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Another RSP thread - with a twist

11781 Views 77 Replies 12 Participants Last post by  Xmedina
First time poster, spent far too long reading lots of interesting threads on intake manifolds.

However times have moved on and I thought you might be interested in my little project.

My car is an S1 elise with a K20a2, it currently has the PRB manifold and the RRC wont fit without cutting the bulkhead.

Then I discovered the RSP, not only does it look okay on paper but more importantly it is a modular design so I can take it apart to tweak quite easily.

I have seen several posts on here where people have cut the back off and welded it up and done similar to the trumpets, even seen people CNC machining new trumpets but all of that is either beyond my skill set or budget.

So this evening I set up a mini photo studio

Resulting in 109 photos of the trumpets, example below

I then imported those into 3DF Zephyr and after an hour or two of custom making image masks I was able to generate this splodge:

The rings of blue dots are the camera positions and the image mask process basically cuts out the block of wood the trumpets are balanced on. The wood is on a lazy susan (turn table) on an oversize clear plastic protractor. After each photo I rotated the part by exactly 10 degrees and took another photo, after 36 photos I rotated the part.

The splodge is a point cloud, the software has compared all of the photos an identified specific points that it can see in all (well some) of the photos and it uses these points to understand the 3d model.

Next step is to refine the point cloud:

And then generate the model

This can then be exported into your modelling software of choice. Because I am on freeware that would be Blender:

Its pretty lumpy and bumpy.

Next step is to basically trace it in the modelling software so that the whole thing is tidied up and then tweak it so it matches the dimensions of the actual part. Place particular emphasis on bolt holes etc.

I've found this to be a nice easy way to get to a 90% model that can be refined because freehand measuring things like curved surfaces can be tricky.

So why bother, where is this going?

First step will be to 3D print a new plenum back plate that does away with the resonance chamber, I can run this a while to check/prove reliability as it will be easy to see if its damaged.
Second step will be to print a set of 20mm trumpets with nice elliptical edges to open up the top end.
Third will be to experiment with different trumpet designs / angles to see if flow can be evened out / improved.
Then try and find a way to add a dual plenum to the back plate to feed the plenum.

Once the design has been tested (including dyno time) and reliability proven the last step will be printing a custom flange and runners as part of a full working printed inlet manifold to maximize.

I'm expecting all of this to take some time but i'll keep this updated if there is interest :)
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Welcome to mrluke!

First step will be to 3D print a new plenum back plate that does away with the resonance chamber, I can run this a while to check/prove reliability as it will be easy to see if its damaged.
Second step will be to print a set of 20mm trumpets with nice elliptical edges to open up the top end.
Third will be to experiment with different trumpet designs / angles to see if flow can be evened out / improved.
Then try and find a way to add a dual plenum to the back plate to feed the plenum.
Amazing what can be done out of digital pictures. I am interested in the quality. Did you measure the bolt pattern in the model and compare it to the real bell mouth bolt pattern?

A very good basis for your design approach is this paper here: RET_Bellmouth_Sept.pdf (
...64.6mm in the model and 64.6mm in real life.
Seems you rises a bit more of my interest with that result. Just the radii weren't build up correctly, especially at the bell mouth. What did happen to the parallel cut on the bolt bores? De-rendered?

But for a couple of hours in CAD i'm pretty happy with where I have got to.
I assume this is a test case for you to investigate the approach for further projects, isn't it? Wouldn't you be faster by to measure and draw it into 3D with that bell mouth flange? We used a 3D laser scan tool (80 kEur) to transfer a head into 3D CAD, the effort to render it correctly would so high we skipt it, maybe we had the wrong technology. But on free form shapes ghost differ, there the real advantage lays to dig out with such technologies. Would nice to see how it works there, e.g. on a more complex runner design of the RSP IM.

I'm not going to spend much more effort on this design because I don't want/need to print trumpets that duplicate the originals. This is good enough to get the dimensional accuracy, next step is to create an elliptical profile in both stock height and shortened lengths.
Very good, I hope the Blair paper helps on that.

