74 SF2 cam options?

angonemoto

angonemoto

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Location
Chicago, USA
Hi all,
Time for a major tune up for my 74 SF2. Carb rebuild and electronic ignition, valve adjustment, synch it all up after the points are replaced. So now I am to put a different cam profile on the list. What kind of options are out there for a little more pull on the street? Im going to stick with the stock carbs to go with the new or old cam unless I get talked out of it here.

Thanks,
Bob
 
Hi Bob.
I think this has been discussed in a thread earlier. Absolutely no point going with a warmer cam (eg 1974 SFC 5/C, a very 'usable' cam profile if set up properly) unless you plan to add the factory 2-1 or similar and probably go to a higher compression ratio. From everything I've heard, the std 7/1 SF2 cam is the best of all profiles for a street bike with a predominantly standard exhaust. They have strong midrange and a very healthy top end.

The factory designed the 5/C to operate in a motor with over 10:1 compression ratio and a the tuned 2-1 exhaust system (which is so loud, you'd struggle to use it anywhere without attracting plenty of unwanted attention).

My SF motor runs high compression, a 5/C cam and a factory 2-1 with a baffle on the megaphone section going into a baffled open 'muffler' beyond the megaphone section. With the open factory 2-1 and reverse cone the tuned effect is very noticeable, with strong power from about 5-8000 RPM. With the muffling system I use it drops a little from the obvious top end, but still works. For rego purposes I fitted a twin SF system (no crossover). It is choked to the point of major losses in torque and power over even a dead stock GT. It just doesn't work. I also tried the 2-1 without the reverse cone. It was outrageously loud and lost about 30% of its power everywhere!

If you want more 'pull' right through the rev range i'd just go with a higher compression ratio - 10:1 is safe. Forget 'hot' cams unless you plan to design the entire engine system to suit.

 
Totally agree with Quentin, the 7/1 is the sweetest of all.  The /C cams are all-out race cams and weren't intended to work with mufflers.  A slight bump in compression and a little tidying of the ports is a lot more worthwhile for a road application imho.

You might even stay friends with most of your neighbours, too.

piet
 
The difference between a crisp and flat SF motor seems to be in the compression ratio. I have measured several SF2 and found around 8.5:1, where they are specced 9.5:1 (iirc). They also usually have about 2mm squish whereas ideal for them is 1mm, and 1mm equates to roughly one compression point with that bore. Of course altering this requires checking all the relevant clearances. The other thing that helps is slotting the chainwheel so that the cam timing can be dialled in, I run a few degrees advanced from std, gives more midrange and that is what most of us use most of the time. Careful setting up makes a lot of difference.
 
I use a 6/C cam with a smaller radius on the rocker foot. CR is 10.2:1 - motor will pull from 2500RPM and is so torquey. I do have adjustable cam timing and can time each cylinder separately. Most 750's have the std timing miles out, but do run surprisingly OK. - Regards - Phil
 
phil37 said:
I use a 6/C cam with a smaller radius on the rocker foot. CR is 10.2:1 - motor will pull from 2500RPM and is so torquey. I do have adjustable cam timing and can time each cylinder separately. Most 750's have the std timing miles out, but do run surprisingly OK. - Regards - Phil
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What exhaust system do you run with it, Phil? I seem to recall pics of a 2-1 with a parallel open baffle?
 
Tippie said:
The difference between a crisp and flat SF motor seems to be in the compression ratio. I have measured several SF2 and found around 8.5:1, where they are specced 9.5:1 (iirc). They also usually have about 2mm squish whereas ideal for them is 1mm, and 1mm equates to roughly one compression point with that bore.
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Probably from using 1200 pistons.

They look very similar. I have a pair of hc ASSOs from a 1200 that I was going to put in a 750 GT, LOL.
 
  What exhaust system do you run with it, Phil? I seem to recall pics of a 2-1 with a parallel open baffle?
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Hi Quentin - I used an old Gibson back part on modified "Marving" downpipes - has a standard type baffle in it - although not that quiet - does work well - before that i used a standard SFC 2 into 1 with a home made "baffle" when running as a 360? and a different state of tune - Regards - Phil
 

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"Probably from using 1200 pistons."
Standard SF Mondials in unfettered motors.
Phil?s posts back up what Piet and Q said. Higher comp and a tuned exhaust if you want to use an SFC cam.
 
Gerrards/Mallory 

for you to be that tight in, there must be someone ahead that you are about to pass, a favourite spot for me as well.

CLEM
 
Thanks for the feedback and following conversation. I'll leave well enough alone then and see whats what after the electronic ignition and other maintenance goes in. Good call on the vernier drilled cam wheels, why didn't I think of that? :)
 
Basketcase said:
Probably from using 1200 pistons.

They look very similar. I have a pair of hc ASSOs from a 1200 that I was going to put in a 750 GT, LOL.
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That ain't gonna work, 1200 pistons are 3mm too short... at the important end. :D

I have modified 1200 flat-top pistons in my long-stroke GTL, more than enough compression there!

piet
 
sfcpiet said:
That ain't gonna work, 1200 pistons are 3mm too short... at the important end. :D

I have modified 1200 flat-top pistons in my long-stroke GTL, more than enough compression there!

piet
Click to expand...
How did you increase the stroke, Piet? Stepped eccentric crankpin?
 
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