Theorical question about big-bores

Started by urymoto, Nov 10 2008 08:22 PM
20 replies to this topic

  • urymoto

Posted 10 November 2008 - 08:22 PM

#1


I was thinking, when you install a big bore, the carb isint pumping any more fuel to power the higher displacement cylinder, so how come it produces more power?
You would need more fuel no?

Will i have to change my jetting in terms of pilot jet? I think the main has to be reduced a little...

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  • SparksXR426

Posted 10 November 2008 - 08:30 PM

#2

Just bored my old Xr out to 426. I asked the same question and the reply I got was because the motor pulls more air it also will naturally draw more fuel. The point where it becomes a problem is when the jets get to be too small to supply enough fuel to keep the ratio correct. So if you were rich before boring you might be close after as was in my case. Good power gains and never touched the jetting. That was my experience anyway.:thumbsup:

  • blt

Posted 10 November 2008 - 08:45 PM

#3

I think the Venturi effect comes into play.

http://demonstration...eVenturiEffect/

  • Fast_Richie

Posted 11 November 2008 - 12:33 AM

#4

Power is in very basic terms.. Size of bank X Number of bangs
Number of bangs being RPM and size of bangs being CC's. Its not so much your getting JUST more fuel in you are getting an increase in air/fuel mixture in. Basically more mixture to go bang..
Pilot circuit is less effected by a big bore kit but would still need some attention. Main jetting can vary on other things, cams, compression, carb, airbox mods etc etc..

  • Hoelio

Posted 11 November 2008 - 12:54 AM

#5

Carbs never pump gas in the cilinder. The cilinder sucks feul and air into itself. If there is more displacement, there will be more sucktion...
Here in The Netherlands I don't change my setting with a BigBore.

  • urymoto

Posted 11 November 2008 - 05:03 AM

#6

of course!! i totally forgot the engine is the one sucking in more fuel!!

thanks all

  • GreenHornet450

Posted 11 November 2008 - 06:03 AM

#7

Hoelio said:

Carbs never pump gas in the cilinder. The cilinder sucks feul and air into itself. If there is more displacement, there will be more sucktion...
Here in The Netherlands I don't change my setting with a BigBore.
I would agree, EXCEPT Carbs with an AP Do Squirt fuel into the Cylinder at the opening stages of the carb & the amount of fuel can be adjusted. Otherwise SUCKING is the way it is...Did I say SUCKING????? I can see where this is going to go.......:banana:

  • urymoto

Posted 11 November 2008 - 09:54 AM

#8

hmm so would i need to increase the duration?

thanks

  • GreenHornet450

Posted 11 November 2008 - 10:00 AM

#9

urymoto said:

hmm so would i need to increase the duration?

thanks

Go into the Technical forum, look under carbs & you will find a short WMV about adjusting the AP Squirt Duration. The AP works from a closed throttle position & only has an effect from closed & should last about 1 second. This is where the BOG can be corrected or lessened. You must use the o-ring or better buy the Heavier AP Spring (Works GREAT!!)

  • iowaBClivewire

Posted 11 November 2008 - 02:49 PM

#10

I think your thinking to hard

Eddie has figered it all out for us .. so all we have to do is buy the parts

get a good pipe and the bigger 39MM FCR carb than add some hot cams with a BBkit ---- most of us will be happy with this combo

if not than the 60HP monster 470stroker is what your looking for

I for one with no money am quite happy with a pipe re-jet the stock carb and some cheep used "E" cams

  • Hoelio

Posted 12 November 2008 - 01:01 AM

#11

readjw said:

EXCEPT Carbs with an AP Do Squirt fuel into the Cylinder

:thumbsup: Yups, right, forgot that feature...:doh:

  • Fast_Richie

Posted 12 November 2008 - 11:10 AM

#12

The fuel from the carb float bowl is actually pushed through the jets by atmospheric pressure not sucked through buy the engine. The air is sucked not the fuel.

