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Another RSP thread - with a twist

11782 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!

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 (
Thank you and thanks for the paper :)

The accuracy is really very good, you're talking about fractions of a mm really. The biggest issue is my photo quality isn't great so the software struggles to find the edges of the dark grey object against the slightly darker grey background. I scaled the model to an overall length of 250mm and checked the widest diameter across the trumpets, 64.6mm in the model and 64.6mm in real life.

The picture below shows my model and the cad scan overlayed so you can see the differences

As you can see its mostly around the edges with little pockets on the surface finish. I think I may have more success using a green screen as the background rather than black so will try that in the future.

And here's the model looking slightly prettier. Renders aren't my forte, I just make and print the parts lol.

There's a bit more work to do before its a finished part, the edges need tidying up so that it looks smooth rather than faceted. But for a couple of hours in CAD i'm pretty happy with where I have got to.

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.
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I like it. Really neat approach instead of fancy 3d scanning and proprietary software.
Thanks :) much cheaper than a scanner but I am very jealous of what they can do with them, the real time scanning is pretty cool. Not very accessible for a hobbyist though.

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.
The software struggled to work out where the edges of the trumpet were which reduced the accuracy.

What did happen to the parallel cut on the bolt bores? De-rendered?
The model has two parts, the first is the underlying geometry and the second is a texture file. On the image its the texture file that hasnt been compiled 100% correctly, its overlapped the holes slightly to make them out of round. Again this has been from the software struggling to identify where the metal stops. Ultimately this is easily overcome because we know that the holes need to be round, we can measure the diameter from the part, When I add the holes to the model I make sure there are all at the same height, the scan will indicate the horizontal position and then its just a case of double checking against the actual part to make sure they are perfectly placed. But again we are into fractions of a mm here.

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?
I'm not quite following. The new bell mouths I will draw free hand and add to the model as my first tweak but things like matching the curvy flange are much easier to do with this technique than from the part with a ruler :).

The plan is to 3d print a functional set of trumpets that can be installed on the car. This will be by outsourcing the printing to a large company that have the latest tech available at affordable prices.

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 can see a head being somewhat difficult to render if you are trying to include all the cooling galleries etc. If you are just looking for airflow then it shouldn't be too bad, depending upon whether you want to accurately recreate the variances in each cylinder/port or just do one and mirror it 4 times till you get a good fit.

Very good, I hope the Blair paper helps on that.
I'll use their profile for the bellmouth

Do you have an idea how long you want to have the runners, for the runner length tuning?
From what I have seen there are multiple ways to calculate ideal runner length and the outputs vary enormously. I have come to the conclusion that the calcs will only land you in the ball park and dyno testing is required to get the perfect length.

My thought is that the issue with the RSP manifold is that runner #1 blocks the TB opening. Fine at lower rpm but at the top of the rev range it makes the flow path too difficult. Hence why people cut them down to a 25mm runner length and regain the top end.

If I can add a dual plenum onto the back of the RSP and cover up the existing throttle opening then maybe the stock length trumpets will show a benefit over the shorter lengths. Given my use case i'm probably aiming for a peak HP at around 7.5k.
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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.

Once I have got all of the elements scanned I will start looking at CFD to narrow down the options.
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Little more progress on the next part, model is a little more interesting this time. Still with the basic photogrammetry setup and my rubbish slightly blurry photos.

This shouldn't take too long to get to a part I can print so that I can see whether the manifold will fit into the lotus or not. Might need an extra spacer on the throttle body to help that clear.
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Buddy of mine used to build engines for Mugen UK, his input on the RSP manifold was that removing the freeze plugs on the resonance chamber wall was worth a couple hp. Same guy brought my RSP manifold over when he moved stateside ;)

I've studied more SAE papers than I can count related to intake and exhaust design, and spent a few months playing with 1-D GT Power models getting my masters. My conclusions:
  • Intake manifold design is not something that can be magically calculated with a single formula.
  • Runner diameter directly impacts ram tuning for high engine speeds, large diameters = lower flow velocities / less ram tuning. Small diameters increase velocity, but if you approach mach, flow decreases.
  • Runner length is the largest contributor to resonance tuning. IM Runner length bumps VE in bands at several engine speeds, based on 1st/2nd/3rd etc. order reflection timings, and valve event timing.
  • Choosing a runner length aimed at your peak power engine speed may give you that impressive peak number, but almost definitely have large torque losses, resulting in less power under the curve
The RSP manifold came on the K20Z4, rated power at 7800 rpm, so if you're expecting a lower peak power speed than that, I'd personally keep the same length, if not go slightly longer on the trumpets. I generally work under the assumption that Honda/Mugen have done their homework on manifold design ;)
I agree 100% with every point you make in your post (!).

