried to keep up with the 80mph wyoming traffic, the EGT's would start to build up. My clubwagon is a brick, but that RV is driving a cinder block.

Now factor in the added airstream weight and wind resistance, and you'd have lots of EGT issues for sure. And you could just turn down the pump, but then what's the point? You lose what little boost that Hypermax kit will put out, even with the WW2.

I'd wager your safer bet is the 4.10's with the 6-speed, 90cc pump, ww2 + hypermax kit, UPGRADE that exhause to have straight-thru high flow muffler and that'll hopefully keep your EGT's at a 1000 deg max, but who knows. Maybe try an innercooler if you have the real estate for it. I've been putting that off on my IDI van for ages. I need to get back on to that project. That RV is a wildcard when it comes to predictions.

 

[Clemens Kruse]

Hey Nick! Nice to meet you here. I actually planned most of my setup based on yours. . I planned for the exhaust 3.5”. Should I go bigger to keep the egt low?

 

[genscripter]

Hey Nick! Nice to meet you here. I actually planned most of my setup based on yours. . I planned for the exhaust 3.5”. Should I go bigger to keep the egt low?

3.5 will be fine. Remember, the hypermax down-pipe is the bottle neck in their design. If you go more than 3.5, i seriously doubt it would amount to any noticeable EGT reduction. Definitely get rid of the stock pipes. when I had the stock muffler and pipes, my EGT's were 200 deg hotter than when I upgraded to the Walker hi-flow muffler and larger diameter pipes.

But even with that, I suspect your rig will be an EGT machine. That's a lot of mass.

I'm sure you already know this, but your shopping list should include head studs too. The WW2 upgrade will push your hypermax beyond their boost spec, and that would also push your head bolt capacity as well. If you are going to do that job in the engine bay, hopefully your RV's engine walls are wider apart than my van's. I did my studs when I was replacing my heads on an unrelated repair. there were 3 or 4 studs and pushrods that I had to rubberband into their respective cylinder head holes (before installing the head on the block) because I wasn't able to slide them into the holes with the heads in place.

Here's an image showing how freaking tight the space is between the wall and deck (and this is with the motor mount pulled on the other side, and the engine tilted as far as it could go):

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You don't want to have to pull the heads to put your studs in, so hopefully your engine bay is more roomy. Just keep that in mind. If you go with the WW2 and hypermax kit, you probably should consider studs.

 

[Clemens Kruse]

I will start another thread to ask for recommendations to rebuilt the engine. Yes I’m planning to do new head studs, new pistons etc. and it will be outside. The engine is free...

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[u2slow]

I recognize that GM chassis. I had the same in my 31' Winny SuperChief. That's a tight fit for an IDI... the chevy frame is narrower than the Fords. The 454 gas engine was the biggest it needed to accommodate. I like this style suspension. Good ride, yet very rugged, and decent brakes.

You likely have 4.56 or 4.88 gears. I would stick with those for road trials to see how it does. Possibly go up a tire size for fine-tuning.

 

[Booyah45828]

If you're using an allison with a double OD, 4.11 would be the lowest I would go. You lose so much quickness off the line by having it geared for highway mileage in something that big. You'll also want all the gear options you can get if you plan on driving any terrain.