390 Power?

level600a

New member
First you need to make it last, so you need Billet rod and flywheel,flywheel could be optional if you have a real honda flywheel not a clone but i would get the flywheel, then for power, get a flat top piston with a cam around 240 dur keep lift around .300, the 260 dur makes more power but if you use the 260 you will need a valve train, rocker arms,pushrods dual springs, etc.,if 240 dur use heavy duty springs, do a mild port clean up on intake side, I like stainless valves, but honda will work, lastly you will need a carb to feed it, 30-34mm range.
This should get you solidly around 20- 25 hp, There are others who may not agree but I have build the above engine, I haven't dyno it yet,still looking for one near by, it feels solidly in the 25hp (seat of the pants) range. I also have the 260 dur engine and it feels like 35+ hp, but it has good amount of head work done to it and a pumper carb on methanol.
Hope this helps
 
Level600 is right. Just a a reference, a bone stock blueprinted 390 with gov removed, header, and open air filter with adapter with a stock (.830) carb will be just shy of 18hp. Shave the head and put in some more cam with 24mm or larger carb and you will be 23hp and above easy. Billet rod is a must. Stock flywheel is fine but only if its a true Honda. Like he said, dont go crazy with the porting. Compression is key. Good luck
 
What about the balancer? Yay or Nay?

Good question. That big heavy thing is a power robbing son of a gun, thats for sure. It does, however, serve a purpose. If you take it out, it will shake. How much and at what rpm is a variable. Ive seen them shake like a paint mixer, ive seen them rum seemingly smoother. Leave it in if you can, typically. If a billet rod is used, you may have to do some minor clearancing of the balancer for the dipper. If you take it out, either remove the two bearings it rides in or peen them in place with a sharp punch.. You dont want one dropping out (and it will when it gets hot) and getting flung around. If you take it out, getting it balanced is a good idea. About a 58% to 62% balance factor is my suggestion. If you take it out, plan on using loctite or studs with locknuts or wiring all your sidecover bolts. Header too. The lighter the piston, pin and rod is, the better, as far as getting it balanced goes. For max horse power with frequent rebuilds, take it out. If you want a little more reliability, long run time, a season worth of races and sacrificing a horsepower or so, leave it in. Polish it up as much as possible, that will help a little. Good luck.
 

BrandonF260Rem

New member
Would putting a pop up piston be better than milling the head? If left to close the same specs as Level600. Or would you need upgraded valve sizes, bigger Cam, Ect?
 

rkcarguy

New member
Just my .02.
The balance shaft can remain if you'll keep it under 6000rpm.
One thing I DO NOT like, is clearancing the balance shaft when you add a billet rod. Putting a notch in a big off-balance spinning piece of weight at 6000rpm is like scribing a piece of glass in my opinion.
I'm more likely to cut some of the dipper off of the billet rod (sorry ARC) or take the balance shaft out.
 

level600a

New member
Would putting a pop up piston be better than milling the head? If left to close the same specs as Level600. Or would you need upgraded valve sizes, bigger Cam, Ect?
If you are talking about Parson's dome piston, I was told by them it will add 1 point to your compression ratio, so if you had 9:1 to start you have 10:1 with the piston, piston will help even if you don't have any other parts in the engine, because of the compression increase,as for it being better then milling, guess in some ways it is but not for the compression it's better because you have a stronger piston with increase compression. You can get more compression with milling, just remember to check clearances for piston, valve and piston, head.
 

level600a

New member
Just my .02.
The balance shaft can remain if you'll keep it under 6000rpm.
One thing I DO NOT like, is clearancing the balance shaft when you add a billet rod. Putting a notch in a big off-balance spinning piece of weight at 6000rpm is like scribing a piece of glass in my opinion.
I'm more likely to cut some of the dipper off of the billet rod (sorry ARC) or take the balance shaft out.

I agree with this statement all the way up until you said you are " more likely to cut some of the dipper off of the billet rod " that I can't agree with cutting on the rod is a sure way to break it's like you said "like scribing a piece of glass" Really the dipper is not the problem it's the rod bolt that will not clear.
 

71 chevy

New member
i took the balance shaft out of my engine, and it runs smoother, go figure. and this is with a 2lb flywheel instead of 12lb flywheel.
 

level600a

New member
i took the balance shaft out of my engine, and it runs smoother, go figure. and this is with a 2lb flywheel instead of 12lb flywheel.
Every motor that I took the balancer out of shook like a paint mixer, I just run it and make my power below 7000 rpms.
 

71 chevy

New member
Every motor that I took the balancer out of shook like a paint mixer, I just run it and make my power below 7000 rpms.

hmm. when i took mine out i ran it without a clutch and it shook bad. but once the 5.25lb clutch was on it, all shaking disappeared and it actually ended up being smoother than with the shaft in, especially on the track
 
I race mowers which is over 500 with rider. I ran with balancer in it and alot of use run with one in fxs class with gx390. I spinned it up to 7600 once. I was told 7500 and under the balancer wont blow up but over 7500 it will blow up like a grenade.
 
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