Ideal intake runner taper

[galderdi]

My plan is to use a standard tripple manifold with tripple Delortos with all the internals removed and injection added to the manifold.

Why you ask? Well I was given the tripples so this will work out more cost effective.

 

As I mentioned I have removed all the chokes and jets from the delortos so I am left with just a tube and a throttle in each. I am also having them machined out from 40mm to 45mm. I have new 45mm butterflies etc to go in once this is done.

 

Now comes the question. I have conflicting information. On one hand I have been told to aim for a gradual taper starting at the trumpet and gradually reducing until it matches the port size. On the other hand I have been told to retain the restriction in the delortos as it will speed the air flow through the restriction and "charge" the air.

 

While I agree with the concept of the second view, I don't think it would work in my case because the restriction is too far from the port. Currently the trumpets start at 45mm and it continues at 45mm until the butterfly where there is an immediate step down to 40mm, then it steps back to 45mm for the beginning of the runners which then gradually taper down until they match the ports. I think the increased air flow through the restriction is only going to be negated when it hits the slightly larger openning of the runner.

 

My theory is that by machining out the delortos to 45mm I then have 45mm all the way from the trumpet to the beginning of the runners, then it gradually tapers down to match the port. I still end up with a restriction which is right at the port. So the air should still speed up and will continue at that speed into the head.

 

Question 1. What is the ideal taper for a runner?

Question 2. What are your views on these two oposing theories?

[cockerstar]

My theory is that by machining out the delortos to 45mm I then have 45mm all the way from the trumpet to the beginning of the runners, then it gradually tapers down to match the port. I still end up with a restriction which is right at the port. So the air should still speed up and will continue at that speed into the head.

 

This is what I would do.

Get a good set of horns on there and fab up a piece to allow you to inject directly into the throat of the carbs and hide it all in an airbox!

 

The look and sound of oldschool carbs, the tunability of modern efi, and the atomization/mixture of an F1 design all in one neat package. What's not to like?

TonyD bounced this prospect off of me a few months ago, and it's very appealing, in my opinion!

 

They're a little pricey, but the guys over at DIYautotune offer some bungs, here, that would make this pretty straight-forward process. The same thing could be accomplished with the tools Ross Machine offers and some bar stock.

[Leon]

My plan is to use a standard tripple manifold with tripple Delortos with all the internals removed and injection added to the manifold.

Why you ask? Well I was given the tripples so this will work out more cost effective.

 

As I mentioned I have removed all the chokes and jets from the delortos so I am left with just a tube and a throttle in each. I am also having them machined out from 40mm to 45mm. I have new 45mm butterflies etc to go in once this is done.

 

Now comes the question. I have conflicting information. On one hand I have been told to aim for a gradual taper starting at the trumpet and gradually reducing until it matches the port size. On the other hand I have been told to retain the restriction in the delortos as it will speed the air flow through the restriction and "charge" the air.

That is plain wrong and frankly, terrible advice. Why retain a restriction? A restriction is just that, something that hinders flow. So many people develop a vexing fixation on velocity, but I just don't get it. Why? Frictional losses increase proportionally with the square of velocity! This is bad.

 

Rather, look at pressure. The more pressurized the intake, the better your cylinder is filled (higher volumetric efficiency), which boosts the amount of torque your engine can make, therefore we want to maximize intake pressure. Now, velocity does affect pressure (dynamic pressure), but with the side-effect of increasing frictional losses (pressure drops) in the intake. As you can see, there is a fine balance between the two where optimization is needed. Trial and error can determine the balance, but that involves making many iterations of intakes varying runner diameter and testing to see which one best matches your goals. However, changing runner diameter also changes your tuned rpm, so you're playing with multiple intertwined variables here.

 

 

While I agree with the concept of the second view, I don't think it would work in my case because the restriction is too far from the port. Currently the trumpets start at 45mm and it continues at 45mm until the butterfly where there is an immediate step down to 40mm, then it steps back to 45mm for the beginning of the runners which then gradually taper down until they match the ports. I think the increased air flow through the restriction is only going to be negated when it hits the slightly larger openning of the runner.

 

My theory is that by machining out the delortos to 45mm I then have 45mm all the way from the trumpet to the beginning of the runners, then it gradually tapers down to match the port. I still end up with a restriction which is right at the port. So the air should still speed up and will continue at that speed into the head.

 

Question 1. What is the ideal taper for a runner?

