How much of a power increase 3.0

[evan9eleven]

Originally Posted by Henry Schmidt

Phil Slate, one of my builders and I were speaking about your project this morning and both of us thought you might consider a cleaned up CIS.
They really are incredibly versatile, reliable and can be tuned to work quite well for a street hot rod.
The key is to eliminate the ancillary non sense that make it pass smog and eliminates driver input.
Over the years we've been able to build "stock" class CIS engine that really do run aggressively without the constant tuning and poor fuel mileage offer by carbs.
You have the big port runners already so if you build a 3.2 SS (70.4x98) with a 20/21 Cam or a DC19 using your new exhaust their is no reason why you shouldn't see a 40hp increase. maybe more. The best part is that you own it.

Here are a couple of shots of clean version of CIS.

Henry, does your CIS cleanup help any with throttle response? My biggest complaint with the CIS is the "lag" when stomping on the loud pedal.

[Henry Schmidt]

Originally Posted by evan9eleven

Henry, does your CIS cleanup help any with throttle response? My biggest complaint with the CIS is the "lag" when stomping on the loud pedal.

My experience with CIS throttle response is that cam shafts and ignition timing have more to do with throttle response than a single butterfly.
The enhanced cams 20/21 (and copies) create incredible amount of manifold vacuum just off idle.
Manifold vacuum is key to moving the air flow meter and the more the air flow meter moves the more fuel is delivered to the fuel distributor/injectors. Much like an accelerator nozzle in a carburetor, this enhanced manifold vacuum creates an environment for acceleration the key to what people call throttle response. By removing those components that restrict fuel delivery (smog) and exhaust flow ( equal length runners for better scavenging) you allow the engine to breath.
By removing the decell devices on the engine (vacuum operated) you also find a crisper response when trail braking.
Ignition timing will also play a role in getting the engine to accelerate. If you can fill the cylinders (cams) and advance to timing without detonation (recurve dist), the engine will respond to throttle input.

The real down fall to CIS is the nature of air flow over the air metering plate. On hard acceleration the air over the plate is turbulent and this turbulent air tends to move/lift the plate more efficiently than the laminar flow you get when the engine is cruising. This reduced lift causes the engine to lean out and continued lean running in detrimental to an air cooled engine. To remedy this condition a small lip (Gurney flap) can be added to the metering plate to break up the laminar flow creating more lift (richer running condition) and a cooler engine.

This lean running condition is also the reason why CIS engine tend to have great fuel mileage compared to other systems for their time.

[Geneulm]

I think arbitrary horsepower goals are not the way to look at this. For seat of the pants feel, it is torque you want and a big, wide curve. So there are two options: go with Steve W.'s thoughts w an engine swap... a guy who has forgotten more than most will ever know on this topic. Or... go with a proven package that will get you pretty darned close, or at least make you THINK you have 275 on tap (and would be reliable too):

A top-end engine rebuild with
JE pistons to get to 9.5 or so to 1
GE60 or similar S style cams
Carbs or MFI depending on your budget
backdated exhaust
recurve your distributor

... then go have some fun.

If you want to go faster zero to 80, switch to a 7/31 ring and pinion

Faster still, get some weight out of the car -- ditch the seats, etc. In the end, that is cheaper than HP.

This is basically the specs of the 80s privateer Group 3 tarmac rally cars built from the Porsche parts catalogue which beat a lot of group 4 cars that were SUPPOSED to be much faster.

[Dpmulvan]

I'm not looking to break any speed records just want something with a couple hundred horsepower that's reliable, with an uncomplicated fuel system.

[Dpmulvan]

Henry how much would one of those modified CIS systems set me back??

[Josh D]

Originally Posted by Henry Schmidt

The real down fall to CIS is the nature of air flow over the air metering plate. On hard acceleration the air over the plate is turbulent and this turbulent air tends to move/lift the plate more efficiently than the laminar flow you get when the engine is cruising. This reduced lift causes the engine to lean out and continued lean running in detrimental to an air cooled engine. To remedy this condition a small lip (Gurney flap) can be added to the metering plate to break up the laminar flow creating more lift (richer running condition) and a cooler engine.

