280cc kit from Muzzy, anyone have any thoughts or experience.?

[sethl]

I just found that muzzy has a 280cc kit for a decent price, 449 after core. Does anyone have any experience with it, or heard anything good or bad about it? Have any thoughts? Thanks guys

[dirtmachine]

It definetely won't help the valve train last longer. I would spend the money on stronger valves and springs before going BB because otherwise you only are asking for trouble.

[sethl]

Yea, I've allready spoken to Eric Gorr, he is going to install the Kibblewhite stainless valve kit, and do a 262cc BB. I just wondered if anyone had heard anything about the 280 kit.

[RC876]

Ditto on what Dirtrider said.

There is tons of power to be gained with proper porting on this bike. Unless you crash a cylinder or are an A rider I would stick with the stock bore.

:thumbsup:Norman

[dirtmachine]

It'd be cheaper to go to a high comp piston than boring out only 12cc. Yes u will get a little more power with the boring but your wallet will be a little thinner than if you chose the high comp piston. Like RC said, it doesn't make much sense if you don't race or have to get it bored anyway.

[Aaron_Silidker]

Not sure what the power gains would be like on a 4 stroke, but on my KX125 I have had two big bore motors...a 144 and a 160. The 144 was amazing compared to stock and the 160 absolutely blew it away. Both of these engines blew away the 2k in Tom Morgan motorwork done to the stock engine.....there is no replacement for displacement.

Once the valve issues get sorted out, my KXF will get a big bore.

[dirtmachine]

You gotta remember that going up 19cc on your 125 is a 15% increase in displacement whereas going up 12cc on his 250 will only be a 5% increase ( the 280 would still only be 12% larger) lets not even get into the fact that a 2 stroke produces about twice as much hp per cc as a 4 stroke. Therefore you should look at the 300cc kits out there for the same results as Aaron had. However, because of the valve issues I wouldn't recommend boring out the cylinder.

[Rich_Rohrich]

You gotta remember that going up 19cc on your 125 is a 15% increase in displacement whereas going up 12cc on his 250 will only be a 5% increase

The differences you see in performance with a big bore four-stroke extend far beyond a simple percentage increase in displacement. When you increase the bore size without changing anything else you lower the torque peak rpm, increase the intake velocity at the same relative rpm, and gain a more favorable valve area to piston area ratio among other things.

Basically you are in a position to make the same power at a lower rpm which is pretty useful from a reliability perspective, especially in engines that are prone to valve related failures.

The problem is the when you bore a thin wall casting you potentially run into cylinder bore instability issues which can show up as cracked barrels or hhigh percentage ring leakage at high speeds.

We don't have enough field data at this point to know how reliable the RMZ big bores will be. We just don't have enough long term tests done yet. On the Yamaha YZ250Fs a simple 12cc displacement increase has proven to be the single best mod for the dollar. It will take time to know for sure if the same will prove to be true with the RMZ. One thing is for sure, it sure makes a 250F more fun to ride, and it shows how overpriced and overrated most pipes are. ?

[dirtmachine]

I agree with the fact that you'll produce more power down low. but just cuz you got the power down low doesn't mean you won't pin it ??

[sethl]

My motor is going to EG next week for stainless valves and a 262 kit. I just asked about the 280 kit because, like Rich said, I wasn't sure how large these cylinders could be bored before the cylinder walls got to thin. Thanks guys

[mxmike117]

Ordered mine today. I talked with the guy from muzzys and he said that he has already sold the first batch of them, and all the second batch that is supposed to be in in two weeks.

[RC876]

As far as we know we were the first ones to have a big bore stroker RMZ on the planet. It was a beast compared to our 167's last year.

Now we have stock bore stock stroke RMZ's making around 39 hp. All my data comes from a heavy wheel Dyno Jet at sea level. Not what my buddy told me or what I read in some article.

In comparison the difference in a RMZ that makes only 38hp and a RM 167 that makes 41hp is that the RMZ makes about 4hp more than the 167 anywhere from 5000 to 9000 rpm. In other words it makes mad torque even though peak hp is less.

Also I am not a big fan of compression. Unless other motorwork is done. Last year we were actually lowering the compression ratio on our 125's. The hp gains above 9,000 made this worth while if you were on the gas.

If you are going big bore without any porting its really senseless. With the proper porting the 280 kits would be a beast.

