3 broken Rotrex chargers.... - K20A.org .:. The K Series Source . Honda / Acura K20a k24a Engine Forum
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Old 04-11-2015, 09:45 AM   #1
gobbledygook
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Default 3 broken Rotrex chargers....

Hi all, I'm after some advice on my setup and any ideas people have to try and keep my Rotrex C38-91 charger in one piece. I have broken 3 chargers over the last 2 years, and I don't want to break a 4th! All three have broken when the impeller shaft snapped.

My first charger was a C30-94 which lasted 5 years of abuse on track and at the drag strip in which it regularly hit the rev limiter without a problem. I decided to overspin it a bit (8%), after which it only lasted 2 more track days before breaking. No complaints here, as I pushed it passed the manufacturers limits.

My second charger was also a C30-94, which I only span to the maximum recommended RPM. I also added an 'auto tensioner override' bolt (as pioneered by Jaydee), to keep the belt tensioner tight and avoid any slip. This charger only lasted 2 track days before it broke. Rotrex replaced this under warranty so I decided to upgrade at the same time.

My third charger was a C38-91. I decided to upgrade to this as it allowed me to underspin the charger and to tune the top of the power curve to turn over well before the rev limiter, encouraging me to change gear before the limiter and making it kinder to the charger should I hit the limiter. This charger lasted 1 day on the dyno and 1 track day before breaking. I have almost every turn this charger has made on kpro data logs, and it has not been abused! I don't think it hit the limiter once, and has only ever been under spun. Rotrex have replaced it under warranty, but I don't want to break another one as it kind of ruins a track day!

The last 2 chargers were of the new design that required the oil can to be lower than the charger shaft. Are these new chargers weaker than the old ones or have I been unlucky?

Some details on my setup:
UKDM EP3 Civic Type-R
105mm Rotrex pulley, 138.5mm K20A2 crank pulley, 8600rpm redline
Hondata Kpro ECU.
Hondata traction control
TTS Performance SC top mounting kit - modified to fit the C38-91
Turbosmart Kompact Dual Port Blow Off Valve, 25mm inlet/outlet, fitted with blue 'supercharger' spring



My tune. Dotted line is without water/meth injection, solid line is with.




My engine bay. The BOV is just before the throttle body and dumps into the intake after the air filter when on the road, or to both the intake and to air when on track.




You can see the 'auto tensioner override' bolt in this picture, holding the belt tensioner tight to prevent belt slip.



Worried that I might be damaging the chargers with part throttle surge, I fitted a second MAP sensor that could monitor the pressure before the throttle plate, and be data logged by Kpro as the red line below:




So at part throttle I only see a max of 1.2bar boost at 7600rpm. Atmospheric pressure was 0.96bar, so I could plot the results from the above datalog onto the compressor map (green spots):




I'm well to the right of the surge line, so should be OK. I'm no expert at reading compressor maps, so feel free to chip in if I have made a mistake. I get a small spike in boost up to about 1.7bar boost when lifting off the throttle after running up to the redline on WOT, but that's still to the right of the surge line. I can't hear any surge when driving or on the dyno either, so I'm pretty sure the BOV is doing its job. It doesn't appear to be blowing open on full boost, but since I don't need any extra boost at the moment I don't care if it does!

The only thing I have left to check is the belt tension. I have bought a belt tension gauge which I will use when I fit my 4th charger, but if anyone has any other thoughts on what might be killing my chargers, please let me know.
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Old 04-11-2015, 09:59 AM   #2
Alex_CTR
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Default Re: 3 broken Rotrex chargers....

We are on the same boat mate, snapped 2 c38 rotrex impeller shafts in a year and rotrex told me they will not replace the 3rd unit if broken again,I sold the new unit and quit.

Had a tts c30-94 on my fn2 for 2.5 years with no issues and when decided to upgrade to c38 all my joy went down the drain.
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Old 04-11-2015, 10:24 AM   #3
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Default Re: 3 broken Rotrex chargers....

