Well now I've gone and done it...

[marrt]

You got my attention.

Me too! I've had this fantasy about building the ultimate 425. Larger engine, larger displacement wheel motors (I'm going to do that for sure, just haven't had time), suspension seat, etc...

BTW, I thought we concluded the wheel motor circuit does not have any reliefs. The systems depends on wheel spin for relief, or engine stall. Correct.

In any case, VERY cool.

 

[KentT]

BTW, I thought we concluded the wheel motor circuit does not have any reliefs. The systems depends on wheel spin for relief, or engine stall. Correct.any case, VERY cool.

My old PT-425 doesn't have relief valves nor case drains...

 

[J_J]

I am not sure that is true about the relief valves.

Just about any hyd circuit would have a relief valve. You really can not afford to be building up pressure beyond the limits of the hyd components.

There are relief valves in my 1445 VSP pump, and when I go to the tow mode, the TOW bolt on top just opens the relief valves so the fluid can pass through with minimal pressure.

If you have your rpms up and full pedal, and you are stopped and are not spinning tires, then the reliefs are working.

It is just illogical to build a hyd circuit without relief.

You could almost bet that the hydrostatic pump circuit has a relief in the pump or the wheel motor circuit.

What is the brand name and model number on the 425 pump?

 

[marrt]

I did a little homework on this and both the Bondioli & Pavesi NT10 and M4PV21 have relief valves in the pump.

 

[KentT]

I guess I was a bit hasty with my statement -- I meant no external bypass valves in the wheel motor circuits. I have no clue what's inside the pump...

 

[Gravy]

I still don't have time for pics & lots of details, but I've had a bit of time to think about it. I'm pretty sure the Robin big blocks will work in any PT 422/425 without major modifications to the tub. The basic big block Robin engine is really only about 1/2 inch longer than the small block, and is actually narrower. The Robin 22-25hp engines are dimensionally smaller than the Kohlers, so you Kohler guys should have plenty of room.

The Robin big blocks have a flywheel PTO shaft option, so there's hope for MossRoad and others with pumps on both ends of the engine, depending on how the pumps are supported.

The only real challenge in my conversion was modifying the pump adaptor to work with the different pilot diameter and bolt circle of the new engine. I have an old metal lathe, but most folks might need to pay a machinist to handle that. It's pretty simple, and not terribly precise by machinist's standards. +/-.005" is probably fine. It took me several hours, but I'm no machinist.

One thing that I initially missed is that the crankshaft centerline height is considerably different. I had to cut down the engine mounting spacers a good bit to keep the pumps at around the original height so they would clear other components. I also drilled the engine mount bolt holes a little farther back than I should have, so I had to grind a notch on the tub to clear a bump on the fan shroud. I will correct that error so the fan inlet screen will fit.

The rest was pretty straightforward. The electrical system was a straight plug-in.

That's all I have time for now. More eventually...

 

[marrt]

Thanks for the update. Curious to hear about the muffler situation when you get time. Also, this engine has the potential in inject a LOT more heat into the engine compartment, which is already overheated in my view. You may want to consider adding a fan somewhere.

Also, I'd consider adding some screen to cover the air intact holes at the back of the tub. The screen will quickly get clogged with debris in some situations, but the main air intact shroud should still be free to pull air from the engine compartment. I need to do this to my tractor because the engine is getting clogged with leaves.

 

[Gravy]

Thanks for the update. Curious to hear about the muffler situation when you get time. Also, this engine has the potential in inject a LOT more heat into the engine compartment, which is already overheated in my view. You may want to consider adding a fan somewhere.

Also, I'd consider adding some screen to cover the air intact holes at the back of the tub. The screen will quickly get clogged with debris in some situations, but the main air intact shroud should still be free to pull air from the engine compartment. I need to do this to my tractor because the engine is getting clogged with leaves.

The muffler is whatever the local muffler shop had in stock. I asked for the smallest inlet/outlet size, oval shape, QUIET and 18-24" overall length. With an elbow on the outlet to point the exhaust noise away from my ears, I can now hear the actual mechanical noise from the engine. Turns out there's quite a bit of that, too.

The Robin big block comes with both a stationary inlet screen and a rotating screen that attaches to the flywheel/fan. Between the two of them, they add at least an inch to the length of the engine. Once I re-drill/slot the engine mount holes in the bottom of the tub, I should be able to move the engine forward enough to accommodate the rotating screen. I'm hoping that the rotating screen will throw debris off.

Regarding the heat issue: I have an idea that the bigger engine may actually run a bit cooler because the airflow and cooling fin surface area should be greater to be in proportion to the displacement. The 720cc engine was maxed out most of the time while mowing, and was running on the ragged edge of it's designed cooling capacity. The 999cc engine should be relatively loafing under the same load, and have plenty of cooling capacity in reserve. That assumes that the tub allows the hot air to escape adequately. If not, I'll take appropriate measures.

I did get to play just a little bit after work today. With the old engine at full throttle, climbing the steepest parts of my front yard wasn't any problem as long as the PTO was off and I feathered the treadle with a bit of care. With the new engine, it's still not any problem if I feather the treadle a bit...but today, I did it at about a quarter throttle.

It's still early in the game, but my impression at this point is that the big block engine hasn't turned my PT425 into a hotrod unstoppable beast. It will just let the PT do everything it should be able to do without panting and gasping desperately. I'm pleased so far. We'll see how it all works out.

 

[marrt]

Please keep us updated. Would love to do this swap myself. Wish you were closer...I'd stop by to help.

