Alfa Romeo Forums banner

1 - 20 of 28 Posts

·
Registered
Joined
·
53 Posts
Discussion Starter #1
We are running a 105 block in our race car. The race car uses a mechanical tach (a Smiths Chronometric). So we need to know how many revolutions the Alfa 105 tach drive does so that we can hook straight up, or get a conversion box. Also need to know which way the cable rotates - clockwise or counterclock wise.

Anyone have those details?
 

·
Registered
Joined
·
53 Posts
Discussion Starter #5
Well, it seems the engines run clockwise. So I guess the only question is if the two little worm gears change it.

I do think with a magnetic tach, it does seem to make sense that it would be turning clockwise when looking at the cable, and counterclockwise when looking at the back of the tach/speedo.
 

·
Registered
Joined
·
53 Posts
Discussion Starter #6 (Edited)
Ok, in case anyone cares, based on these pictures, if the engine is CW, that the cable would be CW too. I think the Alfa gears are the one on the left. And the Alfa drive picture below is the larger gear in the sketch. I'm happy to be shown wrong.

I flipped it around to have the same orientation as the drive on the front of the engine. The red line is the output.

1630569


1630567


Here are the Alfa parts in a picture I found. Pretty sure as that output rotates CW, the drive will go the same way.

1630568
 

·
Registered
Joined
·
432 Posts
If you stick a small Phillips head screwdriver in the tach and spin it by hand you’ll get your answer.
 

·
Registered
Joined
·
53 Posts
Discussion Starter #8 (Edited)
Yeah. But my problem is that this is not for an Alfa. This is for our Elva race car that has a 101/105 engine. So I don't have an Alfa tach to check.

As an aside, the tach is a Smiths Chronometric. That means it's all gearing - no magnetic disc and no circuitry. Just like a watch with gears. A strange set up - at least by more modern standards. But it's new to me. I'm not yet 100% sure which way our Smiths goes, because Smith made both directions. And since it is all geared, I also don't know if going backwards can harm it.

The tach says 4 to 1 on the front. So I may need an adapter gear box anyway.
 

·
Registered
Joined
·
53 Posts
Discussion Starter #11
That's great to know. If still the same, that would mean we don't need any adapter - as long as we figure out the direction of rotation and it matches. I'm still perplexed by the observable gearing showing CW versus Quadrifolio and that shop both saying CCW.
 

·
Registered
Joined
·
53 Posts
Discussion Starter #13
Yeah, perhaps that is the question.

I note that above Quadrifolio said "Well if your sitting inside the car, the cable will turn counter clockwise."

And the shop emailed me and said "the cable rotation on 99% of cars is counter clockwise."

Maybe Quadrifolio flipped it by accident, and the shop is thinking of the rotation when looking at the back of the tach.

I sure hope so! Thanks for your input on this. Truly a question for Einstein.
 

·
Registered
Joined
·
7,856 Posts
Jay this is out of my zone. I do know the engine turns clockwise when facing the fan which equates to CCW when facing the rear of the flywheel. I've read that some Smith's need an adaptor to go in a specific rotation .. I have no idea what that means. The bike guys deal with it.. You might figure out which way the cable turns and then learn it is not the right direction for your specific tach. I think it has something to do with the construction of the tach and the gearing in it so with the cluster of gears in the tach a CW cable might be converted to a CCW tach and vice versa. So in other words the rotation of the cable might be converted to the opposite direction inside your tach.since it is mechanical and doesn't operate on magnetism. Too complicated for me..
 

·
Registered
Joined
·
53 Posts
Discussion Starter #15 (Edited)
FWIW, here is the back of a gauge showing 1.4, just like jkbartusek said. I guess I can only hope that they stayed constant. All things are possible. So whatever I finally confirm, I can get an adapter made. But right now it seems I don't even need an adapter, and that would be great!

My dad used to complain that the English, particularly Sunbeam, whenever the factory built cars with different transmissions or rear end ratios, which changed the speedo ratio drive, they would build a different, car specific speedometer. Thus there are a whole range of identical looking speedos for cars of the same year, but with different ratios. He said that Ford did that better - they made one speedo, and just changed a gear at the connector on the transmission, which you could easily swap to match your set up. I guess I'll ultimately find out what Alfa Romeo did.

1630636
 

·
Registered
Joined
·
7,856 Posts
Alfa used the same system for speedos.. One gauge fits all on the Giuliettas and the drive gear and sender were matched to the RE ratios.. The tachs were all the same drivers and senders and it didn't matter what engine you used.
 

·
Registered
Joined
·
53 Posts
Discussion Starter #17
Even better. Thank you all for the help. I know it's not an Alfa chassis. But it sure is an Alfa heart!

And it now has a Gordon oil pump. And I just found out it has a Spruell windage tray, which I didn't realize was in there. So look out!😀
 

·
Registered
Joined
·
53 Posts
Discussion Starter #19
Yes I did flip it, sorry for that
No problem! It helped me dig in that much more - and have fun learning about worm gears.

Now that we seem to have it settled (I hope), I'm figuring that the mechanic and you basically did the same thing - when you are testing a tach, and maybe hooking up a drill, it actually has to spin CCW from the mechanic's point of view.
 

·
Super Moderator
Joined
·
11,683 Posts
Good thread!
 
1 - 20 of 28 Posts
Top