This lean running condition is also the reason why CIS engine tend to have great fuel mileage compared to other systems for their time.

I would really like to know more about the Gurney flap on the metering plate. A picture of one would really be great. After reading about the Gurney flap, I understand the theory, but need help with the visual application on the metering plate.

Also interested in the cleaned up CIS. I've already removed the decel valve with no detriment to performance or normal operation. I'm curious as to cold start function without the auxilary air regulator and auxilary air valve. Would you need to backdate to the cold start hand throttle?

[Oddball]

I'm in the market to have my 3.0 with CIS rebuilt too. Henry is the process you describe here something that can be accomplished easily in house?

[Henry Schmidt]

Originally Posted by Dpmulvan

Henry how much would one of those modified CIS systems set me back??

It depends on how much and which version of CIS you own.
Most of the "clean up" is about removing unnecessary components.

[Henry Schmidt]

Supertec has built our share of carbureted 3.0 engines (one even won a British Rally championship) and if you have the budget and the need for a monster, buy some carbs and I'll tell you how we build them.

If you want a really nice street engine that starts every time, pulls hard, looks correct, works with your existing fuel pump & lines and gets incredible fuel mileage (for a 911), then use the CIS system you already own and spend the money you save on a driving school.

[Henry Schmidt]

Originally Posted by Josh D

I would really like to know more about the Gurney flap on the metering plate. A picture of one would really be great. After reading about the Gurney flap, I understand the theory, but need help with the visual application on the metering plate.

Also interested in the cleaned up CIS. I've already removed the decel valve with no detriment to performance or normal operation. I'm curious as to cold start function without the auxilary air regulator and auxilary air valve. Would you need to backdate to the cold start hand throttle?

The lip goes on the top side of the metering plate about 25% of the way around.

It only needs to be about 1/8 " (3mm) and when installed will be positioned directly opposite of the fuel distributor. The lip catches the air spilling over the plate causing a more consistent load. Depending on how your system is tuned (control pressure vs system pressure) the cruise mixture will be richer.

It works just like a Gurney Flap/lip on a wing to increase down-force as the air accelerates. Down-force in this case equals increased fuel delivery.

[timmy2]

Curious about the cold start with the modified CIS as well?

[Henry Schmidt]

Originally Posted by timmy2

Curious about the cold start with the modified CIS as well?

It goes pretty much like carburetors. Hand throttle works best for cold idle speed but unlike Weber carburetors, CIS injection has a warm-up regulator that works like a temperature activated choke.
If installing a hand throttle offer too much challenge, there is an electric solenoid (ac fast idle) that can be added and operated as needed from a switch on the dash.

[Dpmulvan]

Sounds like I need to read up on the bosch system before I can understand the modifications

[timmy2]

Ok, thanks Henry, it is as I thought one would need for the extra air enrichment needed to replace the AAR.
'73.5 to '75 style hand throttle or fast idle solenoid with switch.
I know all about the WUR.

[Josh D]

Originally Posted by Henry Schmidt

The lip goes on the top side of the metering plate about 25% of the way around.

It only needs to be about 1/8 " (3mm) and when installed will be positioned directly opposite of the fuel distributor. The lip catches the air spilling over the plate causing a more consistent load. Depending on how your system is tuned (control pressure vs system pressure) the cruise mixture will be richer.

It works just like a Gurney Flap/lip on a wing to increase down-force as the air accelerates. Down-force in this case equals increased fuel delivery.

Thanks for the explanation. I can now visualize it in my head.

[mradovan]

Here is a link to a thread I started about optimizing AFR for a modified 3.0 CIS car:
Rennsport Systems Community • View topic - Optimal AFR for 1980 911SC

Here you will find quantitative info on my attempts to tune the CIS system with an after market wide band O2 sensor and a modified warm up regulator which allows me to adjust control pressure.

I just did a complete rebuild of the engine and I changed the following:

964 Cam profile on original cams
SSI with Dansk exhaust
re-curved distributor
modified fuel pressure regulator
No Decel valve

I thought long and hard about other changes but concluded that it made more sense to do a 3.6 conversion if I wanted to take a big step in total power output. The car is noticably quicker with much improved throttle response.