How big of an effect is 3 degrees off timing?

JoeMag · 06:06 PM

Well, got the BIG wheel on my motor and checked the timing from when it was setup several years ago. Looks like cam timing was retarted about 3 degrees. By putting timing where it belongs, anyone know about how much torque curve will be shifted to lower rpm? 993 rsr cams.

This big wheel is amazing the resolution. Can easily get to 1/2 degree not too much additional to get to 1/4 degree.

Set TDC using dial indicator directly over top of piston before installing heads, and then with piston stop with heads installed... basically got same number within about 0.12 degrees. Though I did determine length of piston stop so it would be on flat part of piston (3.8 rsr) with an empty head on a cylinder & piston.

btw, is angle of valve on 993 different than 964? I have 964 heads on my motor with 993 cam towers. Noticed intake valve on 964 head is at 30 degrees from vertical and 993 cam tower valve cover surface is at 25 degrees. ...exh valve matches at 25 degrees.

lvporschepilot · 02-25-2019, 05:16 AM

3 degrees translates to what difference in valve lift on the dial indicator? Most cams have a range of lift at TDC where they can be installed. Range is usually around .3mm, so for something like a DC60 it would be 'install at 5mm-5.3mm at TDC'. Not sure what that translates to in degrees at the crank but from the experimenting I've done, 3 degrees at the crank doesn't make a big difference.

Around 2010ish I experimented with Ferrari's P6 cams. The P6 grind was Ferrari's proprietary race grind in the 70s used on various applications. The cam was considered not streetable, which it really isn't. Anyway, I experimented with a 3L 308 2v carb motor using these cams with high compression pistons and a bunch of other intake alterations. I had the car on a dynojet and adjusted the cams upwards of 10 degrees each direction (using custom cogs for quick changes) and saw basically no differences in under 4 degree changes at the cam which translates to 8 degrees at the crank. Very different engine I know, but it is still 2v with a hemispherical chamber. Best number was around 240 at the wheels btw, but the rig was a total 'on or off' switch.

JoeMag · 02-25-2019, 08:25 AM

The cams are supposed to be set at 4.6mm at overlap. I was getting 4.2mm. An additional 3 degrees got them to 4.6mm.

lvporschepilot · 02-25-2019, 10:17 AM

Quote:

Originally Posted by JoeMag

The cams are supposed to be set at 4.6mm at overlap. I was getting 4.2mm. An additional 3 degrees got them to 4.6mm.

That's awfully close isn't it. I wouldn't have as much of a concern if it were on the other side of TDC making the intake valve close a touch earlier. Your current setting means the intake valve isn't closing for 3 degrees retarded which reduces your effective stroke until volumetric efficiency spins the motor high enough to accommodate. Dynamically, I'm sure the motor will be fine at higher rpm given the intake/exhaust works well. Lower end flexibility may take a hit, albeit nominal.

Neil Harvey · 02-25-2019, 03:13 PM

Quote:

Originally Posted by JoeMag

The cams are supposed to be set at 4.6mm at overlap. I was getting 4.2mm. An additional 3 degrees got them to 4.6mm.

If I can be so bold as to say, the cams are never "supposed" to be at any setting. The only "supposed" I can think of, is piston clearance. Opening the valves earlier or later will have an effect on how much "work" the engine will produce and at what RPM.

3° will make a difference and that difference will become larger as the engine becomes more developed. I tell customers who don't get it, think of it in % terms. You let something slide and it amounts to a loss of 10 HP. Your engine makes 350 HP. A loss but not quite the effect 10 HP loss will have on a 160 HP engine. Don't give away the free HP you engine can make by thinking its not worth it. It is.

While you are at the timing checks, check the difference from the left side to the right side when setting the valve lifts at the height. You may, will probably see a crank degree difference there too. What you are now checking is the cam quality and the rocker arm quality. For the sake of doing it, check every lobe on both the Intake and exhaust side of the cam. Don't be shocked at what you see.

They may not be all equal. The idea here is to have the valves open all at the same crank degree and with the other losses known, try to obtain the same power output from every cylinder. Doing some math here can give you an idea of how much your engine could make if each cylinder produced the same power. They won't as this is the holy grail of engine building, trying to get even power from each cylinder. These air cooled engines are the worst, as thermal issues are high with the center cylinders siamesed and the air cooling somewhat hit and miss.