GX390/Predator/clone 420?

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Brettm57

Member
Right up front - I don't know diddly squat about these engines! But from what I've read, I take it most of the internal parts will interchange between these? Is the Honda better built than the Predator/clone?

Here's my situation: I'm thinking about putting one of these engines in a snowmobile for my grandson to ride (Grandpa might want to chug around on it now and then too...). Durability is more important than performance. With that in mind, is the GX390 a better built engine than the Predator/clone versions? Should I cough up the extra money for a Honda over the Predator/clone? This sled could potentially be ridden several hundred miles per year over the next several years, so I don't want something I have to rebuild every year if I can avoid it.

I'm thinking that for my purpose I'd want to swap out the rod for a billet one, and possibly heavier valve springs. I'd probably swap the stock carb for a Mikuni and a fuel pump to make it easier to adapt to a snowmobile. I'm going to use some type of CVT clutch on it, so if that's set up right it should hold the RPM's down. I'm guessing I won't want it to rev much above 3600 RPM with all stock internals? In stock configuration, will it make more HP if I rev it above 3600?

Rather than learn the hard way, I thought I'd ask the experts on this site. Thanks in advance for your advice!
 
I'm a complete motor moron as far as knowing the technical differences, but what I do know absolutely 100 % from working heavy highway construction for 40 yrs is whether it was a water pump, trash pump, tack buggy, generator, pressure washer, the ones with Honda engines would always start easier and last 4 times as long as any other.
 
Happy to chime in. I have run the Honda gx390 and the Vegas VC460 (Predator) in long track road race series for about 4 years. Typical rpm is between 6400-6800. Without doubt the quality of the honda is superior and worth the expense.

If you are planning to keep the revs around 3600 I don't think the difference in quality will be discernible or worth the extra cash. Billet rod is a good idea as the stock rod is the weak link in this type engine.

Keep the oil changed, the lash adjusted and the head bolts tight and you will be okay with either engine.

I would think twice about the heavier valve springs. At 3600rpm the increased internal drag they create will do two things. Rob the engine of power and wear out your camshaft faster.

As far as the Mikuni carb and fuel pump goes, that your preference. They work well in high rpm engines, I'm not sure how they would improve performance in a low rev engine. Check back with us and let us know how it goes.

These big blocks make most of their torque at low rpm. Higher rpm will shorten the life of the rod and decrease the time between rebuilds.
 
I wouldn't buy the Honda, I just don't think it's worth double the price of a Predator or clone, at least not for what you're using it for.
The Honda is gonna be better quality for sure, but the Predator is perfectly fine.

You might want to check out the Wildcat 460cc engine, it costs slightly more than the Predator, but it's 40cc larger and makes more power / torque, should be more durable too. They're out of stock right now, don't know when they'll be back.

I'd advise against the valve springs and Mikuni for the same reasons stated by Compu-kart.
 
These are the kind of things I need to know. Thanks! But it also makes me think of more questions.

If I use the stock valve springs and cam, at what RPM will these engines make the most power? From the info you've given me, it sounds like I want to rev it higher than 3600. What should my target RPM be? Will I have any problems with valve float at that RPM? I need to know that in order to determine what drive clutch setup to use. Unlike a kart, snowmobile clutches should shift out at the peak HP RPM and not let it rev above that.

As for the Mikuni, I'll build my own setup for that. I think the 34mm kits available will be too big for what I want to do. Depending on the size of the stock carb (I've never seen one), I'm guessing probably something in the 28mm to 30mm range will be better for what I'm doing. I've got a pretty complete inventory of Mikuni carbs and parts on hand, I think I can come up with something that will work.

I'll look into the 460. There's no replacement for displacement!

Thanks again!
 
Here is a thread that ends up discussing horsepower vs torque, and the reason for more rpm if possible.

https://4cycle.com/karting/threads/rotational-torque.110040/
Toward the middle of the 3rd page, post #51, is where that discussion starts.

There is a graph of the torque curve for a 212 predator used for the math part of the explanation. This will be somewhat relevant to your application.

I believe the Vegas carts engines are largely intended for golf cart applications, with torque converters. A lot of the legwork, including cam and spring choices for rpm, will have been done already.

Just a thought.
 
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Good Morning, we run mower racers with engines governed @3600 rpm in stock trim without issue. With a stock cam,oem carb bored from 21mm to 24mm jetted, timing set at 30 to 32 and a pipe will run up to 4500rpm before it noses over. We have run them with the stock rod and a Genuine Honda Flywheel in this combination. Yes a billet flywheel and billet rod will give you insurance with the loss of the charging system. Harbor Freight has the NEW 459cc engines in stock now and has pressure oiling also.
 
Another option is a Honda UT2 390 flywheel and digital coil that has a built in rev limiter around 4500 rpm.
 
This will most likely be a summer/fall project, so there won't be any updates right away. But when the time comes I'll be sure to let you know!
 
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