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Green 912 Green 912 is offline
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Join Date: Mar 2001
Location: Falls church Va
Posts: 725
The deal is they have been tested and refined over many years and they work when used correctly.

Exhaust setups are extremely important on non-injected non-computer controlled cars. Your anecdotal experience with one setup was not a good one. Why? Perhaps you had the incorrect cam timing for the header and the setup was causing you to run lean. Lean carbs will fart and pop. No surprise that you didn’t see any improvement if the rest of the motor wasn’t in “tune” with the different exhaust pulse phasing. When the exhaust valve is starting to open the piston has not even come near the bottom of the power stroke. High energy in the cylinder blasts by the opening exhaust valve and creates a high-pressure wave and the high-pressure wave will, after a certain time, create a low pressure. As the piston ends the power stroke and starts back up it pushes the rest of the remaining exhaust gas out. At the same time the intake valve is starting to open well before the exhaust valve closes. If the intake charge velocity (in this case velocity equals pressure due to the mass and momentum of the air/fuel charge) is not high enough the exhaust will be pushed past both valves. This will even further decrease the flow rate through the carb ventures and really lean things out. The reason we want to create this pressure wave that seems to cause problems is when the timing of all 3 is right the high pressure pulse passes a set point and creates a partial vacuum during intake and helps pull MORE air/fuel mixture into the cylinder during the scavenge phase. The valve timing (cam) and exhaust pulse timing (header) set everything.

Many things go into the planning of a performance enhancement to a 356/912 motor. Size and location (RPM) of the power band, planned use of the motor, weight of the car, gear ratios and the list goes on.

I can tell in a big way when my motor is “up on the cam”. The carbs, cam and exhaust are all working in phase, the power is ON, and the motor pulls. If any of the 3 is not in phase then the motor isn’t producing power as well as its design limits will allow. At one track there are noise limits and my “J” pipe exceeds them. I have to use an insert to reduce the noise and the damm thing kills the power band for anything past 6300 RPM. Change something and you effect all to varying degree. Example. Pop in a “hot” high lift 310 deg duration cam, racers use em’ and get lots of power so why not. You just killed your idle and without the correct carb, intake and exhaust setup the motor will not produce much more power. In fact unless you are in the now much tighter and higher power band you will probably make LESS. When you stray from stock it is important to plan what your goals for the motor are and only then modify accordingly.

For lots of good info explained in an easy to read format get “How to make an old Porsche fly” by Craig Richter. It is out of print but can still be found and is well worth the search if you plan to build a 356/912 motor outside the realm of stock.
Old 03-09-2002, 09:05 PM
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