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ST824 Tecumseh-to-Predator Engine Swap

16K views 76 replies 17 participants last post by  Russell Stephan 
#1 ·
A one-take video overview presentation of the effort:

 
#4 ·
The modifications on the OEM unit will get me through the season.
175386
 
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#8 ·
They make belt cover risers for an st1136 that will work for you
Good to know! I'll have to look into that.
 
#9 ·
You will surely want to fix that open cover ... You do not want water and snow in all
the belts like you have it there.

Also, ditch the lawn turf tires, and get yourself some nice XTrac snow tires.
Per mention in the video, a cover replacement or modification will be forthcoming. But, for the time being, the modified original will suffice.

Central Ohio doesn't get the levels of snow to warrant a deep concern or additional expenditures for tire/traction issues. But, it's nice to know there are other options, thanks.
 
#63 ·
Chains on those turf tires still beat out any tire, thats what I did to mine, it barrels thru just about anything no problem, my ST824 has the Snohogs, I'll probably just throw a set of chains over those too. They worked great together on my old Snapper that had Snohogs.
 

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#11 ·
And since it's a completely fresh engine, might as well add a tach/hour meter. No doubt, I'll probably end up putting on a stage one hop-up kit with a 4° advance woodruff key. Might be a good idea to know exactly the engine RPMs.

And from the parts bin, a D handle for the pull-cord.



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#13 · (Edited)
We'll see. First, I have to guaranty all the currently-delivered PTO power makes it into thrown snow. Hitting big drifts doesn't bog the engine like I would expect. It does, however, cause snow to dribble out of the discharge chute. I have a feeling there's a bit of belt slippage going on. The unit does have wipers installed on two of the four impeller blades.

I've done some preliminary research on a possible double belt impeller drive. But, it looks like it will be a rather custom and intensive fabrication exercise. There does look to be some extra belt friction available with some simple modifications to the engagement clutch mechanism -- increasing the throw of the idler pulley into the belt slack. Or, a slightly larger PTO impeller pulley is also a possibility.

It might be better to keep this one as-is and wait for a roached engine chassis to come up on Craigslist or FB Marketplace which will accept the Ariens double impeller belt kit.

I'm already on the lookout for a parts chassis to obtain a chute drive ring for building a taller and straighter example.
 
#64 ·
My buddy put a 420 on his. Its too big and heavy and it keeps burning out belts. It tires him out fast and is a bear to operate. The 301 is more than ample, honestly the 212 on a 24" is already far more powerful than the 8hp Tecumseh ever was and that one is the direct bolt on replacement to the Tecumseh.
 
#15 ·
I'm patiently waiting for the predator 420cc engines to go on sale
at harbor freight and then I'm going to be putting one on my st824.
Pull up the owner's manual *.pdf off the Harbor Freight web site and get the 420cc base dimensions from the back of the manual. Keep in mind, the diagram is not to scale so go by the listed dimensions and measure... The 301cc 8HP engine barely fits on the flat part of the engine deck and the 420cc motor is a bit larger base-wise. In stock configuration, the 8HP Predator does cause some belt slippage when hitting drifts and such. The 420cc would be worse. 8HP is kinda the limit without redesigning the impeller belt drivetrain.

Hell, even a Predator 5HP motor properly massaged can get up there in HP -- via billet flywheel, rod, valve springs, etc. So, the same modifications on the 8HP will eclipse the stock-configured 420cc example.

Eventually someone is going to produce a super-charger for these Chinese screamers.
 
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#17 ·
I've had the Predator engine on the ST824 for about a week and a half or maybe two... I did all the 20 minute break-in runs without the tach/hour meter installed. Well, after last night's snow storm blow-through and the day-long dig out of the neighborhood, I've got 6.1 hours of actual snow blowing on the hour meter... Granted I do about a dozen driveways when the thing is running in addition to a couple of miles of sidewalk passes.

But, some observations... In deep or dense stuff, the belt is slipping. We'll have to start playing with ways to firm up belt friction through a longer clutch pulley throw or larger diameter PTO wheel. Wipers on two of the four blades causes the ejected snow-stream to pulse. I will be adding wipers to the other blades to get ride of the unnecessary vibrations caused by the pulsing.

