Topic: Has anyone actually done a dyno comparison between pods and the stock airbox?

[Gonzowerke]

Once again, the frustrating search function on this forum is telling me there are no threads on this exact subject, so I ask...Does anyone have any dyno results to show that pods do indeed add HP over the carefully designed stock airbox?

Thanks!

Gonzo

[seanbarney41]

I've been trying to get someone to do this for years...my prediction?...airbox will have a nice, flat torque curve...pods will have a little high rpm hp bump ...(and the size of the bump will depend on the skill of the carb tuner)

[Gonzowerke]

I have been wanting to put this debate to bed for sometime now. Pods do away with the stacks, which is only good for upper range WOT running. I can see it being good for a drag racer, but not for a regular racebike, nor a streetbike. I refuse to trade power for looks. My s1000rr lifts the stacks off the throttle bodies at 12K, so they are pulling through the open throat. And when it does that, you know it!

[Bill/BentON Racing]

Some pods have stacks in them.If you flow more air and adjust fuel ratio correctly,i'll surely take that over stock box,just can't get enough air sometimes with stock/modified box.A streetbike pods and rain don't mix well.Explain "stacks lifing off throttle body".Thanks,Bill
May try this so we all know next off season with Brian Livingood @ Livingood Motorsports(our dyno man).

[simon#42]

bill the bmw has two inlet trumpets per injector body [ one on top of the other ], at low revs they are together [ making one long trumpet ] above 6000 revs the top trumpet is moved away from the bottom [ by a servo motor ] just leaving the shorter bottom trumpet [ which is better for top end power ]  on the race bike we just throw it all in the bin !

[Retro Rocket]

I've been trying to get someone to do this for years...my prediction?...airbox will have a nice, flat torque curve...pods will have a little high rpm hp bump ...(and the size of the bump will depend on the skill of the carb tuner)

That will also depend on the quality of the pods and if they have stacks inbuilt or not. If you expect performance from emgo pods then you will be sadly mistaken.  Getting more air into the stock airbox is the way to go as there is nothing directly interfering with the stack mouths. I have considered running a couple of ram tubes into the filter section form over the top of the engine to give more flow at higher speeds, like some of the modern sports bikes....?

[Gonzowerke]

I've been trying to get someone to do this for years...my prediction?...airbox will have a nice, flat torque curve...pods will have a little high rpm hp bump ...(and the size of the bump will depend on the skill of the carb tuner)

That will also depend on the quality of the pods and if they have stacks inbuilt or not. If you expect performance from emgo pods then you will be sadly mistaken.  Getting more air into the stock airbox is the way to go as there is nothing directly interfering with the stack mouths. I have considered running a couple of ram tubes into the filter section form over the top of the engine to give more flow at higher speeds, like some of the modern sports bikes....?

I've been thinking about this too, using some sort of ducting. Aircooled VW beetle heater hose is about right, but looks like crap.

[TwoTired]

Be aware that the SOHC4 carbs's jet flow relies on the pressure differential between what is in the carb throat and outside atmospheric pressure.

While more air available inside the air box is regarded as a good thing, ram air or air supplied under pressure can be transferred to the carb throats and will reduced fuel flow through those metering jets, leaning the A/F ratio.

While the engine wants more volume of air and fuel to make more power, don't ignore the pressure effects at whatever air volume is supplied.

Now, with fuel injection and a closed loop system of adjusting the a/f ratio, this can be automatically accounted for without detriment.  So, careful what you "learn" from modern bike technology.

Cheers,

[Gonzowerke]

Be aware that the SOHC4 carbs's jet flow relies on the pressure differential between what is in the carb throat and outside atmospheric pressure.

While more air available inside the air box is regarded as a good thing, ram air or air supplied under pressure can be transferred to the carb throats and will reduced fuel flow through those metering jets, leaning the A/F ratio.

While the engine wants more volume of air and fuel to make more power, don't ignore the pressure effects at whatever air volume is supplied.

Now, with fuel injection and a closed loop system of adjusting the a/f ratio, this can be automatically accounted for without detriment.  So, careful what you "learn" from modern bike technology.

