Engine swap?

[Gravy]

After 5 years and approaching 600 hours of hard use and abuse, along with several catastrophic air filter failures in extremely dusty conditions, the Robin EH72D 720cc/25hp engine in my PT425 has just about had enough. The blow-by is now overwhelming the additional crankcase breather separation/filtration system that I added last year. I also cranked up the governor to a maximum speed of somewhere between 3800-4000 rpm to compensate for lost compression. (It was out of warranty, and I needed the power...)

I don't think a rebuild is practical. The engine has ingested a lot of dust, and I suspect the wear is systemic. Also, it's snow season, and disabling the PT for more than a few days could be a problem - we have a long driveway.

On top of that, there are some improvements I'd like to make over the original Robin engine: I want MORE POWER! I also want less noise and much better fuel efficiency. (My employer has a Cat 416C backhoe that weighs on the order of 10 times what my PT425 does, and can lift and dig proportionally. It does all that while using LESS fuel per hour than my PT.)

I'm starting to look at replacement options. There are a bunch of choices for replacement engine. There are two that a pretty much no-brainers:

1: An exact replacement Robin EH72D. Pros: Exact fit with no worries. Cons: It was underpowered, excessively noisy, and a fuel hog from the factory.

2: The new Robin fuel injected EH72FI. Pros: Probably exact fit, since it's the same basic engine with the addition of EFI. Also, it's rated to run at 4000RPM with an increased HP rating. Cons: The claimed power and fuel efficiency improvements are pretty trivial (maybe 4-11%). Also, the ECU is incorporated in the throttle body, which leads to concerns about heat and vibration damaging the electronics. In addition, I haven't seen any claims of noise reduction.

Those are the easy ones. After that it gets complicated. Honda, Kohler, Briggs, Generac and Kawasaki all offer a range of air-cooled V-twins, some of which *might* physically fit. Some of them are fuel injected. Some have greater displacement, some have higher compression ratios. Some have extra noise controls.

But wait, there's more! Even on the basics, comparing apples to apples isn't easy. Some manufacturers quote "SAE Gross" or "maximum" HP & torque specs. Others quote "SAE Net" specs, which always give lower numbers for the same engine. (Not to mention DIN or ISO specs).

So aside from the implied rant, I do have an actual question: Does anybody know of a replacement engine that will be an overall improvement? If I have to, I will buy an exact replacement, but I'm really hoping for better.

Anybody got a spare "Mr. Fusion" laying around?

 

[BobRip]

Gravy, you might want to mention some of your technical skills and what you are willing to change. Can you weld. Would you be willing to cut the tub and lenghten the engine compartment? Sounds like no, since you need it turned around quickly.
Some other issues - Will the pumps and wheel motors take the additional speed and torque without shortening the life too much. Somebody did an engine upgrade (I can't remember) and did extensive modifications to the machine, only to have it damaged by turning on the PTO without anything attached.

Good luck with this. I sure would like the extra power for my 422. Although by this time I have gotten use to it.

Consider buying a 1430 and making an adapter for your existing attachments.

 

[KentT]

Well, a Kohler Command CH 25 will obviously fit in the pan...

Moss, I, and others are running them. Is your configuration with all three pumps on one end, or are there pumps on both ends of the engine. I haven't checked to see what the size of the new, larger Kohlers are....

Personally, I'd be tempted to try the 24HP Honda... quiet and reliable.

 

[RegL]

When my engine gets tired ( hopefully in the distant future ) I think I will seriously look into converting the PT to electric. In most cases, if I could get a couple of hours run time out of a charge, It would do me fine. For jobs that need more energy and time, like field mowing, maybe one could hang a portable generator off the rear. If you have ever driven an electric golf cart you know they can be a real pleasure. Quite and torque. A silent tractor would also be cool.

 

[ponytug]

How electric are you thinking?
Electric motor(s) driving hydraulic pumps?
All electric?
Electric drive power?
Electric PTO?
Electric control (lift, curl, Q/A)?
Electric steering?

Just curious...
All the best,

Peter

When my engine gets tired ( hopefully in the distant future ) I think I will seriously look into converting the PT to electric. In most cases, if I could get a couple of hours run time out of a charge, It would do me fine. For jobs that need more energy and time, like field mowing, maybe one could hang a portable generator off the rear. If you have ever driven an electric golf cart you know they can be a real pleasure. Quite and torque. A silent tractor would also be cool.

 

[RegL]

How electric are you thinking?
Electric motor(s) driving hydraulic pumps?
All electric?
Electric drive power?
Electric PTO?
Electric control (lift, curl, Q/A)?
Electric steering?

Just curious...
All the best,

Peter

Electric motor driving the hydraulic pumps like the gas or diesel engines do now. That's why I think it could be an easy conversion. Could probably just run the motor at a constant speed.

 

[ponytug]

You might want to consider using a pressure driven variable speed control to minimize power usage. That way you don't needless drive hydraulic fluid through relief valves, or dump it back to the tank.

All the best,

Peter

Electric motor driving the hydraulic pumps like the gas or diesel engines do now. That's why I think it could be an easy conversion. Could probably just run the motor at a constant speed.

 

[BobRip]

Electric motor driving the hydraulic pumps like the gas or diesel engines do now. That's why I think it could be an easy conversion. Could probably just run the motor at a constant speed.

The Chevy Volt has a 500 pound battery to drive it 40 miles. One gallon of gas (5 pounds also) will drive it 40 miles. The energy density of the battery is 1/100 of gasoline. The Volt goes to extremes to not waste energy. The PT pumps run all of the time pulling some power even when sitting still. I think battery drive in this case is not very feasible. If you drive every pump with it's own electric motor, that would help, but still very wasteful. One of the reasons the PT uses so much fuel is the pumps running all the time. Just my opinion after developing battery powered devices for 15 years, but not vehicles.

 

[ponytug]

Bob,

Exactly. Which is why you would only want to run a pump to do work, and then shut off.
I think it is worth doing the math about how many watts(HP) for how long, derated by efficiencies of the motors, hydraulic fluid, the batteries and temperature to get a battery size.

All the best,

Peter

The Chevy Volt has a 500 pound battery to drive it 40 miles. One gallon of gas (5 pounds also) will drive it 40 miles. The energy density of the battery is 1/100 of gasoline. The Volt goes to extremes to not waste energy. The PT pumps run all of the time pulling some power even when sitting still. I think battery drive in this case is not very feasible. If you drive every pump with it's own electric motor, that would help, but still very wasteful. One of the reasons the PT uses so much fuel is the pumps running all the time. Just my opinion after developing battery powered devices for 15 years, but not vehicles.

 

[BobRip]

Bob,

Exactly. Which is why you would only want to run a pump to do work, and then shut off.
I think it is worth doing the math about how many watts(HP) for how long, derated by efficiencies of the motors, hydraulic fluid, the batteries and temperature to get a battery size.

All the best,

Peter

Yes, separate motors for each pump and variable speed would help a lot. I am thinking that even 1/2 hour of run time can do a lot. A small generator hung off the back or in a trailer (hard to manuever though). One of my big learnings about batteries was to minimize consumption when not doing anything. This can be a surprising help. If you are using the PT to haul wood and loading by hand and even unloading by hand then most of the time you are not doing anything. You also rarely need full power so go slow with the motor when possible.

I doubt if Gravy plans to go this far. So we are really getting off topic.

 

[MossRoad]

You could put an electric motor at each wheel and drive the steering and lift with hydraulics like our electric fork lift does... except it only has one electric motor on a differential that drives the two front tires...

I think you are re-inventing the wheel if you try to convert a PT to electric. Too much to change for little benefit.... unless you just like to make cool stuff!

 

[Gravy]

Gravy, you might want to mention some of your technical skills and what you are willing to change. Can you weld. Would you be willing to cut the tub and lenghten the engine compartment? Sounds like no, since you need it turned around quickly.
Some other issues - Will the pumps and wheel motors take the additional speed and torque without shortening the life too much. Somebody did an engine upgrade (I can't remember) and did extensive modifications to the machine, only to have it damaged by turning on the PTO without anything attached.

Good luck with this. I sure would like the extra power for my 422. Although by this time I have gotten use to it.

Consider buying a 1430 and making an adapter for your existing attachments.

Good questions and comments.

I can weld, fabricate and machine at a pretty basic level, but I don't want to get into anything like modifying the tub. Those kind of projects tend to spin out of control. I'm hoping to find something that doesn't require more than drilling a few holes and splicing some wires.

I'd love a 1430, but unless Santa leaves me a bag of 100's, that's not gonna happen.

 

[Gravy]

Well, a Kohler Command CH 25 will obviously fit in the pan...

Moss, I, and others are running them. Is your configuration with all three pumps on one end, or are there pumps on both ends of the engine. I haven't checked to see what the size of the new, larger Kohlers are....

Personally, I'd be tempted to try the 24HP Honda... quiet and reliable.

Kent,

I'm not entirely sure that the Kohler will fit. Mine has all 3 pumps in front of the engine. I think there was a pretty extensive redesign when PT changed engines and wheel motors (and maybe some pumps).

I'm very tempted by the Honda engines, but I REALLY want more power. Unfortunately Honda quotes net HP and Robin quotes gross HP. Apples & oranges...

 

[Gravy]

Some other issues - Will the pumps and wheel motors take the additional speed and torque without shortening the life too much.

I don't worry much about this. The relief valves should protect the pumps and motors from excess pressure. Higher speeds might increase wear rates, but I'm hoping to find an engine that makes more power without spinning faster.

 

[Gravy]

I would also love an electric version as an additional machine. It would be great for those quick hauling & loading jobs that don't last long enough to warm up an internal combustion engine. I'd probably convert a PT180 and change attachment plates to share buckets.

 

[MossRoad]

PJ's PT425 has a Kohler engine in the back and all three pumps in front of it, so a Kohler can be mounted in there. The hard part will be finding the correct mount for the variable volume pump to mount to the engine. You might want to consult Terry at Power Trac on that to see if it would be hard to swap a Kohler for the Robin.

 

[MossRoad]

Here are some observations I made about PJs machine back in 2006.


It was nice to see an updated model of the PT425. Paul痴 is, I believe, about two years newer than mine. Some observations I made were:

- His tractor is several inches longer than mine.
- It is much, much quieter than mine with the same Kohler engine. I believe this is due to the mid mount muffler and insulating wrap around that muffler.
- His motor is turned 180 degrees from mine and set at the very rear of the engine compartment so that the cooling fan is pointed out the back.
- In front of the engine is his tram pump, which is hydraulically operated VS my cable operation.
- In front of the tram pumps are his two PTO pumps. Mine are behind my engine.
- His fuel tank is in the hood. It was very easy to open the hood. I was very surprised as I expected it would take more force.
- The battery and hydraulic filter are mounted way up high, where my fuel tank would be. This looked like a much superior setup than mine. Everything was much more accessible.
- His treadle assembly is the hydraulic version and the foot pedals require a very light touch compared to mine, which I have to stomp down on due to the spring tension in my hydroback assembly.
- His wheel motors are mounted much diffently than mine, They are on individual brackets that are bolted to the frame, whereas mine are mounted on a different type of bracket that is welded to the frame.
- He also has the higher lift height. I noticed the front shelf of his model where the lift cylinders are attached is deeper than mine, with a large recess in the front.

 

[KentT]

Don't know if you're aware of this place -- Padgett's Small Engine Warehouse. They have some of the best prices I've seen on new engines. I've not bought one this size, but I did buy a 6.75 HP lawnmower engine from them at one heck of a good price.

They have 3 pages of different horizontal shafts in the 24-27HP range.

 Engine Search by Small Engine Warehouse

 

[Gravy]

I stopped by Power-Trac and talked to Terry and Keith. They encouraged me to stick with the 25hp Robin on the grounds of it being a direct fit and because of the horsepower input limits of the pumps. Terry did say that I could probably get away with 2 or 3 more HP without too much risk.

On the other hand, Honda has a new GX690 engine that looks like it might fit with just a little finagling. According to the drawings, the Honda looks like it is a little bit longer and a little bit narrower.The main engineering issue might be the bolt circle for the pump adapter. I haven't looked at that in detail. I need to find better drawings for the Robin.

Translating the "net" vs. "peak" power ratings, it looks like it probably would be called a 27 or 28HP engine compared to the Robin 25. The displacement is a bit less at 690 vs. 720cc's, but the compression is significantly higher at 9.3/1 vs. 8.3/1. It looks like it might make as much power at about 3200RPM as the Robin does at 3600RPM. I'd love to be able to get the job done at lower RPM's.

I've loved every Honda engine I've ever had - they start easily, run quietly, and don't seem to break as long as they have oil. The Honda GX690 seems to be priced about the same as the Robin EH72D. If it will physically fit and is actually available for the advertised price, I will probably try it.

 

[tracdoc]

Well, I'm sure impressed, Gravy. Even considering an engine transplant in a PT is leagues beyond what I would take on. The only "consideration" I could possibly offer relates to our going where no one had gone before in the missus' race car a few years back. We took a perfectly good full race prepped Mini Cooper S (supercharged), and for reasons that are thankfully cloudy now that time has passed, decided to twin-charge it by adding a turbocharger. The R&D was very hard on the pocketbook, since different race engine shops had radically different ideas. In hindsight---what a flail!!! We eventually realized that a car with front wheel drive and the aerodynamics of a barn door was not able to effectively deal with 300 wheel HP. Terry's advice may be the most sage of all....