Maximizing Velocity in Two-Stroke Engine Intake Manifolds: Why Speed Beats Volume

[IceRocket1286]

Well alot of them you do this on now, both water and eddy braked ones,
Super Flow
Dyno jet
Go Power systems to name a few

Its mostly up to you as a tuner/control of the dyno that limits what pulls that is possible/want to do

I agree.

Sorry to threadjack. Looks like another quality part from the Wax racing crew!

 

[waxhead]

I agree.

Sorry to threadjack. Looks like another quality part from the Wax racing crew!

Feel free to threadjack, I’m actually enjoying the discussion turning towards dynos. It’s an interesting situation since I don’t know anyone currently dynoing two-stroke ski engines. I have my own thoughts on why it might not work. I remember years ago, Dan Lamey mentioned that his high-performance engines couldn’t be put on the dyno because the load was too much and it would destroy them.

 

[kevbo]

Ski engines run over peak torque rpm so the mixture would be too lean at max torque during a slow sweep. A custom dyno ignition curve would need to be developed. Plus the power at high rpm comes mainly from the pipe. Any high port engine without a pipe will be low on power. Ski engines are very easy to tune once you realize they must get on the pipe very quickly. I have watched many owners do a lot of bad things to their engines (increase compression and timing) when they should have just reduced the pump load a little.

 

[503701]

A lot of rhis is over my head so forgive the dumb questions. So back on the original topic, if I run 44's on a wax manifold will I be loosing velocity because the difference in diameter between the carbs and manifold? And that would be detrimental to velocity because the change in diameter right?

 

[waxhead]

A lot of rhis is over my head so forgive the dumb questions. So back on the original topic, if I run 44's on a wax manifold will I be loosing velocity because the difference in diameter between the carbs and manifold? And that would be detrimental to velocity because the change in diameter right?

You have to understand that any manifold has to be designed to accommodate the largest size carbs that might be used on it. It's not economically viable to produce a different manifold for every carb size—nobody does that. However, by keeping the intake consistent in area all the way down, you maintain the best possible speed. By keeping it as small as possible without restricting flow, you increase velocity and improve throttle response.

Ideally, an intake manifold with a 7-degree included angle that gradually narrows as it goes in would be optimal. But imagine trying to sell that idea to the jet ski crowd! This is why we keep the intake consistent—not decreasing and certainly not increasing.

 

[wiikstromm]

Ski engines run over peak torque rpm so the mixture would be too lean at max torque during a slow sweep. A custom dyno ignition curve would need to be developed. Plus the power at high rpm comes mainly from the pipe. Any high port engine without a pipe will be low on power. Ski engines are very easy to tune once you realize they must get on the pipe very quickly. I have watched many owners do a lot of bad things to their engines (increase compression and timing) when they should have just reduced the pump load a little.

There is no need to pull long slow sweeps, thats only for 4strokes and 2stroke racebikes imo
Dyno ski engines might not be for have it tuned, but a reference that is actually can produce this and that alot of HP the builder say they do.
And yes you can fool the dyno with correction numbers and this and that but thats just then shows that the builder who he is

 

[kevbo]

There is no need to pull long slow sweeps, thats only for 4strokes and 2stroke racebikes imo
Dyno ski engines might not be for have it tuned, but a reference that is actually can produce this and that alot of HP the builder say they do.
And yes you can fool the dyno with correction numbers and this and that but thats just then shows that the builder who he is

It’s easy to fool the customer with a high port and short pipe since this will produce high hp numbers. The greatest advantage of large engines is their ability to turn large pumps. Adding volume increases thrust at a higher rate than adding pressure does. Pump load also rises more rapidly with pressure than it does with volume. This is why a big pump can make a lot more thrust with a little more hp.

 

[Flash-FX]

The Boyesen manifolds were used on 1000 and 1200CC motors. Do You need that much intake volume on smaller CC motors?? Probably not. Here's what I did a few years back...

To increase the velocity on my 2-SS865 motors with FS 48mm carbs (past tense), I epoxied the intake tracts full length with epoxy. The starting point of the "stock Boyesen manifold" was about 48mm, and my finished one is 44mm. The only other epoxy treatment I've seen on the Boyesen manifolds was some JB weld poured into the outside radius when the manifold was propped up for curing, reducing some volume and making a "short cut" into the reed cage entrance. I didn't like the looks of that and went in this direction instead. My batch of epoxy (west systems) had some aluminum powder as filler and was runny. I intentionally stirred it up to create bubbles that would later be cut into and roughened up after curing.