Cheers,

I'm not an engineer, but I thought that routing the carb bowl vent to the box would equalize the pressure differential so they would work as normal.

[TwoTired]

Be aware that the SOHC4 carbs's jet flow relies on the pressure differential between what is in the carb throat and outside atmospheric pressure.

While more air available inside the air box is regarded as a good thing, ram air or air supplied under pressure can be transferred to the carb throats and will reduced fuel flow through those metering jets, leaning the A/F ratio.

While the engine wants more volume of air and fuel to make more power, don't ignore the pressure effects at whatever air volume is supplied.

Now, with fuel injection and a closed loop system of adjusting the a/f ratio, this can be automatically accounted for without detriment.  So, careful what you "learn" from modern bike technology.

Cheers,

I'm not an engineer, but I thought that routing the carb bowl vent to the box would equalize the pressure differential so they would work as normal.

The air box normally runs at slightly lower than outside atmospheric pressure.    The source of the low pressure is the engine cylinder, which isn't equalized until you follow the inlet path to some distance outside of the air box entrance.  Air (a fluid) only moves when there is differential pressure present.

Consider this.  If the bowl vent were at the same pressure as in the carb throat, there would be no fluid movement across the fuel jet orifices.
It won't matter how much volume of air is traveling through the carb throat, if the pressure differential is the same on both sides of the fuel jet orifice, no fuel will flow.  Moving the bowl vent to the air box allows pressure effects from the engine cylinder to alter the vent reference, as would a ram air pressure effect, make vent pressure variable under different conditions.

I could work up an equation if that would help.

Also understand that in our gravity feed fuel systems, altering the carb bowl chamber pressure also has an effect on inlet fuel flow, particularly when the head pressures are low due to low fuel level either at stand pipe height or at base of fuel tank.  (Assuming fuel tank is also vented to atmosphere, of course.)

Cheers,

[scottly]

It won't matter how much volume of air is traveling through the carb throat, if the pressure differential is the same on both sides of the fuel jet orifice, no fuel will flow. 

Cheers,

The pressure differential in the throat of the carb is directly related to the volume of air passing through the VENTURI.

[TwoTired]

It won't matter how much volume of air is traveling through the carb throat, if the pressure differential is the same on both sides of the fuel jet orifice, no fuel will flow. 

Cheers,

The pressure differential in the throat of the carb is directly related to the volume of air passing through the VENTURI.

The venturi is only part of the pressure equation (and it's the speed, not the volume).  The engine cylinder is still creating a negative pressure source.  Even so, it still does not invalidate my statement.  Place the pressure in the carb throat (whether it be from venturi effect or cylinder source) on the carb vents and no fuel flows through the fuel jet orifices.

Cheers,

[scottly]

Have you never heard of a "blow-through" supercharger configuration? As long as the bowl vents are at the same pressure as present at the carb intake, the carb will function normally. (Of course, in this case, a fuel pump will be required.)

[scottly]

The venturi is only part of the pressure equation (and it's the speed, not the volume).  The engine cylinder is still creating a negative pressure source.  Even so, it still does not invalidate my statement.  Place the pressure in the carb throat (whether it be from venturi effect or cylinder source) on the carb vents and no fuel flows through the fuel jet orifices.

Cheers,

The velocity of the air through the venturi is directly related to the volume. Yes, if you place the bowl vents in the venturi, you will have no flow. Duh.

[lucky]

CB750- Pods over stock airbox is about  +5HP.

On a modern bike it is about +9HP.

But the added HP is because of the larger jets and needle position that
goes with the increased air.

[Retro Rocket]

CB750- Pods over stock airbox is about  +5HP.

On a modern bike it is about +9HP.

But the added HP is because of the larger jets and needle position that
goes with the increased air.

