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Mohawk

Yoshi Style Velocity Stacks revisited

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So I was just re-reading this old thread, wow was it that long ago ?  I forgot to add some good reference material, for those frustrated engineers.

 

Old thread is here

https://www.vfrdiscussion.com/index.php?/forums/topic/80640-yoshi-style-velocity-stacks/&page=3

 

Additional tech info here http://profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

Which explains why I did this, the pulse bulges out of the open pipe/velocity stack end & the dual stack upper catches the pulse & carries it up its own pipe, hence the spacing between the upeer & lower is critical as demonstrated by HighSideNZ's adjustments.

 

The calc for runner length is 84000/desired RPM for a result in inches, times by 25.4 to get it in mm.

So 84000/9000=9.33” or 237mm, the 5th gen actual is 225mm (rough measurement by me) which gives peak torque at 8600rpm. So it’s a fairly close estimate for 4th reflection tuning length.

 

Enjoy.

 

 

 

 

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Engines are very sensitive to intake changes, and velocity stacks, and neck openings are something all their own.  If you look at some of the older first generation Honda 750 - 4's  ( not the VFR ) you will see that the velocity stack stuck out well into the stock airbox.  Any changes to their length affected the engines performance.

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Yep correct, shorter makes more HP, longer more torque, these get the best of both worlds 👍😀

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I’m still amazed by how well thought-through your trumpet setup is, Mohawk. 

 

A lot of engines are now running variable length length stacks powered by servos and clever ecm programming. Yours achieves the same result with a clever and frankly simple double-stack design. 

 

Very elegant. 

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Not as elegant as one may imagine.

I imagined "variable" to be some arrangement that infinitely varies the intake tract length over a range, based on rpm and throttle position. But it's not like that.

At a specified high rpm, regardless of throttle position, the upper stacks pop up.

I've done lots of individual runner velocity stack R&D.

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Some bikes use a form of factory dual stack now & others dual length. The problem with the dual length is they are really on/off not variable, but variable is very hard to do as there is always a step & the step disrupts the pressure pulse & also hurts the airflow around the circumference of the runner. 

 

At at some point this year I hope to try out a modified Throttle body & see if that improves things. 

 

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On 3/5/2019 at 10:12 PM, marc99 said:

Not as elegant as one may imagine.

I imagined "variable" to be some arrangement that infinitely varies the intake tract length over a range, based on rpm and throttle position. But it's not like that.

At a specified high rpm, regardless of throttle position, the upper stacks pop up.

I've done lots of individual runner velocity stack R&D.

I meant that Mohawk’s design is elegant in its simplicity. 

 

The stacks stay in the same position but airflow changes as the engine sucks harder. No moving parts to break and no disruption to airflow as changes occur. Perfect! 

 

It’s really very clever. 

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Your pics seem to have evaporated from the original thread... any chance you could re-post?

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Here ya go

 

post-24524-0-12789700-1427925341.jpg

 

post-24524-0-49344300-1427925415.jpg

 

post-24524-0-34287300-1427925596.jpg

 

I’ve just updated this setup but still working on the fuelling.

I have replaced my Y2K TB’s with a set of reworked 1999 versions. I have taper bored each intake to 42mm to remove the step left by the removal of the plastic trumpets from the intake, you can see this lip in the first picture. I used a spare airbox & have done away with the mushroom lips on the intake, which reduces the intake length by 10mm. I have rounded the airbox to intake & matched them, so now have a runner length of 185mm from valve to base of airbox, plus the dual stack. 

 

I have not not taken pictures of this yet as I like to test my work before releasing it. But I have to play with the fuel pressure regulator, so will take some pics then.  So the intake at the base of airbox is a 42mm opening tapering to the butterfly at 36mm. According to intake testing data I’ve seen this is close to the perfect intake configuration for taper. Once the fuelling issue is sorted I’ll get it Dyno’d & see what the result is.

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This is definitely cool. I don't know that I'll ever wind up doing anything with it, but I like the possibilities. 