Question 2. What are your views on these two oposing theories?

Let's forget about the restriction nonsense, and focus on intake design and taper. Your idea on tapering down to the head is better than putting a restrictor into the intake. The head port is still a restriction (not a good thing!) but there are compromises there that we cannot avoid (packaging).

 

So, why taper the runners? It's all about resonance tuning. Well what the hell is resonance tuning, you may ask!

 

When your intake valve opens, the mixture begins to flow into the cylinder. This propogates an expansion wave up the intake tract. Think of it this way: the intake runner is a 4-lane highway, the valve is a traffic signal, and the cylinder is a mall parking lot after the signal. The cars on the highway waiting for the light represent the air-fuel molecules. When the traffic signal turns green (valve opens), cars begin to flow into the parking lot (cylinder). Looking down at the highway (intake runner) with a bird's-eye view, you can see the front-most cars moving first and creating a gap between themselves and the cars behind them. Keep looking down and you'll see that this gap travels down the line of cars on the highway until it reaches the last car and the last car begins to move. This is your expansion wave propogating up your intake tract!

 

Wave dynamics state that if a wave travels through a pipe and hits a discontinuity (a change in diameter, like the end of a pipe) then it is reflected back down that pipe with the opposite effect (negative to positive - an expansion wave gets reflected/inverted as a compression wave). Let's get back to our traffic analogy where we left off, imagine that the reflection happens when the wave hits the last car in line on the highway and it begins to move. The line of cars has now expanded after the light turned green, so the last car in line will work to compress it. So, the last car in line gasses it and rams the car in front! They now go down the line, ramming each car in front of them until they've compressed themselves into a big hunk of metal upon reaching the parking lot (cylinder). This is the compression wave reflecting down the runner and squeezing back into your cylinder! This compression wave increases intake pressure, which fills more of the cylinder (an increase in "volumetric efficiency"), which is what ultimately increases engine torque.

 

Silly analogies aside, those are the fundamentals.

 

Back to our intake and cylinder, we can see that it takes time for the expansion wave to travel up the intake, reflect, and travel back down as a compression wave. In order for our intake to be "tuned", the compression wave must reach the valve just before it closes so that we can squeeze in that last extra bit of air-fuel mixture without reversion of the mixture back into the intake. You can see now, the connection of intake tuning to valve timing and intake length. The longer the intake runner, the longer it takes for the compression wave to travel back to the valve. This means that when you increase runner length, your tuned rpm drops to a slower engine speed, where the wave has enough time to get back to the valve. If you change your valve timing, e.g. closig the intake valve later, then you raise your tuned rpm to a higher engine speed since the wave now has extra time to get back to the valve before closing.

 

What does this have to do with intake taper?

 

Think back to the expansion wave hitting a discontinuity in the pipe. A discontinuity causes an inverted reflection of the wave. If your intake is straight from air filter to valve, you get one big, fat reflection but only at one discrete engine rpm. At other engine speeds, the timing of the reflection will be off. This gives your torque curve a pronounced peak but no benefit from intake tuning off that peak. What a taper in the pipe does, is introduce a bunch of tiny, little discontinuities along its length. Meaning, as the wave travels through the taper, the diameter is continuously changing. This allows some of the wave to be reflected back down the intake, while the rest of it keeps moving out. What you have now is a series of smaller reflections but at a broader range of engine speeds. This results in a less peaky but more flat and robust torque curve.

 

From what we've discussed here, it can be seen that a short, agressive taper will make the reflection stronger but last for a shorter span of rpm. The opposite goes for a shallower, longer taper (broader tuned range, but with a smaller bump in performance).

 

Taper location also plays a role, once you think about it. Since the wave takes time to travel up and down the intake runner, the further you space the taper away from the valve, the longer it will take for the expansion wave to reach the discontinuity. This allows you to "activate" your taper at different rpm. If you start the taper further out from the valve, then you delay relfection, which then shifts your torque curve down. At higher rpm, the compression wave simply won't have the time to reach the valve before it closes.

 

In conclusion, there is no ideal intake taper! You must figure out your design intent (peaky or flat torque curve; low, mid or high end torque) from which ideal runner size can be found.

 

Sorry for the long post, but I felt compelled to give a more thorough answer. Hopefully, you can get some benefit out of it!

