Extracting more power from 2.7 CIS

[jess p]

The 911 2.7 CIS engine is rated at about 175 h.p. The limitations
on this unit is airflow. Weber makes a bigger throttle body
for this engine. To match more fuel with the increased airflow
one simply has to increase the mixture screw. I've talked to
people who have done this and the power increase is
substantial.





77 911
66 912 chevy V8
68 912 2.2L CARBURETED WITH NITROUS

[john walker's workshop]

substantial is pushing it a bit. minimal would be a better word. the mixture screw only effects idle and real low rpm. the rest of the fuel delivery is predetermined. wimpy cams, low compression, CIS, that's the limitation.

[IROC]

....and you can't simply put a bigger opening (larger throttle body) on a pipe (the rest of the engine) and increase it's flow rate. There's alot more involved. Now, if the throttle body was truly the "bottleneck" in the system, then you might be on to something. Porsche wouldn't have left that hp laying on the table, though.....

I've heard from a pretty well-respected guy that - on a CIS 2.7 - if you don't also go in and modify the heads, a larger throttle body is essentially worthless.

Mike

[jstgermaine]

...my thoughts (which for the most part are useless and often mistaken for banter), take it out and give it away...the weight savings alone will allow you to roll down hill at a rate as quick as stock acceleration.

[raceman77]

what will work for a noticable power gain on the cis is to bump up the fuel pressure.l had a clogged return line on my 2.7 that was bumping up the fuel pressure and it was the fastest stock 2.7 911 l had been in.lt is possible to open the reg and modify it so it can be adjusted..

[Adam]

I can vouch that NEW injectors make a world of difference. Mine were the original '76 units and had lasted until last year. Now the car has some real punch.

Put a nice exhaust on it and enjoy. As Wayne said, "the SC's 3.0 ('78) is rated at 180bhp and the 2.7 is rated at 175bhp, but with a superior torque curve..."

A nice exhaust should add around 5-8 bhp (or SSIs which would add around 20-25) which should put you above the SC's range with a slightly better power-to-weight ratio.

[JTO]

So if Adam is correct, what is my engine putting out? Its a 75 2.7 L, fully rebuilt. Its got SSIs and a GHL dual in/single out. A bump in CR to 9.0:1, polished intake runners and slightly massaged heads, new injectors, among other things. I'm just curious what it might be putting out at the crank.

Thanks,
Troy

[dd74]

Originally posted by JTO
So if Adam is correct, what is my engine putting out? Its a 75 2.7 L, fully rebuilt. Its got SSIs and a GHL dual in/single out. A bump in CR to 9.0:1, polished intake runners and slightly massaged heads, new injectors, among other things. I'm just curious what it might be putting out at the crank.

Thanks,
Troy

I'd venture to guess 210, maybe 215 hp.

[jess p]

Porsche installed CIS units on their cars in the 70's, which was
adequate to meet newer impending emission regulations. Take a
look at the 1982 Chevy corvette with the new crossfire injection.
That 350 small block is rated at 190 H.P. The limiting factor
on that engine is the intake manifold. It restricts airflow. An engine
is nothing more than a pump. Restrict airflow and you restrict power.
The solution to that car's problem is the x-ram. A redesigned intake
manifold. I will be acquiring the larger weber throttle body soon. We
will see just how much more power my rebuilt 2.7 puts out.
I've spoke to a fellow with who had an older vw that installed the
weber larger throttle body and he indicated that power was up quite
noticibly.

[jluetjen]

what is my engine putting out?

I'll Play -- 2.7 normal or S? What does "slightly messaged" heads mean? How big did you make the ports or did you only polish them?

[JTO]

Hi John,
My car is a 911S. The intake ports were polished, but the port size is stock. I'm just curious what this thing is putting out.

Thanks,

Troy

[Randy Webb]

I'd bet on an extra 5 to 10 hp only with the GHL, etc. Polishing is not gong to help much at the rpms the CIS can achieve.

For the bigger throttle body -- do 3 dyn runs before and 3 after - on the SAME dyno. An engine dyno will be a lot more accurate. Then post the results.

[jluetjen]

OK here are my estimates; 2.7S (with 35 mm ports) made 175 HP at 5800 RPM and 173 lb/ft at 3800 RPM,

* SSIs and a GHL dual in/single out. (add ~10 HP)

* A bump in CR to 9.0:1, (no measurable change to peak HP, but I would expect the torque curve to be a bit "fatter")

* polished intake runners and slightly massaged heads (no change - in the absence of a cam change, the stock heads with 35 mm ports flow good enough for up to about 200 HP. Aside from the cams, the CIS meter is the big obstruction in the intake)

* new injectors (they were new when the car made 175, I doubt that it will make more now as a result of this)

Survey says... 185 HP.

[JTO]

Thanks jluetjen. It feels like an SC but a bit more. Probably due to the lower weight.
Troy

[jluetjen]

There's another thread right now that asks pretty much the same question for a 3.0 SC. The examples I gave there apply here too.

For comparison:

2.7S, 185(?) HP at 5800 RPM through 35 mm ports equals 7,830 liters/minute of air or 91% of the air of a 2.7 RS for 88% of the HP.

[Wayne 962]

Noah's 100% right. Jess, you need to do a little more research on this subject. Firstly, the 911 engine is *not* a Chevy or VW engine - the same principles do not apply. Secondly, your 2.7 engine is not limited by airflow - at least not in the same manner that the Chevy engines might be. Extensive dyno testing has proved this.

To improve the performance of a CIS engine, you really need to make the following changes:

Stage I:
- Higher compression pistons
- 964 or similar camshafts
- Early (1974 or earlier) exhaust
Maybe about 200 HP


Stage II:
- MFI 2.7RS Spec or Weber 40s.
- Higher compression JE pistons (higher than 8.5:1 RS-Spec)
- Early exhaust
- Flowed heads
Probably about 220-230 depending upon other mods.

The same thing goes for the 3.0L engines too.

You might want to pick up a copy of my new book, it explains a lot in there:

http://www.101projects.com/rebuild.htm

-Wayne