In the 80's, there was an option box you could tick to add a Isuzu Diesel motor, in Normally Asperated, or Turbocharged, to your Classic Motorhome.
Those that didn't tick that box, are 350 or 454 gas powered.
There are numerous people here, that either have a wish to put a Diesel in to replace the gas motor, or have done it, or a previous owner has.
As someone who is planning to put a diesel in, I thought it was time to have a thread where we can get all the relevant info in one place.
The venerable Chevy 454 V8 gasoline engine
Let me start by discussing the commonly fitted Chevrolet Big Block 454/7.4L V8 fitted to our Motorhomes.
This motor is an Icon.
It is about as simple, powerful, torque laden, and as long lived as gas engines come. The Truck/MH block features the better/stronger and more stable, 4 bolt main bearing caps, cast iron heads, with small port(Peanut port) intake runner, that enhanced low RPM torque.
Earlier engines came with a Quadrajet carburetor, and later models had TBI injection.
As fitted to our Motorhomes, I believe the motor weighs in at about 600lb, and power output is specified by GM, and Airstream at 230hp/360ftlb Torque.
Remember those numbers... they are important!
Interestingly though, in a recent article, Car Craft Magazine(March 2018 edition), ripped an original, untouched/unmodified 454, from a 1986 Suncrest Motor-home. This engine had an indicated 35,000 miles from new.
After they removed it, they put it in a dyno cell pretty much untouched, with its original Quadrajet carb, but used headers, and ran it as was, to get the baseline scoop.
They were surprised(and me too!) that the motor put out 335hp and 485ftlb torque!
Quite different to the official figures!
Link to that article, and watch the video... its kinda fun
http://www.hotrod.com/articles/bolt-...ome-big-block/
Given its task, of dragging up to 16,500lb (Airstream 345/350) around, it performs well enough, and most owners report about 6 to 8 mpg is general use for the early models with carb and 3 speed trans.
Later models had TBI and some had the 4L80E 4 speed Overdrive transmission. I have seen reports of 6 to 10 mpg from these.
Its fair to discuss modifications and tuning of the 454.
Having spent countless hours, mulling the 454, and possible improvements I reached the following basic conclusion...
On Carburetor, the venerable Quadrajet, when running right is tough to beat for power and economy. Some people hate 'em, and that is fine, but the experts say that the combo of the small bores for economy and larger bores for power is unbeatable. The correct "Motorhome" unit flows 800cfm, and I gather is special, rare, and right.
I'm also told that the stock, cast iron intake is better in the MH than any other.... an expert told me that the aluminum ones are not suitable, and even though they flow better, they hurt the torque and economy...
Same goes for heads.... The smal port heads are setup for torque. Im sure larger oval port heads(049, and 781 castings) make more power, but not where we need it! It may be that some valve work, and bowl blending on the stock head would help flow.
The stock cam is setup for torque, and there are numerous "RV/Towing" grinds out there that might add something.
The one area that is PROVEN to add power and economy is the exhaust manifolds and exhaust.
Banks Power quoted that their package which consists of Intake, long tube headers, and exhaust system adds about 100hp. It maybe that it did... compared to the official figures...
There maybe gains be had in putting on a modern, aftermarket Fuel Injection system, such as MSD, or Holley, or others. These kits cost $1000 up, and maybe very good for power, economy, and consistency, as well as consistancy.
TBI models can be improved slightly too, taking into account all the above comments!
One area that is worthy of comment is transmissions....
The standard transmission behind the Motorhome 454 is the TH475. This is a version of the famous and bulletproof TH400, with stronger, straight cut planetary sun gears, and normally a tailshaft mounted parking brake.
It is a 3 speed, and works well.
There is opinion that it does hurt economy, due to its lack of gears, lower gearing, no Overdrive, and slight slippage due to Non-lockup Torque Converter.
Many people have added a Gear-Vendors OD unit behind this transmission with good results.
There are some motor-homes fitted with Allison transmissions too, and I believe these are 4 or 5 speed OD units.
The modern solution would be a 4L80E or 4L85E, and indeed these were fitted to some later TBI Injected units. These are 4 speed, electronically controlled OD, Lock-up transmissions.