[galderdi]

Thanks for taking the time for such a detailed reply. That makes a lot of sense to me. I am aiming for torqey mid range power eg 4000 - 5500. The intake is a cannon manifold which has logish runners. The the Delortos which add to that length and the trumpets which are 65mm from memory. I haven't done the math yet but I understand the resulting length will be more suitable for power low down so I am hopeful it will support my goal.

 

I need to retake the measurements so I can plug them into the resonance formula.

[ctc]

So many people develop a vexing fixation on velocity, but I just don't get it. Why? Frictional losses increase proportionally with the square of velocity! This is bad.

 

Here's WHY,

 

More velocity is good as is Total pressure. You should attempt to increase both. Fictional losses are an order of magnitude or more less than the gain for the increase in velocity.

 

VE is dependant on the mass of air that gets into the engine. Mass flow is a function of area, desity and velocity. Mass flow can also be increased by increasing Total Pressure (what a turbo charger does).

 

What you want to say is you need to increase momentum (mass times velocity) of the fuel air charge. This will increase torque, both peak and the rpm band it operates over.

 

I agree with your statement on the pressure wave, but the term you are lookin for is inlet column inertia. Newton's second law, once the air gets moving it does not want to stop so it contiues to compress behind the intake valve until that valve opens. This is driven by "tuning" the length of the intake runner.

 

I haven't brushed up on my physics, but would like to understand the background for the infinate discontinuity taper argument. I have always understood from fluid mechanics that the taper does three things; 1) it fights the frictional losses by increasing velocity (again more gain from the velocity than loss from friction) 2) it prevents the air from detaching from the walls and choking the intake flow (effective flow area) 3)prevents the fuel from falling out of suspension.

 

Buy the way, on some engines, empirically about 7 degrees of taper is the maximum to prevent the airflow from detaching from the walls. So some where between no taper and 7 degrees is where you want to be.

[Leon]

Here's WHY,

 

More velocity is good as is Total pressure. You should attempt to increase both. Fictional losses are an order of magnitude or more less than the gain for the increase in velocity.

 

VE is dependant on the mass of air that gets into the engine. Mass flow is a function of area, desity and velocity. Mass flow can also be increased by increasing Total Pressure (what a turbo charger does).

 

What you want to say is you need to increase momentum (mass times velocity) of the fuel air charge. This will increase torque, both peak and the rpm band it operates over.

 

I agree with your statement on the pressure wave, but the term you are lookin for is inlet column inertia. Newton's second law, once the air gets moving it does not want to stop so it contiues to compress behind the intake valve until that valve opens. This is driven by "tuning" the length of the intake runner.

 

I haven't brushed up on my physics, but would like to understand the background for the infinate discontinuity taper argument. I have always understood from fluid mechanics that the taper does three things; 1) it fights the frictional losses by increasing velocity (again more gain from the velocity than loss from friction) 2) it prevents the air from detaching from the walls and choking the intake flow (effective flow area) 3)prevents the fuel from falling out of suspension.

 

Buy the way, on some engines, empirically about 7 degrees of taper is the maximum to prevent the airflow from detaching from the walls. So some where between no taper and 7 degrees is where you want to be.

I wish they were fictional losses!

 

Yes, I understand the relationship of velocity to momentum. My point is that people tend to focus completely on velocity, not realizing that frictional losses are present, not to mention that they increase with the square of velocity. I mentioned this in the exact line that you quoted.

 

Here's how it works, in a nutshell. The engine is essentially a pump with the air (and fuel) as the working fluid. When talking about frictional and velocity effects, we must speak in the same units of measure (e.g. don't compare momentum to pressure).

 

A good way to go about this is using Bernoulli's energy equation. Pressure losses depend on V², with the rest of the numbers being constants (pipe length, diameter, friction factor). Dynamic pressure, i.e. the pressure contribution from velocity, is also dependent on V², with the constant being density. In fact, frictional losses is what our engine (pump) must fight against. These are the so-called pumping losses, and they're responsible for preventing 100% VE at all engine speeds. Clearly, the effect is greater at higher engine speeds, and actually overshadows the ram effect even at high speed. Heywood's "Internal Combustion Engine Fundamentals" has a fantastic graphic showing this, Figure 6-9. The air filter, throttle valve and intake valve are the big losses, but don't disregard the friction in the runners. Plus, the throttle valve size is dependent on runner diameter!

