Timing a carbed 2.7

[Headrage]

I just finished installing a MSD/Pertronix ignition on the motor. Now I need to time it. The motor is in a 914 so I had the rebuilder index the flywheel at tdc and 35 btdc so I wouldn't have to use mirrors or cut a hole in the firewall.

I seem to remember someone saying that the timing was set at 35 btdc @6k rpm.

Seems awfully high to be spinning a unloaded motor.

Any advice?

Thanks...

[klaucke]

Better to spin it unloaded and verify the timing, than to run those RPM's and have a detonation problem. I used to be skeptical about this procedure as well, until Grady Clay gave the nod.

[pjv911]

Your engine (if healthy of course) will easily handle a sustained 6k free rev. I think 35 deg total advance may be a little high. I would look for another persons advice on that spec. I recall setting one of my past 2.7 engines at 31 deg total advance.
Keep in mind that too much timing advance actually reduces horsepower and pounds the rod bearings and wrist pins. Let alone higher combustion pressures/temps.
One of the benifits of twin plugging is less horsepower loss and cooler combustion temps due to less total timing advance. For instance I run my twin plug 2.6 at 24/25 deg total advance.
Its a very often mistaken theory that you should run as much timing as possible without pinging. Pinging can easily go undetected by ear.
I would definetly play it safe and get some solid advice for your timing spec. If you dont get a reliable answer just set it at 30 deg total. There you should be safe. Im assuming you have 8.5 comp ratio and stock cis cams.

Kurt Williams

[A Quiet Boom]

Running 35 here on a CIS pistoned 3.0SC with carbs. I am NOT running the factory CIS advance curve though, I use an MSD programmable timing computer.

I've built a lot of non-porsche engines and I've never heard that less timing provided more power, true we ran high speed retards of about 4 degrees in high gear for high MPH through the traps on the 1/4 mile but it usually hurt ET a touch. I've run as high as 38 degrees on a full race V8.

The whole key to ignition timing is to build max cylinder pressure just after TDC thus producing the most power. Detonation occurs when max pressure is before TDC which hammers the rods, crank and pistons.

[pjv911]

If your mixture is lit to early the expansion force will work to slow the piston down before it passes tdc. The goal is to light the fire early enough that its pushing down precisely as the piston is at tdc. Since the fuel mixture ignition speed does not change we need to light the fire a little earlier at high rpms to compensate for a faster reaction. That why timing advances with rpm.
Keep in mind that 38 degress total advance would be fine if you have high dome pistons that require more advanced timing due to poor flame travel across the top of the ineffecient piston crown. Also the heads are water cooled in a chevy V8.
Your setting of 35 deg on your 3.0 may be the correct sweet spot.
All engines have a different sweet spot. Things like piston pin height, rod length, piston stroke, and most importanly piston and combustion chamber shape will play an effective role in the needed timing for max power.
Also note that due to the engine design of the aircooled 6 the spark plug location is position in a very poor place(on one side of the dome).
The idea of twin plugging is to light the fire from both sides so you can run more retarded ignition timing while burning the fuel mixture just as fast. The end result is more horsepower, cooler combuston temps and the abilty to run higher compression (higher domed) pistons.

Kurt Williams

[A Quiet Boom]

Kurt,

The advantage to twin plugging is not that less timing make more power per see. It's the fact that you're starting the flame front from two place instead of one so you can fire later. The advantage here of course is that max cylinder pressure is reached much more quickly and the piston traveling up the bore doesn't have to fight against the flame front as much. The other more obvious advantage is less chance of detonation due to less timing advance. Also twin plugs provide a more complete burn, redunancy, and better emissions. I had a 4cyl Nissan truck with twin plugs, there was a very noticable power difference with one set of plugs shut off.

So yes twin plugs make more power because of less advance but that doesn't apply to single plug. Also you are correct that max power would be if max pressure where reach right at TDC, the problem however is all the variables that make running right on the ragged edge of detonation too dangerous. I blew up more than one expensive drag motor trying to squeeze a few extra ponies out that way.

My old two vehicle had a warmed over 460 that would pull my trailer like the dickens, since I had MSD on it I installed their dash mounted timing control knob which allowed me to advance or retard the timing depending on the load. At freeway cruise I'd go for max advance around 40 but I could pull it back to 25 on a steep grade or in hot weather. This also allowed me to run junk gas without fear of detonation. I've considered installing one on the 911 if for no other reason than a little protection on very hot days or when the gas is questionable. As it stands I have a switch to the timing computers high speed retard with a 4 degree pill so I can pull 4 degrees out of it if need be without leaving the drivers seat.

This is the computer I run with my 6al http://www.msdignition.com/tc_3.htm

I had to lock the mechanical and vacuum advance in the stock SC dizzy.

