BIG BORE VS SMALL

[stroking]

When is a bigbore pipe needed ?

 

would a 421 cub or vito benefit from a bigbore?

 

what would the bike do if you put bigbore pipes on a smaller motor ?

 

lose lowend/topend/etc

[okbeast]

Some builders porting runs better on the 421 as others with the big bore, some use the small bore.

 

I'm not really sure what the cutoff point is to go from small bore to big bore though. If you have someone porting them ask them.

[mailman]

The best answer is "It Depends". You really need to try a set of both on your setup and take the bike to the dyno, hill, or drag strip to compare. Every motor is a little different and there are a lot of factors that would influence your choice between small bore and big bore pipes.

With that said, I have tried both on a few different motors up to 521 cc's and the small bores always did best in my comparisons. I have known a few people that say the big bore worked better for their 421 cubs so again, it is something that really needs to be tested. If you had to guess and could not test then I would say that for a 421 the small bore will be the best most of the time.

The big difference between the two is the size of the stinger opening. The big bore is larger and doesn't hold as much heat in the motor. I think some builders put these on their customers bike just to be on the "safe side". I have yet to see a pipe out perform Matt Shearer's oof sb/sf pipes on the 4 mils and 10 mils that I have tested them on. I don't think the big bore pipe has a strong enough return pulse for the smaller motors and therefore doesn't push as much of the charge back into the motor.

 

I have attached a dyno sheet of three pipes that I tested on a 4 mil. The two best runs are essentially the same pipe and are inframe small bore shearers. The weaker run is with big bore inframe shearers. You can see that "in this case" the big bores were weaker from about 8800 to 10500 rpms when they recovered. The big bore was clearly not the best choice for this motor which was 472 cc's.

 

Many feel that 500 cc's is the magic number when a motor should switch from small bore to big bore pipes. Well, it is not a bad guideline but there really is no magic number. Testing is your best bet but if forced to make a decision with a 421 then go small bore.

 

I hope this helps,

 

Mailman

 

[DUMBASS]

When is a bigbore pipe needed ?

 

would a 421 cub or vito benefit from a bigbore?

 

what would the bike do if you put bigbore pipes on a smaller motor ?

 

lose lowend/topend/etc

....i prefer the medium bores

[dajogejr]

Matt Shearer will tell you he built the big bore for the 535 cheetah and bigger.

I've had bigs on my 10 mil cub (465 cc) and they've done well.

My builder, HJR, would prefer small bore.

 

As said...there is no clear cut answer, it's all in the application you're using them for and your builder's specs and setup.

[J-Madd]

If you choose CPI, you can't go wrong with the unstamped big bore, imo. As far as Shearer's - I had the impression there was a "new style" small bore (some may call it the medium bore) ; but that is not true. I don't know where or how that rumor got started. Matt Shearer said all his stamped SB pipes are the same. It was only the old hand coned ones that were different, and that was several years ago.

 

As far as stinger i.d.; I have SB/SF, BB/BF and BB/SF and the stinger i.d. is the same on all. Go figure

[mailman]

 

As far as stinger i.d.; I have SB/SF, BB/BF and BB/SF and the stinger i.d. is the same on all. Go figure

 

 

The smallest part of the divergent section of the exhaust is what I was actually referring to. This is often smaller than the end of the pipe.

On my Shearer sb/sf oof silenced pipes, the OD is 1" at the smallest part of the convergent section and only about .905" ID there. The silencer fits over this and therefore has a measurement of 1" ID at the end of the pipe.

When we had the inframe Shearer big bore and small bore pipes, the big bore did have a larger ID measurement.

If you think about it, the big bore is designed for larger cc motors because they flow more exhaust due to the larger displacement. If they had the same id as a small bore then the heat would back up in the motor and cause some undesirably high piston crown temps.

I don't know what CPI is running for IDs on big bore or small bore.

[srp]

Projected heat and flange inside diameter will dictate the angles and body diameter of a correctly designed pipe. A high Bmep 421 and a low Bmep 510 will take a similar pipe. On engines such as a high Bmep 510 Cheetah will need a bigger stinger to bleed of heat . Sniper will offer this type of mod for big bore and hill shooting, both needing to bleed of heat to combat power fade. We dynoed with the small bore small flange out of frames Mailman was talking about on 6/3/2010. The inframe Assassins dynoed 5hp stronger with 2 more foot pounds of torque on a 421 cub.

[J-Madd]

The smallest part of the divergent section of the exhaust is what I was actually referring to. This is often smaller than the end of the pipe.

On my Shearer sb/sf oof silenced pipes, the OD is 1" at the smallest part of the convergent section and only about .905" ID there. The silencer fits over this and therefore has a measurement of 1" ID at the end of the pipe.

When we had the inframe Shearer big bore and small bore pipes, the big bore did have a larger ID measurement.