My intention at the moment is to remove the resonance chamber, smooth it off and bolt it back on as a rounded plenum. Are you saying that it would be beneficial instead to keep the extra volume from the resonance chamber?

From other peoples dyno experiments my views on runner diameter are that too narrow quickly becomes a restriction at higher rpm whereas runners that are slightly too large don't appear to cost power elsewhere in the rev range so in short, too big is safer than too small. However for now my runner diameters are pretty much fixed by the RSP.

There also appears to be a relationship between runner length and diameter i.e. a runner that is short and narrow may be okay, but as you make it longer you need to increase the width otherwise it chokes. I'm thinking of it as runner diameter being a course / rough adjustment with the length being more of a fine tune.

This channel has been a fantastic resource, all testing no bs.

If I can hit peak at 7,800rpm that would be lovely. Limiter is around 8,300rpm i'm not planning at pushing much if any higher than that right now.

The only issue with the runner length is the throttle body inlet location

I think this is why people are seeing benefits from reducing the runner length. At the top end its too much of a restriction to the flow.

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.
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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.
Thanks, I have read through the thread a few times, its the main reason i'm sharing my progress on this forum :)

I could only really see one before and after dyno of a modified RSP.

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.

BTW and again...
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.
Given my use case i'm probably aiming for a peak HP at around 7.5k.
If I can hit peak at 7,800rpm that would be lovely. Limiter is around 8,300rpm i'm not planning at pushing much if any higher than that right now.
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.

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 :) Or for those already with an rsp maybe it unlocks some performance for less than the cost of a new manifold.
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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.


Back plate:

Plenty of tidying up to do but not far off ordering.
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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.
Yes, the overlap will be resolved, at the moment I have only drawn one of the trumpets, the software is duplicating it across the other holes, this means that I can still make changes to the profile and they will all be identical. To make the cuts I need to first make each trumpet into its own object which is one of the last steps :)

The trumpets and the flange will all be printed together as one solid piece. This will be easier and more reliable than trying to join the trumpets to the flange afterwards.

The volume at the moment is the same as oem with the resonance chamber removed. Alot of the papers on intake volume are workarounds to having a restrictor fitted, but its easy enough to print some spacers to play with the volume and see what difference it makes :)

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.
How do you mean that. What should be restricted and in which approach?
Many competitive race series require an inlet restrictor to limit maximum power.

You can see here pre-turbo

And here on an NA fsae engine

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.

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...
There are lots of factors that wont be accurately modeled by CFD, but that doesnt mean it has no use to us at all. What i'm really looking to gain from it is equalizing the air distribution between the intake runners.

There's no point making something that has perfect runner length, diameter, taper etc if the flow looks like this:

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I don't believe the PRC is a modular design? So somewhat trickier to modify in the way that I can the RSP.

So last night I pushed the button on the trumpets and a new back plate.

2-3 weeks until arrival, depending upon Brexit.
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I'm pretty sure the bolts will be accessible one way or another, there's plenty of room under the bells and the holes shown are 8mm to allow for inserts whereas the bolts are 6mm (well 5.8).

As suggested can always change to cap heads :)

That said these are only a mockup, i'm not expecting them to be a final product but hopefully I can mount them and start testing for durability.

But I will happily bet you a beer that I can install the bolts without too much bother.
M6 threads.

I dont want bolt the plastic directly so it will have metal (likely brass) insert which is much more robust. Therefore the holes are oversized to allow for installation of the inserts.

And FWIW I'm aware of unmodified elise's with the RSP manifold fitted, we'll see.
I love delivery day! Wasn't supposed to be here until Tuesday :D

I did manage to get all the bolts but the bottom two were a bit awkward to tighten. 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.

Blown away by how accurate the hole placement is.

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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.