Also there is the cool factor. If you have a big bore and your buddy doesnt thats cool. What is costs doesnt matter. If he still smokes you on a stock bore with some mild motor work and tuned properly then he will probally rag you every chance he gets.

Norman ?

[YZDAD1]

Now we have stock bore stock stroke RMZ's making around 39 hp. All my data comes from a heavy wheel Dyno Jet at sea level. Not what my buddy told me or what I read in some article.

Do you know what your bike dynoed at before your mods, and what type exhaust are you using?

[Pontificus Maximus]

I am wanting to go big bore but I am a little hesitant because I rode a YZ250F bored to 262cc and although it had tons of power right off the bottom it was noticably slower than my KX250 on top.

Has anyone done it yet and can give some feedback on how it worked for them? Also, I am curious about going the porting route. What does it cost and do for your power band... Thx!

[RC876]

Do you know what your bike dynoed at before your mods, and what type exhaust are you using?

YZDad1, a RMZ will make about 30.5 hp in stock form. One can easily have a bike the makes 1hp less or 1/2hp more.

With a few minor mods/proper tuning you can make above 32+ with the stock exhaust.

Believe it or not the stock headpipe is a good piece. For some time we didnt see a headpipe beat it except for way up top and for this high rpm power gain you lost allmost everywhere else.

At one time tht stock headpipe and the pro circuit can was the cheapest bang for your buck. Now some pipe builders have done their homework. The Yoshi with thye long headpipe seems to be doing well.

Norman

[dirtmachine]

ya a stock rmz makes 30 hp. Riiiiiiiiiiiiiiiiiiiight, and pigs fly and lead floats. Was that 39hp with a street tire or a knobby?

[RC876]

Float, EEXXAAAACCCCTTTTLLLLYYYY!

When one big bores their cylinder and has no combustion chamber work it raises the compression ratio. With more compression the initial hit is amazing. Then its kind of flat everywhere else. Keep in mind that this also changes some per model and what you stock compression is to begin with.

Its my opinion that trying to squeeeze more cc's through a port design thats not working anywhere close to where it could be turns a bike into a TORQUE SPONGE somewhere right after youm get this initial hit that amazes you so much.

A proplerly ported RMZ with stock cams will make more useable power that just throwing on a big bore.

But, If you go big bore and dont cheese out on the porting then the package will work as it is suppose to and make power everywhere.

Our first big bore stroker RMZ by all rights should have lost some power at some point to gain in others. It made more torque off Idle and across the board as compared to our 250cc mod. This is hard to do especially on the first attempt.

? Norman

[RC876]

ya a stock rmz makes 30 hp. Riiiiiiiiiiiiiiiiiiiight, and pigs fly and lead floats. Was that 39hp with a street tire or a knobby?

HMMMMM I am confused first you wrote 30hp.

Are trying to say a RMZ makes more power then 30 in stock form>??

Cause in 2003 it was advertised that a RM125 made 31.5hp in stock form. I know of no one who got over 29hp in stock form and most were around 28.5. This was first conformed for us when me and Willie Manning was biting our tongues not wanting to tell each other what we got till the other said it cause we wanted to see who was telling the truth.

Dont allways believe what you read. AS I mentioned my data comes from personal time on a Dyno jet. Not from some hyped up article.

AS far as the street tire knobby scenario why would one waste their time and dyno a bike with a knobby when we keep a wheel with a street tire mounted on it just for dyno runs??

If you have some data to share on this please do. Please keep in mind I have lots of Dyno time on RM's

Peace Norman

[dirtmachine]

Actually excuse me for jumping the gun, are you guys talking about pure engine hp or are you talking real world hp (ie, at the rear wheel)? If ur talking engine hp then yes your figures are correct (at least close)if at the wheel, you're aiming somewhat high.

[RC876]

Dirtmachine you are one of those single digit guys are you??

Real wheel Hp on a stock RMZ will be around 30 give or take 1/2hp or so. Considering that they are advertised to have about 33 I think that the 30 hp figure would be within reason.

Also we have dynoed many stock RMZ's. Thi figure has allways been within a few points.

If you have some other data I would be more than interested seeing it.

Tha data I gave on modified engines were actualy dyno tests. On a dyno jet with a heavy wheel at sea level.

Last year our camp went through about 40 RMZ's/KXF,s along with many other bikes.