Big bore benifits

What are the benifits of building a large motor, over a small motor?

No replacement for displacement.

Power at lower rpm

Yep. The bigger the motor, the lower the powerband, everything else being the same.

What Mark said...If you need a bore, and how many bikes of our' age don't? short of machining cases and replacing liners, why not go as big as possible? OE bits will cost as much as wisco. Good clearance is the key to long longevity, at nearly 50k miles my thou is still stock, and hp isn't my aim. still, if I were to go for a bore, It would be as large as feasible. 2cents G

Here are a few of my thoughts on the subject, perhaps you can add alittle more to it. I know this is only part of the differances.

Big Bore:
Yes, the upfront cost is a lot more.

It is better to buy more motor, then you think, that you will need. Trying to patch together an upgrade, never really works out. Either the motor will be short lived or you will not be happy with the performance outcome.

Big motors do not Rev as high, therefore increasing their life.

As the bore dia get larger, the cylinder wall is moved away from the valves. This will unshroud the valve therefore increasing the flow into the motor.

On a large motor there is no going home, once the cased are bored, it's a big bore motor forever.

A big bore, has a larger/heavier rotating mass.


Small Bore:
The cost of a small motor is on the backend.

Small motors Rev higher, therefore they will not last as long. They must be rebuilt more often.

The small bore dia puts the cylinder wall, very close to the valves decreasing the flow area.

A small motor can be returned, to it's stock condition.

A small bore, has a smaller/lighter rotating mass.

A small motor can be a good project, for 1st time builders.

Both motors:
Both motors can last for years with the proper care and limited throttle abuse.

Both have the same stroke, therefore both have the same windage.

Both have the same stroke, therefore both have the same ring friction on the cylinder walls.

I know the list can go on and on, please add to it!

As a preface to the following comments, I have an AAS in Mechanical Engineering Technology and I'm working toward a BS in the same. I will admit that I am not correct all the time and in all situations. I could and may be in fact completely wrong on one or all of my answers.

I am also using Z1 based engines in my examples with "small bore" being defined as 66mm-76mm for 903cc cylinder blocks and 70mm-76mm for pre-J model 1015cc blocks. Anything larger than that a big block is recommended and I considered as "large bore" engines.

Now that I got all the legal jargon and CYA terms covered here's what I think...

Big Bore:
the upfront cost is a lot more.

I don't know what your definition of "a lot" is but the difference between a 1197 motor and a 1397 motor is only about the cost of the big block itself. They both need piston kits so that should be a wash, they both need to have the upper case machined for the larger sleeves, and all the other supporting items should be the same ie. cams, HD studs, etc. However if you are talking about a 1045cc or 1075cc kit then yes there will be a substantial price difference as the stock sleeves will be sufficient for a 1015cc block.

Big Bore:
Big motors do not Rev as high, therefore increasing their life.

But they still have bigger pistons which means more mass to start and stop at TDC and BDC. They are still hard on the connecting rods.

Big Bore:
As the bore dia get larger, the cylinder wall is moved away from the valves. This will unshroud the valve therefore increasing the flow into the motor.

Yes and no, if the head supports the displacement it can help but if the ports further upstream are the same size it will still only be able to pull so much volume through the runners.

Big Bore:
On a large motor there is no going home, once the cased are bored, it's a big bore motor forever.

You should still be able to install a 903cc or 1015cc block back onto a set of cases that have been machined for larger sleeves.

Small Bore:
Small motors Rev higher, therefore they will not last as long. They must be rebuilt more often.

Redline depends more on cam selection and valve springs than on piston size. An example would be a Ex250 and a Zx14. Does the Ex250 need rebuilt more than a Zx14 because it spins faster...probably not. It all depends on your right hand and what components are hidden inside the engine.

Small Bore:
A small motor can be a good project, for 1st time builders.

