25 replies to this topic

[luismx123]

just wondering if there's a number of added bhp/Torque per overbore on a BBU or SBU?
say +20 is approx. a 4bhp increase, +40 is 7bhp etc.

I just want to know, keeping all things the same except for the bore diameter. i.e better head, uprated cam remains the same on each overbore level

cheers

[unburntfuelinthemorning]

Basically the percentage increase in torque/power is equal to the percentage increase in cubic capacity.

 

In reality although the increase in torque may be around this, the increase in power from a capacity increase alone, all other factors being equal, will be a lower percentage.

 

This is mainly due to the volumetric efficiency or breathing ability of the engine remaining the same.  If the breathing is also improved then the percentage power increase could be the same or even higher than the percentage capacity increase.

[DeadSquare]

just wondering if there's a number of added bhp/Torque per overbore on a BBU or SBU?
say +20 is approx. a 4bhp increase, +40 is 7bhp etc.

I just want to know, keeping all things the same except for the bore diameter. i.e better head, uprated cam remains the same on each overbore level

cheers

 

 

NO.  because It depends on how big the piston is.

 

If you imagine the extra volume gained by a rebore as a key ring, there is a lot more metal in a 5" key ring than a 1" one.

[Dusky]

Bigger bored also intresse friction, the head etc all stay the same to, so they don't "grow" with the increased displacement. Over boring is one of the most expensive ways to gain bhp and imho is only there for track use ( and ofcourse if you NEED to go up a size because of engine wear)

[Ethel]

Power comes from burning fuel, how much you can burn depends on how much air there is for combustion in the correct ratio. A bigger cylinder can contain more air, but it still has to get in so valves, cams and all parts of the induction and exhaust have an effect.

 

If all those factors stay the same, the effect will be a larger bore hits the same gas flow rate sooner so the peak torque will occur at lower rpm.

 

More swept volume, without other changes, would also increase the compression ratio which improves thermal efficiency and economy - provided there's no pre-ignition.

[Retroman]

Contrary to some above I would say overboring is probably the best and cheapest route to more go.

 

There is no substiute for CC's

 

Its not comon to go much over +40 without other mods

 

Combined with a better head / cam / breathing you can double the standard power on a small bore, and still be perfectly drivable. 

 

On a big bore going to 1330 [+60] gives substantial gains even keeping a standard head and carb etc

 

The problem with the 1300 block is a later cam change means engine out, split and strip.

 

So it makes sense to change the cam at the same time

[Spider]

Contrary to some above I would say overboring is probably the best and cheapest route to more go.

 

There is no substiute for CC's

 

Its not comon to go much over +40 without other mods

 

Combined with a better head / cam / breathing you can double the standard power on a small bore, and still be perfectly drivable. 

 

On a big bore going to 1330 [+60] gives substantial gains even keeping a standard head and carb etc

 

The problem with the 1300 block is a later cam change means engine out, split and strip.

 

So it makes sense to change the cam at the same time

 

I gotta agree with Retroman on this.

 

There is no substitute for Cubic Capacity.

 

The other often well over looked aspect is there's less shrouding around the valves with the more open type of headed (eg all 1275 heads and the 12G295 small bore head), which also gives a small but measurable increase in power.

[Turbo Phil]

It’s a law of diminishing returns. Yes there’s no replacement for displacement, but the bigger the cylinder the harder it is to fill. The standard head is asthmatic & going to 1380cc etc and expecting huge gains when the breathing isn’t improved is fantasy. Sure you’ll see some improvement but nothing like what could be achieved with the correct head, cam exhaust and carburation.

Phil.

Edited by Turbo Phil, 03 August 2019 - 10:42 PM.

[nicklouse]

Just look at the power a bike headed engine makes when compared with a same spec 5 port. A third more just with a better head.

[DeadSquare]

The O.P. was simply wondering if there is a "Straight line graph" value for power, co relation to a simple re bore.

[unburntfuelinthemorning]

He was but isn't this all much more interesting than just saying "No" and I think he must have worked out that "No" is the simple answer by now.

[Ethel]

Actually the simple answer is probably - Yes.

 

If you were to do any single modification there'd be a measurable effect, but I agree with you it wouldn't that useful to know.

 

Engine's are very much the sum of their parts. Modify anything and you can be certain modifying something else will be required to see the maximum benefit and very likely in different ways for different goals.

[luismx123]

ah don't mind the conversation. gets the grey matter working ;)

what I was wondering is basically this. if I took a 998 and rebored to 1138, would I gain 50% of the difference between a 998 and 1275s power? (splitting the middle between a 998 and 1275 is + or - 138.5cc)

if so, then it would be fairly linear. more cc= more power, so if I was to rebuild my 998, I could await a certain increase in power for every step I went wider. this is all theoretical of course... :/

[DeadSquare]

ah don't mind the conversation. gets the grey matter working ;)

what I was wondering is basically this. if I took a 998 and rebored to 1138, would I gain 50% of the difference between a 998 and 1275s power? (splitting the middle between a 998 and 1275 is + or - 138.5cc)

if so, then it would be fairly linear. more cc= more power, so if I was to rebuild my 998, I could await a certain increase in power for every step I went wider. this is all theoretical of course... :/

 

 

Good on you for volunteering to keep boring out your engine and measuring the power increase.

 

Please keep us all informed of your results !

[ACDodd]

In theory yes the more capacity you have the more work can be done.

However there comes a point where the breathing capacity of the head and valve train cannot feed the needs of the bigger capcity, the result is higher rpm horsepower suffers more than peak torque.
I machined a little 850 a few years back and stretched it to 1150cc. This engine want from stock 33hp to 71.5bhp. in ordert o get that far it needed a 12g295 head fitted with. 1275inlet valves and twin hs2 carbs. That being said the original 850 block went on to produce 72lbft of torque up from the original 44lbft!
Capacity wins here everytime.
For fast road applications think that 68 to 70lbft per litre is possible. The more litres you have the more grunt you have.

Now for the overlooked bit.
Most used engines are well past there best ring seal conditions so people don't realise that a simple rebore will restore a lot of lost power and torque. So a simple 0.020" rebore and new pistons can feel like twice the engine! The point is it's not the increase in capacity that gave the massive increase in performance. Is the restoration of the lost seal.

Ac