It seems like it is not boost related or belt vibration related but rather a material or design issue. Where exactly is the shaft breaking? Same spot every time? Can you send me a shaft and the parts rotating with the shaft? I can crunch some numbers for you and figure out a shaft that can take the torsional load.
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Old 04-11-2015, 10:34 AM   #4
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Default Re: 3 broken Rotrex chargers....

I'll answer that, can't disassembly rotrex unit if you want to claim a new unit under warranty.

It's simple, shaft impellers on c38 range can't copy with impellers weight especially on 81/91 units were impellers are bigger and heavier.

Hard rev limiter brings also too much stress to impeller.

If I wanted to continue with Rotrex I'll might went for other ecu.

http://youtu.be/Dc6p7jyPZTs
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Old 04-11-2015, 11:06 AM   #5
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Default Re: 3 broken Rotrex chargers....

Quote:
Originally Posted by AWOC View Post
It seems like it is not boost related or belt vibration related but rather a material or design issue. Where exactly is the shaft breaking? Same spot every time? Can you send me a shaft and the parts rotating with the shaft? I can crunch some numbers for you and figure out a shaft that can take the torsional load.
The shaft always breaks at the point where it comes into contact with the back of the impeller, some pictures of broken shafts and discussion as to why they break can be found here:
https://www.k20a.org/forum/showthread...=86410&page=61

Alex_CTR is right that replacing the shaft would void the warranty, and as the units are precision built it's unlikely that we would be able to get something else to fit exactly right on the first attempt.
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Old 04-11-2015, 12:24 PM   #6
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I also run 2 rotrex oil coolers, and datalog the oil temperatures, which have never gone over 35C when the car has been moving.

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Old 04-11-2015, 12:51 PM   #7
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Default Re: 3 broken Rotrex chargers....

How often did you "bounce off the rev limiter"?

Post up a screen shot of your timing map if you can.

When using Hondata products, if you do not go into the timing map and set up a "soft" rev limit you will kill Rotrex blowers. This happens because of shockloads when the power is on/off/on/off in extremely small increments of time. This is actually bad for any motor and all other belt driven blowers but the big Rotrex blowers seem to have the lowest tolerance to this abuse. The c30-94 can take more abuse than the c38's.
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Old 04-11-2015, 01:03 PM   #8
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Default Re: 3 broken Rotrex chargers....

Had a similar problem on my bottom mount c38 kit. I think it was the connection from the rotrex output to the intercooler beeing to short causing the mountig brakets to bent slidely during engine movements. upgraded to the tts intercooler with longer silikone hoses and had no issues for 17k miles now.
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Old 04-11-2015, 01:35 PM   #9
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Default Re: 3 broken Rotrex chargers....

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Originally Posted by 94ex-vtec View Post
How often did you "bounce off the rev limiter"?
With my first C30-94 I hit the limiter many hundreds of times.
With my second C30-94 maybe 50 times.
With my C38-91 no more than 5 times.

Quote:
Originally Posted by 94ex-vtec View Post
Post up a screen shot of your timing map if you can.

When using Hondata products, if you do not go into the timing map and set up a "soft" rev limit you will kill Rotrex blowers. This happens because of shockloads when the power is on/off/on/off in extremely small increments of time. This is actually bad for any motor and all other belt driven blowers but the big Rotrex blowers seem to have the lowest tolerance to this abuse. The c30-94 can take more abuse than the c38's.
This is my timing on the 40degree high cam, the one at which I would reach the rev limit. 50degree is very similar. You can see that the timing has been turned down to achieve a 'soft cut', which is why the power curve in the dyno plot in my first post turns over like it does.



I only had the soft cut setup on my C38-91 charger, but obviously it wasn't enough to save it.

One thing I was wondering was the influence of the Hondata traction control. It is quick to take out timing when traction is lost, then add it back when traction is regained. There are no real 'spikes' or 'dips' in the engine RPM logs, but they are a little 'lumpy'. Not sure if they would be worse without traction control though, if the wheels spun up and then regained traction.
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Old 04-11-2015, 01:38 PM   #10
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Default Re: 3 broken Rotrex chargers....

