Jerry 87 928S4

First of all, I did read the archives and the manuals. Just trying to make sure I understand my options.

Just replaced my AC compressor and realized that my smog air pump is making quite a bit of noise. Since it is an 87, I just removed the belt.

My understanding is that it feeds air prior to the cats to increase their efficiency. However, on my last emmissions test, the numbers were so low that I passed on the abbreviated test!

I am thinking of the leaving the belt off and see if it will still pass the inspection, if not then I guess I have to replace it.

After all my reading, I can not see any other negative consequences of just leaving the belt off - am I missing anything?

Rufus Sanders

Jerry, for what it's worth, I had an air pump freeze up on me, snap the belt and disable the fan on my '82. It was in early March in Minnesota, so I made it home.

My solution was to "gut" the air pump, replace the bearings and just use it's pulley to tension the belt for the fan. That was almost a year ago and no problems so far. This was suggested by the shop I take it to, but we also don't have emiss. testing in MN.- Ruf

Thom1

Hey Rufus,

Can you provide part numbers, manufacturers, and supplier contact information on your smog pump bearings please? I have tried to find that information to no avail for a long time.

Jerry,

There is some controversy about adverse effects of removing the smog pump. However, I concluded that expert conclusions pointed to no adverse effects.
<img border="0" alt="[burnout]" title="" src="graemlins/burnout.gif" />

BrianG

Thom, that assumption isn't quite correct. The adverse effect will be on your emissions output. You will be choking your CATs. If there is no O2 pumped in, to light them off and "self clean", they will eventually accumulate enough "effluent" to restrict their flow, then you will lose power. If you don't care about your effect upon the atmosphere, and do disconnect the AIR pump, you should also remove the CATs..... although that's only legal for off-road use.

Thom1

deleted

BrianG

Thom, I certainly meant no offense. <img border="0" title="" alt="[Eek!]" src="eek.gif" /> It was simply a statement of fact, as regards the answer to your question about a particular component of an OEM system on our cars. Do you really think Porsche put the CATs and the pump that feeds it the O2 they need to function there for nothing?
I find it offensive and presumptuous for you to suggest such a thing of my favorite car manufacturer. <img border="0" alt="[ouch]" title="" src="graemlins/c.gif" />

Lighten up <img border="0" alt="[hiha]" title="" src="graemlins/roflmao.gif" />

ViribusUnits

Depends on what emisions your talking about.

The pump is vital to lowering hydrocarbon and CO emitions.

Our engines tend to run a bit rich all the time. This is a good thing. It keeps the detonation away, keeps NO down, lowers the exaust temp, and helps protect everything if you've got a vacume leak. This is expecaly true of engines with out the O2 sencor. They're running on a calculation, without the O2 sencor to tune the mixture. As a result, the engineers in control of the thing had to go with the safest numbers. The richest setting.

This rich setting prevents full combustion of the fuel. As a result, there are extra hydrocarbons and CO, which are the results of there not being enough O2 in the mixture.

The smog pump puts air into the exaust, infrount of the cat. In the heat of the exaust, the extra fuel is mixed with air provided by the smog pump, and is burned. The CO also combined with the O2, reduceing CO emitions. Finaly, the heat of the burning of the above is used to heat the catalysts, allowing them to convert NO2 into N2 and O2. That O2 further assistes is getting rid of extra CO and hydrocarbons.

Without the smog pump, the extra hydrocarbons will "stick" to the catylists. There the hydrocarbons will "cook" with out the O2 to react with, and will turn into basicly pure carbon, and clog the system up. You can't have the cat, w/o the pump.

That is typicaly not good for the enviroment.

The pump can still be of use without the cat. The cat is only needed to reduce NO2 emitions, the pump's O2 is what oxidixes the hydrocarbons, and the CO. All that are needed is O2, and the high temps of the exaust.

Do rember though, the system is realy only needed to clean up after a messy engine. If the engine is perfectly tuned, almost all the hydrocarbons will have burned, and it will not have the combustins temps high enough to worry about NO2, or the problem with detonation.

However, the later cars, with the anti-knock sencors are forced to use less efficent settings if they are running lower octane fuel. Retarding the ignition, and richening the mixture result in no detanation, but more hydrocarbons, and more CO. They can realy use the emition system.

(btw) the pump and cat are there because US law demanded it. In Europe where they were not required, Porsche didn't include them, because they eat at horsepower, and add cost.

BrianG

ViribusUnits: Now please don't take this personally, because it's just a discussion point around your comments, and my understanding of CAT function. I was given to understand that the first CATs/AIR pump were designed to deal with the left-over Hydro-carbons. Then in later days, after the "leaning philosophy" you mentioned came into vogue, they went to "dual-bed" CATs which had a second type and location of catalyst that dealt with the NOx issue. Further, isn't there a 3-bed CAT now widely in use to deal with yet another emissions issue?

With respect to your last paragraph: Are you saying that the car met all of the US emissions standards of the day, without the CAT's and AIR-pump, and DOT made them put them there because they liked the look of them?

Rich9928p

"(btw) the pump and cat are there because US law demanded it. In Europe where they were not required, Porsche didn't include them, because they eat at horsepower, and add cost."

&gt;&gt; This was true up to the time when the EEC required cats. They were an option in the '85 - '86 years in many European countries as was outlined in the first Forum 928 magazine. I'm thinking that even in Europe cats were requried from '87 onward, though I could be wrong on the date. All the new European cars have cats.

Erik - Denmark

ViribusUnits,
Good explanation except -You wrote:
(btw) the pump and cat are there because US law demanded it. In Europe where they were not required, Porsche didn't include them, because they eat at horsepower, and add cost.
---
Not totally correct, up to about 1987 (I am 100% sure of the year) we had the pump but not the cat on 928's in Denmark - Non of my two cars have the cat but bought have the pump
But I totally agree, without the cat we do not need the pump

Normy

My '85 S2 had no cats from the factory [I've since added a pair of S4 units], and no place to mount an O2 sensor.

I'm interested in maintaining my air pump while replacing the engine driven fan with an aftermarket or Mustang electric unit. That means I will need a different belt for the air pump...does anyone know which belt will work after this modification?

Normy-
'85 S2 5 Speed

Rufus Sanders

Hmmm. Well, sorry I mentioned running without an air pump... Um, I think the bearings for the air pump are the same as for the alternator on an '82 anyway. But I don't know the exact part number.

I think 928 Int'l actually sells the bearings, the pump and (please nobody jump on me for this...) the tensioner that replaces the air pump on Euro cars. There. 'nuff said. I'll stay out of this now. - Ruf

PorKen

I have the fan, air pump, and AC pump removed.

I have idly wondered if I should run an idler in place of the air pump, to balance the belt tension on the crank bearings from the alternator and PS pump?

bcdavis

Darnit!

I have a Euro, with no air pump, but with cats...

Grrrrr....

I suppose I have to punch out those cats now!
(just kidding... )

(Maybe...)

<img border="0" title="" alt="[Wink]" src="wink.gif" />

Jerry 87 928S4

I ordered the air pump. Amazing, I seem to keep all the mechanics original and modify everything else

Thanks for all the information and advice.