If you think about it, the big bore is designed for larger cc motors because they flow more exhaust due to the larger displacement. If they had the same id as a small bore then the heat would back up in the motor and cause some undesirably high piston crown temps.

I don't know what CPI is running for IDs on big bore or small bore.

 

 

I was referring to the very end of the stinger being the same on my pipes, but yeah - I know the BBs are bigger in places than SBs and do get rid of heat better.

[PassionRE]

Projected heat and flange inside diameter will dictate the angles and body diameter of a correctly designed pipe. A high Bmep 421 and a low Bmep 510 will take a similar pipe. On engines such as a high Bmep 510 Cheetah will need a bigger stinger to bleed of heat . Sniper will offer this type of mod for big bore and hill shooting, both needing to bleed of heat to combat power fade. We dynoed with the small bore small flange out of frames Mailman was talking about on 6/3/2010. The inframe Assassins dynoed 5hp stronger with 2 more foot pounds of torque on a 421 cub.

 

 

 

What he said^^^^^....I would add that some high bmep engines in this class range will produce more HP on the dyno with small bore pipes but truley need the big bore pipe for sustained HP. This is critical in hillshooter engines along with any type of high drag coefficient enviroments. Big numbers on the dyno at times are only flash peak numbers. The engine builds heat fast, big numbers, and then power fade. Keep that engine under real world load and you might not have as much power as you think....Jim

[SonOfSand]

Just trying to learn some more, what does "BMEP" stand for and what does it mean?

 

Thanks.

[srp]

Just trying to learn some more, what does "BMEP" stand for and what does it mean?

 

Thanks.

 

BMEP is brake mean effective pressure. It is a way to calculate potential engine horse-power before the engine is actually built and will tell you if an existing motor is putting out maximum horse-power. A well built 2 stroke on gas will be around 11 bmep and on alcohol can be close to 13 bmep. The equation for for two stroke bmep is 75.4*torque(lb-ft)/displacement(Ci)/14.7

[mailman]

I think those numbers are way too low. There are many different derivations of the formula but here is the formula I use. HP X 6500 / L X RPM where hp is max horsepower, L is the engine displacement in liters, and of course RPM is the revolutions per minute where the engine produces max. hp.

 

For my gas engine: I have 492 cc's or .492 Liters (1000 cc's = 1 liter), I put out 105 hp at 10700 rpm so my bmep would be:

 

(105 X 6500) / (.492 X 10700) or about 130 for a bmep which is actually not a very good number for a twin two stroke motor.

 

I see bmep as a function that relates the hp you put out with the cc's you have and at "what rpm". The easy way to make hp is just to spin the motor faster. If a person, and most do, just compare max. hp numbers then they would never understand why two motors with the same hp would not be equally fast in a race if the two motors have different bmep's or in other words make the max hp at different rpms. The one with the higher bmep or making the hp at a lower rpm should always win assuming all other things being equal. BMEP is a more effective measuring tool than just comparing max hp since it takes into consideration the rpm where the max hp is made and of course the cc's. I like to think of bmep as "how highly developed" a motor is.

 

Hope that makes sense,

 

Mailman

[SonOfSand]

Thanks for the explanation guys.

[srp]

I think those numbers are way too low. There are many different derivations of the formula but here is the formula I use. HP X 6500 / L X RPM where hp is max horsepower, L is the engine displacement in liters, and of course RPM is the revolutions per minute where the engine produces max. hp.

 

For my gas engine: I have 492 cc's or .492 Liters (1000 cc's = 1 liter), I put out 105 hp at 10700 rpm so my bmep would be:

 

(105 X 6500) / (.492 X 10700) or about 130 for a bmep which is actually not a very good number for a twin two stroke motor.

 

I see bmep as a function that relates the hp you put out with the cc's you have and at "what rpm". The easy way to make hp is just to spin the motor faster. If a person, and most do, just compare max. hp numbers then they would never understand why two motors with the same hp would not be equally fast in a race if the two motors have different bmep's or in other words make the max hp at different rpms. The one with the higher bmep or making the hp at a lower rpm should always win assuming all other things being equal. BMEP is a more effective measuring tool than just comparing max hp since it takes into consideration the rpm where the max hp is made and of course the cc's. I like to think of bmep as "how highly developed" a motor is.

 

Hope that makes sense,

 

Mailman

The scientific standard is calculated in bars (9 bmep low) (11bmep high). Your formula divided by 14.7 will convert to bars. The 492 is somewhere around 8.85 bmep. If a 492 is tuned to 11 bmep@10700 it would make 131 horse-power. This shows more tuning can be done to extract more power. The engine in my drag bike is a 10.88 bmep Passion 421 Cub on gas making 105 horse-power@10150. This shows my engine is very close to maximum output. By your comments you seem to be knowledgeable and on your game. The first number I look at when rating an engine is bmep and then horse-power second.