Interesting thread!

-I asked about the differences on a stock motor because I've had both. I just sold an 83 coupe that I did back-date with ssi's, sport exh etc...-and stock CIS. That car ran perfectly once warm, but did not like to warm up, and needed alot of throttle feathering when it was cold.

-My current project is an 81, with a stock 3.0, but converted to Weber 40's by the PO. It has the same SSI's and sport exh as my 83 had. This car fires right up after a quick prime, and runs great when cold-much better than my CIS car had. I however don't really feel like this car is making more power than the CIS car, throttle response is great, sure, but it doesn't seem to pull as hard. I haven't put many miles on it yet, so I really don't know for sure what I have yet, but I am thinking of pulling the carbs, and putting back the CIS (EURO setup) (-O2 sensor)

I am wondering if I need to just get the carb's re-jetted/tuned, or if you need a hotter cam to get carbs to work well on SC motors?

I also think Andy's correct about the better flow. The engine is an air pump, and the less restricted the intake and exhaust systems are, the more air you will pump and the more power you will make.

Consider when you will likely be able to take advantage of this however. If the car is a track car, and spends a sustained amount of time at high RPM, then horsepower is important and cold start, driveability and economy less so.

If the car is a street car and a daily driver, then this becomes much less important when you think about getting up in the morning, starting up, driving through town to the freeway, and onwards. I'm going to submit that for a street-driven car, torque is much more important than horsepower, unless you flog the pi$$ out of the car on every shift and drive around at or near reline.

For these cars that are making more HP with carbs, I'd be very interested in comparing the torque curves with those of the stock CIS.

I still think an engine swap 2.7 -> 3.0,3.2 is a much better plan than buying a set of PMO's though...

ianc

Interesting discussion - thanks for all the input guys. . .

My initial question centers on the 2.7L CIS-equipped '74 engine. I believe this has somewhat restrictive cams that could probably benefit from some modification (any recommendations?)

The '74 is also smog exempt (one of the reasons I specifically sought out a pre-'75 example). Here in SoCal, I suspect cold starts to be a relative non-issue, so the carbs have a certain amount of appeal. If nobody has firm dyno #'s regarding this swap, I may take it upon myself to blaze the trail (i.e. stock-with-CIS=XXX RWHP, stock-with-PMOs=YYY RWHP, modified-cams-with-PMOs=ZZZ RWHP where I suspect ZZZ will be >>> YYY >>> XXX).

Gotta' think about this one - I may have other things to do to the engine first, but if it's ultimately going to be an appreciable difference, why the heck not?

Jeff, why bother?

I'm going out on a limb and no doubt will get lambasted for it, but, out of the factory, the 2.7 was the worst engine Porsche ever built.

Why not just do a swap and have the best of both worlds? Why continue to dump cash in it for questionable results?

ianc

Ian - note his smog issue.

Even if smog is a non issue, you'll still get more power with a 3.0 or 3.2 (with the rest of the bennies of keeping FI) for about the same money as buying PMO's...

ianc

That's a thought, although I wanted to keep the original engine so I'd have matching #'s. I suppose that doesn't mean I can't keep the original on a stand in the garage while I tear around on a 3.6 though (although 3.6s are frightfully expensive - I haven't seen one for under $12k)

3.2 might work. I think those can be picked up for around $5k or so. . .

With smog out of the way, it's time to hotrod that 75! Find a good, used 3.0, put on headers or SSIs, a set of PMOs and have some FUN!

FYI, a friend of mine who just got done setting up his early SC (big port 3.0) for the Toyo spec series that EagleDriver mentions found 210 HP at the wheels (measured on a dyno while tuning the carbs) with 40mm Webers, SSIs, and a Dansk 2-in 2-out.