I see all of these posts debating which is the best big bore kit for the 400EX. In these posts, there are always these guys talking about things like "the 416 revs faster than the 440, but the 440 makes more low end." What I want to know is...Where is the proof?!? Seems like people are stating this stuff like it is fact just because they heard someone else say the same thing. Has anyone actually seen dyno runs comparing the different sizes?

I cannot believe that the relatively small (and I mean very small) difference in piston weight could have anything to do with the powerband of the engine. True, the weight of the OVERALL rotating assembly will have a lot to do with the speed at which an engine will accelerate (build RPM). But, this weight includes the weight of the crankshaft, the connecting rod, the flywheel, and yes, the piston. Compared to the weight of the other items, a 1 or 2mm larger piston is just not going to make a sizeable difference.

The types of things which will influence the powerband of a 4-stroke engine are things like:
1. Bore and stroke ratio - typically, an engine with a larger bore than stroke will rev pretty well, and an engine with a larger stroke than bore will make lots of low-end torque.
2. Camshaft profile - valve lift and duration
3. Cylinder head ports - both size and shape. Remember that velocity can be just as important as overall flow numbers.
4. Valve sizes
5. Compression ratio
6. Carburetor design and size
7. Exhaust system design

I guess what I am trying to say is that unless you really understand these types of things and the effects they have on a 4-stroke engine, stop putting information up on the board which is not really going to help the person asking the question. If someone has seen some dyno charts showing the differences between the 416, 426, and 440, please share them with the rest of us.

My answer to which size to go with depends on what you want and how much you want to spend. Think about this first, the majority of the power increase (from any of the pistons) is going to come from the increase in compression ratio. If you think you are going to need frequent top end rebuilds, start with the 416. This way, you will have room for future rebuilds using the stock cylinder sleeve. This route also makes sense if you do not want the extra expense of a cylinder sleeve change. If you don't mind the extra expense of a new sleeve, go for the 440. It's a proven combination (XR400's have been using them for a long time), and remember...there is no replacement for displacement.

 

That all sounds very nice, but which pipe should I buy?

 

I'm pulling your leg.......

 

I hear SuperTrapp has a new pipe out. It's called the BYR2 series(back yard racer). It optimizes power output for those times you need too "smack down" the other guy too prove your point.

It comes in carbon fiber or chrome and retails for about $750 bucks. A small price too pay for a 25HP gain.

We will see what happens the next time someone steps foot in my backyard.

 

Opps....Double post.

 

What in the world are you talking about? Going from stock 28 horspower and jumping to 53 with just a pipe? Are you for sure of this statement, or are you just pulling $hit out of your a$$? PLease write back and infor me, because i would love to double my horsepower.

 

That was to long. Every little bit helps though. If you save a pound or half a pound here inside the motor, then you will be doing better. Of course I may be truly stupid on this subject since I am more of an automobile mechanic, but they all still work on the same principles I would think. I agree with what you say, but I still think that every little bit of weight you can cut down on with exchanging parts, then you will gain more HP and higher revs, because every little bit will help it out.

 

First let me say...finally, an intelligent response.

Next, you are right, every little bit of weight saved in the reciprocating assembly will allow the engine to build RPMs faster. But, you are not talking about saving 1/2 of a pound. These pistons are weighed in terms of ounces or grams, not pounds. You are probably only talking about a difference in weight which could be stated in milligrams, and don't forget that while you may be saving weight, you are also giving up displacement. Your automotive experience should tell you that the more "over-square" the bore to stroke ratio is, the better the engine will rev.

 

Slyder, I think you were reading my mind when you started this topic. My theory is that when people install a 440 kit and finally ride their machine the power is unexpected and they get off the throttle and proclaim that it revs slower. But in reality the increase of power only startles them so they believe the engine is turning slower rpm's even though the power output is greater at a lower rpm. How does this sound as a plausable answer?
Norm

 

I am taking my bike to Gt Thunder Mon to have a 440 kit installed with a 11:1 piston, and cam. I think the 440 is the way to go for the type of riding I do. Although what your sayin Slyder makes sense. I cant see why they would say the smaller kits (416, 426) rev so much faster. Slyder it sounds like you have about the same set up. How do you like it? What do you think about the Edelbrock carb? Guy from Gt Thunder didnt have to many good things to say about it. Not sure why.