Later Allison units with 5 and 6 speeds are good too!
They will perform better and save gas.
So, lets talk costs, and do some simple math(Maths for Brits)...
Say you drive your 454 powered Motor-home 10,000 miles, in a year or whatever.
Let split the difference of 6 and 8 mpg, and call it 7mpg.
10,000 miles Divided by 7mpg is...... 1,429 gallons of gas.
Today, according to AAA, average USA price of gas was Nationally was:
Regular 87ron= $2.60/gallon.
Mid Grade 89ron was = $2.88/gallon.
Premium 91ron was =$3.13/gallon.
So, if you use regular, your fuel costs for 10,000 miles would be 1,429 gallons x $2.60 = $3,714.
Using that formula.
1 mpg improvement in economy saves you around $357...
2 mpg is about an $825 saving.
3 mpg is about an $1,114 saving.
Of course, if you use Midrange, or Premium gas, or you live somewhere like California, where gas is more expensive than the average, your numbers and savings may differ!
You can use the above formula, to figure out if that Fuel injection setup for
$1500 or that OD transmission will gain you mpg and how long it will take to pay off the investment!
Maybe!
Longevity of the 454 in our motor-homes, is a varied subject, with reports of engines expiring at about 50,000 to 70,000 miles, and some are still alive and kicking at 180,000 miles, so my guess is that its all about maintenance.
My info tells me that when a 454 wears out/expires, the cost of replacing it is very subjective.
People do what they want/afford, and the cost is all over the place.
I hear of people who spent $3k on a short block and reused everything else, and others who bought a GM crate 502, @ $8k and had it installed professionally with all the other bits, they are maybe $12k plus....
So....
1/ Why put a diesel in?
There is a good reason why larger/heavier trucks are Diesel powered.
A/ Torque. Diesels have a higher compression ration, plus the compression added by a Turbo. A diesel always has more torque per CC/CI than a gasoline engine, and so they get the weight moving.
B/ Longevity. Diesels engines generally last longer. in use due to their beefier construction, and the natural lubrication in Diesel fuel.
C/ Efficiency. Pound for pound, Diesel fuel produces more power than gas, and so better fuel consumption.
My research tells me that the Isuzu Diesel installed in a 310, and driven in the same way as the 454 gives between 11 and 15 mpg.
Isuzu engine peeps please post your numbers, so I can confirm, update???
I gather, Cummins 6BT powered rigs are reporting similar.
So, lets apply the same math we did for the 454, but skewed for Diesel.
10,000 miles, split the difference at 13 mpg.
Diesel is more expensive Nationally @$3.00/gallon.
10,000 miles Divided by 13mpg is 769 gallons... x $3.00 = $2,308.
By my simple math, a Diesel will save you $1406 per 10,000 miles.
Same logic dictates that:
1 MPG increase saves $165
2 MPG increase saves $330
And so on.
Not enough to justify the investment in logical terms, of course.
But there is another factor.
Almost all Diesel vehicles command a premium because of the perceived increase in MPG and longevity.
I believe, based upon the sale prices recently, it might be $7,000 to $15,000 over a similar 454 powered MH.
Interesting, eh?
2/ What Diesel engine should you choose?
There are many many options, and it is all about what fits, and what is proven to work.
Here is a basic overview of the Diesel engines that have been fitted, that we are aware of:
Isuzu
As already mentioned, this was the engine option from the factory, so maybe the easiest swap, either by buying the engine and drive train from a crashed Airstream, or replicating its configuration.
I will edit in the basic info when i get feedback from the experts here!
Cummins
The Cummins 6BT, a straight 6,
12v Turbo Diesel, was fitted by Airstream, to just ONE known 345 in 1990. There is at least one other almost Identical engine conversion, and several others that use the same engine in different configurations.
The 4 cylinder version of this engine, known as the 4BT, might also work very well, in smaller lighter Motorhomes.
It seems to be the most common swap for many reasons.
GM Duramax V8
There is at least one guy who has put this motor in his Airstream 280 in Holland, and another in Florida.
I will be asking for his permission to post his pics and knowledge.
Linear Mode