 

[J_J]

If the engine rpm is the same, you will have the same speed as before.

If you want more speed, get larger diameter tires.

What you have is extra HP above what is required to power all pumps at maximum.

 

[marrt]

What you have is extra HP above what is required to power all pumps at maximum.

J.J., I'm not sure this is true. In other words, I'm not sure 25 HP will power all pumps to full pressure and speed WHEN the wheel motors and attachment motors are at full design load. For example, there's a hill on my property that has a moderate slope. If I attach the mower, but leave it off, the tractor will climb the hill fine. However, if I turn on the mower and the grass is fairly tall, I can't make it up the hill due to the engine not having enough power. I believe MR has described this behavior in the past too. Since the tractor and mower are driven by different pumps, this suggests we don't have enough HP.

In addition, I can stall the engine by placing the tractor against an immovable object and slowing flooring the tram. This suggests a) my reliefs are set too high in the VV pump or b) the engine doesn't have enough power to break the reliefs before it stalls.

And finally, I've notice others claiming a large increase in "power" from installing a larger engine. In theory, it shouldn't make any difference in power if the engine was originally sized to take advantage of the maximum capacity of the pump and other components.

 

[ponytug]

I would agree; I think that the machines do not have enough power to run all loads at maximum slope, or we would be able to charge hills at full throttle. Therefore, more HP will make a difference to the mower load/slope/speed tradeoff. One way or another, it is all load, aka power, i.e. HP.

I think that this is a fantastic project. Hats off to Gravy for doing it and sharing it with us.

All the best,

Peter

J.J., I'm not sure this is true. In other words, I'm not sure 25 HP will power all pumps to full pressure and speed WHEN the wheel motors and attachment motors are at full design load. For example, there's a hill on my property that has a moderate slope. If I attach the mower, but leave it off, the tractor will climb the hill fine. However, if I turn on the mower and the grass is fairly tall, I can't make it up the hill due to the engine not having enough power. I believe MR has described this behavior in the past too. Since the tractor and mower are driven by different pumps, this suggests we don't have enough HP.

In addition, I can stall the engine by placing the tractor against an immovable object and slowing flooring the tram. This suggests a) my reliefs are set too high in the VV pump or b) the engine doesn't have enough power to break the reliefs before it stalls.

And finally, I've notice others claiming a large increase in "power" from installing a larger engine. In theory, it shouldn't make any difference in power if the engine was originally sized to take advantage of the maximum capacity of the pump and other components.

 

[J_J]

The machines that are designed for slopes should have enough power to do the job at the advertised angle.

As your machine get work/older, it will not meet the specs.

I think some of you are asking the machine to do more than it is designed.

This is just my opinion, but why would you not designed for less HP for the pumps requirement, plus some extra.

Have you ever heard of 110 percent.

Maybe PT is not telling the full story. Like will my 425 climb a 30 degree hill with mower running full out and not stall?

 

[KMA]

FWIW, when I called Terry a while back to ask if my PT422's pumps would handle the EH72 FI (25-28hp) vs the original EH65 (22HP), he said "yeah, no problem".

 

[J_J]

You can turn a hyd pump with any size engine that is above the HP requirement of the pump.

10 GPM at 3000 psi, requires about 21 HP. If that pump /HP was designed for flat ground, then you can expect it to not perform climbing a 30 degree hill.

 

[KMA]

I noticed a marginal improvement in climbing hills with the finish mower engaged. It wasn't night and day, but a bit better. That being said, the bigger engine won't turn it into something it's not.

 

[MossRoad]

Yeah, my 425 will not climb steep hills with the mower engaged. If I shut it off, up we go. So, not enough power to turn the wheel motors and the mower deck motor at the same time in that situation. Is that bad? I don't think so. If I put larger displacement wheel motors, I think I could climb that hill and power the mower at the same time, but I'd sacrifice top speed, which I do use often to get from one side of the place to the other. Although, thinking about it, I wouldn't mind giving up 2mph top speed. On flat land, the PT scoots just fine at full speed. But out in the woods, or hauling a trailer of mulch, etc... I never go top speed. If I ever lose a wheel motor, I may consider a swap, but until then, I'm keeping the thousand bucks in my pocket. :laughing:

 

[MossRoad]

And to be clear, I really have no need to mow that hill. I only brush hog a trail on it a couple times a year. I can do that in the down direction. Its not like I'm mowing vast expanses of lawn on a hillside.

 

[KentT]

I understand JJ's point, IF we were dealing with fixed displacement pumps, matched closely to the output power of the engine...

But, since the tram pump is variable displacement a larger HP engine provides the potential to produce more torque (pressure) at the wheel motors BECAUSE the factory-supplied engines cannot provide both maximum flow and maximum pressure at the same time. Flow (speed) and torque/power (pressure) is ALWAYS a balancing act. We have all experienced the need to "back off on the treadle" to climb a slope, for example, especially when running a PTO implement such as a mower. With a larger HP engine, you would not have to back off the treadle as much... Similarly, when pushing into a pile of dirt with a bucket, the engine would not start to drop RPMs (and eventually stall) as soon...

You would feel the power difference -- though how significant is TBD...

 

[MossRoad]

Makes you wonder if you had more HP available, and you did not back off the pedal as you would with the lower HP engine, would the pump be able to handle the added HP or blow up? Or bypass, I mean. Since none of us have apparently ever hit the bypass on the tram pump, lets add some more HP and push them till they bypass, then back off the HP. Right? :laughing: