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yes the multi valves breathe much better due to larger valve area but you're still fighting the battle of getting X airflow in at Y RPM. you don't need a 290 degree cam like in a pushrod engine but the 944S desperately needs more duration to make any more power.
side note on lift, rule of thumb is there isn't a whole lot to be gained (diminishing returns) once valve lift = 1/4 of valve diameter, since the "curtain area" at that lift is equivalent to the diameter of the valve itself.
like hooking a 6" pipe to a 2" pipe and expecting the 2" pipe to suddenly flow more...
944S is already over that 1/4 rule at stock 11mm lift and 37mm valves.
Maintaining a 7000 redline with HP peak at 6400 RPM or something close to that is preferable to me, I'm not going competitive racing with this. I do not want to go to the expense of solid lifters. Venturi size will move peak HP up and down rev range but a cam profile that compliments the peak HP RPM is my goal. I have no need to toss the OEM headers since they are plenty good enough for my meager tuning goal. I expect a "Street Performance" cam is what this would be called and a camshaft provider should be able to generate a suitable cam grind to suit or have a profile already available.
Plan for intake manifolds is individual radiused tubes (exhaust header bends) and shaped to match oblong ports in heads, just long enough to put carbs horizontal. They will therefore be rather short, probably a 6 inch total path length length from base of carb to port on the head plus another 3 inches to waist of main venturi. From my sizing formulas I see 53mm throttle bore with 42mm venturis.
Air cleaner will be remotely located and feed an aluminum plenum around the intake air horns. Deleted power brakes (dual master cylinders), A/C, power steering and pop-up headlights plus low-mounted alternator make room for air cleaner and plenum once OEM intake plenum and air flow contraption are tossed.
Here is a copy of a Swedish Weber handbook
My experience is to go big on carbs bore, and small on venturis. To make the car more drive able in low revs. DCO 55, Vent. 40mm'ish
Dose anyone have experience/dynos on different cams?
Interesting project. One note though, you still need to retain almost all of the factory EFI hardware to retain spark control. However since the 944S has a factory 60-2 trigger wheel, you can cheaply move to a stand-alone spark control ECU.
looking at a dyno graph i'd say the 944S hits 100% or higher VE right around 4000 rpm where the torque peaks and the intake runners tune in...
T, you gonna run some real headers?
OP says "stock headers" in his project, so don't want to venture into that too deep, but yes, up until now, with this latest 16V project, we always run the stock header setup with 2 x enlarged diameter downstream past the 2/1 merge to help with clearing the cylinders. Going with custom modelled header this time though, too much left on table with stock when all your components change.
Look at the closure of the exhaust valve on stock cams.
The opening of intake too.
Both events are dead at TDC.
Momentum of exhaust leaving the cylinder via still open exhaust valve causes a negative pressure that not only cleans residual gases that displace fresh air/fuel, but this supercharges the incoming cycle as it's "pulled" in.
Will be back to comment later if it stays on OP's intended thread purpose.
To the OP, I once set all this up, but with throttle bodies from a Suzuki Hyabusa 1300 cc.
Was also on the 8V head.
I have pics of the stock/cut manifold, throttle bodies mocked up, spacing etc. and the mandrel bent alu tubing I bought to create the project.
Being as you won't have much length, and with carbs you may not need at all, but I had to create a chamber that mimicked a virtual manifold for MAP.
Still have all the parts if anyone is interested in picking it up where I went a different direction.
I love the idea of the carbs, hope you go full 100% with it.
Yes to 55 DCO Webers and 40 or 42 mm venturis if purchased but I have a design for throttle bodies of my fabrication using Weber 55 DCO auxiliary venturis and using spill tubes for fuel supply to eliminate complication of designing/fabricating float bowls.
What "stand-alone spark control ECU" is recommended? I'm a simple, analog guy and laptop tuning spark advance is a bit daunting.
This is a planning stage for me and actual build is some time away.
Wow Spenser, that's induction ****. I still love carburetors for their simplicity, atmospheric pressure always works Four Harley Davidson CV (constant velocity) carburetors have always been kicking around in the back of my head for my S2. The HD CVs provide great throttle response at low rpm and are easy to jet for use over a wide range of temperature. Mid sixties Hot Rod Magazine installed 4 Honda motorcycle carburetors on a Ford Mustang's 170 cubic inch inline 6 cylinder. Spigots to mount the Honda units were brazed onto the factory log style manifold. Getting them synchronized (all those cables) was a bit of a challenge but some horsepower was gained. Those old Honda carburetors left a lot to be desired, I had a 305 Scrambler and could kill the engine by snapping the throttle wide open at low RPM. It was jetted for the straight pipes and would blow blue flames at night. The CV's slide being engine vacuum controlled only opens as much as the engine can take under the operating conditions. You can play with slide weights to customize throttle response. YMMV
03-04-2019, 07:31 PM