[Headrage]

I'm running an MSD 6al, Blaster2 coil and Pertronix if that makes any difference.

I went out and putzed with it a little by myself and noticed that after about 3500 - 4000 rpm it the 35 degree mark would settle and any rpm increase had no affect.

[A Quiet Boom]

Headrage,

I've got my timing computer set to have all the advance in by 3500 rpm. On a watercooled motor I'd normally have it all in by 3000 but figured I'd give myself a little insurance. I'd still set the timing at 6000 though. Also pull the cap and rotor, under the rotor is a felt plug that needs to be lubricated with oil so the advance mechanism doesn't stick.

[Headrage]

Originally posted by A Quiet Boom
Also pull the cap and rotor, under the rotor is a felt plug that needs to be lubricated with oil so the advance mechanism doesn't stick.

What king of oil shoul I use?

[ruf-porsche]

Originally posted by Headrage
What king of oil shoul I use?

Regular motor oil not unless you want to get fancy with synthetic.

[RoninLB]

Only theoritical bs for max power as our engines are not ideal.

"For starters, here's what happens during an "ideal" combustion event. The spark fires at about 20 to 25 degrees before top dead center (TDC), depending on the engine. The fire starts, and takes a little time to get going. At first, the flame front moves at a very slow rate, only about 35 fps. It starts burning in earnest at about TDC, and reaches maximum pressure (roughly 800 PSI) and maximum flame front speed (about 150 FPS) somewhere around 15 to 20 degrees past TDC. Once that peak pressure occurs, pressures and temperatures fall off rapidly. At some point
before the exhaust valve opens, combustion is complete, the fire goes out and only cool gases remain. "

[A Quiet Boom]

Originally posted by RoninLB
"....The spark fires at about 20 to 25 degrees before top dead center (TDC), depending on the engine. ..."

Like you said our engines are not ideal. I doubt you'll see 20-25 degrees BTC on anything but an engine with a small bore and a plug dead center. This is why twin plugging is so effective, you're starting the flame on two fronts and decreasing the time to max pressure. There are other things to take into account like bore to stroke ratio, longer strokes need more timing due to higher piston speed. An ideal high RPM HP engine would have a short stroke, large bore, and 4 valves so the plug is right in the center of the cylinder head. Well at least in my opinion. Arguments have been made that suggest larger bores for more HP and longer strokes for more torque with a square (equal bore and stroke) being a perfect compromise. I experienced this firsthand with a stroker 351W Ford engine, with a 4" stroke and 4.040 I had a 410" engine. This engine made just shy of 100HP per liter and would have made more if I could have gotten it to breathe better. Even with 2" primarys on the headers, race ported heads, race ported intake and a huge carb I could not kill the traction robbing torque the engine made. I could spin the wheels in high gear at 55 mph just by flooring it! Even with 13.5:1 compression I needed 36-38 degrees of timing due to the very high piston speeds that result from such a long stroke.

911 engines are over square (larger bore than stroke) yet mine requires 35 degrees of timing for best power. I feel part of this is the CIS pistons which are less than ideal for carbs and thus slow down the flame front. Again that's my opinion but I can't see any other reason for it. One thing that has always annoyed me about the 2.0 I used to own and the 3.0 I currently own is the under lack of proper timing marks on the pulley. I had to mark my pulley for degrees by figuring the circumferance and then degrees per inch. When I pull the engine this winter I'm gonna put the pulley on my rotary table and scribe actual degree markings in it.

[pjv911]

Christian, I agree with everything you said.


Kurt Williams

[RoninLB]

I need one of these




"Looking Inside The Cylinder
You need to understand a little about how GAMI gathers this data to appreciate the quality of what you're seeing. They use in-house proprietary software, hardware, and test equipment to gather this data in ways that have never been done before.
Driven by George Braly (rumors of whips and chains and piteous cries late in the night from converted tornado shelters abound) GAMI has developed a portable 128-channel data acquisition system that can be installed and operated. It is entirely independent, with its own power source. Data can be captured to a laptop computer's hard disk for later processing, and can also be displayed in real time, up to 50,000 samples per
second (all channels). Using specially-modified sensors, GAMI can capture real time data from the ignition system, crankshaft angle, fuel flow, a large array of temperatures (including several
points around an individual cylinder), and anything else that can be sensed.

Interesting Most amazing of all to me is that this system can
record actual instantaneous pressures within the combustion chamber at those same data rates! GAMI accomplishes this little trick by drilling a tiny hole down inside the barrel of the spark plug, and inserting a proprietary sensor that can measure
pressure very accurately and at very high sample rates, sufficient to record every detail of the combustion event in real time.

For the first time, we can have a look right inside the very heart of an operating engine, and watch and record what happens when we change anything."

[pjv911]

Originally posted by RoninLB
I need one of these

Where is the link and sample data ?