Quote:
Originally Posted by rsamd View Post
Had a similar problem on my bottom mount c38 kit. I think it was the connection from the rotrex output to the intercooler beeing to short causing the mountig brakets to bent slidely during engine movements. upgraded to the tts intercooler with longer silikone hoses and had no issues for 17k miles now.
That's interesting to know. My system has a charge cooler mounted above the gearbox, so there is nothing attached to the Rotrex that can't move with the engine. The mounting is very solid and over engineered, but I'm looking into making it even stronger. There is no noticeable 'give' in the mounts, but I have to try something.
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Old 04-11-2015, 02:14 PM   #11
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Default Re: 3 broken Rotrex chargers....

This isn't mounting related nor timing related. The acceleration from the engine moving around is nothing compared to the wheel speed acceleration the rotrex sees. The mounting points are more than adequately strong.

I'd recommend getting a shaft remade with better precision and stronger material. I would chamfer the back of the wheel to allow for the shaft to have a big radius where the shaft keeps breaking. A new shaft will be cheaper than a new unit. Has anyone done a Rockwell test on the shaft?
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Old 04-12-2015, 01:58 PM   #12
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Default Re: 3 broken Rotrex chargers....

Join the clan of broken Rotrex superchargers

We are on our 5 or 6th, lost count of the broken shafts on the C38-91's over the past 2 years. Expensive exercise, but its hard not keep using it due to power delivery and the results we have had with our cars

Apparently Rotrex has since revised the 91 and 92 late last year and we are now on the latest version. Physically looking the same, the changes were made inside the unit to make it more "reliable". We have noticed better boost pressure with the new unit as well.

My advice is to get a good belt and have maximum belt wrap (run a big crank pulley with big s/c pulley). We have since remove the fixed tensioner and so far its been working OK. Lets see how it goes with a few more trackdays

Having said that, we got a unit on our street S2000 and its seems to hold together OK. Its been running close to 1 year in the car and a few track-days with occasional drag days in between.
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Old 04-12-2015, 05:10 PM   #13
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Default Re: 3 broken Rotrex chargers....

Anyone willing to sell me 1 or 2 broken units? I would like to dig into this a bit more.
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Old 04-12-2015, 11:11 PM   #14
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Thanks, its good to know my new charger might be more reliable. I use Dayco belts as I can get them in any size I need. I have a larger crank pulley from a k24 engine that I think I'll fit, as this will also let me run a larger charger pulley.
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Old 04-13-2015, 02:55 AM   #15
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Default Re: 3 broken Rotrex chargers....

2nd broken unit was made on 12/2014 and lasted less than a month, only few dyno pulls and aprox 800km.
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Old 04-13-2015, 08:50 AM   #16
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Default Re: 3 broken Rotrex chargers....

I've said this for a long time,, this is a serious problem and I don't think rotrex or their distributors are doing enough to solve the problem and give the confidence back to the potential market.

If mine fails then ill order another and if that fails I will turbocharge.. and never buy any product from rotrex again.

They need to come up with a proper solution. Letting customers do all the R&D isn't ideal.
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Old 04-13-2015, 04:27 PM   #17
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Default Re: 3 broken Rotrex chargers....

Well this isn't very reassuring! Just about a month on my -91 with about 1200 miles put on it so far hitting the limiter maybe 50 times. No destruction yet, but this is on a S2000 which drives the charger with a cog belt and fixed tensioner.
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Old 04-13-2015, 11:16 PM   #18
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Default Re: 3 broken Rotrex chargers....

Quote:
Originally Posted by AWOC View Post
This isn't mounting related nor timing related. The acceleration from the engine moving around is nothing compared to the wheel speed acceleration the rotrex sees. The mounting points are more than adequately strong.

I'd recommend getting a shaft remade with better precision and stronger material. I would chamfer the back of the wheel to allow for the shaft to have a big radius where the shaft keeps breaking. A new shaft will be cheaper than a new unit. Has anyone done a Rockwell test on the shaft?

Firstly I would like to say that I have been using Rotrex since 2006 on many extreme vehicles and most of time all engine parameters have been datalogged. I have emailed Rotrex during my twins development and they have been very helpful with technical information. We have had zero issues until we had a tensioner overide failure on the dyno. Unaware of the override failure we broke a -91 shortly afterwards.

