Horse Power Question

[Az Shadeguy]

If a Honda gx390 and a gx340 have the same bore and stroke.
How come the 390 has 11.7 hp and the 340 has 10.7 hp?
What increases the hp?

[KE4AVB]

According to the shop manual they not the same bore. HP figures here are Gross HP

[Az Shadeguy]

This must be a misprint on the 340

https://engines.honda.com/models/model-detail/gx340

https://engines.honda.com/models/model-detail/gx390

[bobodu]

That's interesting...every link I clicked on has the 340 listed as 389 cc....

[38racing]

definitely somebody goofed on pubs
http://m.engines.honda.com/models/model-detail/gx390
http://m.engines.honda.com/models/model-detail/gx340
or has honda decided the model number will no longer match displacement.

[KE4AVB]

Well fellows looks like we can put this to bed now. I just got back email from a Honda dealer in AB, Canada which contacted Honda directly and they say it is incorrect info. The GX340 does indeed has a 82mm bore.

Hopefully who ever stated the info gets it straighten out. This what we get when non-mechanics just do what they are told without fact checking things. It leads to a lot mistakes. Just think if one of us had ordered 88mm piston and rings for a GX340 just to find out we had the wrong parts after having to wait a week for the parts.

I did ask if they were using a restrictor plate instead which could have explain the HP differences but that is not the case here.

[wristpin]

There was a time when a ride on mower sold in the UK used the same Vanguard Vee twin with some badged 14Hp and some 16!

[KE4AVB]

Same with the Briggs 420000 Opposed. 18.5 19.5 and 20.5 all with the same model and type numbers. All on Craftsman mowers.

[Az Shadeguy]

I found myself back at this website with the wrong info
https://engines.honda.com/models/model-detail/gx340
So I emailed them and this is there response

Hello Paul,

Thanks for writing in with your question regarding the differences between the new GX340 & GX390. The website is correct in advising that the displacement, bore, and stroke are the same between the two. The GX340 has less horse power and torque due the tune that the unit was designed with. We hope this information is helpful.

Cordially,
Honda Engine
Customer Relations


Now I am really confused. How could a tune be different?
It looks like they maybe changed something in 2010
"The GX340 is all new for 2010, featuring more power, quieter performance, lower fuel consumption, and lower emissions."
From this page

https://www.lawnmowerhosp.com/product/h ... engine-539

[bgsengine]

If you are able to engineer the amount of air and fuel going in, you're gonna be able to change power output even with the same displacement, though you might need to change things like valve dimensions, lift duration, timing, etc. So it is entirely possible to have a single universal displacement with varying amounts of power output by changing other things... with a closer look at all the relevant parts involved you may be able to spot what is different in the engineering..

However, I think it rather stupid since Honda has historically had their engines "named" after their cc displacement.. (389cc rounds up to GX390, etc) This is probably just exactly why you should not pay a whole lot of attention to the model, and look up your Honda specs and parts based on the engine's serial number and type number.