As I suggested 😉.

Are you going to do a back to back test on the dyno with it?
I think you said it wouldn't fit ;)

Surface roughness is actually slightly smoother than the aluminium in the runners. The phone flash isn't very complimentary to it on the close up but its actually pretty good.

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.

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.

I also need to make/fit the brass inserts before these are permanently mounted....still awaiting delivery of brass tube.

Next major step is getting the rest of the manifold body scanned and modelled
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There must have been a lot of work in getting your inlet manifold made up!

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.

For comparison the actual print cost for the trumpets is under £50.

I'm not sure if this nylon is going to be the final material, waiting to see how it deals with the temp. However I have a fallback option of Nylon 12 CF which can be heated to 143c under 18bar load before it deforms by 0.25mm. Which I am more confident would be up to the task. Failing that there are things like Ultem.

Even the plain nylon can hold 4.5bar at 150c so we're certainly in the ballpark of possibility.

Details on the Heat Deflection Test:
Heat Deflection Temperature: Definition & Values at 1.8 Mpa (264 psi)
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.
That's my next step when my brass tubes arrive. The Brass tube will be cut to match the thickness/depth of the flange then you use a flanged bolt to tighten. This means that the torque is transmitted by the brass tube from the bolt head through to the underlying metal fixing point. The plastic is then held in place by the bolt flange.

This is pretty standard in many OEM plastic parts, however they dont seem to be available in quantities <1,000s so i'm resorting to making my own. I imagine as this sort of printing becomes more common you will be able to buy "compression limiters" in much smaller numbers, like you can with threaded inserts.
I haven't figured out specifically what the manifolds are printed in yet, but I did find this:
Higher melt nylons: PA12 → PA6/66 → PA6
High T engineering plastics for engine and tooling: Ultem → PEEK → PPSU

PA12, followed by a sealing process called "Imprex"
Wow i'm shocked that PA12 is holding up to that sort of abuse! Ultem, Peek etc i've been looking at.

I'd be interested in where the steel inserts come from, are they bespoke made for each project?

The Imprex seems to improve porosity so something to keep an eye on.

Interested in any other info you have :D Thanks for the contribution!
So slight update to my "studio" now using a box painted with black 3.0 rather than a piece of wood. Idea being that its easier for the software to tell what is the part and what is background.

Which appeared to work quite well, i'm getting better at this now.

First two pictures are in the software:

And this is the exported textured mesh in Blender.

Really impressed with the quality, there's a couple of small holes to fill in where I didn't get good enough photo coverage but generally the whole piece is much crisper.

The runners have about the first 50mm which is accurately modelled then after that they go solid. I dont really see this being a major issue and was actually expecting the runners to be solid pretty much the whole way.

Now I have to remake this as a simplified and tidied up model, the actual generated mesh is too dense to work with as is.
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Small update.

This is by far the most complex part I have modeled and its taking a little while.

I'm not particularly interested in the outside features as this is only really for CFD.

Modelling to do list/WIP
- Runners are a constant radius
- Flanges to be squared up
- Overlength runners to be trimmed
- Plenum interior is only a mock up, needs tidying

This is the current state of play:


Just the model



And a bit of a catch up.

You've probably spotted it in the model but I have chopped up the flange on the manifold to fit my k20a2:

I've also spent far too long cutting up little brass spacers for the bolt holes:

That'll be okay for these prototypes but definitely something I need to find a better solution for going forward. Ideally i'll find some similar sized unthreaded spacers and then adjust the flange thicknesses to suit.

Printing is mounted up and i've added some temp strips that will show the maximum temperature they have reached.

One on the bottom closest to the printed parts and likely to get hotest

And one on the runner because it will be easier to see when the manifold is installed

Printing should be okay up to about 150c. These strips measure from 70c to 110c. In reality I don't think I will even get a reading on this top mounted one.

I'll add hotter strips if need be.

Manifold should be going on the car this week to start durability test while I finalise the CFD model.

Once the plenum model is up and running i'm going to work on improving flow distribution between the runners by trialing slightly angled trumpets. Once I have a few that work well they'll be printed for back to back testing.

@mrluke amazing work. I am looking forward to the digital version. Are you going to add a wall to the surface?
I'm not sure what you mean by adding a wall to the surface?
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