Wow, you come out with some good ones Lucky, most all pods bar the few that use velocity stacks are crap and do nothing but make the bike hard to get some semblance of  tune, use more fuel and give no improvement over stock, the second part of your statement is even worse, even MotoGP bikes use an airbox, there is a very good reason for that. If the increased air you speak of is turbulent it does nothing positive to the state of tune at all and makes tuning problematic, there are far to many variables when using pods, especially at speed, like cross wind rain and an inconsistent supply of air, airboxes solve all of these and is the reason they are still engineered and used {on every production bike as far as i know}, if pods were the be all and end all of tuning they would be on every bike made, particularly seeings though they are miles cheaper to produce than an airbox. Pods don't always increase airflow either, some are just plain crap and made for looks only.

[Old Scrambler]

+1 with Retro..............I ride a 201cc single-cylinder Tiger Cub push-rod 4-stroke at the Bonneville Salt Flats..........My 4 x 6-inch air filter is 7 inches removed from my carburetor.  The connector is a 3-inch diameter tube with an elbow leading to an enclosed rolled bell on the carb.  I covered the air-cleaner area with vinyl to keep it clean and protect it from passing (turbulant) air at 84mph. My direct competitor runs 48 more cc's and a larger carb but with a pod.............I hold the record!

[MRieck]

While I believe it was "carefully" designed it is also compromised by having to fit into a particular space. Modern airboxes work very well ( I like them too) primarily because of the increased volume. Engines are in a much, much higher state of tune and require large amounts of air. Even modern boxes can be improved (meaning increase in air volume) by removal of flapper valves and installation of a BMC or other race type filter. Tuning with a Power Commander can result in a straight A/F line and no loss of low end torque (the reason for the flapper valve. I have before/after dyno charts to prove that for my Busa and a CBR929 I did quite of bit of work to.
 If you did test a CB750 you would want one in stock tune. The minute you start increasing engines demand for charge via porting/larger valves, increasing displacement, higher lift/longer duration cam, less restrictive exhaust etc the stock unmodified box will run out of volume.
 The only other thing I would is if somebody did do it is to use a bike with an electronic ignition. Point ignitions do not do well when subjected to multiple runs on a dyno.

[simon#42]

mike would it be possible to put a stock air box on your flow bench and see what it actually flows ?   a quick comparison with a stock heads flow should prove how restrictive , or not it is .

[MRieck]

mike would it be possible to put a stock air box on your flow bench and see what it actually flows ?   a quick comparison with a stock heads flow should prove how restrictive , or not it is .

I guess I could do that Simon but I would need a 4 into 1 manifold......my bench can pull about 250cfm. I'm sure it would quickly show how much air it can pull. Unfortunately it will have to wait until I'm retired (about 7 years) secondary to scheduling problems.

[Gonzowerke]

My '78K is dead bone stock as far as the engine goes, except for 4/1 Jardine exhaust. I just need to put the bike back together so I can Dyno it!

Note to self: Start thread on engine dyno's.

[bwaller]

mike would it be possible to put a stock air box on your flow bench and see what it actually flows ?   a quick comparison with a stock heads flow should prove how restrictive , or not it is .

I guess I could do that Simon but I would need a 4 into 1 manifold......my bench can pull about 250cfm. I'm sure it would quickly show how much air it can pull. Unfortunately it will have to wait until I'm retired (about 7 years) secondary to scheduling problems.

Now Mike you know I told you not to let work get in the way. 

[lucky]

CB750- Pods over stock airbox is about  +5HP.

On a modern bike it is about +9HP.

But the added HP is because of the larger jets and needle position that
goes with the increased air.

Wow, you come out with some good ones Lucky, most all pods bar the few that use velocity stacks are crap and do nothing but make the bike hard to get some semblance of  tune, use more fuel and give no improvement over stock, the second part of your statement is even worse, even MotoGP bikes use an airbox, there is a very good reason for that. If the increased air you speak of is turbulent it does nothing positive to the state of tune at all and makes tuning problematic, there are far to many variables when using pods, especially at speed, like cross wind rain and an inconsistent supply of air, airboxes solve all of these and is the reason they are still engineered and used {on every production bike as far as i know}, if pods were the be all and end all of tuning they would be on every bike made, particularly seeings though they are miles cheaper to produce than an airbox. Pods don't always increase airflow either, some are just plain crap and made for looks only.

