2.7 for Competition?

[Cory M]

I’ve got a stock 2.7 from a 1977 911S. I am considering using it in a racecar (no street driving ever, no smog, etc). My main motivation for using this engine and not going to a later, larger displacement engine is to stay in the race class I want to be in and to be able to run at a very lightweight. I could stay in the same class and switch to a 3.0 from an SC, but I would have to run about 325 pounds heavier and I don’t really want to do that, plus I already have the 2.7. The things I can do to the engine are very limited. I could add weight and make more modifications, but that kind of defeats the purpose and puts me further away from my lightweight goal. The engine must use the original CIS, going to carbs or a different fuel injection set-up is not an option (big points penalties). I plan on running headers and an open exhaust. I want a reliable engine, not a hot motor that needs to be rebuilt all the time.

Right now I can do ONE of the following without having to add any weight: raise the compression, change the cams, increase the displacement by .25 litres, or increase the valve sizes and port & polish. If I want to do anything else I have to add about 150 pounds to the car for each mod.

What mod(s) would you do and why?

Is there another modification on the list that you feel needs to be done and will offset the 150 pounds I’d have to add by doing it?

How much realistic power/torque can you expect to get from a 2.7 in the configuration you selected?

Should I even bother with the 2.7 or just add a lot of ballast and run the 3.0 for the improved reliability?

[jluetjen]

Can you change the gears? If so I'd close up the ratios in the box and change the cam. You'll most likely give away some driveability below 3000 RPM, but who cares if you're doing better from 3000-6800 RPM and you're geared to use it!

As far as the other options, to be honest, 911 motors don't seem to respond a great deal to changing the CR by itself. Increasing the capacity by .25 liters will bring you up to 3 liters which is not that cheap to do with the mag case. You'd do better to swap in an SC motor. Increasing the valve size might be useful if you were changing other things, but with the stock cam and capacity the valve sizes are not an issue, while a "port and polish" isn't going to net you much since 911 ports are pretty good to start. You should be able to get up to around 200 HP with the stock ports, but I don't think that you'll get there with a cam alone.

Off the top of my head, something like a 964, Webcam 20/21, 993 Supercup or Camgrinder's Mod-E cam might be good choices. He may also have a better option based on your requirements.

[304065]

I'm with John on the cams. A modern grind will get the valves open quicker and shut them quicker.

[jpnovak]

Can you lower compression? Do you get more points back to do the gear changes John Suggested?

I know this goes against the grain. Consider this. Mill valve pockets into your CIS pistons. This will lower compression. At this point I suspect you would be able to use a S or DC 40 style cam. Who cares if it doesn't idle with CIS. Its a track-only car. Just set the idle high enough that the low speed reversion doesn't mess with the Air valve. This would provide maximum results.

[Cory M]

I am allowed to change the gearing and plan on doing it, probably just 3-4-5 though.

No points for lowering the compression, just raising it..