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Old 07-27-2016, 06:48 PM   #1
Brian Boyer
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Default Big port head vs small port heads?

Well i have the Lifan,.. Its closer to a GX390 than a Loncin Predator,.. The 2011(ish) and on GX390's are UT2,.. CDI ignition AND small port heads vs the previous UT1 big port heads,..

The Lifan utilizes the CDI ignition,.. But from what i gather DOES NOT share Honda's thinking in going to the small port head,..the Lifan is still the large port head..

What are ya'll thoughts on this?,.. I'm in cam selection mode and just when i had settled into a decision, NR's 280TQ, i stumble upon the head information,.. Obviously this needs consideration in cam choice,.. Thoughts?

(And thank you)
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Old 07-28-2016, 06:44 PM   #2
Brian Boyer
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Default Re: Big port head vs small port heads?

No takers? Come one come all,.. Any input welcomed
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Old 07-28-2016, 07:59 PM   #3
sho305
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Default Re: Big port head vs small port heads?

I would guess most all the performance parts were engineered for the old style GX390s, not the new one. I looked at that cam too but not sure, however NR has been nice about answering questions when I asked via email. Since the cams are mostly for carts or mowers I would want to know the powerband rpm and so on. I figure I would just get VCs cheater cam its cheap, get a rod in and run that for the time being and see how it works. The larger cams require typically all new valvetrain as well. I'm not really sure how the cam would work in my cart mostly, with the CVT, is why I would hesitate. Certainly a TQ cam or marine would likely be better in a cart, most of the race cams are big rpm.

Yeah honda pretty much trashed aftermarket use of their v twin as well.
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Old 07-28-2016, 09:02 PM   #4
Brian Boyer
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Default Re: Big port head vs small port heads?

My motor ideally operates just ever slighlty above you all's max sustained rpm, but we run for lllong periods.

4,700 to 5,000 rpms or so on the top end,.. Need to make power from 2,000 through 5k.
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Old 07-28-2016, 09:13 PM   #5
TX Foilhead
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Default Re: Big port head vs small port heads?

In theory the small head should be more responsive down low but not as strong on the top end. The only thing that I would question is weather or not most of the simple carbs can use this as an advantage. A couple of tenths on the race track is a difference, 10 seconds going around the neighborhood isn't noticeable.
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Old 07-28-2016, 09:20 PM   #6
Brian Boyer
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Default Re: Big port head vs small port heads?

Quote:
Originally Posted by TX Foilhead View Post
In theory the small head should be more responsive down low but not as strong on the top end. The only thing that I would question is weather or not most of the simple carbs can use this as an advantage. A couple of tenths on the race track is a difference, 10 seconds going around the neighborhood isn't noticeable.
I guess the 389/420cc= 390 is 92.6% of the size engine as the 420,.. So that may be why Lifan stuck with the big port head,.. Wish i could see the two head ports side by side.

Ah well, my heads on& paid for guess i better do my best with it,.. Heck i'm gonna stay 280TQ cam and see how it does.

If it doesn't work/ scratch my head later.
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Old 07-29-2016, 07:44 PM   #7
sho305
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Default Re: Big port head vs small port heads?

Honda updated the whole engine, the idea was to make it more efficient and lower emissions. Optimizing it for 0-3600rpm and efficiency makes smaller ports a likely thing to do.

My clone stock cam and ported/cut head, 4 degree advance, 1.3 rockers, header, stock carb, would run just over 5K on 22s to 38mph on gps. So said the $15 tach that no longer works. Most of the time it runs 4K on the CVT as it seems to pull hardest there, so I set it to shift there.
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Old 07-30-2016, 06:46 AM   #8
slonomo
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Default Re: Big port head vs small port heads?

Just get the Vegas Carts 460 head, cam, and rod and be done with it. Port it on the short side radius, take that big bump out. Clean the rest up smooth. Polish the combustion chamber. Add the cam and rod. Cruise at 5500 all day all night, and enjoy the added midrange power you just added. At $227 you'll enjoy better performance and reliability for the cost of dinner and drinks and a movie. The difference will be very noticeable.
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