First off, turbo's don't add CC's.

Second, with a big bore and stock exhaust, you will maybe see a gain of 3 hp. Add a big bore with a performance exhaust, and you will see upwards of 6 hp.

You get more bang for your buck that way. I think that is what most people are trying to argue.

But why are you arguing CC's on a 250 motorcycle?

 

because a 351 big bore kit is almost 50% larger??????

 

1. 3 HP, are you kidding??? You been breathing to much thin air. I think you just like loud toys.

Who is trying to argue? lets do the math.... 350/250 x 15HP =21HP
a gain of 6 HP.
You get 6HP out of an exhaust? I don't think so.

2, I disagree, turbo's or superchargers do add cc's as when you put more air & gas into the cylinder, is that not increasing cc's? The increase in boost can significantly increase the amount of air & fuel an engine uses and the amount of power it produces. But adding a turbo on a bike is just crazy.

 

I've got the 351 with stock exhaust and stock air filter. It's just perfect for me. The 351 has given me just what I wanted. More power, and a really big smile on my face. If for some reason my stock exhaust fails or get's damaged, then I will upgrade.

 

durielk, I think the reference to a turbo and a supercharger is just showing an example of not everything having to be displacement related. However, even in your reference to how a turbo/supercharger increases performance by increasing the amount of air and fuel, I'll bet you don't see many applications of those devices where an equally improved exhaust isn't present to take advantage of them. No one is saying that displacement isn't highly beneficial. Perhaps it's just more beneficial if you give it an exhaust that takes full advantage of it.

I think the OP's original purpose for the question may have been strayed from here a bit, and I'm equally guilty of helping it stray. I think what some of us are trying to say to the OP is that installing a big pumper carb on a stock KLX may not be the best, first move in the modding order. I only mentioned the exhaust as another logical step before the carb. No one has to take any of the advice given here, and the OP won't blow up his bike by doing the carb first...as long as he jets properly. We're just trying to discuss the reasoning here.

As to the turbo and supercharger references, I don't think anyone is suggesting or contemplating one on a KLX. Your statement that displacement rules...PERIOD...may have been taken in a general sense when I think you meant strictly in the case of the KLX...and maybe generally with dirtbike engines.

 

Well the KLX300 has larger displacement and produces more power with basically the same exhaust as the 250 if you enlarge the exhaust outlet on the muffler.

I don't think anyone is discounting the benefits of a better exhaust system on these bikes, i think the point trying to be conveyed is that for the money spent on mods vs. actual power increase the big bore kit produces the most results dollar for dollar.

 

This whole thing started when someone sayes, "The exhaust and rejetting should come before a carb or big bore any day." A blanket statement like this whether it applies to the KLX or all vehicles, sounds like a rule to me & rules are made to be broken. On a drag bike or high speed racer, the exhaust would probably move up in priority, but we don't have cubiic inch restrictions, so that is the path.

Lets take a look at this. In my opinion, the order of best bang for the buck, for seriously riding fast on the trails, for the 250 KLX.

1. Remove extra crap from the bike, tail piece, silly hand guards, etc & install replacement parts as needed.
2. Big Bore, carb (if $ available), re-tune.
3. Racing Tires (if you plan on riding in the rocks).
4. Front suspension redue. Rear suspension too if your over 175#.
5. Exhaust
6. ? (What is my next project?)

 

Thank you all.
I have not yet committed to the carb yet still have to sell the stock tank, and a shot gun for the funds. I have seriously thought about the big bore mod, but not in a bolt on kit. I have a friend who can do spec bore and hone. Said he would do it right for me when I get the parts. However not fully sure thats what I'm after. I will know better tomorrow gonna take a 1995 KLX250 (Stroker Modified 331cc with a pumper carb) for a test ride.

 

Rules are made to be broken? You rebel, you!

No, there are no regulations about the order, but I think you'll find in the vast majority of performance hop-ups on motorcycles, hotrods, etc., the exhaust is done before or at least at the same time as increases in the displacement or big mods to the intake and fuel delivery...especially in the case of small OEM style exhausts. The soda straw exhaust on our KLX is fairly restrictive any way you look at it. OMG!...I said "any way you look at it". Sounds like a rule.

Relax, durielk. We can mod our bikes, cars, and trucks in any order we want and as money allows. And we can have reasonable opinions about it. My reasoning is not flawed. I'm not crazy about loud toys...except maybe at the race track. And I'm not breathing thin air. However, I will be breathing thin air this August, and apparently so will you. So I will refrain from stating any opinions during that period.

 

i will say if you are looking for better response and take off....the carb upgrade will definitely help...i was surprised after testing a dirt bike before and after the carb upgrade...power just feels lot more instant...the one things i didn't understand was the difference between a "pumper carb" and regular carb...the shop i was talking to was trying to talk me out of the "pumper carb" and was recommending a different carb that they said was way better...it was about $300 for the carb, but forget the name of the company that makes a direct replacement for the klx250...if somebody mentions some names, i'll probably remember....it was not mikuni or keihin...