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RBL · Posted: Mon Feb 09, 2009 11:01 am Post subject: Vernier cam pulleys

I'll probably be skimming the block to raise the compression on the new 8valve 1.7 engine I'm building. So to keep the cam timing spot on, I'm looking for a pair of vernier pulleys.

Anyone?
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Porsche 550 spyder replica with Alfa Sprint 1.7 engine

paulhide · Posted: Tue Feb 10, 2009 9:48 am Post subject:

What are vernier pulleys?
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bobbber · Posted: Tue Feb 10, 2009 10:59 am Post subject:

I think Vernier Pulleys are high quality machined cam pulleys (generally with spokes to keep the weight down, rather than a solid disc).

Bob
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Fiat Bravo 165 Sport

RBL · Posted: Tue Feb 10, 2009 11:26 am Post subject:

The standard pulleys only allow the cams to be timed at a fixed position (fixed number of degrees in relation to the position of the piston). This gives optimum performance on a standard engine.

A vernier pulley allow you to change the angle of the cam in relation to the position of the piston, effectively changing the valve timing.

As I understand it, to raise the compression ratio, a small amount obviously needs to be skimmed off the face of the head or the block. But when this is done the distance between the crank and the cam is reduced, shortening the belt length and in turn changing the specified angle of the cam and altering the valve timing. (one cam would be rotated incorrectly clockwise and the other cam anticlockwise).

So if you're going to increase the compression ratio, ideally the cam pulleys needs to be adjustable to get the valve timing back to what it should be.

I'm not bothered about lightened and polished, just adjustable. I might even be able to fabricate a pair from a couple of sets of regular ones.
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Porsche 550 spyder replica with Alfa Sprint 1.7 engine

bobbber · Posted: Tue Feb 10, 2009 12:08 pm Post subject:

RBL... great explanation.

(Note to self... stop trying to be smart Laughing

)

Bob
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Fiat Bravo 165 Sport

RBL · Posted: Tue Feb 10, 2009 12:59 pm Post subject:

Bob, you're right - they are normally high quality machined pulleys, usually for people building high spec race engines. I just want a bit more power while staying with standard road cams.

Here's a pair on a 16valve:

http://www.ahmotorsports.co.uk/photos/Race%20Parts/16Box/16vCam%20Pulleys.jpg
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Porsche 550 spyder replica with Alfa Sprint 1.7 engine

RBL · Posted: Tue Feb 10, 2009 4:36 pm Post subject:

I spoke to AH Motorsports and a cheap option is to modify the original pulleys. By slotting the hole in the pulley that the cam dowel fits in, the cam can be re-timed. Not ideal, but an option that can to be used.
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Porsche 550 spyder replica with Alfa Sprint 1.7 engine

Rich B · Posted: Wed Feb 11, 2009 4:07 pm Post subject:

Thought it would be useful to show a piccy of vernier pulleys, here are mine:

As you'll see, the pulleys have a series of extra bolts around the outside with a slot, as RBL explained, these allow you to adjust the cam timing more accurately and are particularly useful on an engine with lots of mods. By advancing or retarding the timing on the camshafts you can gain quite reasonable amounts of power.

Hope that helps

RBL · Posted: Thu Feb 12, 2009 11:13 am Post subject:

Thanks for those pics Rich.
Are they standard pulleys which have been modified? I can't quite see how they work - is the toothed section welded to the silver disc we can see at the front? (with the back part of the pulley fixed to the cam with the dowel?
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Porsche 550 spyder replica with Alfa Sprint 1.7 engine

Rich B · Posted: Thu Feb 12, 2009 5:13 pm Post subject:

No probs.

I think the pulleys are from AH Motorsports but not sure as I bought them from someone I know, he tends to only buy the very best so theres a good chance they are. I could take some better piccies if you like, those were done in a bit of a rush...

RBL · Posted: Thu Feb 12, 2009 5:38 pm Post subject:

Thanks. a couple of better pics would be great if you get time. They do look similar to the AH motorsport ones.
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Porsche 550 spyder replica with Alfa Sprint 1.7 engine

Oggie · Posted: Thu Feb 12, 2009 8:23 pm Post subject:

They are earlier version of the vernier pulleys, they can loose the black finish with time, i understand that Kent Cams now do pulleys if your interested , may be cheaper?.
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PHAT 33

rsfruitbat · Posted: Fri Feb 13, 2009 1:53 pm Post subject:

Vernier pulleys are in two halves that bolt together. The inner section bolts to the cam and uses the existing drive, woodruff key.
The outer toothed section is driven by the cambelt.
By slotting in one face and bolting through it to the other face the two halves are clamped together.
By slackening the clamping bolts cam or crank can be turned relative to the other then retightened to lock up the "adjustable " pulley in it new position.
If you have a lathe they are fairly easy to make but you will need two sets of pulleys so that you can make an outer and an inner.
Remember to make the clamping bolts always face forward else its just as hard to adjust

rsfruitbat
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