As stated above there isn't a big difference in a big bore and small bore engine. You still have to make sure all the tolerances are in spec., all the fasteners are torqued properly, and cam timing is correct.

Both motors:
Both have the same stroke, therefore both have the same windage.

But the bigger the bore the more crankcase pressure the engine will make as the volume under the pistons@TDC has increased just like the volume above the pistons@BDC has increased.

Both motors:
Both have the same stroke, therefore both have the same ring friction on the cylinder walls.

The rings do have to move the same distance for both small and large bore engines but as cylinder bore diameter increases so does the circumference. The bigger the circumference the more area the rings are pressing against.

Later, Doug

From what I have read here my asumption is that we are talking about oil cooled motors. It seems a few problems with BIG motors on the STREET stem from overheating. Oil cooler or not. I am kinda surprised no one pointed that out?
I have had a few BIG street motors that were only mildly built. No big cams [395 lift], nor big compression [11:0]. On the negative side of things are : blued cam bearing journals, scuffed cam bearings, scuffed piston skirts, damaged cylinder walls, minor oil leaks, and eventually oil consumption, etc.
And before someone thinks, lack of maintance, or bad rebuild I assure you only the best oils and filters were used @ 2000 mile intervals, and air filters were installed 100% of the time, and the builder [not me] as 30 yrs. expierence with Kaws. And ALL of the internals were BRAND NEW top quality. He even told me of these problems BEFORE he rebuilt them. But did I listen ? NOOOOO !!! As well as oversized oil coolers.
These problems were on 900/1000 big block conversions, up to 1327, and 1260. And we are talking about complete rebuilds. I don't beleive in cutting corners.
After 2-3 unsucessfull rounds with the bigger motors I have done some checking around. It has come from more than one reliable source that anything over 1075 on the street seems to have overheating issues.
I can tell you my current 1075 upon cold start, heats up rather quickly compaired to my OEM 1000. I can't imagine what the 1327 did in comparison.
I agree that a larger displacement has benefits such as more torque, lower RPM's, and yes they are surely quicker.
But from my expierences, the smaller displacment motors actually have LESS down time, fewer rebuilds, and on a daily bases need less maintance. And I would rather be riding a trouble free bike than one thats not.
I am thinking that 1-2/10ths of a second in the 1/4 mile means nothing on the street. So why put yourself threw all the agravation ?
So thats my story, and I am stickin' to it...LOL

I don't know if maybe you missed the point with the big bore motors as the big block doesn't cool as efficiently and since we are dealing with air cooled engines here it would stand to reason that bigger bore equals more heat and less cooling capacity as the space between the pistons that used to be for cooling is not there because it is stuffed with big pistons. Any way you slice it the more you demand from the engine the less life it will have in the long run and if you build the bike and don't throttle the crap out of it it's kinda like being with a hot blooded woman and not riding her hard and putting her away wet. She won't like it and neither will that kaw between your legs so do whatcha gotta do and forget about the specifics.
I think maybe a smaller displacement for the street would be more "streetable" as it has better manners and doesn't heat up as much as the big block will. But either way should be lots of fun to ride. Why would you build a big motor and not ride it hard. Kinda like defeats the purpose of building a bigger motor.

StreetfighterKz wrote:

As a preface to the following comments, I have an AAS in Mechanical Engineering Technology and I'm working toward a BS in the same. I will admit that I am not correct all the time and in all situations. I could and may be in fact completely wrong on one or all of my answers.

I am also using Z1 based engines in my examples with "small bore" being defined as 66mm-76mm for 903cc cylinder blocks and 70mm-76mm for pre-J model 1015cc blocks. Anything larger than that a big block is recommended and I considered as "large bore" engines.

Now that I got all the legal jargon and CYA terms covered here's what I think...

Big Bore:
the upfront cost is a lot more.

