2.4 w/ 2.2S pistons, S cams; 24deg. advance at 6000rpm

[berettafan]

Any thoughts on this? reference materials seem to indicate early 911 motors s/b around 32-35 BTDC @6000 rpm.

[Grady Clay]

berettafan,

The issue you are facing is the compression ratio (CR). The calculations have been done before but your (very desirable) 2.2S pistons were 9.8:1 CR with the 66 mm stroke crank. The installed CR is higher with the 70.4 mm stroke crank. Look under the cylinders and see if there is only one 0.25 mm base gasket.

Timing the spark at 24º @ 6000 rpm is probably way too conservative. Even with twin ignition the norm is 26-28º. With your configuration (10:1+ CR) I would stay at 30º @ 6000 rpm.

The real issue here is the octane of your fuel. I would start with very high octane (110) race fuel and tune the engine for best running. I would very carefully reduce the octane, searching for any sign of detonation. This will involve lots of ‘cold’ sparkplugs (NGK BP8ES or BP9ES) and careful observation.

I suspect you will eventually settle on something around 100 octane. This is expensive but worth the effort to get the performance built into the engine.

The alternative is to go back to the ‘puffer’ low compression (8.5:1 CR) pistons and do as Porsche did; increase the displacement with 90 mm pistons and 2687 cc as the 911/83.

Best,
Grady

[berettafan]

thank you Grady!

I will shoot for 30deg. and see how she runs. We don't have a Sunoco w/ race gas in the immediate area but i'll start looking for one.

At some point i hope to borrow a boroscope to confirm the 2.2S pistons. (specs are per PO who was a bit of a check writer and thus far i've been unable to contact the builder; shop was Doc Emard's but PO was unable to get further details from his widow and i am not inclined to bother her over what would surely seem a trivial thing vs. the loss of her husband).

Can you point me to a general discussion on 'cold' vs. 'hot' plugs and how they interact with octane and compression ratio? I need a better understanding of this.

[Grady Clay]

Don’t move the timing to 30º until you have known high octane fuel. It may be the PO or shop put the timing at 24º just to prevent detonation with pump premium. Detonation is something to be VERY cautious with. You can destroy your engine – BIG TIME.

There is a lot on Pelican about detonation and the related issues; octane, CR, ignition advance, spark plug heat range and much more. This is a very complex issue and central to internal combustion engine performance.

The basic issue is that higher combustion pressures from higher CR generate more power out of the same fuel mixture (more hp/lb). The downside is detonation. This is prevented with higher octane fuel (read more expensive and in limited supply).

Because of the high cylinder pressures and temperatures from higher CR, you need to use ‘colder’ spark plugs. That is where the temperature of the electrode and insulator operate at a lower temperature by greater heat conduction to the head and outside world. The downside of ‘cold’ plugs is they are more sensitive to fowling.

Used spark plug condition is a good indicator for signs of detonation. Your engine will tolerate some detonation but very little and only for short periods. Detonation is destructive to an engine. With leaded race fuel, detonation can show up as tiny (<0.1 mm) grey ‘balls’ on the spark plug insulator. This only is true with a ‘cold’ sparkplug or the signs will ‘burn off’. The top of the piston will appear ‘metallic grey’ as opposed to the flat grey of proper combustion.

Testing for detonation with lowering octane has risks. Be very careful to not destroy your engine.

Best,
Grady

[berettafan]

Grady i have no idea what timing was set at prior to having the dizzy rebuilt (did i mention i'm learning as i go here?) so i can't say for sure what they had done. i have been looking to find video/sound clips of detonation so i know what to look for when i'm tweaking the timing/carbs.

thank you again for taking the time to respond.