Other than those issues, the Predator engine absolutely sings. Once its bed-in time gets beyond 10 hours, I'll up the max throttle setting to 4,300 RPM or so.

FYI, the go-kart crowd does same-engine races with the small 5 HP Predator. They install the engines right out of the box and set the max RPM to 4,800 and race to test the drivers and not their hardware.
 
#18 ·
Im curious how a mod like a billet flywheel would have any effect of a continuous load application like a snowblower?

I also want to hear more about the 8hp.predators. Im on the fence between engines to replace my 8hp tec that's not strong enough for my use. I can get a 10 hp tec but I'm concerned it's also not enough and will still have the same annoyances of the 8hp tec (terrible carb etc ). But, it is a drop in replacement.

To me the carb and governor on the hondas and clones seem much better than the tec. Will the 8hp predator be that much stronger than an 8 hp tec that's in good shape?

The belts concerned me as well. Im also using an 824.
 
#19 · (Edited)
Im curious how a mod like a billet flywheel would have any effect
of a continuous load application like a snowblower?
A billet flywheel because the OEM cast one may and has disintegrated under higher RPMs of modified engines.

I also want to hear more about the 8hp.predators. Im on the fence
between engines to replace my 8hp tec that's not strong enough for
my use. I can get a 10 hp tec but I'm concerned it's also not enough
and will still have the same annoyances of the 8hp tec (terrible carb etc ).
But, it is a drop in replacement.
The 8HP Predator modified ST824 suffers zero engine power issues. The engine hardly changes RPMs at all. But, there is a lot of belt slipping when large quantities of snow or wet/heavy stuff are tackled.

The stock Predator is just fine until I come up with a way to reduce significantly the impeller belt slippage.

To me the carb and governor on the hondas and clones seem much better
than the tec. Will the 8hp predator be that much stronger than an 8 hp tec
that's in good shape?
From my experience, the Predator engine is far smoother, quieter, and just sounds so much more well put-together. In a little more than nine days I've put 7.9 hours of actual snow blowing on the machine. I'm very satisfied.


The belts concerned me as well. Im also using an 824.
As mentioned above, the belt is the current limiting factor in getting all that engine power into thrown snow.
 
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#21 ·
A billet flywheel because the OEM cast one may and has disintegrated under higher RPMs of modified engines.


The 8HP Predator modified ST824 suffers zero engine power issues. The engine hardly changes RPMs at all. But, there is a lot of belt slipping when large quantities of snow or wet/heavy stuff are tackled.

The stock Predator is just fine until I come up with a way to significantly reduce the impeller belt slippage.


From my experience, the Predator engine is far smoother, quieter, and just sounds so much more well put-together. In a little more than nine days I've put 7.9 hours of actual snow blowing on the machine. I'm very satisfied.



As mentioned above, the belt is the current limiting factor in getting all that engine power into thrown snow.

Are you completely sure your belts didn't get wet?
Are those belts raw edge? From what I read last week raw edge belts grab better and can transfer more power. The original belts were raw.
 
#20 ·
Interesting... Stumbled on this electro-magnetic clutch while what-iffing the belt slippage problem:

 
#22 ·
Then, I wondered if anyone has put a charger and/or electric start on the Predator 301cc engine:



That's all I need... Another big time/money sink project.
 
#23 · (Edited)
Are you completely sure your belts didn't get wet?
Can't say for 100% sure. But, the slippage is pretty constant and predictable when hitting bigger snow bites or heavy street plow cast-off.


Are those belts raw edge? From what I read last week raw edge belts
grab better and can transfer more power. The original belts were raw.
Doubtful. I just sourced the larger size needed for the engine swap from the local NAPA. Looks like I'm a gonna have to source a raw edge example.

Thanks for the pointer!
 
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#24 ·
Can't say for 100% sure. But, the slippage is pretty constant and predictable when hitting bigger snow bites or heavy street plow cast-off.