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Just a quick. note to any UK VFR owners, if you want a spare airbox top to try a big airbox mod. I find that I have two, happy to ship in UK for postage. 

 

B946016B-4473-47E9-8750-0088E0A93743.thumb.jpeg.cea7b3c70e74e618ae7fa614df9abcfb.jpeg

 

They have been de-flapped already. 

 

For others info here's a pic of a foam cast I took of of the area above my airbox top filter.

 

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This is the area where the flapper & snorkel bulges live, but it my case the whole area is now the space above the filter. 

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So having forgotten to take pics of the Mk2 airbox, I was fitting a new FPR today & remembered to take some.  Pics above show Mk1. Below id the Mk2. I used a spare airbox & rounded the intakes to a 6mm radius, which is the minimum recommended intake radius. I then adjusted the dual stacks to suit the new intake height.

 

49E06A34-4DBB-4B8A-93C3-BFDE0265B02A.thumb.jpeg.6622fa7285589a60ce85c5de059002c8.jpeg

 

5E8AA2A9-BB17-431F-A02A-90DF26ABC3B6.thumb.jpeg.4d875ecff69b0d24ba6ce171de5ca344.jpeg

 

3E1A7D7B-FF25-4C4A-A025-661C7673B36A.thumb.jpeg.5f58d2ad3c620e2129a28b49dfe0f4bf.jpeg

 

5604EFAB-8854-4C0E-89DA-023413662E3E.thumb.jpeg.53529e0ea0e10ee71936ce62cf00a914.jpeg

 

This is matched to reworked throttle bodies which now taper from 42mm to 36mm. matching the optimum intake taper angle. These are fitted with CBR929 4-hole injectors, which require 51psi at idle or 4bar (58psi) peak. 

 

I will put a few miles on the bike to get the fuelling sorted for the new setup, then get it dyno'd & see what we have. 

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So no dual stacks anymore? Just single stacks set just above the base of the airbox?

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I put a radius on the inlets at the base of the box, so they are effectively the short or primary stack. The dual or secondary stack is the one above with the mushroom lip. So still dual stack. Plus the inlets are wider now & tapered to the butterflies, so should widen the torque curve further whilst improving top end flow & the new injectors should provide a better atomisation at all rpm, which is supposed to help torque, power & mpg. 👍

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Mr. Mohawk,

 

Owning a carbed bike, I've never messed with injection. Having wrenched on my own stuff for over 50 (!) years, I'm curious as to why you are using CBR 929 injectors with only 4 holes when most conversations are about the advantages of the 'newer' 12 hole vs the 'older' 6 hole.  

 

Thanks for your response.

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So the 5th gen VFR only has a single pintle injector. The 4 hole is a major improvement over this & the CBR929 injectors are a drop in fit. The 6th gen uses 12 hole injectors, but the research I have done seems to indicate from empirical evidence that the change from 1 to 4 hole is really good but the step from 4 to 12 less so. The later larger hole count injectors seem to be more about reducing emissions than improving performance. 

 

If there was a readily available 12 hole that would fit then I would have tried them. There are some rather expensive ones designed to fit Hayabusa's which used the same single hole injectors as the VFR. but $500 seemed a bit steep for an experiment !  Maybe if the 4 hole prove to be a good upgrade then I might entertain those expensive ones but not if they don't at least double the gain from the 4 hole injectors.

 

I just did about 50 miles today & as per the last time I used s zero map on the RBR  the bike was very fluffy at the beginning. After the 50 miles of O2 auto fuel correction, the seat of the pants feel is really good. I'll get another 50-100 mes on it then get it dyno'd. Looking at the fuel correction tables, where the stock fuel pressure was adding up to 35% more fuel, the new setup has reduced many cells, so injector delivery is working as intended now. But importantly the cells that matter in the mid range to highend are adding or not removing fuel duty cycle, so more air must be flowing. The bike picks up really well now in any gear from any revs in sny gear, where ss with the lower fuel pressure it always seemed to lag a bit ! 

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