The first thing most people do when upgrading their car is to remove the Air Pump and throw on some under drive pulleys to compensate for the reduce contact patch. Even with under drive pulleys install the contact patch on the water pump pulley is still too small and most find that they can turn the water pump pulley with their fingers even with the belt tightened above stock tension. What most people then do is over tighten the belt to stop the slipping.

The rotary engine uses hydrodynamic journal bearings to support the eccentric shaft. The way it works is by flowing oil under pressure between the bearing surface and the shaft and basically “floating” the shaft through the fluid dynamic effect. Increasing the lateral load to one side on the shaft reduces how well the journal bearing can do its job of “floating” the shaft and results is metal on metal contact and accelerated wear. Over tightening the belt creates just this type of lateral load.

Pictured below is just one example of excessive bearing wear caused by running under drive pulleys without the air pump and over tightening the belt to compensate. The first picture is of the front bearing on two motors with a similar history. Both ran Mobil 1 10w30 most of their life and were autocrossed. The 99,800 mile motor stayed in stock form until it met its end due to a water jacket o-ring failure. The 66,000 mile motor had the air pump removed and under drive pulleys install around 53,000 miles. It also met its end to a water jacket o-ring failure.

The front stationary gears are in their correct orientation so what you are looking at is the top of the bearing as it sits in the motor. The copper color area is the wear.




The next image is of the rear stationary gear bearing. The picture is upside down to the way it installs in the motor so you are looking at the bottom of the bearing surface.



Close up of rear bearing wear:


As you can see the bearing wear is considerably worse on the 66,000 mile motor and the wear pattern on the bearing clearly shows the extra load applied by the lack of an air pump or idler in its place is the cause.

There are 3 idler pulleys on the market right now that resovle this problem. You will have to search to find them as I am not allowed to link to them.

Dan

 

As I said on the other forum, Great write up Dan! I look forward to see what you come up with for the single turbo guys. I'll prob. end up getting your kit for my FD with twins as well.

 

That's an interesting find. I'll have to purchase an idler pulley then.

 

very informative thread , thx a lot ... it has been stressed in the past that over tightening will cause this but this is a very good visual representation of that

thx for taking the time to post it

 

Good information.

That's retarded. Isn't the Bush era over?

 

that extra wear was in only 13k miles of over-tightened belts. holy crap.

 

Yes, once I saw that I developed an idler pulley right away

I just read the rules again and I can have a link to the group buy interest thread in my signature so there it is For the other two search for Pineapple and Garfinkle

Dan

 

where can i buy!

 

Bout time David Let me know when you get them all ready i will be ready to buy

 

Guys, the group buy interest thread is in his sig. For the lazy:

https://www.rx7club.com/forum/showthread.php?t=807734

 

Great thread Dan, definitely interesting to see that kind of accelerated bearing wear...... Ihor and I have seen quite a few cases of bearings being in pretty bad shape upon teardown, I'm going to start examining them more closely for this pattern.

 

Thanks Rich, I have some that look even worse but don't have the complete history of the motors to know for sure what the cause was. What is interesting is the 66,000 mile motors had extremely low wear on all the other internals including the rotor bearings.

On a side note, are you guys headed to DGRR this year, would love to meet you in person.

Dan

 

Even the OEM waterpump setup look sketchy to me. For now i'm keeping it as it is but once the enevitable happens i'll go single turbo with an electric water pump.

 

It cleans up the engine bay and is not needed if you're not running the main cat.

 

It reduces parasitic load below 3000 rpm, weight and cleans up the engine bay as RX7 Rage said.

Dan

 

Could not find a air pump removal kit on their website, do you have a link?

Dan

 

Great thread!

 

I have actually seen a e-shaft break due to the belt being over tightened.

 

Isnt there no way of designing a better belt, to be honest ive notice that i can turn the water pump quite easily and i already realised that if it start over tightening the belt it wil apply more load on the bearings in the verticle plane. The back side of the belt has no grip at all if it was coated with a rubberised compound i bet it would grip much better. just an idea....

 

excuse my ignorance, but if you just remove the stock airpump and keep all the other factory pulleys, this causes the waterpump to turn slower and the belt to slip? Does it always turn slower? I am an FC guy and I know on FC's (which don't have serpentine belts) you can have slippage problems from removing the airpump.

The reason I say this is because I have a friend with an FD who removed his airpump (not sure if he kept all the other pulleys on or what he did) and also has been having some cooling problems despite a bigger rad, fan mods, etc.

 

Yes, that is a possibility. If you simply remove the airpump and use a smaller belt (without different pullies), the new belt may not have enough of a contact patch on the waterpump pulley to reliably turn it. The aftermarket pullies for airpump removal kits increase the size of the waterpump and alternator pulleys to increase the contact (which also underdrives those accessories).

 

I tried a belt dressing on the back of my belt that was sticky; problem was that it wore off to quick to be really useful. Cost of manufacturing a new belt for our limited use would be prohibitive.

Dan

 

This quote i find very hard to believe. are u joking?

It is not necessary to over tighten the belt, many have eliminated the airpump and used an aftermarket pulley with no over heating issues or premature wear.

 

With a brand new properly designed water pump pulley and a new belt I don't disagree. Problem is the nice edges on the groves of the water pump pulley wear off with time and the back of the belt gets smoothed out and hard.

You could keep treating the symptoms by replacing the belt and/or pulley or correct the actual problem of having to small of a contact patch by adding an idler pulley.

If you feel comfortable with your setup, more power to you, but don’t mind me sticking to what I know from personal experience.

Dan

 

I would love to use this in my car. Unfortunately, I don't think it will work with my long runner manifold with my single turbo.

Eddie