turbo 400 tranny

If you are willing to spend money and want to increase your mileage, install fast burn heads. Increase the compression to about 10:1. The optimum most efficient point to push on the piston is 14 degrees after TDC. Any push before or after that point, is not as effective in pushing the piston down. This being understood, if the burn is faster, more energy can be focused to occur at that optimum point in the engine rotation. This significantly increases the efficiency of the engine. If you look at engines running fast burn heads, you will notice they require a lot less timing advance - like 10 degrees less. This is because the flame gets going that much faster. Not only is it more efficient, but it also means you can run higher compression, as there is less time for random uneven burn - detonation. So the higher compression also helps efficiency. That is one of the big reasons why the new modern engines get so much better fuel mileage.

A lot of what you are asking hinges on the vintage of the Isuzu diesel that is in the coach. The original 3.9 liter Isuzus only had about 135 hp, whereas the newer 5.2 liter models have about 205 hp and over 400 torque which would provide better performance than the original 454 gas V8 and good mpg as well.

I know. If I can get the fuel mileage up to 10 MPG I would be ecstatic. If I can get this 454 up to 10 MPG I will keep it. I have a friend that has a school bus, his son wanted the engine that was in it to put in his truck sold the truck, now the bus is sitting without the engine, He wants to trade me the diesel engine for my 454. I am tempted. The diesel was hooked up to the same transmission that is in my motorhome I wonder if it would fit into the space. I most likely will keep the 454, but I got to get the mileage up. Somehow.

I didn't run the numbers, but my feeble little brain suggests to me that the cost (and effort) to do a conversion of this type would never be recovered in fuel savings. Here in PA diesel is about 40-50 cents per gallon higher than gas before you even look at any savings from increased mpg.

OK, we can speculate on paper all over the place, but the first place to start is to weigh your front axle. Old motorhomes are notoriously over weight, so you need to know if you have any space to work with, as a diesel will be a bit heavier.

Being that you are coming from a 454 which had 220 HP, a 190 HP diesel from a power stand point is not a bad choice. I assume your coach is older, so I'm guessing 16K lbs chassis?? If you are going for mileage, I would convert to the 4L85E, assuming you are not over weight. The 4L85E is a few hundred lbs heavier. TCI makes a controller for around 600 bucks, which is what I am using. It is perfect for that application, as you can tweak the TC lock up point, to just stay locked up most of the time. That will give you the mileage and the gearing you are looking for. I would also lift one of your wheels off the ground, and spin the axle and count the rotations to figure out what gearing you have, to make sure you are in the ball park of were you want to be.

As far as dropping in the 4L85E, the trans is about 2 inches longer, so the drive shaft will need to be cut, but the is easy - about a 100 bucks. You will want to order the trans with a different output shaft, so the drive shaft yoke will fit. TH400 output shafts will fit inside the 4L85E, so no big deal, you just need to tell them what you are doing, so they know what you need.

I wish I could fit a DT466 in that motorhome,, I had a schoolie that got 10 MPG at 60. That was nice.

what series is that 190hp Cummins? I may see if an overdrive would be a good idea. I would like to get the rpms down if I am driving on the flat. I would like to get it at 10MPG I would be happy. Can I get a higher rearend ratio? any other ideas for better fuel mileage? God this country needs to be up in arms over these fuel prices. Am I the only one that is tired of this public abuse? sorry for the soapbox just filled my Ford ranger today. Plus put in 10 gallons of gas in the motorhome. it was high.

How much HP is the JD? What is redline? 2 incredibly important issues that must be answered before anyone can give much valid advice. If this engine is designed for a tractor, it is very likely it has a very, very low red line. That being the case, you end up with 2 problems. First off, the gearing will be wrong, and you may be running wide open at redline to hold 60 mph if you can even get up to 60. Second issue is that if the redline is very low, that will mean the power band will be extremely narrow and a 3 speed trans won't cut it. The engine may have enough power, but is likely designed for a much different application.

On Edit: Went back and looked at what JD 4 cyl diesels engine specs. Stock gearing for a P30 chassis is going to want to see ~2800 RPM @ 60. With a redline of 2400 RPM, you will be a traffic hazard. Not only that, but the highest HP rating is 140 HP, not enough for a motorhome. Most people complain about the 190 HP Cummins not being enough. JD is just the wrong engine choice for so many different reasons.

My desire would be to put a 4 cylinder John Deere diesel engine in. I remember using a John Deere tractor on a horse farm. it was huge and moved really fast in high range with a big round bale of hay. I am thinking that that tractor weighs more than my motorhome,, and this huge tractor had a 4 cylinder engine in it. I think it would not be worth it. but pondering it. you know what I mean. Not sure how good a 454 is.. but if I can get 14 MPG with a diesel I would certainly sacrifice power.

Not sure he bought the unit yet. I think he is just trying to figure out if this modification was a good idea or not, and are the compromises worth the mileage. Really, the only way to know is to drive it and see if it has enough power to move the coach or not. more than likely, the question that should be asked is if the diesel can be cranked up to give acceptable acceleration.