 

There is also an interdepenence of resonance tuning and pipe length, and thus friction. To take the biggest advantage of resonance, you'd have to tune to the first harmonic, giving you in the range of 5 foot long intake runners. This clearly increases frictional losses, especially if there are bends in the 5 foot long intake pipe. Friction is always a consideration and is not to be tossed aside as if its contribution is "magnitudes" less than dynamic pressure. This is simply not true. In fact, this is the biggest design challenge to an engine designer. Fuel and spark are easy, but getting as much air into the cylinder as possible is what's difficult.

 

Another key point you must realize is that resonance tuning is not the same thing as "ram" (intake inertia) tuning! The principles on which these phenomena operate are entirely different. Resonance tuning uses pressure waves (as I discussed in some detail), and ram tuning utilizes momentum/dynamic pressure. What you are talking about (ram tuning) depends on air mass, and how much of it rests in the intake runner waiting for the intake valve to open. In my highway analogy, ram tuning depends on the amount of cars and resonance tuning depends on how long the line is. Summing this up, resonance tuning drives runner length and ram tuning drives runner volume. It is imperative to have a solid understanding of this.

 

One last caveat on resonance vs. ram tuning is that ram tuning only has an effect at high rpm, where there is enough air velocity. This means that in order to take advantage of ram tuning, valve duration must be increased, which then hinders low-speed opertion. Resonance tuning can be tailored to any valve timing/duration without that compromise.

 

 

In regard to your taper questions, I'll answer in the same format that you used:

 

1) it fights the frictional losses by increasing velocity (again more gain from the velocity than loss from friction)

I've basically answered this earlier. That is not true, a taper does not and cannot "fight" frictional losses.

 

2) it prevents the air from detaching from the walls and choking the intake flow (effective flow area)

Not really, a straight runner would prevent flow separation as well.

 

3)prevents the fuel from falling out of suspension.

Somewhat, yes. This is one of the compromises of a "wet" intake runner. A direct-injected or port-injected engine design does not have to worry about fuel in the runners, thus you can go bigger (and more creative: think BMW's variable-runner systems of the past, a prime example of using resonance tuning!) without having the side-effect of more wetted runner area (larger Tau layer). Having to keep fuel atomized presents a compromise only when speaking about TBI or carbureted engines.

 

As far as theory goes...

 

As mentioned, a wave gets reflected at a discontinuity (diameter change). A straight, open pipe has one discontinuity, which is located at its exit. Now, if we add a step in the pipe such that the diameter changes to a bigger pipe before exiting, we have 2 discontinuities. The pressure wave will still travel the length of the pipe, but now there are two reflections at two distinct lengths (at step and at end). However, these reflected waves will now be smaller in amplitude. This decreases the magnitude of the resonance effect, but now stretches it out to 2 distinct engine speeds.

 

Look at the taper as an infinite series of steps. At each point, the diameter is getting bigger in infinitely small steps. The wave travels the length of the pipe, but gets reflected at each point along the taper. The longer your taper (more steps) the more you decrease the amplitude, but broaden the range. As with anything, it's a trade-off! Your overall goals determine whether you value amplitude (peakier torque curve) or range (wider, but less pronounced torque curve) more.

 

Yes, 7 degrees of taper is close to maximum before you get flow separation in the intake, thanks for mentioning that.

 

The overall goal of a properly designed intake is to minimize losses while extracting as much performance as possible. Resonance tuning and ram tuning both aim to achieve this, while working against flow reversion, charge heating, and friction.

 

References:

(1) Internal Combustion Engine Fundamentals by Heywood

(2) Introduction to Fluid Mechanics by Fox, McDonald and Pritchard

[Leon]

Thanks for taking the time for such a detailed reply. That makes a lot of sense to me. I am aiming for torqey mid range power eg 4000 - 5500. The intake is a cannon manifold which has logish runners. The the Delortos which add to that length and the trumpets which are 65mm from memory. I haven't done the math yet but I understand the resulting length will be more suitable for power low down so I am hopeful it will support my goal.

 

I need to retake the measurements so I can plug them into the resonance formula.

No problem! I've been working on a resonance tuning spreadsheet based on Heywood's book, all I need to do at this point id take accurate measurements of my actual setup to see if the numbers come out right. I think the Cannon manifold is as long as it gets, as far as triple carb manifolds go. Long air horns will give you more low- and mid-range but make sure you don't shroud them.

[Tony D]

I skipped everything and will respond to the original post/question.