Do you have an idea how long you want to have the runners, for the runner length tuning?
So I have modelled the elliptical profile from the paper:
However they aren't going to fit the RSP manifold.
They are also 1 diameter high so circa 50mm which means they will obstruct the TB opening as well.
Welcome to the engineering world. Theory and Practice, here the engineering began to span the bridge between both: clearance limits and theoretical recognitions to implement. If you read carefully the paper you will find the that Blair has a heart for engineers, as he already foreseen issues and made general design rules to follow. I would follow that.

I am no friend of 3D CFD, as the results are too depended on modeling approaches of boundary layers and mathematical requirements to get it started or to be able to calculate it. It doesn't include the fuel droplet spitting and it's effect of vaporization during that back and forward process. So I wouldn't mind about doing 3D CFD. BTW, most CFD codes look into stationary conditions, but the conditions here are by far not stationary. Modelling this correctly will cost some days or even weeks to calculate one single inlet event with respect to Plenum pressure and mass movement :D.

Regarding the length approaches. Here simple approaches like Speed of Sound wave front calculations up to 1D-engine simulation are rational. The interesting is, the first differ not too much from the last and every misses the reflection flexation and efficiency. The last was also not included by Blair, he just looked at the flow properties, not into the oscillation properties. Something, many approaches misses. What engine speed is in your specification?
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Plan is to have a few different trumpet sets to run on the dyno to try and see what is going on. Will be interesting to compare standard length trumpets with a side and rear inlet.
There are tons of tests comparing RRC vs RSP in different bell mouth lengths on different setups you can find here: RBC vs RSP(euro type R) intake manifold | Honda / Acura K20a K24a Engine Forum, especially I like the comparison of member 6spd_EK on pages #9 and #10. For my experience it isn't worth a penny to test different lengths below 30 mm difference. That said the there are 2 maybe 3 test cases: original, shorted by 30 or 60 mm. The effects what 6spd_EK found was coming from different sides: better inflow condition into the plenum, better inflow into the runners by well designed bell mouths and increased peak power engine speed with an aligning higher peak power. So maybe the 30 mm shortened runner isn't worth the effort too.

BTW and again...
What engine speed is in your specification?
As Scider said, length is the most important factor regarding peak power settlement as well as the other harmonic orders down low the revving band, therefore my introducing question.
I have thought about that as well, in regards to TB placement. I would love to test out a blanking plate and modify the lower resonance chamber to adapt the throttle body, probably in a dual plenum design but really just looking for improved throttle body angle of attack.
drtye, you may remember the 6spd_EK RSP plenum? You find them here: (1) RBC vs RSP(euro type R) intake manifold | Page 34 | Honda / Acura K20a K24a Engine Forum . I never saw the results, I lost the contact to 6spd_EK. Maybe you have the FB contact or any other? He lives in the mountains of the sate of NY. Not to far away from your place if I am not wrong 🤔?!
Which gives fairly minor gains from 5,750 through to 8,500 with the caveat that the low cam tune wasn't altered to suit the RSP.
That is what it is. Reality is a challenger 😉

A peak anywhere between 7.5k and 8k would be good, i'm not specifying an exact target because I haven't seen any calculations yet that have proven to be that accurate.
Like Scider mentioned above, exactly that is what the series RSP already does, it let the 2 Liter engine peak at 7800 rpm. More displacement = lower peak power engine speed.

The whole idea was around getting a manifold to perform as well as the RRC but still fit into a lotus without cutting a hole in the bulkhead :)
The RSP plenum flange bolt pattern causes the clearance demand of around 275 mm head to plenum flange end. No room to reduce that. The firewall/bulkhead has to be moved. What you could do is what I've done, doing a complete own IM. Otherwise, aftermarket could supply some more or less well developed IM which are definitely less challenging in clearance. But think of this, moving the bulkhead is a DIY job of around 100 Euro and two days of work. Better do this then landing in a dead end road.

