Higher Compression timing change, venturi size, etc...

[914magazine]

I'm sorry if this topic has been discussed at nauseum but I need to know and can't seem to find a thread with enough info. So, here goes

I have an Autocross car with a 2.7 from a 76 911s and am running E cams. Stock crank. I had RS spec P&Cs and Weber 40IDAs. RS 011 distributor. Very fast and responsive.
I put this engine into a different car which is slightly heavier and was hoping for a bit more umph to push her around so...
Call me silly but I have installed a Carrillo 12.5:1 piston set, twin-plug and Electromotive ignition. I made no other changes.
110 leaded fuel used at all times

Questions:

1) Ignition timing - should I be changing the ignition timing to differ from the ignition timing on the 011 distributor? Suggested timing curve or where to look up reliable info?
2) Venturi size - Does this compression change and possible timing change require or benefit from a venturi change and subsequent jetting change?
3) AFR - What AFR am I looking for at WOT under these conditions?
4) Cam timing - Does cam timing need to be changed when increasing compression and going to twin plug?

I am in the San Francisco Bay Area (Santa Clara), in case there is a guru you can point me to as well.

Thank you in advance for the time and consideration you may take in helping me learn and advance my skills. It is truly appreciated and will not go unnoticed.

Mike

[zztot]

You obviously jumped off the deep end.
My sources go back a long way but Jerry Woods is there right???
Check 914world and hit up Sergio…..for potential source (PCAGGR7) if I recall.
I still ask him all my dumb questions…..should no who might be good resource.

[Rosco_NZ]

Here’s a few thoughts ..
- Compression ratio, port size and cam timing is a set which needs to work together.
- From the outset, you need to design the engine to use ‘pump gas’ or ‘race gas’. Keep a normally available fuel in mind. No point trying to run a race only engine on the street.
- The more CR, the more cam you need as the dynamic compression may go off the chat and blow sky high. A ‘T’ cam with next to no overlap could ruin your engine and your day.
- When you modify design from stock, put in highest octane fuel for first start & initial tuning. Dial the ignition timing right back by say 5 degrees and find a good dyno tuner.
- Your tuner should have venturis, emulsion tubes and jets to suit if you have chosen the right carb for the application.
- Modern programmable ignitions much easier to work with in adjusting ignition curves.
- If you have to run mech spring advance, put in a stiffer spring initially. Spring tension a compromise in setting timing curve when compared to programmable.
- Twin plug ignition curves look much different to single plug, much less advance required, typically maxing out @ 25 deg full.
- E cams @ 12.5 .. I’d think this is not possible single plug on ‘pump gas’.
- Much time ($$$) can be spent getting your fuel and ignition curves sorted.
- Get the fuelling right with conservative ignition first, then optimise ignition.

[914magazine]

ZZTOT - Thanks, I replied to your PM

Rosco_NZ - I believe you misunderstood the questions but thank you for the reply.

I appreciate the feedback, nonetheless.

Mike