Engine swap?

[BAGTIC]

When my Subaru siezed last year and needed to be replaced I looked at other engines that could be used. I found a couple (Kohler and B&S) that would have offered significant improvement and fit the Subaru footprint. The problem was that they were a little TOO HIGH. If a person welded and was willing to deepen the engine compartment by that much they would have offered far more power and more efficiently.

Ignore the max horsepower ratings. They are irrelevant as they are at max continuous rpm where sfc drops off considerably. That is what eats the gas.

Check the respective engines performance curves. Look for a larger displacement engine and compare its horsepower at max torque (approximately 2/3 of max rpm). A larger engine derated by lowering rpm will still produce considerably more USABLE hp and at its most efficient rpm speed it will be much more fuel efficient. The lower rpm should also make it quieter.

An engine in the 40 hp max range running at max torque will use less gas than a '25 hp' engine running wide open while still providing better fuel consumption, more torque, and better sfc.

 

[jaquetarr]

I've used Small Eng W'house several times and find the engines / service / shipping very reasonable. I've probably got 3-4 of their engines and do not hesitate to recommend them. Understand that the engines they sell may not be direct replacements. For my uses, they are suitable
Rgds,
tim

 

[Gravy]

When my Subaru siezed last year and needed to be replaced I looked at other engines that could be used. I found a couple (Kohler and B&S) that would have offered significant improvement and fit the Subaru footprint. The problem was that they were a little TOO HIGH. If a person welded and was willing to deepen the engine compartment by that much they would have offered far more power and more efficiently.

Ignore the max horsepower ratings. They are irrelevant as they are at max continuous rpm where sfc drops off considerably. That is what eats the gas.

Check the respective engines performance curves. Look for a larger displacement engine and compare its horsepower at max torque (approximately 2/3 of max rpm). A larger engine derated by lowering rpm will still produce considerably more USABLE hp and at its most efficient rpm speed it will be much more fuel efficient. The lower rpm should also make it quieter.

An engine in the 40 hp max range running at max torque will use less gas than a '25 hp' engine running wide open while still providing better fuel consumption, more torque, and better sfc.

You are touching on a lot of the issues that I am trying to figure out.

"Ignore the max horsepower ratings". I've been trying to, but it's not easy. It's apparent that there are various hp rating standards in use - Max, Continuous, Recommended, Peak, and Net, not to mention SAE or DIN or ECE. Unfortunately, unlike the automotive industry there does not seem to be any consensus standard that allows "apples to apples" comparison.

"The problem was that they were a little TOO HIGH." Oddly, height has been the least of my concerns. I replaced the OE air filter on my Robin with one that is easily 3" or 4" taller, and still have quite a bit of room before I have to worry about hitting the hydraulic oil cooler.

"SFC" doesn't ring any bells. Specific Fuel Consumption? Anyway, I like the idea of a derated larger displacement engine. Manufacturers other than SubaruRobin make "big block" engines that run around 1000cc's displacement as opposed to the typical 700-750cc's of a "25hp" engine. The problem I have found is that they are generally too wide and/or too long to fit in the tub. The Robin engine is unusually short front-to-back at about 12.5" (not including the crankshaft stub). Going by memory, the big-block Briggs, Kohler, and Generac engines run more in the 15-17" range. They are wider and taller as well.

Raising the doghouse would be relatively easy. Lengthening and/or widening the tub is a whole different can of worms, especially if the bottom mounting holes don't line up. I can dedicate a weekend to replacing the engine and doing some minor modifications. I'm not up for re-engineering the whole rear platform.

At present I'm leaning towards either: the fuel injected version of my old Robin (somewhat more power at 3600rpm, significantly more power and a higher redline at 4000rpm [where I might be overspeeding the pumps], somewhat better fuel economy, supposed to start much better in cold weather); or the new Honda GX690 (Honda reputation for being quiet and reliable, higher compression ratio, slightly smaller dimensions, maybe higher hp and flatter torque curve depending on how you compare old Honda ratings to the current ones. [The GX670 used to be rated at 24hp. Now it's rated at 20.5hp.] Interpolating from that and the current ratings, the GX690 should have something like 26-28 "peak" hp).