Maybe you do a DIY plenum for the PRC IM, sort of central feed plenum, which would likely improve power significantly. This IM is the only one of the row of OEM ones which fits without any issues in a '02-'05 Elise chassis.
Bit more progress, would like to get the back plate ordered up this weekend and probably a set of trumpets for mock up / fitment check.
Very nice 👏.

Are you going to cut out the overlapping bell mouths? I would recommend to cut them at the touching. As they are symmetric they will form at the cut line exact the same shape, which makes it flow-wise much better. Just in case you do 3D print outs, on a single bell mouth fab approach, you would do that automatically.

How much volume to you provide for the plenum? The plenum volume is beside the shape of it the 3rd free parameter. You can design it via Helmholtz-resonance (cylinder displacement vs plenum volume) or do an 1D calculation method approach. In any case it is always power support vs. throttle response. If you don't race slalom races, where throttle response and control is most important, a rule of thumb would be around 2-4 Liter are ok for most applications. Maybe that helps.
To make the cuts I need to first make each trumpet into its own object which is one of the last steps
That makes sense. Very good.

The trumpets and the flange will all be printed together as one solid piece
Good approach, 3D printing does make it possible 🆙.

Alot of the papers on intake volume are workarounds to having a restrictor fitted,
How do you mean that. What should be restricted and in which approach?

Next stage will be getting the last piece of the manifold scanned in which then opens up CFD fun for what its worth. I'm really curious to see what the flow distribution is like and whether there is room for improvement.
I am curious about your results. I have my doubt the 3D CFD is any near reality as you simulate only moments of an whole work cycle and you likely don't have a model for the fuel film and vaporization, which has an huge influence on VE of the engine. Anyway, I am curious what it will show as results...
Many competitive race series require an inlet restrictor to limit maximum power.
Understood. You talk about an intake restrictor.

...What i'm really looking to gain from it is equalizing the air distribution between the intake runners.
Via e.g. a center feed plenum, like I suggested above (PRC plus customized center feed plenum)

Because the air feed into the plenum is restricted, there is benefit in having a much larger plenum volume as it effectively stores air mass post restrictor. On street cars we obviously dont need to work around a restrictor. I'm not saying plenum volume is not important, just that its use / requirement will be different.
A stored mass is only effective for one or two working cycles. What I mean is something which is always there. The alternation of load process is a complex system of more then one drivers. One of the bigger supporters are resonances, which squeeze out the maximum of energy available in the pressure oscillation during an work cycle. The resonance of plenum und chamber is one of them. You might be look into the CFD if they use an acoustic approach to model the pressure energy or if they include a temperature-pressure-correlation to model those oscillation. It is not natural that CFD codes include this as stationary flowing doesn't focus on that.
I don't believe the PRC is a modular design? So somewhat trickier to modify in the way that I can the RSP.
Yes, it is no modular design, but it has a way better clearance design as basis for your requirement to fit into the Lotus engine bay. As an extreme example you can find this here: Aggressive as fuck RSP custom "center feed" plenum | Honda / Acura K20a K24a Engine Forum

So last night I pushed the button on the trumpets and a new back plate. 2-3 weeks until arrival, depending upon Brexit.
Looks nice. But maybe you need to push the button a second time. How do you want to place bolts into the flange or do you place threats with screw nut. Anyway both will get challenging. I would design bigger bore diameters around the bolt axis in vertical to get the screw's mounted.
Allen socket head bolts with a tool like this might be able to reach around the bells,
Definitely drtye. This allen socket approach would safe some clearance, but the bore diameter of the cut out of the bell mouths is the same as the flange bore. The screw head is always bigger to give a surface to be able to transfer the bolt force right down. I assume at least a 2 mm bigger radius. With the actual design it won't be possible, it has to be reworked. Just my two cents.
But I will happily bet you a beer that I can install the bolts without too much bother.
I am not sure if it will arrive due to the new mess around the Brexit 😇 . These are M6 or M8 bolt threads?
Nice progress mrluke 🆙! Are you satisfied with the surface roughness of the bell mouths? How do they align with the runners? I can't see it clearly in the picture.