Since then 5 track test days, 4 days dyno development and no problems.

So what do I do that is different? My twin charged setup creates much higher loadings on the charger and belts than any single so we are very careful with the bypass valve and belt layout. I have had endless problems with pressure spikes, etc until I fitted what all the eaton or twin screw SC use. It's a throttle butterfly bypass driven by a diaphram. No hysteresis so that no tuning problems, no pressure spikes or leakage. These problems were solved instantly.

http://www.kennebell.net/KBWebsite/A...passvalves.htm

Belt slip will occur with the Honda K series tensioner or with a manual tensioner. This is because a 7 rib belt stretches under load so the Rotrex drive is inconsistent. The sudden acceleration forces then exceed the shafts capacity. I have measured 30mm+ stretch. If you don't believe me then video your belt and tensioner at 8000rpm shifts. (I have 2 new custom 8 rib Dampers if anyone wants one and manual overrides so just PM)

The only belts to use are Gates Heavy Duty Green Stripe belts.....period.
These are 50% stronger at the under bonnet temperatures in a loaded environment which also means they stretch 50% less.


Use a Gates belt tension gauge and set the belt to 80K to 90K on the Gates gauge.

The Honda tensioner requires an override as it only supplies 1/2 the necessary tension. The override must be capable of allowing the auto tensioner to move as the belt stretches under load.

I am working with Gates on a Heavy Duty tensioner design for our K installations but this is 6 months away.

I don't use Hondata and the Autronic SM4 rev limiter is soft and kind on the motor. Back in 2006 I used a KPro with Motec ADL3 data logging. With the KPro limiter set to 8300rpm I was logging 8750rpm on the Motec Datalogging even though the KPro was not in the rev limiter. This was constantly happening. This is just an observation for you too not believe what you log.

According to Rotrex 99% of their charger issues are on Honda Kseries motors. I have no reason to doubt the technical guy I am emailing with.

You are correct and it is my opinion that the shaft is not strong enough from the seal to the rear of the impellor. The way the shaft fails is right at the raised flange that the impellor sits against. This indicates the shaft has been bending at this point. This bending is from axial torsional loading and the inertia loading. So belt tension, bypass valve pressure loadings, surge and Rev Limiter and TC strategies all play a part in creating this load.

Several possiblities.
1. Improved material for the shaft with appropiate heat treating
2. removing the sudden shoulder size reduction on the shaft that the impellor sits against which is concentrating the stress. The impellor acts as a stiffening sleeve on the end of the shaft. Behind the seal the shaft is supported so currently the highly loaded stressed section of the shaft is very short. As with a gearbox or rear half shaft design, manufacturers have found smooth radius cross sectional changes and longer sections with progressive flex have a much longer service life. So I suggest a progressive 10mm taper down to the threaded shaft diameter with a mate-ing taper machined into the impellor.
3. As the bending load on the shaft has increased with the mass of the C38-91 and 92 units heavier impellor I suggest a lighter impellor material choice if this is possible.

I am aware that Rotrex are working on a solution and are expecting to launch this product in the Northern Hemisphere autumn this year.

This situation is painful but I would like to state that Rotrex have been solid with warranties.

This will be my only post as currently I cannot spend the time to explain in more detail.
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Old 04-14-2015, 07:51 AM   #19
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Default Re: 3 broken Rotrex chargers....

Hey Jeff the Kenne Bell Link you posted isnt working... Thank you as always for this great useful rotrex knowledge!
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Old 04-14-2015, 01:58 PM   #20
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Default Re: 3 broken Rotrex chargers....

Thanks for the info Jaydee, as always it's been very useful. I now have a Gates Heavy Duty green belt on its way to me in the UK from the USA at great expense. I've also purchased a butterfly recirculating valve as used on the supercharger on a Mini Cooper, which I will run together with my turbosmart piston dump valve. I contacted Kenne Bell by email to enquire about their valves and they weren't very helpful, so hopefully I can get the BMW valve I have bought to work correctly.

If I get my new belt session right and stay off the limiter I'm hoping this setup will be more reliable.
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