Camshaft timing question

hobieboy · 06-15-2006, 08:01 PM

I am in the middle of timing the SC cam in my 930. The specs that I got is to time it to 1.4-1.7mm @ 0.1mm valve clearance.

Two questions -
a) I read somewhere that if one times it to the low end of the spectrum, it favours torque/power at the top end (RPM)? Why?

b) I ended up having it at 1.38mm at the moment. Is that good enough?

thanks.

DonE · 06-16-2006, 05:29 AM

With your specs, 1.4 is retarded and favors low end (little overlap). With 1.7mm (advanced), that will favor higher end power. Think of your power band as a sliding scale within your operating RPM. Advance the setting and you will advance the power band. Retard it, and you will retard the power band.

On a 930 with SC cams, I would favor the higher end of the settings since the SC cam conbination is so good.

hobieboy · 06-16-2006, 01:33 PM

Thanks for the response Don... looks like I should re-do it to make it closer to 1.7mm then.

Eagledriver · 06-16-2006, 02:30 PM

Don has it backward. Think of it this way. When the cam is advanced (1.7mm) it opens earlier and allows the air in earlier. This helps the cylinder fill faster when the engine is turning slower. When the engine is turning faster the air going in takes time to "get going" and the piston is moving so fast that it can start moving up before the cylinder is finished filling. In this case you need the valve to stay open longer to get all the air in that you can. To keep the valve open later you need to retard the cam (1.4mm).

In truth most have found that a cam set toward the advance side (1.7mm) feels better on the street because you get more torque sooner. I would think that would be extra good with a turbo charged engine.

-Andy

Eagledriver · 06-16-2006, 02:34 PM

By the way overlap is fixed on these cams. You don't get more overlap by advancing the cam. Overlap is just the amount of rotation that both valves are open at the same time not where in the rotation they are open.

sand_man · 06-16-2006, 05:45 PM

I did my SCs at 1.65MM. I wanted to stay just shy of 1.7MM in case things like assembly lube on the cam lobes might be playing into the tolerance mix. My car runs great. Still breaking it in, but the lag is a thing of the past.

hobieboy · 06-16-2006, 06:09 PM

Andy, yes - I thought about it more after I responded and checked a few more sources and came to similar conclusion.

I have set my cams at 1.44mm because from past experience (with cams at 1.57mm) I had more than enough torque/power at the lower RPM range. I use the car mainly on track (yes - I should have gotten a different cam but...) so I've decided to try the other end of the spectrum & see if I can squeeze a bit more out at the 6000 rpm range.

DonE · 06-16-2006, 06:17 PM

Quote:

Originally posted by Eagledriver
Don has it backward. Think of it this way. When the cam is advanced (1.7mm) it opens earlier and allows the air in earlier. This helps the cylinder fill faster when the engine is turning slower. When the engine is turning faster the air going in takes time to "get going" and the piston is moving so fast that it can start moving up before the cylinder is finished filling. In this case you need the valve to stay open longer to get all the air in that you can. To keep the valve open later you need to retard the cam (1.4mm).

In truth most have found that a cam set toward the advance side (1.7mm) feels better on the street because you get more torque sooner. I would think that would be extra good with a turbo charged engine.

-Andy

I apologize - you are correct. Bad day....