what are the pros and cons of the big bore kits? who makes the best kit hpd or aaen? is there alot of difference between the pump gas head and the race fuel head? if you or someone you know got this kit please respond about it.

 

This topic seems to come up often, A search of the archives might reveal more posts.

Here is one I copied rom another forum:

"Some time ago, I posted a long response to a question very similar to this one. In the interest of time, I'll try to sumarize that posting here.

I have had Aaens 430 kit (86.9mm piston = 415cc) and HPDs big bore kit (86.5mm piston = 411cc). Lots of overbores are available, from 86.5mm to 88mm (88mm = 426cc). My experience has always been positive. All else being equal, a good big man will always beat a good small man; all else being equal, the bigger piston will always breath a bigger air fuel mixture. The big bore will make more hp and torque everywhere, but mostly in the middle. It will pull better on launch, on top, and everywhere else. But remember, you need to compare apples to apples, ie., don't compare a high end ported std bore to a trail ported big bore. I am talking similar porting, carbs, pipes, etc.

Sometimes, bigger bores are not more powerful than smaller bores. The best example is the comparison between a twin and a triple of equal displacement. A 600cc triple will make more power than a 600cc twin, because the smaller bores of the triple breath better than the bigger twin bores. This gets complicated, but basically the smaller bore requires less time for the air fuel mixture to travel from the transfer ports to the center of the combustion chamber (smaller bore=less distance to travel), and therefore the smaller bore can fill itself faster (higher rpm potential) and more complete (faster scavengeing).

However, when comparing a big bore to a small bore when displacement is not the same, the losses attributed to the heavier piston (less rpm) and the longer travel distances for the mixture (less rpm) are much smaller than the gains for the bigger absolute fuel mixture breathed by the bigger bore. Net result is more hp everywhere (albeit most in the middle).

Cons of the big bore are; more potential to overheat (thinner sleeve), cost to bore the jug and buy the overpriced piston, possibility that the carb and pipe may not be large enough.

I have never overheated mine even with no electric fan installed, and I found that my 38mm tmx and hot seat pipe are a great match."

Jim Dostine:'96 Scrambler 4X4

Here are my thoughts,

Basically the BB kits are designed to be a high performance racing application. As stated above there are cooling and reliability issues that need to be resolved to make the kit worthwhile for a trail machine. The ratio of money spent to performance gained is minimal, unless to already have the parts to maximize the new found HP.