To create this "fill" I used even weights of modeling clay for each runner. The formed clay inside the runners is to mimic the final epoxy pour. I made the clay fill layer thicker on the outside radius more than the short side radius. Then cut some plexiglass to use as dams on each end of the runners, using 2 part blue soft silicone to fill the center void of the tract. After the silicone cured, the silicone plugs and clay was removed. (The silicone "plugs" were cast in place by using acorn nuts bolted to the plexiglass dams for centering repeatability on other manifolds). Drilled a fill hole in the top plate of plexiglass for filling, mixed up the epoxy and poured it in. The silicone plugs were removed through the more rectangular "reed cage" end. The final finish was a rough carbide burr opening up the surface layer of bubbles cured in the epoxy. Sorry, NO dyno testing to prove this lost HP...But it ran good with 0 hesitation or lag bog. One of these epoxied manifolds is on my Dasa 850 today.

 

[waxhead]

The Boyesen manifolds were used on 1000 and 1200CC motors. Do You need that much intake volume on smaller CC motors?? Probably not. Here's what I did a few years back...

To increase the velocity on my 2-SS865 motors with FS 48mm carbs (past tense), I epoxied the intake tracts full length with epoxy. The starting point of the "stock Boyesen manifold" was about 48mm, and my finished one is 44mm. The only other epoxy treatment I've seen on the Boyesen manifolds was some JB weld poured into the outside radius when the manifold was propped up for curing, reducing some volume and making a "short cut" into the reed cage entrance. I didn't like the looks of that and went in this direction instead. My batch of epoxy (west systems) had some aluminum powder as filler and was runny. I intentionally stirred it up to create bubbles that would later be cut into and roughened up after curing.

To create this "fill" I used even weights of modeling clay for each runner. The formed clay inside the runners is to mimic the final epoxy pour. I made the clay fill layer thicker on the outside radius more than the short side radius. Then cut some plexiglass to use as dams on each end of the runners, using 2 part blue soft silicone to fill the center void of the tract. After the silicone cured, the silicone plugs and clay was removed. (The silicone "plugs" were cast in place by using acorn nuts bolted to the plexiglass dams for centering repeatability on other manifolds). Drilled a fill hole in the top plate of plexiglass for filling, mixed up the epoxy and poured it in. The silicone plugs were removed through the more rectangular "reed cage" end. The final finish was a rough carbide burr opening up the surface layer of bubbles cured in the epoxy. Sorry, NO dyno testing to prove this lost HP...But it ran good with 0 hesitation or lag bog. One of these epoxied manifolds is on my Dasa 850 today. View attachment 450478View attachment 450479

That’s awesome, and you basically expressed what we were trying to say in a better way. I did want to build a taper down to increase speed and maintain airflow, and a 7-degree included angle is the commonly recognized standard for that. Making a 44 manifold wasn’t an option for us because someone will inevitably want to run 46 or 48 manifolds. If I were to build a perfect manifold for a 44, I would have done pretty much what you did.

 

[Storbeck]

I've had my eye on using 2/3 of a kawi 900 triple mani on my next kawi project. Compared to the twin it has a little less total angle to it, overall a little shorter runner, bigger inside radius, the opening into the reed area is smaller, and no crossover to fill. They only used kehins so would have to modify the mounting holes for mikunis. MAYBE could be opened up for 44's but probably it's only big enough for 38 or 40mm carbs so not much interest for big power.

 

[wiikstromm]

It’s easy to fool the customer with a high port and short pipe since this will produce high hp numbers. The greatest advantage of large engines is their ability to turn large pumps. Adding volume increases thrust at a higher rate than adding pressure does. Pump load also rises more rapidly with pressure than it does with volume. This is why a big pump can make a lot more thrust with a little more hp.

It will give you high rpms and a narrow powerband but power is power, it takes power (kw) to move water and you always have a correlation between power and torque thats just mathematical.
My point was that no one in standup industry dyno anything and thats beacuse they say it cant be done, wich is just bullpoop, it can be done and will give you true numbers if done correctly is my point.

 

[wiikstromm]

The Boyesen manifolds were used on 1000 and 1200CC motors. Do You need that much intake volume on smaller CC motors?? Probably not. Here's what I did a few years back...

To increase the velocity on my 2-SS865 motors with FS 48mm carbs (past tense), I epoxied the intake tracts

Ive seen Xscream also do this to there intakes, the did tho fill the elbow a bit in the intake to straighten out and reducing volume in the intake.
I use one of those modded boyesen intakes on my dasa 1100 wich i also reduced crankcase volume in, for me it looks like good results.

