924Board.org

[heisian]

Hi, I have a '77 924 engine that i rebuilt and just dropped back into the chassis. This is my first porsche that i've ever worked on. So i'm having trouble getting the alternator mounted, the air intake scoop collides with the headers, plus it also looks like the pulley is a few millimeters misaligned with the water pump and crank pulleys. I took off the air intake scoop and mounted the alternator, but then I can't get enough travel space to fully tension the drive belt because the alternator hits the engine bay frame.
I have the proper belt to connect the 3 pulleys (OEM 10x888).

I'm wondering if anybody has a clear picture of how their alternator is mounted because I'm probably mounting it incorrectly and I can't fidn a picture of it in the haynes manual.

Here is a picture of my engine before i dropped it in:


thanks, Tim

[Chrenan]

Well, your upper alternator mount appears correct. Do you have the correct lower rubber mount as pictured below?



Is the alternator V-belt the correct size? All I can think is that the belt is too long, and that is giving more slack than can be taken up. As for the snorkel on the back, I would cut the snorkel off to provide room for the headers, but mount the remainder of the snorkel backing onto the alternator to keep dirt off the wiring connections. Many people run without a hose connected to the alternator snorkel (me included) without any problem, so you shouldn't have an issue.
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1987 951 - M193 Version for Japan

[Ozzie]

My belt is very difficult to get on even with all the travel of the alternator pushed back.
If yours goes over the pulleys easily then it is too long.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[-nick]

I believe your water pump pulley is on backwards. Likely the root of your alignment problems.

nick

[heisian]

Thanks for the responses,

I do have the correct lower rubber mount, but I dont know about the bracket that goes on top of it connectgin it to the alternator.

I ordered the belt OEM size for waterpump/alternator, that is what i got. Common sense just says get a shorter belt, thanks for the input.

I thought the water pump pulley was on backwards too, but the way I have it is the way the FSM shows and the pulley woudl be completely off from the alternator if I flipped it. The way I have it now, it looks like its a millimeter or less off from the alternator pulley.

[John Brown]

The alternator cooling shroud does interfer on the headers; when headers are used. Delicate reshaping with a light ball-peen hammer has been adequate at this location. Particularly with headers I prefer to retain the shroud if possible as headers dump much more heat onto the alternator than the stock cast iron manifold and down pipe. FWIW, for a street driven 924 I would retain the stock setup except in the case of a cracked manifold; at which point a replacement manifold costs more than headers.

The water pump pulley is correct. The silly rubber bushing mounting used by Porsche almost guarantees imperfect alignment at best. So you do need to replace any and all of the bushings and washers both top and bottom as needed. Note that the close fit between the water pump pulley and the timing belt idler is the reason you MUST rotate the idler the correct direction or the water pump pulley will stop them both from turning. Also, anyone looking at a turbo will find the water pump pulley different.

Installation is a bear. It's much worse with headers.

BTW, Fram does not come off well in online research for filters. Can't miss with the Mahl OEM. And the teflon additive is completely debunked so far as I know.
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

[Lizard]

-nick, there are 2 different positions for the alt, the way he has his it is run off a smaller pulley which puts less hp drain off the crank at lower RPM (but means that the car doesnt charge in the low RPMs)
it is also possible that you are reading the belt size for the other position which does need to be longer.

as for the backplate for the ALT, I would remove it and not think twice,
I ran my car with headers and no cover on the back of the alt the entire time I had the car and it never had any trouble with the bearing or diodes in it.
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3 928s,

[gohim]

Lizard, you have the alternator size/load figured backwards.

Porsche went to the smaller size pulley on later cars to increase alternator output at lower rpms. This will actually place a larger load on the alternator at lower rpms, since there would be a higher load on the engine (same amount of work over a shorter distance equals a higher load).

Another reason that the pulley size was changed in the later cars is that the later cars have bigger (higher wattage) defrosters in the rear window, which can cause a battery discharge condition at lower rpms when the alternator output is not sufficient.

The problem caused by the alternator pulley size switch is is the fact that it makes it even harder to adjust the belt tight enough to avoid slipping, and the additional stress on the belt makes the alternator mounting bushings deteriorate faster (if you are still using the original type rubber bushings).

[heisian]

thanks, i think i got it figured out
i moved the upper alternator bracket forward, using the other set of mounting holes in the bracket

I flipped the water pump pulley around
I found another upper alternator bracket (that bolts to the water pump) that is offset more so it allows the alternator to mount a few inches forward.
also found a spacer for the water pump pulley (i have a lot of spare parts from a few different 924's) and now all 3 pulleys line up within < 1mm

The belt is now being driven by the forward most crank pulley groove, instead of the smaller diameter groove in the back

I got the headers and adjustable cam gear at the junk yard for $30 bucks total, so I decided why not use them

Also got that vacuum pump with the big aluminum pulley from the same junkyard car but the size of it worries me

[Lizard]

rereading my post you are correct Gohim, thanx.
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3 928s,