I don't know what your definition of "a lot" is but the difference between a 1197 motor and a 1397 motor is only about the cost of the big block itself. They both need piston kits so that should be a wash, they both need to have the upper case machined for the larger sleeves, and all the other supporting items should be the same ie. cams, HD studs, etc. However if you are talking about a 1045cc or 1075cc kit then yes there will be a substantial price difference as the stock sleeves will be sufficient for a 1015cc block.

Big Bore:
Big motors do not Rev as high, therefore increasing their life.

But they still have bigger pistons which means more mass to start and stop at TDC and BDC. They are still hard on the connecting rods.

Big Bore:
As the bore dia get larger, the cylinder wall is moved away from the valves. This will unshroud the valve therefore increasing the flow into the motor.

Yes and no, if the head supports the displacement it can help but if the ports further upstream are the same size it will still only be able to pull so much volume through the runners.

Big Bore:
On a large motor there is no going home, once the cased are bored, it's a big bore motor forever.

You should still be able to install a 903cc or 1015cc block back onto a set of cases that have been machined for larger sleeves.

Small Bore:
Small motors Rev higher, therefore they will not last as long. They must be rebuilt more often.

Redline depends more on cam selection and valve springs than on piston size. An example would be a Ex250 and a Zx14. Does the Ex250 need rebuilt more than a Zx14 because it spins faster...probably not. It all depends on your right hand and what components are hidden inside the engine.

Small Bore:
A small motor can be a good project, for 1st time builders.

As stated above there isn't a big difference in a big bore and small bore engine. You still have to make sure all the tolerances are in spec., all the fasteners are torqued properly, and cam timing is correct.

Both motors:
Both have the same stroke, therefore both have the same windage.

But the bigger the bore the more crankcase pressure the engine will make as the volume under the pistons@TDC has increased just like the volume above the pistons@BDC has increased.

Both motors:
Both have the same stroke, therefore both have the same ring friction on the cylinder walls.

The rings do have to move the same distance for both small and large bore engines but as cylinder bore diameter increases so does the circumference. The bigger the circumference the more area the rings are pressing against.

Later, Doug

Thank you for your answer! It was nice of you to share with us, what you have learned over the years......

edspring1 wrote:

From what I have read here my asumption is that we are talking about oil cooled motors. It seems a few problems with BIG motors on the STREET stem from overheating. Oil cooler or not. I am kinda surprised no one pointed that out?
I have had a few BIG street motors that were only mildly built. No big cams [395 lift], nor big compression [11:0]. On the negative side of things are : blued cam bearing journals, scuffed cam bearings, scuffed piston skirts, damaged cylinder walls, minor oil leaks, and eventually oil consumption, etc.
And before someone thinks, lack of maintance, or bad rebuild I assure you only the best oils and filters were used @ 2000 mile intervals, and air filters were installed 100% of the time, and the builder [not me] as 30 yrs. expierence with Kaws. And ALL of the internals were BRAND NEW top quality. He even told me of these problems BEFORE he rebuilt them. But did I listen ? NOOOOO !!! As well as oversized oil coolers.
These problems were on 900/1000 big block conversions, up to 1327, and 1260. And we are talking about complete rebuilds. I don't beleive in cutting corners.
After 2-3 unsucessfull rounds with the bigger motors I have done some checking around. It has come from more than one reliable source that anything over 1075 on the street seems to have overheating issues.
I can tell you my current 1075 upon cold start, heats up rather quickly compaired to my OEM 1000. I can't imagine what the 1327 did in comparison.
I agree that a larger displacement has benefits such as more torque, lower RPM's, and yes they are surely quicker.
But from my expierences, the smaller displacment motors actually have LESS down time, fewer rebuilds, and on a daily bases need less maintance. And I would rather be riding a trouble free bike than one thats not.
I am thinking that 1-2/10ths of a second in the 1/4 mile means nothing on the street. So why put yourself threw all the agravation ?
So thats my story, and I am stickin' to it...LOL

I agree with you.........ride that story, until the wheels fall off!