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Detailed look at a Franco variable cam gear
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ideola  



Joined: 01 Oct 2004
Posts: 15427
Location: Woodstock IL

PostPosted: Thu May 08, 2008 9:41 am    Post subject: Detailed look at a Franco variable cam gear Reply with quote

Crap. Edited the wrong post. Will fix this later.
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Last edited by ideola on Fri Apr 02, 2010 7:37 pm; edited 3 times in total
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emoore924  



Joined: 13 Apr 2004
Posts: 2788

PostPosted: Thu May 08, 2008 12:14 pm    Post subject: Reply with quote

SCHWEEEEeeeetttt!

Whassit do?
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Thu May 08, 2008 1:30 pm    Post subject: Reply with quote

This is a variable cam gear (not to be confused with an adjustable cam gear). In a nutshell, it is a mechanical ignition timing control device. It provides 10 of timing adjustment based on RPM. At low RPM, you want torque, which is helped by advanced ignition timing. However, it's not a good idea (especially on forced induction) to run too far advanced at high RPM because it can induce detonation.

This nifty little device allows you to set your timing with 7 of advance. However, it begins to mechanically retard the ignition timing starting at about 3500 RPM, up to -10 at about 4200 RPM and above. Since the gear housing is not directly locked into position via the woodruff keyway (which is cut into the "drive plate"), the centrifugal weights fixed within the gear housing cause it to rotate by pivoting on the needle bearings, effectively retarding the timing at higher RPMs. Clever.
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Min  



Joined: 04 Nov 2002
Posts: 2365
Location: Vernon, British Columbia, Canada

PostPosted: Thu May 08, 2008 2:18 pm    Post subject: Reply with quote

ideola wrote:
This is a variable cam gear (not to be confused with an adjustable cam gear). In a nutshell, it is a mechanical ignition timing control device.


I was under the assumption that this device provided cam timing changes, rather than ignition timing ones(which it would also provide if you kept your distributor)

Min
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RC  



Joined: 25 Mar 2007
Posts: 2612
Location: Australia

PostPosted: Thu May 08, 2008 7:35 pm    Post subject: Reply with quote

Min wrote:
ideola wrote:
This is a variable cam gear (not to be confused with an adjustable cam gear). In a nutshell, it is a mechanical ignition timing control device.


I was under the assumption that this device provided cam timing changes, rather than ignition timing ones(which it would also provide if you kept your distributor)

Min


Me too Min!

As I understand it, at minimum RPM this device provides the maximum (depending on which version or setting) CAM timing, which theoretically permits greater low end torque to be developed by a stock to moderate duration/ overlap camshaft. As the RPM increases, the centrifugal force acting on the weights permits the cam timing to be retarded up to the maximum permissible depending on the design and spring rate. With the same cam in a retarded position (relative to the crank) more torque should be generated at high revs than would otherwise be with the cam at the same timing.

Since cam design for all but a flat out race engine is a compromise between low, mid or top end power this device is intended to provide the best of both. The theory is simple and is the basics behind variable valve timing technology.

With the stock distributor being gear driven from the camshaft any variations in cam timing will also affect ignition timing. This is in reality not a desirable feature. Optimum ignition timing does not generally involve retardation at higher RPMs irrespective of engine load, ie. MAP. This device may be beneficial altering the valve timing only with an independent programmable ignition or EMS triggered off a crank or flywheel sensor and an input for manifold pressure to determine load.

Good timing on the prospective MS purchase Dan, as it will provide total control of both your fuel and ignition requirements with a crank driven pick up, EDIS or not.

Roger
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Thu May 08, 2008 8:31 pm    Post subject: Reply with quote

D'oh...total gaff on my part, you are correct, it should state "cam timing" not "ign timing". Thanks for the correction!
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924RACR  



Joined: 29 Jul 2001
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PostPosted: Thu May 08, 2008 9:43 pm    Post subject: Reply with quote

Well, of course, it does both, since the distributor's driven by the cam... except I suppose on DITC 931's, which will ignore this variation, as it's triggered off the flywheel.