What I really care about is torque in the 2000-3200rpm range. I no longer mow with my PT, so I'm not out for maximum main PTO flow rates to spin the blades. I just want to keep on pushing and climbing without breaking pumps or motors or my bank account.

Oh yeah - I finally found more information on the Chinese V-twin diesels. They are apparently available in the US. The 20hp ones *might* fit. The bigger ones won't. I need more power, not less. If I had a 418 or a 180 or even a 422, I might consider one. Also, it looks like they are a good bit louder than what I have now (Yikes!).

I've probably missed some critical points. I am wide open for suggestions. I really hope to get this figured out in the next month or so.

Cheers! Gravy

 

[woodlandfarms]

Only question I had was about the Diesel V Gas. Isn't HP on a diesel not an accurate assumption of the engines capability. Meaning a 26HP Gas has no where near the torque of a 20HP Diesel?

 

[MossRoad]

Only question I had was about the Diesel V Gas. Isn't HP on a diesel not an accurate assumption of the engines capability. Meaning a 26HP Gas has no where near the torque of a 20HP Diesel?

I've been lectured on this several times. So here goes...

HP is HP no matter where it comes from. A diesel or gas engine properly and accurately rated at 25 HP will both deliver 25 HP. HP is the ability to move a weight a distance in a given time. So both will be equal at that point. However, the diesel has the ability to stay in motion under load better than the gas engine because it has a larger piston and longer crank arm. Once you get that mass and leverage going, it is harder to stop it, thus, it can use that momentum to stay moving. That is why a diesel with the same HP rating as a gas has a much wider torque curve. If you notice, a diesel and gas rated at the same HP, the diesel is usually a much larger engine in displacement, too.

 

[jaquetarr]

Mr G,
Could I ask for a link to your China diesel engine?
Rgds,
tim

 

[Gravy]

Only question I had was about the Diesel V Gas. Isn't HP on a diesel not an accurate assumption of the engines capability. Meaning a 26HP Gas has no where near the torque of a 20HP Diesel?

Oversimplified answer: not so much.

That reputation is mostly based on the fact that old school heavy duty engines are big and slow and make a bunch of torque at low RPMs. The reason most of those heavy duty engines are diesel is because diesels are historically more fuel efficient and less maintenance intensive than gasoline engines.

Horsepower is a calculated value based on torque x rpm. An engine that makes 20hp at 1800rpm has more torque than one that makes 26hp at 3600rpm. That "20hp" engine likely displaces and weighs twice as much as the "26hp" engine.

When you compare engines that make their rated HP at 3600 rpm, the torque differences are much more subtle. For our purposes, it's probably more meaningful to compare torque vs displacement among the available engines that will fit.

 

[Gravy]

I've been lectured on this several times. So here goes...

HP is HP no matter where it comes from. A diesel or gas engine properly and accurately rated at 25 HP will both deliver 25 HP. HP is the ability to move a weight a distance in a given time. So both will be equal at that point. However, the diesel has the ability to stay in motion under load better than the gas engine because it has a larger piston and longer crank arm. Once you get that mass and leverage going, it is harder to stop it, thus, it can use that momentum to stay moving. That is why a diesel with the same HP rating as a gas has a much wider torque curve. If you notice, a diesel and gas rated at the same HP, the diesel is usually a much larger engine in displacement, too.

Again, not so much.

While diesels often have a longer stroke and heavier components, that is not a defining characteristic. What you are describing is really just a heavy duty/high torque design that can be achieved with either a gas or diesel engine. "Make it big and turn it slow."

The diesel advantage has more to do with maintenance requirements, fuel efficiency and compatibility with forced induction.

 

[Gravy]

Mr G,
Could I ask for a link to your China diesel engine?
Rgds,
tim

Here ya go:

A1biofuel.net

I haven't investigated this stuff beyond a quick look at the website.

I do have a little bit of experience with an engine very similar to the 10-13hp models on that site. It was painfully noisy. I can have a conversation standing fairly close to a 13hp Honda gas engine. I'd have to yell loud over the Chinese diesel. I don't like to think of what the big ones sound like.

Also, they are physically too big to replace my Robin 25hp. Still, I'd sure like that diesel fuel economy.

 

[jaquetarr]

You know these are Tier III engines - not.
Rgds,
tim