Doubtful. I just sourced the larger size needed for the engine swap from the local NAPA. Looks like I'm a gonna have ta source a raw edge example.

Thanks for the pointer!

I'll help anyway I can.
Im tempted to put the hmsk100 on it because it's stronger but like I said before it'll still have the same crummy carb etc. I've been around many hmsk engines and they all run the same (not quite right).

The belts aside, your impression is the 8hp predator is noticeably stronger than the 8hp tecumseh was? I assume the tecumseh was in good shape?

Just trying to get a good comparison.
 
#25 ·
The belts aside, your impression is the 8hp predator is noticeably stronger
than the 8hp tecumseh was? I assume the tecumseh was in good shape?
Yes. The Predator should also take to running a bit faster in stock configuration better than the OEM Tecumseh. And hop-up parts are easily obtained for the Predator series.

Raw edged belt ordered. Although, it won't get here until March...
 
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#29 ·
As stated previously, the 420cc footprint is slightly larger. When installing the 301cc engine I did have to grind angles on the bolt head hex flats to avoid interference with the 45° bend in the engine deck at the edge allowing the studs to stand straight. If someone has done it previously, then obviously, it can be done.

As for getting all that power transmitted into throwing snow, Ariens did have issues with their greater-than 9HP setups necessitating the double-belt upgrade kits. No such kit exists for the ST824. So, we're on our own engineering wise unless someone has documented a fix. Power that can't be transmitted is the same as no additional available power.

There's only one way to test whether or not all that power is making its way into throwing snow. Stuff the nose of the blower into a big heavy pile of fresh street plow cast-off and see if it stalls the engine. If the engine doesn't stall when overloaded, the choices are: The belt is slipping. The impeller design is too small. The test isn't inputting enough heavy snow.

Also, see previous post concerning a YouTube presentation on installing charging coils and an electric starter on the 301cc engine.
 
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#30 ·
As stated previously, the 420cc footprint is slightly larger. When installing the 301cc engine I did have to grind angles on the bolt head hex flats to avoid interference with the 45° bend in the engine deck at the edge allowing the studs to stand straight. If someone has done it previously, then obviously, it can be done.

As for getting all that power transmitted into throwing snow, Ariens did have issues with their greater-than 9HP setups necessitating the double-belt upgrade kits. No such kit exists for the ST824. So, we're on our own engineering wise unless someone has documented a fix. Power that can't be transmitted is the same as no additional available power.

There's only one way to test whether or not all that power is making its way into throwing snow. Stuff the nose of the blower into a big heavy pile of fresh street plow cast-off and see if it stalls the engine. If the engine doesn't stall when overloaded, the choices are: The belt is slipping. The impeller design is too small. The test isn't inputting enough heavy snow.

Also, see previous post concerning a YouTube presentation on installing charging coils and an electric starter on the 301cc engine.
Agreed.
There is a benefit to using a stronger engine than the belts can handle. It could be run a lower rpm which would both be easier on the ears, and the engine.

A 3.5" pulley which I would expect should grab better than a 2.75 would give 1100 rpm impeller speeds at 2800 rpm and it would also give 1400 rpm at 3600 rpm if the need is there. Unless of course that single belt just can't do it.
 
#31 ·
Agreed.

There is a benefit to using a stronger engine than the belts can handle.
It could be run a lower rpm which would both be easier on the ears, and
the engine.
Design and engineering are always a, "Pick your poison," proposition of trade-offs...
 
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#33 ·
@Russell Stephan So I’ve been dealing with some belt slippage on my snapper 1030. The engine is plenty healthy and torquey and the impeller gearing is quite low. But in heavy snow throwing distances suffer even with impeller seals.

My though is why does my Honda with a big motor and high impeller speeds have ZERO slippage. It might have something to do with the large drive pulleys having greater surface area or maybe Honda uses better belts or BOTH.

Honda uses a bando w800 belt for the auger system. It certainly works great, but there is one higher line of belt the w1000. Once you dial in the most appropriate belt length, maybe try a w800 or w1000 belt of the correct size and see how it performs.

 
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