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Big Bore Kits VS Stock cc

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What are some of the main benefits of a big bore kit?

What are some of the downsides?

Do you sacrifice reliability when you put a big bore kit on a bike?

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The downside to a big bore is they tend to sign off really early in the RPM range. They tend to run a little warner and as a result they run the compression lower. And over all they dont make much more power for the cost.

Typically a good port job and matching cam will make more power with better rideability for less money. And you wont loose the primary balancing of the engine because of the huge piston.

The CRF engine (as most modern MX fourstroke engines) arent bore/stroke challenged. Meaning that the bore vs. stroke is way way over bored already. The head is designed to flow with the 96mm piston.

A lot of guys love their big bores otherwise they'd never sell the kits. Its not an overall power adder though since they dont make that much power over stock if any. If youre going to do a big bore Id suggest getting a cam that matches the setup so everything comes back into symbiosis.

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You have a big bore and do not know what it does for your bike? I am confused here.

I know what it does for my bike, just curious what others think.

It's a forum... 👍

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Do you have to run higher octane with a big bore kit? and if the kit does not add power, what is the point of it?

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Do you have to run higher octane with a big bore kit? and if the kit does not add power, what is the point of it?

To say it doesnt add power is a misnomer really. It usually adds bottom end power, but over all, the peak power sometimes ends up less than stock. But the bottom and mid range grunt where a lot of guys ride them is meatier. But with the larger displacement through the same intake port and cam the engine runs out of air quicker.

Usually they lower the compression ratios on big bore kits just so you dont have too many issues with the fuel detonating. Precisely so you dont have to worry about race gas. Whether or not you need race gas is kind of up to the tune in the setup youve got.

LIke if you were trying to ride single track stuff with a 498cc bb kit and a 13.5:1 compression ratio, id definitely suggest race gas just to deal with the heat that engine would produce to prevent detonation.

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To say it doesnt add power is a misnomer really. It usually adds bottom end power, but over all, the peak power sometimes ends up less than stock. But the bottom and mid range grunt where a lot of guys ride them is meatier. But with the larger displacement through the same intake port and cam the engine runs out of air quicker.

Usually they lower the compression ratios on big bore kits just so you dont have too many issues with the fuel detonating. Precisely so you dont have to worry about race gas. Whether or not you need race gas is kind of up to the tune in the setup youve got.

LIke if you were trying to ride single track stuff with a 498cc bb kit and a 13.5:1 compression ratio, id definitely suggest race gas just to deal with the heat that engine would produce to prevent detonation.

Thanks for clearing this up! So it creates a lot of torque compared to stock?

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yes because of more inertia and weight and air flow at low RPM. i dont like big bored 450's because they dont rev out like they should. if you need more torque, get a good bottom end cam and get some mild porting done, possibly a slight bump in compression. wont run near as hot as the big bore, and it'll rev out like a 450 should.

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yes because of more inertia and weight and air flow at low RPM. i dont like big bored 450's because they dont rev out like they should. if you need more torque, get a good bottom end cam and get some mild porting done, possibly a slight bump in compression. wont run near as hot as the big bore, and it'll rev out like a 450 should.

Adding rotating weight/increasing inertia does not add torque, what they do is make it harder for the engine to change RPMs, both up and down, just like adding a flywheel weight. Depending on the application, this can be good or bad. If you ride off-road and are looking for traction and smoothness in on-off throttle situations, it is good. If you're racing MX and want the engine to rev quick for coming out of corners and attacking jumps, its not so good.

If you change cams and port for bottom end you'll, still lose some top end and over rev too. At the end of the day, it comes down to airflow and velocity of the air/fuel charge. Whether you go big bore or tune the stock bore/stroke for bottom end, you're still increasing the charge velocity across the board and will run out of flow capabilities sooner in the RPM range. On the flip side, the reason high RPM tuned bikes don't run well down low is because they loose intake charge velocity for a given RPM. Making HP and torque isn't just about CFMs, charge velocity is an important factor as well.

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my 08 is bored out to a 490 and even before the sprocket change i and everyone who rode the bike noticed a huge power change.

change sprockets and add a bigger camshaft and you're golden.

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I have added a big bore to 3 bikes - KX250, KTM 525 and CRF450R. None of them ever worked as good as the stock set up. They lost overrev overheated more often and are not as reliable. I put the Ice Cube in a CRF 450R and my friends stock was faster and more reliable. I have learned to stay OEM as the factory has put in the R&D $ on what works. Maybe add exhuast, Tune jetting and a few bolt on items. I will never do any big bore again on a dirt bike. Not worth the money - time and issues.

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A good motor builder can make the current 450's faster than most people can handle using the stock cam/cams and valvetrain on non-oxygenated fuel. The right port work, proper cam timing and a good map will suprise most 450 riders.

To make a big bore really work you still need the head work and mapping.

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Wow, talk about some bad experiences. I have had and rode several BB bikes and I always liked them. Really, if you are buying a kit you need to look at whom you are getting it from and how he is designing the motor for YOU. I also see so many kits out there that have no thought put into their design so I guess I should not be surprised to see this thread.

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If you increase the displacement of an engine without also increasing it's ability to breath more, it won't make much more power, but it will take the power band and move it down the RPM scale. The fact that it will make big power in the mid range makes it feel like it has more power.

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If you increase the displacement of an engine without also increasing it's ability to breath more, it won't make much more power, but it will take the power band and move it down the RPM scale. The fact that it will make big power in the mid range makes it feel like it has more power.

I did a 511 for a guy. HotRods stroker crank and a Cylinder Works big bore. I ported the head, it had a Yosh Pipe and to be honest, it was flat scary fast. The thing that blew me away was it never signed off. It just kept pulling like a stocker does but way way harder.

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I did a 511 for a guy. HotRods stroker crank and a Cylinder Works big bore. I ported the head, it had a Yosh Pipe and to be honest, it was flat scary fast. The thing that blew me away was it never signed off. It just kept pulling like a stocker does but way way harder.

That's because you ported the head. If you would have left the head stock, it probably would have signed off.

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Pistons and gaskets always cost more and are often limited in supply or choice of brands. Sometimes you get stuck with no piston available or trying to find one.

if your talking crf450 specific? there is an magazine test online where they build it about 4 different ways (bore cam compression etc) and riders give their impressions.

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Pistons and gaskets always cost more and are often limited in supply or choice of brands. Sometimes you get stuck with no piston available or trying to find one.

if your talking crf450 specific? there is an magazine test online where they build it about 4 different ways (bore cam compression etc) and riders give their impressions.

This is true. Looking to rebuild the top end this winter and my options are limited and much more pricey. So much so I am thinking about going back to oem cylinder and piston. I bought my bike used it had the ice cube big bore on it already. Another item i just learned (According to the local dealer) is that you cannot do anything to the cylinder other than send it back and have it relined. another costly item.

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I had a ape head polished and ported on the crf498 ice cube rpm kit and it was weak compared to my friends stock crf450. The valve bend and ruined the top end with 20 hours on new build.

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