  • Eddie Sisneros

Posted 12 November 2008 - 11:16 AM

#13

sort of.the pressure differential that the moving air creates going thru the venturi dictate how easily the fuel flows out of the bowl the fuel up.

the bigger the motor the more air its move.the more it moves the greater the pressure differential.the greater the differential the easier the fuel moves.

this is why bigger motors can typically use leaner jetting.

  • camman

Posted 12 November 2008 - 07:57 PM

#14

I am almost tempted not to respond to this question about BB and power because I can just see all of the coming questions. In simple terms, unless the bigger bore “unshrouds” the valves, just increasing the displacement will only increase torque and not power. This occurs because the port flow and valve lift curve dictate the amount of air flow capacity capable of passing through the engine (ie hp). The amount of air that a cylinder can “gulp” in one stroke is the torque. Thus, a larger displacement engine can gulp more air at lower engine speeds (hence more torque) but the air flow per time (hp) is limited by the port flow and valve timing. If anyone is interested please read the seminal SAE Paper called “Mean Inlet Mach Number”. It covers the above commentary in detail.

  • urymoto

Posted 12 November 2008 - 09:04 PM

#15

amazing...

i would imagine that SOME hp might be gained before limitations come in..no??

  • iowaBClivewire

Posted 12 November 2008 - 11:02 PM

#16

camman said:

I am almost tempted not to respond to this question about BB and power because I can just see all of the coming questions. In simple terms, unless the bigger bore “unshrouds” the valves, just increasing the displacement will only increase torque and not power. This occurs because the port flow and valve lift curve dictate the amount of air flow capacity capable of passing through the engine (ie hp). The amount of air that a cylinder can “gulp” in one stroke is the torque. Thus, a larger displacement engine can gulp more air at lower engine speeds (hence more torque) but the air flow per time (hp) is limited by the port flow and valve timing. If anyone is interested please read the seminal SAE Paper called “Mean Inlet Mach Number”. It covers the above commentary in detail.
yes yes your ded on

but most guys change the valve timing buy adding better cams

if you stock the to stock "S" or "SM" cams you choke over 5 HP out of doing a BBkit

so if you have a low milege motor it is not worth replacing a good stock cylder with out doing cams furst

  • Eddie Sisneros

Posted 13 November 2008 - 05:18 AM

#17

camman said:

I am almost tempted not to respond to this question about BB and power because I can just see all of the coming questions. In simple terms, unless the bigger bore “unshrouds” the valves, just increasing the displacement will only increase torque and not power. This occurs because the port flow and valve lift curve dictate the amount of air flow capacity capable of passing through the engine (ie hp). The amount of air that a cylinder can “gulp” in one stroke is the torque. Thus, a larger displacement engine can gulp more air at lower engine speeds (hence more torque) but the air flow per time (hp) is limited by the port flow and valve timing. If anyone is interested please read the seminal SAE Paper called “Mean Inlet Mach Number”. It covers the above commentary in detail.


the big bores on drz's most definately unshroud the valves.

that is why they respond so nicely to it.

but as noted adding cams and increasing head flow also compliment nicely.

  • Hoelio

Posted 17 November 2008 - 01:32 AM

#18

Can someone explain "unshrouding the valves"? I really can't make anything of it...:excuseme:

  • Eddie Sisneros

Posted 17 November 2008 - 06:11 AM

#19

when you move the cylinder wall away from the valves on a drz the head flow better.

  • Fast_Richie

Posted 17 November 2008 - 12:42 PM

#20

Hoelio said:

Can someone explain "unshrouding the valves"? I really can't make anything of it...:excuseme:

Imagine a tube of similar diameter around the valve head. Even with the valve open it still has to flow between the valve head and the tube. The larger the tube around the valve the easier the air will flow.
As the bore gets larger the part nearest the valve head moves away allowing the mixture to flow better in that area creating a better cylinder fill.



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