Many things you say are true.
But air boxs are also designed to cut down noise on modern production motorcycles.
For instance the Kawasaki 1500 classic (carbed), has 9 bends in the air box system before the air reaches the carb. One of the bends is 180º.  Bends in the intake system slow the air down. That cuts down noise. Dyno runs have shown an increase in power when this air box system is eliminated.

Plemnum chambers on fuel injection motorcycles is a completely different subject.
Back in the day when these carbed  CB750's were racing I never saw them with stock air boxes. I did not see any of the bikes with stock air boxes.

Modern fuel injected motorcycles are a different thing.

[ThomasD883]

I've decided to use a small scuba tank under my seat to push the air in turbo style, do you think i should still use the stock air box or use one for each carb? Maybe a leaf blower would be better, more cfm?

[TwoTired]

But air boxs are also designed to cut down noise on modern production motorcycles.

I expect noise probably IS inclusive of the overall design goal.  But, I don't see that as evidence of a primary design goal.  I feel confident in saying that overall performance in the majority of driving conditions, as well as peak performance in a confined application space, was a main selling goal for Honda.  Does this mean some design trade-offs?  I expect so.

For instance the Kawasaki 1500 classic (carbed), has 9 bends in the air box system before the air reaches the carb. One of the bends is 180º.  Bends in the intake system slow the air down. That cuts down noise. Dyno runs have shown an increase in power when this air box system is eliminated.

OK, but that is a dyno run for a completely different design.  Implying that two induction system designs behave in an identical manner, is rather like saying two glass containers hold the same amount of liquid, regardless of shape, size, or construction.
Besides, isn't the OP looking for dyno comparisons, before and after, on the SOHC4?  I'd like to see those, too!  (Particularly if the jetting was optimized both before and after the pod conversion.)

Plemnum chambers on fuel injection motorcycles is a completely different subject.

How is that different from the Kawasaki 1500, or the various forms of SOHC4 induction systems?  (This is actually a rhetorical question just to draw attention to the basic comparison dissimilarity.)

Back in the day when these carbed  CB750's were racing I never saw them with stock air boxes. I did not see any of the bikes with stock air boxes.

I hear/read this argument a lot.  I assume the base assumption being that if it is good for a racer, it is good for a street bike.  However, I feel the assumption fails in the majority of cases, imo.

What unlimited racer is not willing to sacrifice an engine that would normally run for 100,000 miles, if it would win him 5 races of 20 miles each?
How popular would a street bike offering be if the engine only lasted 100 miles between complete rebuilds?  Racers routinely sacrifice the machine to win the prize.  Is that really the model you wish to emulate?  Are you racing? ...Or going on a road trip?  Or, do you want to drive the machine to work and back for 5 (or more) years?

While there are some race mods that make the transference to production models, certainly not ALL race mods do.  The mods may be task based, where that task is not routinely applied to, or needed for, a reliable general transportation vehicle.
I feel pretty confident in generalizing that the stock induction offers NO detriment to engine performance up to 2/3 (quite possibly much more) of the operational engine RPM band between idle and red line RPM.  Would a race purposed bike be concerned much about this lower operational band?
Would a race purposed bike be willing to sacrifice 50% of performance bellow 2/3 of red line to make a 5% gain nearer to and above red line?
Would that same 5% gain coupled with a shorter engine life be accepted for a race purposed bike?  If it lasts to end of race, the answer is mostly yes!
The point is, if you adopt any race bike mods, you also accept the trade off made for that mod.

I could make a conjecture that pods on race bikes became popular simply as a means for quicker jet size changes, in order to eek out max power for the altitude, barometric pressure, and temperature of the track where the race bike was being run.
If it was done solely for that purpose on a race bike, does it really make sense to put pods on a street bike for that same purpose?  Can you imagine optimizing the jets and carb tune each day before going to work? 

Still, I'd like to see reproducible data for the SOHC4, showing where a particular pod conversion has improved performance and in what RPM band it occurred.

Cheers,