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ideola
Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI
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Posted: Thu May 08, 2008 9:41 am Post subject: Detailed look at a Franco variable cam gear |
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Crap. Edited the wrong post. Will fix this later. _________________ erstwhile owner of just about every 924 variant ever made
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: 2815
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Posted: Thu May 08, 2008 12:14 pm Post subject: |
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SCHWEEEEeeeetttt!
Whassit do? |
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ideola
Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI
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Posted: Thu May 08, 2008 1:30 pm Post subject: |
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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. _________________ erstwhile owner of just about every 924 variant ever made |
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Min
Joined: 04 Nov 2002 Posts: 2368 Location: Vernon, British Columbia, Canada
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Posted: Thu May 08, 2008 2:18 pm Post subject: |
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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 _________________ Custom means it didn't come from a box.
1980 n/a with EDIS and Megasquirt II Injection. 7 different colors and counting. |
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RC
Joined: 25 Mar 2007 Posts: 2636 Location: Australia
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Posted: Thu May 08, 2008 7:35 pm Post subject: |
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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 Posts: 15548 Location: Spring Lake MI
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Posted: Thu May 08, 2008 8:31 pm Post subject: |
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D'oh...total gaff on my part, you are correct, it should state "cam timing" not "ign timing". Thanks for the correction! _________________ erstwhile owner of just about every 924 variant ever made |
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924RACR
Joined: 29 Jul 2001 Posts: 8794 Location: Royal Oak, MI, USA
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Posted: Thu May 08, 2008 9:43 pm Post subject: |
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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! _________________ Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype |
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fiat22turbo
Joined: 18 Jan 2006 Posts: 4040 Location: Portland, OR
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Posted: Fri May 09, 2008 4:23 am Post subject: |
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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) _________________ Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose) |
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ideola
Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI
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Posted: Fri May 09, 2008 4:39 am Post subject: |
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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 _________________ erstwhile owner of just about every 924 variant ever made |
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bass gt
Joined: 02 Dec 2004 Posts: 971 Location: Johannesburg for now!!
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Posted: Fri May 09, 2008 5:00 am Post subject: |
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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 _________________ Front Wheel Drive is the Devil's work. |
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moone924
Joined: 05 Feb 2003 Posts: 867 Location: Douglas Wyoming
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Posted: Fri May 09, 2008 6:36 am Post subject: |
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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/ _________________ Ryan Moone
Always shopping for a nice s2 931 to baby.
New philosophy : one car project at a time. |
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bass gt
Joined: 02 Dec 2004 Posts: 971 Location: Johannesburg for now!!
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Posted: Fri May 09, 2008 6:42 am Post subject: |
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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 _________________ Front Wheel Drive is the Devil's work. |
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Martijnus
Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands
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Posted: Fri May 09, 2008 8:11 am Post subject: |
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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 _________________ "Rule: Turbo's make torque, and torque makes fun." (C. Bell)
924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment |
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gegge
Joined: 27 Jul 2007 Posts: 1124 Location: Sweden
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Posted: Fri May 09, 2008 8:44 pm Post subject: |
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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. _________________ Carl Fredrik Torkildsen
924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs |
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AppleBit
Joined: 16 Nov 2002 Posts: 1516 Location: Minneapolis, MN
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