Good article, though!
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fiat22turbo  



Joined: 18 Jan 2006
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PostPosted: Fri May 09, 2008 4:23 am    Post subject: Reply with quote

Now, someone just needs to take one apart and take measurements so that some of use can make one ourselves.

Specifically the size and shape of the weights and the spring rates (diameter of the metal in the spring and the number of coils)

Using eMachineshop (http://www.emachineshop.com/) or a local machine shop, one could reproduce these for your own personal use without too much hassle using an old cam gear as the basis.

Obviously, if someone wanted to make these for sale to the public you'd have to get permission from Franco.

Changing the ignition timing at the same time is an unfortunate drawback to driving the distributor off the camshaft. Luckily, that isn't too hard to change thanks to Ford's EDIS ignition system and MegaSquirt (just use it to control the ignition if you aren't ready to go full on EFI yet)
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Fri May 09, 2008 4:39 am    Post subject: Reply with quote

Piper are already looking at doing precisely that. Don't know about getting permission from Mr. Franco...he seems to have gone somewhat incommunicado. James at Piper indicated they had made units like this in the past for VWs and were very curious about these units and what the results were on the 924. We'll see.

As for dismantling one, I'm not about to tear mine apart. These things are rare as hens teeth, replacement parts would have to be fabricated, and I don't want to mess with it. You might ask Steve B if he'd be willing to sacrifice one of his
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bass gt  



Joined: 02 Dec 2004
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PostPosted: Fri May 09, 2008 5:00 am    Post subject: Reply with quote

ideola wrote:
You might ask Steve B if he'd be willing to sacrifice one of his


I'd rather sell my children to medical science
Wait a mo, i don't have any!! DAMN!!
Actually, i'm going to speak with my machine shop to see what they think about making them, cost wise ect. Could be made from T6061 Ali ect. The springs wont be hard to source, just need to dyno test them to get the specs

Steve
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moone924  



Joined: 05 Feb 2003
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PostPosted: Fri May 09, 2008 6:36 am    Post subject: Reply with quote

Last I checked AppleBit had two Francos, one he sent to Mr.Franco to fix or whatever, so parts might be available.

here's a interesting site too...

http://www.selenengineering.com/xFranco.aspx

Franco himself?

http://thorin.adnc.com/~figf/
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bass gt  



Joined: 02 Dec 2004
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Location: Johannesburg for now!!

PostPosted: Fri May 09, 2008 6:42 am    Post subject: Reply with quote

moone924 wrote:
Last I checked AppleBit had two Francos, one he sent to Mr.Franco to fix or whatever, so parts might be available.


Not any more he don't
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Martijnus  



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PostPosted: Fri May 09, 2008 8:11 am    Post subject: Reply with quote

wonder how much difference it will make. Downside is that it's way better to change in/out timing seperately, which only can be done with dual overhead cams..

still I guess it will add to the performance...hard to tune/tweek for optimal performance I think, but it's a real clever device nicely designed
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gegge  



Joined: 27 Jul 2007
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PostPosted: Fri May 09, 2008 8:44 pm    Post subject: Reply with quote

Agree, I really like the idea of the variable camgear.

Higher dynamic compression on low revs becacause of Inlet Valve Close earlier. Better mpg and efficiency. More torqe. Early Exhaust Valve Opening improves response on a turbo engine. Helps ilde on high duration cams.

Stock on high revs.

BUT, would not put the "home-made" gear on an interferance engine like the turbo. I would be interested in a high-quality CNC item though...

If I understand it correctly, some benefit under 3500 rpm and the trade-off is reliability, noice and maintenance.
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AppleBit  



Joined: 16 Nov 2002
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PostPosted: Fri May 09, 2008 11:53 pm    Post subject: Reply with quote

I just sold BOTH of mine. They were AMAZING for power!

Although, now the amazing power is located here: (as I steal the thread) http://www.924board.org/viewtopic.php?t=13183&start=90
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