OK, decision time: 28 or 35

[marrt]

Ok, something ain't right. So, let me post some additional details.

If you go to tewarehouse.com and look up these engines, you can download a high resolution image of the bolt circle under the "Specifications" tab for each engine. For the image I posted of the 25 bolt circle, you can see in the orange highligted area a spec of 163.5 for the pilot flange. Assuming this is mm, that's 6.43701". In the image below, you can see a 6" scale on the motor mount. The machined flange is clearly larger than 6" and is probably 6.44" (I only have a 6" caliper).

As far as I can tell from the spec, the 35 uses a 5.75" flange. But Gravy said, if I'm reading his message correctly, that he needed to enlarge his motor mount flange to fit the pilot flange mount. So, ether I'm not understanding his comment, or his old 25 motor mount is significantly different than mine.

Now, my mount will fit the 35 bolt circle, assuming it's 5.75", but it won't take advantage of the engine's pilot flange to accurately position the mount. Given the natural "play" in the coupling, this may or may not matter. I have a milling machine with a DRO that will accurately position a new bolt circle in my existing motor mount. Would this be "good enough" and I can just ignore the pilot flange altogether?

My existing motor mount with a 6" scale:

Here's where it mounts to my 25" (highlighted with a black marker):

Here's a pic of the tube (just for fun):

 

[Gravy]

Hopefully this will explain the modification. The new bolt circle is larger and unevenly spaced, while the new pilot is smaller.

 

[Frank Sorbello]

Gravy, Does it idle down nice? Do you have to run it at full throttle? Is this engine noticeably louder than the EH72? How many hours do you figure it took to swap the engines?

Thanks
Frank

 

[Gravy]

Frank,

It idles just fine. I only run it at full throttle for mowing uphill or hard digging. I built a custom super quiet muffler, so it's hard to compare the noise level. Two notes about that: Once the exhaust noise is under control, the hydraulic whine is amazingly loud; and building a custom muffler is much more trouble than it's worth. Buy a Nelson instead.

As far as how long it took, my memory of chronology is spectacularly bad, but I think it amounted to about a weekend to get it running and a couple of evenings of improving details. That includes machining the pump adapter and cutting/welding the exhaust manifold outlet to fit through the notch in the engine cover. The custom muffler took another couple of days.

 

[MossRoad]

Marrt,
Where's the sawzall and torches? :laughing:

Very interesting project. Thanks for the pics. I'll be following it. :thumbsup:

 

[Marks481]

Hmm... no one's done the Kawasaki before. I know I'm curious about how'd it turn out.

Scrag did.

http://www.tractorbynet.com/forums/p...-modified.html
http://www.tractorbynet.com/forums/power-trac/110268-pt-429-project.html

He had problems fitting it all in, and ended up mounting the radiator above the engine lid, making the back end of his PT425 about a foot taller. There has got to be a cleaner way to shoe-horn it all in, since the base engine (sans radiator) is basically the same size as a EH72.

I seem to recall he had like a 150 page thread on blow-by-blow of his conversion, but couldn't find it.

 

[marrt]

View attachment 433921 Hopefully this will explain the modification. The new bolt circle is larger and unevenly spaced, while the new pilot is smaller.

Thanks Gravy. That clears it up for me.

Tom

 

[joshlee]

Thats interesting about the hydraulic brakes working better at full throttle. I've heard it said many times that an engine is cooler when at full throttle, but I don't believe that. Since putting this Subaru muffler on and the fan, I took many, many temp readings on the engine head, crankcase, and different places. I'm not saying its a good thing to lug the motor because of not giving it enough throttle,I'm just saying the temp rises drastically if you do a test when it is idling for about 8-10 min; then do the same thing when its at a higher RPM. Although, its not necessarily a bad thing, but the engine gets hotter, no doubt about it.

Couple reasons. First, airflow over and around the engine is diminished at lower throttle which can lead to overheating. Second, hydraulic braking and response is best with full engine power.

 

[marrt]

Ok, let's run through some numbers. The Robin EH72 is rated at 25HP. This is using the old J1940 standard. Using the new J1995 standard like Kohler and other engine manufacturers, the advertised HP would liking by reduced to about 23.5HP. Even Subaru recommends running the engine at 20 HP for continuous use. So, the recommended continuous use is about 80% of the gross rating. Stay with me.

The B&P 21cc pump's maximum GPM at 1500 PSI and 3600 RPMs is about 20 GPM. This requires 20 HP. So, tramming the 425 down the road at full speed will already "uses" up the full recommended HP of the engine. This isn't really surprising to me. I noticed that running the PT at full speed on level ground puts a decent load on the engine.

Now, what about mowing? The PTO is rated at 8 GPM at 2500 PSI. This requires about 13.7 HP. So, when mowing, you have very little reserve capacity to drive the tractor. Further, these are optimal efficiency numbers and should be reduced, I'd guess, by at least 15% due to drag caused by fittings, hoses, control valves, etc...

Now, most of us don't mow at full speed and don't require maximum pressure and flow rates from the PTO for mowing. However, some of us do have hills and, based on the numbers above, we don't have enough HP available to climb hills while mowing without over-stressing the engine. I'd say the 25 HP engine is fine for non-PTO uses. However, once the PTO is switched on, you quickly run outside the recommended HP range of the engine. Said another way, if you want to stay near the maximum recommend continuous use ratings of the tram and PTO pumps and the engine, you need a larger engine.

Now, let's look at the 35 HP engine. Subaru doesn't list the recommended HP for continuous use. However, using the same 80% metric above, we'd get about 27 HP for continuous use. So, assuming reasonable mowing speeds (not full tram) and a reasonable PTO load (not full flow and pressure), the 35 HP engine seams much better suited to the 425's tram and PTO pumps.

Why bring all this up? Isn't it obvious that more HP is always better? Well, not necessarily. If you dropped a Chevy small block in there, you'd quickly max out the tram and PTO capacity and run around with the bypass valves fully open. This wouldn't be good for hydraulic temperatures.

In researching this, my main concern was the 35 might have too much HP and push the tram/PTO pumps into bypass too often. While this is still a risk, the 35 seems much better suited to normal PTO use without over-stressing the engine. Further, it appears that PTO use for mowing on hills is almost ALWAYS over-stressing the 25 engine.

BTW, I'm not looking to start a flame war. Just having a little fun on a Saturday afternoon.

 

[Gravy]

This fits my experience since I swapped engines. Where the original engine struggled to tram uphill at all while running the mower, there is now enough power to mow and move as fast as my back and kidneys care to bounce. If I were in my 20's and not too bright, I might even go for the 40hp version. It would probably take somewhere around 50hp to simultaneously push the tram and PTO circuits to bypass while mowing thick grass up a steep hill .