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  1. #1
    Platinum Member
    Join Date
    Jan 2005
    Posts
    650
    Location
    SW Virginia
    Tractor
    PT425

    Default Engine swap?

    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?

  2. #2
    Elite Member BobRip's Avatar
    Join Date
    Sep 2004
    Posts
    4,260
    Location
    Powhatan Va.
    Tractor
    2000 Power Trac 422

    Default Re: Engine swap?

    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.
    Bob Rip
    Tell me and I will hear.
    Show me and I will see.
    Let me do and I will learn.
    Let me fail and I will understand.

  3. #3
    Elite Member KentT's Avatar
    Join Date
    Mar 2005
    Posts
    2,648
    Location
    Sevierville, TN
    Tractor
    Power Trac PT 425

    Default Re: Engine swap?

    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.
    Power Trac PT-425 w/numerous attachments
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  4. #4
    Platinum Member RegL's Avatar
    Join Date
    Aug 2003
    Posts
    616
    Location
    Northwestern CT
    Tractor
    PT 1430

    Default Re: Engine swap?

    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.

  5. #5
    Veteran Member
    Join Date
    Mar 2007
    Posts
    1,171
    Location
    Bay Area, CA
    Tractor
    Power Trac PT1445

    Default Re: Engine swap?

    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
    Quote Originally Posted by RegL View Post
    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.

  6. #6
    Platinum Member RegL's Avatar
    Join Date
    Aug 2003
    Posts
    616
    Location
    Northwestern CT
    Tractor
    PT 1430

    Default Re: Engine swap?

    Quote Originally Posted by ponytug View Post
    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.

  7. #7
    Veteran Member
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    Mar 2007
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    1,171
    Location
    Bay Area, CA
    Tractor
    Power Trac PT1445

    Default Re: Engine swap?

    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
    Quote Originally Posted by RegL View Post
    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.

  8. #8
    Elite Member BobRip's Avatar
    Join Date
    Sep 2004
    Posts
    4,260
    Location
    Powhatan Va.
    Tractor
    2000 Power Trac 422

    Default Re: Engine swap?

    Quote Originally Posted by RegL View Post
    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.
    Bob Rip
    Tell me and I will hear.
    Show me and I will see.
    Let me do and I will learn.
    Let me fail and I will understand.

  9. #9
    Veteran Member
    Join Date
    Mar 2007
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    1,171
    Location
    Bay Area, CA
    Tractor
    Power Trac PT1445

    Default Re: Engine swap?

    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
    Quote Originally Posted by BobRip View Post
    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.

  10. #10
    Elite Member BobRip's Avatar
    Join Date
    Sep 2004
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    4,260
    Location
    Powhatan Va.
    Tractor
    2000 Power Trac 422

    Default Re: Engine swap?

    Quote Originally Posted by ponytug View Post
    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.
    Bob Rip
    Tell me and I will hear.
    Show me and I will see.
    Let me do and I will learn.
    Let me fail and I will understand.

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