Ideal exhaust port width?

[CCarlisi]

A friend of mine has had 3 motors fail on the dyno running 13psi with afrs in the 10s and ignition starting around 10 and going up to 16 at redline.  The symptoms for all 3 are the same; once the boost is raised to 13 psi the idle vac decreases with each dyno pull until the car finally stalls.

I have an apex seal out of motor number one in front of me and noticed that the worn part of the seal is just about 6cm across.  Is that normal for a ported exhaust?   I can imagine that if the port is too wide the apex seal is more likely to be damaged as it sweeps over the final edge of the port.   With this in mind what is a good width to shoot for?

I appreciate any input.  

Thanks,

Chris Carlisi.

[Carl]

Hi CCarlisi,
It is not so much the width that is important,although I think that maybe the ports are a little too wide for my liking.
It is the port roof\'s finish that is important.
It should be chamfered so as to aid the seal in port closing.It also is important to give a slight radius to this edge.
If you think that the exhaust gas is under very high pressure when the exhaust port is opened so it really helps itself to evacuate to the atmosphere,well because of this that is why the exhaust port does not have to be really big.
What apex seals are you using?
Hope this helps.
Kind regards
Carl

[CCarlisi]

Thanks for the reply Carl.   According to the engine builder the edge of the port has a 2mm bevel, but I have not had a chance to look at it.   I\'m not sure about the port shape, but based on the pictures I\'ve seen of other motors he has built it appears he uses a relatively flat/rectangular port shape.  

Motor #1: mazda 2 piece 2mm seals
Motor #2: mazda 3 piece 2mm seals
Motor #3: mazda 2 piece 2mm seals

I have read about people having similar problems with the Atkins seals, but never with factory seals.

[Carl]

Hi Chris,
On the port roof on the engines I build that are ported I always put a radius in to create a scissor closing action for the seal,I also do a bevel.
I have not found this to create the problem as you describe it.
what sort of revs are you using?
Bear in mind that a new Mazda seal is crowned,that means that the sealing surface is higher in the middle than at the ends.  
If you would like to see what I mean then put 2 new seals against each other,maybe the porting work is taking some of this off because of how it is being done?
If you think about it that means with a big hole to drop into when the seal runs over the exhaust port that the seal will drop into the hole even further than a stock port,this would give the opportunity for it to take alot of the seal surface off with each revolution.
I guess that your seal now looks concave?
Kind regards
Carl

[CCarlisi]

I\'m thinking along the same lines.  As you can see from the picture the crown has been reduced to a narrow ridge.  

I spoke to Mazdatrix about the problem today.  They believe it may have been caused by high oil temps.  Apparently, when the temp gets to about 250F the apex seal springs loose their tension, allowing the seals to chatter.   Would seal chatter cause wear like this?  Many of his symptoms are consistent with this hypothesis.   From what I understand the oil temp is higher in the front than the rear rotor.  While rotor seals in both housings have been damaged the damage to the front seals is worse than the that to the rear seals.  Second, his 3540 turbo has develop significant thrust bearing play after only a handful of dyno runs.  I imagine that could also be attributed to overly high oil temps.

[Carl]

Hi Chris,
Show us a pic of the rotor housings.
The seals are black from blow by.Hi Oil temp may do this,it is actualy lack of cooling to the rotors.
Kind regards
Carl

[Timrxmotors]

The apex seals from my 12A were all black in the middle,they were warped.Are yours warped,or worn away?The cause of mine warping was probably not enough housing lubrication,too much boost,too many rpm,and no intercooling.Interesating what you say about the GT35/40 turbo,mine has loads of play in it,don\'t know why.

[Carl]

The thrust bearing play could be because of not having a blow off valve to relieve the boost pressure on sudden throttle closure(Gear changes).
If this is not released then it reverts back to the compressor and creates tremendous thrust pressure on the compressor wheel.
Kind regards
Carl

[CCarlisi]

My friend has a BOV, but it\'s on the outlet side of the IC core.  The turbo and motor problems may not be related.  Unfortunately, both were upgraded at the same time so it\'s difficult to ascertain whether one is having an effect on the other.  Even more unfortunately, both problems are very costly :o

I have been talking to a few people about the oil metering system.  My friend\'s MOP has 120,000 miles on it.   Maybe the pump\'s output has been diminished to the point that it cannot provide enough oil line pressure to allow oil to be metered intoo the motor at 13psi.  I assume to do so the oil line pressure would need to be 13psi+  Is the damage to his seal consistent with this hypothesis?

[Carl]

Running a premix would solve that problem,but it should not be necessary.
There is something else wrong with this,maybe Mazdatrix are right about the oil temp getting too hot.
It seems to me also though that maybe the engine is also getting too rich which could create a bore wash situation,too much fuel washing the oil that is getting into the engine for seal lubrication away?Does the oil in the sump smell petroly?
If you had seal chatter like has been suggested then it would be obvious on the rotor housings.
What timing are you running under max boost?
Kind regards
Carl

[Timrxmotors]

Good idea to check the MOP,mine was putting out virtually nothing.My GT35/40 has a bov just before the throttle body.It spins for ever when the engine is off,with no oil going to it,I think maybe this is why the bearings are worn,as I use it every day.

[spanner]

Hello there,

HOw are you guys doing up there? Carl, I need a little advice about a Weber 45DCOE on a stock 12A motor.  I took one of the carbs you sold to Mac , the Weber 45 and wanted to know if you would be able to suggest jettings that I can run on a stock 12A motor. I presently have the stock Nikki carb on it, but plan to pull it off very soon to put on the side draft carb. It presently has 38 Chokes, F11 emulsion tubes with 150 air correctors and 160 mains, with 55 F11 idle jets.
Any assistance would be appreciated. Carl you can also visit //www.ausrotary.com and check out the pics from this years Jamboree in Sydney.

[Timrxmotors]

I think you will find jetting recommendations in the book\'how to modify your RX7\'.Back to the apex seals,if the seals are spinning at 13,000 rpm,the MOP only supplies enough oil for 7000 rpm,that is wide open,so at 13,000 rpm the  seals are definitely not getting enough oil,they get too hot,and fail.Modify the MOP to deliver more oil.

[Timrxmotors]

What rpm did the engine go to?

[Carl]

Originally posted by Timrxmotors
Back to the apex seals,if the seals are spinning at 13,000 rpm,the MOP only supplies enough oil for 7000 rpm,that is wide open,so at 13,000 rpm the  seals are definitely not getting enough oil,they get too hot,and fail.Modify the MOP to deliver more oil.

Hi Tim,
Where did the 13000 rpm come from?I think you are muddled with 13psi boost?
Bear in mind that the OMP oil does not cool the apex seals,only the contact with the rotor housing and the oil sprayed inside the rotor does.
I agree with you though that there needs to be more oil for lubrication which can be increased through higher oil pressure.That increase would also flow more oil into the rotors which would aid cooling if you also had uprated oil coolers.
Kind regards
Carl