I would skip the rebore to 45mm. 40mm ITB is PLENTY for the street.

I have done this on old style Toyota Mikuni 40PHH's... they utilised a 40mm bore, but a 44mm set of main venturis. They necked them down with sleeves which all the racers removed to give a 44mm bore (and 44mm trumpet) to the main choke.

 

When converting them to EFI, I copy the original spacer appropriately lengthening it to accomodate the original choke length as well, with the 44mm OD, but with a taper from the gasket flange to the 40mm bore.

 

If you look at your EFI Speed throttle bodies in OZ, you will see they offer this option on their 'race' ITB's.

 

This is what I would do with your Dellortos. Don't spend the money to bore them to 45mm. Leave them at 40. Make a tapered piece that will accommodate the length from back of the choke to front face of the carbie as a tapered section. Won't cost any more than the boring and you can then return the carbies to carbies whenever you want. This gives you a lead-in taper almost directly to the throttle plate, which you can then continue in the manifold to the head.

 

Really where you increase the diameter doesn't matter. FIA Homogolation has the gasket surface of the head at 35.5mm on the intake ports or something like that. So porting the head (gas flowing) shouldn't have you larger than that. Set the manifold to taper from 35.5 to 40mm at the intake manifold mounting flange, and from the back of the choke to the front of the carb body run whatever taper is appropriate to fill the space with a spacer that is held in place just like the stock venturi and booster.

 

The airflow from 40mm ITB's kills, well easily into the 300+HP range N/A on a 2.8L L6. And turbo? Who knows, but it's well over 700HP...WELL over 700 HP!

 

How much are you shooting for that you think you need 45mm bores and the bother that that entails?

[Tony D]

I have to apologise to the board, ending of last year there was a Japanese Magazine that did EXACTLY what is being discussed in this thread: Gutting the Carbies and running them as EFI ITB's. The performance bump was noticable and across the board compared to the carbs appropriately dyno tuned.

In the last 8 months I've had 20 days 'off' and of those 19 were vacation days to see my father for Christmas so he didn't spend the first holiday after my mom's passing alone, and going to Frank280ZX's place for Spa Weekend. I just haven't had enough time to scan the article and post it up. Even as Japanese article, it's photos are worth their weight in gold. This conversion in Japan is becoming so common now there are companies offering bolt-on TPS adapters for any of the popular side-drafts.

 

Don't even get into what I saw there earlier this month... I'm still in shock/recovering!

 

Maybe I'll quit work and have all the time in the world to post these cool magazine articles. But since I haven't all I can say is "sorry guys" -- I'm sure someone would take something away from the article, if even looking at the photos and before/after dyno charts!

[bigbreak_2000]

interesting discussion. I was asking someone the other day about ideas to improve the intake manifold on my 3.2L stroker. The motor is fuel injection. Although the N42 intake had been port matched, and I'm running a 70mm TB on it, it actually lost power (10% down from the previous street cam) after we put the bigger cam in. So the conclusion drawn is with the bigger cam (duration/overlap) the motor really needs more air, and even distribution of that. So he hinted at the idea of custom fabrication - taking a tripple carb manifold and convert it to FI. Has anyone actually done this stateside and have dyno results to show? Tony please scan that article, I am very curious just to see a picture.

Edited May 9, 2012 by bigbreak_2000

[Tony D]

So he hinted at the idea of custom fabrication - taking a tripple carb manifold and convert it to FI. Has anyone actually done this stateside and have dyno results to show?

 

Do you have the book "How to Modify your Nissan OHC"?

 

If so, you have a photo of just such a modified triple intake manifold. The Cannon (actually Nissan Comp) Triple manifold was used as that was what all the porters (most to this day) used as a standard 'best flowing combination' which was then flanged to some bent tubing (undoubtedly with full radius trumpets on the end) and a rubber tube isolator that connected that tubing to a plenum with a single throttle body.

 

That setup is shown in several different pictures from several different angles in the above mentioned book.

 

BTW, I just got my EMS Post from Japan Monday---many more books from Nostalgia Two-Days. I've overspent...I'm broke, and heading back to Japan on Monday!

[bigbreak_2000]

Finally have the ITB conversion complete. Dave did a great job, here's a picture of what the intake looks like. Now just need the brake setup completed, then its back to enjoying the car.

[Leon]

Finally have the ITB conversion complete. Dave did a great job, here's a picture of what the intake looks like. Now just need the brake setup completed, then its back to enjoying the car.

