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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. |
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. |
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.
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Henry how much would one of those modified CIS systems set me back??
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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? |
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?
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Most of the "clean up" is about removing unnecessary components. |
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. http://forums.pelicanparts.com/uploa...1431797195.jpg http://forums.pelicanparts.com/uploa...1431797213.jpg |
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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. |
Curious about the cold start with the modified CIS as well?
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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. |
Sounds like I need to read up on the bosch system before I can understand the modifications
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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. |
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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. |
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