Cap heads would work but now I have the physical part, making the cutout larger doesn't look as bad as it did in the 3d model so i'll do that.
As I suggested 😉.

Are you going to do a back to back test on the dyno with it?
Surface roughness is actually slightly smoother than the aluminium in the runners...They align well with the runners, if you look carefully at the bottom of the far right bellmouth there is a little bit of lip showing but that could be resolved by moving them slightly and re-tightening.
Sounds good. For the first approach of it fits very well. My pencil shadow draw technique with a paper and the head to get a better blue print basis for creating a intake manifold flange needed 2 subsequent laser cut outs of a 1 mm alu sheet to get it aligned finally for the CNC flange.


This picture shows the 1st CNC flange attempt and the alignment. The flange CNC cut quality was awfully as you can see. The double sided cut direction was not aligning, there was an offset of 1.2 mm and the surface smoothness was draft level only. A 760 Euro failure working with the wrong people. If the 3D plastic would hold higher temperatures of around up to 180 °C without any change of dimensions and shape than it would be more interesting for IM fabing.

There will be a back to back dyno run but mostly i'd like to get them fitted so I can start a longevity / reliability test. If that goes well i've got a few different options i'd like to run on the dyno.
Great 🆙, I am looking forward to it.

Next major step is getting the rest of the manifold body scanned and modelled
I stay tunned 🆙.
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Would this fit the bill?
Ultrasint PA6 FR (Flame Retardant) 3D Printing Material
Looks like it's melting point is 201C and flame retardant. Limited by printer bed size.
Wow, you are up early and well informed. Thanks for the link. I skimmed it, many properties of it looks promising. My feeling says I would still need notched brass inserts to enforce the bolt bores to spread the blot forces far wider into the material. I am not sure about this.

There must have been a lot of work in getting your inlet manifold made up!

It took some effort, yes. Costs for my prototype was around 2000 Euro only for fabrication and material, a bit expensive, but worth the results. Made already 180 [email protected] rpm (or 180 [email protected] rpm at the wheels), not bad for a 2 Liter NA engine shortly after VTEC.

For comparison the actual print cost for the trumpets is under £50.
That's another world, the same of alu would be under 50 GBP when producing as cast type. Sounds very good!

The benefit of the nylon part is that if needs be it could be sanded fairly easily to improve the port match, it might even be worth making the ID slightly too small so that they could be hand finished.
So it doesn't smear when it get sanded?

Wow, thanks for the link. It seems it may worth a deep dive into it for the next IM, which is planned for raced 87x99 NA engines.

Thanks for the inspiring links guys!
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This is by far the most complex part I have modeled and its taking a little while.
Very nice. Thanks for sharing, looks like an amazing work you have done there!

I'm not sure what you mean by adding a wall to the surface?
Just giving it a wall thickness over the surface model.
Unfortunately it isn't that easy, the scan has a very high number of points, in the region of 1 million. This makes it very difficult to work with. It's like working on photoshop with a 1 pixel brush.
That's a huge number of data points.

BTW, how did you solve the bolt clearance thing?
Going forward I'll order cap head bolts to minimise the impact on the trumpets. They'll also be 5mm shorter so they don't have to go through the trumpets to put them in the holes.
Sounds like a solution. I like your idea of using a temperature indicator. Would be nice to know which surface temperature mine has. BTW, the Testo company is just a one and a half hour away from my place. I've used many emission and temperature measurement tools of them, they stand for higher quality equipment for a more fair price. Many of the service guys of my former engine company I worked for use them to tune the emissions correctly during service of the gas engines. Anyway, a good idea to use such an indicator.

Which process did you use to place the bronze spacers? Looks like a thermal heating process, but I am curious about the details :).
Place the spacers in location, put a soldering iron through them and wait until they get to ~200c at which point they melt through the nylon. Take the heat off when there is about 1mm left to go and press it on a flat metal surface for the last part. Helps to make it flat.
Thanks for the process explanation 🆙.
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