 

Very nice, looking forward to dyno results!

[galderdi]

Yeah, very nice

 

I keep procrastinating on my project. I don't want to risk the car not being available for all the events I plan to attend.

 

By the way my 240 is road registered but only to get to and from the track.

[johnc]

One data point...

The intake manifold built for my old race car was designed for torque.  Plenum was exactly 1/2 of total displacement (1.5L plenum for a 3L engine), runners were 7.5" long and tapered from a 1.75" air horn inside the plenum to the port size.  The tape started 2" from the plenum so the actual length of the taper was 5.5".  TB was 65mm.  The engine made at least 200 ft. lbs. of torque from about 4,000 to 7,500 rpm.  Torque peak was about 275 but I forget where - its posted somewhere here on HybridZ.

 

[SlowRob]

Motorcycle manufacturers utilize variable intake horns to maximize intake tuning effects.  In simplest form, they have movable velocity stacks.  At low RPM, the velocity stacks are in place, lengthening the compression wave, maximizing low end performance.  As RPM rises above a set threshold, the stacks are removed - a servo lifts them off the throat of the runner - increasing high end performance.

 

Keep in mind that they're working with 600 - 1000cc engines that spin to between 12k - 16k RPM.

 

I'd still be interested to see if a gain could be found using this idea on these [much] lower reving motors.

[clarkspeed]

I saw a formula SAE car once with variable runner length on 4 trumpets. A simple trumpet tube sliding in a custom intake runner. The trumpets were mounted on a movable wall of a plexiglass box with linkage and a servo controlling. I think the box prevented leak by on the tubes. It looked pretty cool when they reved the engine.

[bigbreak_2000]

I don't have the dynosheet with me at the moment but I think it was 230 (maybe240?) ft.lbs of torque at 4000 and peak just over 290 at 5700rpms. Max HP was at 6700 so its a very decent power band, and should give good speed in the gears...I say "should" because I haven't driven the car on the road yet after the upgrade, as I'm still waiting on the rotor/hats from Coleman. But that power is with the airbox on. What is interesting is that the motor, without the airbox, would make the same power but peak torque at 5200 rpm. So the goal is to get a good street tune with the airbox on, and ideally move the airfilter as close to the TB as possible (but still keeping the intake air at a decent temperature). I'll post some runner measurements soon and compare to what you've got John. Keep in mind mine is slightly larger capacity at 3.2L and have the 70mm TB.

Edited January 18, 2013 by bigbreak_2000

[Leon]

Motorcycle manufacturers utilize variable intake horns to maximize intake tuning effects.  In simplest form, they have movable velocity stacks.  At low RPM, the velocity stacks are in place, lengthening the compression wave, maximizing low end performance.  As RPM rises above a set threshold, the stacks are removed - a servo lifts them off the throat of the runner - increasing high end performance.

 

Keep in mind that they're working with 600 - 1000cc engines that spin to between 12k - 16k RPM.

 

I'd still be interested to see if a gain could be found using this idea on these [much] lower reving motors.

 

 

I saw a formula SAE car once with variable runner length on 4 trumpets. A simple trumpet tube sliding in a custom intake runner. The trumpets were mounted on a movable wall of a plexiglass box with linkage and a servo controlling. I think the box prevented leak by on the tubes. It looked pretty cool when they reved the engine.

 

We contemplated doing the same on our FSAE car at the time but ended up keeping a fixed length and tuning the intake and exhaust to the desired RPMs. Our engine didn't really have the RPM range to require movable trumpets. I would have loved to see that FSAE car!

[Leon]

I don't have the dynosheet with me at the moment but I think it was 230 (maybe240?) ft.lbs of torque at 4000 and peak just over 290 at 5700rpms. Max HP was at 6700 so its a very decent power band, and should give good speed in the gears...I say "should" because I haven't driven the car on the road yet after the upgrade, as I'm still waiting on the rotor/hats from Coleman. But that power is with the airbox on. What is interesting is that the motor, without the airbox, would make the same power but peak torque at 5200 rpm. So the goal is to get a good street tune with the airbox on, and ideally move the airfilter as close to the TB as possible (but still keeping the intake air at a decent temperature). I'll post some runner measurements soon and compare to what you've got John. Keep in mind mine is slightly larger capacity at 3.2L and have the 70mm TB.

 

Sounds like your airbox is undersized.