 

[kevbo]

I've had my eye on using 2/3 of a kawi 900 triple mani on my next kawi project. Compared to the twin it has a little less total angle to it, overall a little shorter runner, bigger inside radius, the opening into the reed area is smaller, and no crossover to fill. They only used kehins so would have to modify the mounting holes for mikunis. MAYBE could be opened up for 44's but probably it's only big enough for 38 or 40mm carbs so not much interest for big power.

The 900zxi manifold is 38mm all the way through. Big pin 750 manifolds are 40mm with a crossover tube. A stock 750 keihin carb and intake work very well but the throttle cable mount may need to be modified. Stock small pin manifolds used on the 92-95 XI are larger and ideal for 44mm carbs. They get larger going down and only need the inlet hole bored to 44mm. I’m not sure how critical it is to get manifold volume down. I recently put 44s on a 46 manifold and it still needed much smaller jets than anticipated. The extra volume under the carb didn’t attenuate signal very much.

 

[Quinc]

It will give you high rpms and a narrow powerband but power is power, it takes power (kw) to move water and you always have a correlation between power and torque thats just mathematical.
My point was that no one in standup industry dyno anything and thats beacuse they say it cant be done, wich is just bullpoop, it can be done and will give you true numbers if done correctly is my point.

Tim from tnt claims that he used a dyno when creating his chambers. Factory pipe also said they used a dyno, and Joey from HighSpeed just purchased a land and sea dyno. He hasn't posted any numbers or videos yet.

 

[Storbeck]

Tim also said that you can't get much from a dyno and you have to water test.

 

[wiikstromm]

Tim from tnt claims that he used a dyno when creating his chambers. Factory pipe also said they used a dyno, and Joey from HighSpeed just purchased a land and sea dyno. He hasn't posted any numbers or videos yet.

Yes they claim, but where are the dyno sheets?
And "Tim says" you have to water test, well yeah its a jetski with a jetpump, just like say a snowmobile with clutches that needs tuning, but a dyno will get you numbers of what the motor is capable of producing in power, im just fascinated thats it not a common thing to do in this industry where we have extremly expensive high end engines from good builders.

 

[JetManiac]

Ive seen Xscream also do this to there intakes, the did tho fill the elbow a bit in the intake to straighten out and reducing volume in the intake.
I use one of those modded boyesen intakes on my dasa 1100 wich i also reduced crankcase volume in, for me it looks like good results.

Yes, Chuckie at XS did epoxy the boyesen manis to straight the flow path and volume adjust. Flash is running an XS motor so may have been inspired by the XS mani mods. The PHP mani fixed the issues with the Boyesen intake so no epoxy or modding needed.

 

[Quinc]

Yes they claim, but where are the dyno sheets?
And "Tim says" you have to water test, well yeah its a jetski with a jetpump, just like say a snowmobile with clutches that needs tuning, but a dyno will get you numbers of what the motor is capable of producing in power, im just fascinated thats it not a common thing to do in this industry where we have extremly expensive high end engines from good builders.

but then people might find out most of these engines dont actually make 200+ hp. =)

 

[kevbo]

but then people might find out most of these engines dont actually make 200+ hp. =)

Consumers would have to make sense of port heights and tuned pipes before they could get much useful information from a dyno chart. I dont think this group is that sophisticated. Its like mixing letters with numbers, not many want to understand it.

 

[waxhead]

When we designed the Wax Racing intake, we focused on keeping the volume as low as possible and ensuring the intake didn't increase in volume as air moved through. This was specifically done to maintain high airspeed, which is essential for optimal engine performance.

The Importance of High Air Velocity in the Intake System​

Maintaining high air velocity is key to maximizing performance in any intake system. Here’s why it’s more beneficial than simply increasing intake volume:

• Improved Throttle Response: Higher air velocity ensures quicker air delivery to the combustion chamber, leading to faster throttle response—especially at low RPMs when instant power is needed.
• Better Fuel Atomization: High airspeed allows for better mixing of air and fuel, leading to more efficient combustion, which boosts power and overall engine efficiency.
• Increased Low-End Torque: Engines with high air velocity at low RPMs experience improved cylinder filling, resulting in stronger low-end torque. This is critical for applications requiring quick bursts of power.
• Consistency Across RPM Ranges: While larger intake volumes may benefit high RPMs, maintaining high airspeed provides more consistent performance across all RPM ranges, improving throttle response and power delivery throughout.

Conclusion:​

By prioritizing high air velocity in the Wax Racing intake design, we’ve ensured better throttle response, superior fuel atomization, and stronger low-end torque, making it the ideal choice for most performance goals.