ras62

An old subject I know but given recent threads on choice of camshafts, changes of ECU's etc I am looking at more something a little more mundane. Air filters!

Post engine build and the motor is running great. Dyno run showed 283 rwhp and torque is strong everywhere. As winter approaches I got to thinking about intalling a larger throttle body which in turn led me to look at differences between the 964 and 993 intake systems. The only differences of note are slightly larger inlet ports and a much larger air box. I stumbled across this site https://streetandcircuit.biz/engine-...-filter-small/ and started doing some basic calcs. The dimensions of the standard filter shows as 243 x 249mm to the outside edge but looking at the usable area on the engine side of the filter those dimension actually fall to 200 x 210mm. The already marginal sizing of the standard filter now looks like a restriction when looking for more power. For reference the 993 filter is 240 x 325mm or approximately 25% bigger. The 3.6 996 filter is 50% bigger to support 315 bhp.

It looks like Porsche had this figured out all those years ago hence the increase in airbox size with each new model and rise in power. At a guess I would say the standard 964 airbox will support a little over 300 bhp depending on what filter is used in the standard air box. I will do some filter mods to see if I can drop the differential pressure across the restrictive standard filter and see where that takes me...

cobalt

The heads of the 993 are different than the 964. Different alloy and larger intake port size hence the larger filter size. Many use the cup box or drilled box approach but there isn't much gain to be had. 283rwhp sounds a bit optimistic without significant engine modifications. Possibly 283BHP?? Dyno numbers really don't mean much as a mustang will read less than a dynapac by 16-18% and it all depends on the operator. They are a good tool to tell if minor changes like filters make a positive or negative difference. It is not clear what you are trying to achieve. Increasing the filter size shouldn't gain much as there are other restrictions in the system.

ras62

The intake port is 2mm bigger on the 993 but the inlet stacks and intake runners ID are the same size on both models. Small beer whereas the increase in airfilter size is a whopping 25%. I dont see any other restriction on the intake other than the filter.
To be clear my heads were port matched for 993 inlet runners so are now 43mm. Internally cams are different and the car made 283 bhp/265 ft/lbs at the wheels after mapping on a fresh engine. Freeing up breathing through the filter should improve on those numbers.

cobalt

So you increased both intake and exhaust port size from 41.5 to 43 what did you do with the exhaust? A lot of this has to do with the Vram as well and harmonics on the 993.

We can do the calculations but I would love to see the charts. 335bhp with porting heads and cams otherwise stock 3.6 seems optimistic to me. I am curious what else was done. I would have to assume lightened rods and an increase in RPMS as well?

I am curious what you discover. It would be nice to see before and after charts for each type of filter. I have found others who played with this before and found in some cases a loss and never a gain but good luck.

ras62

I only ported the inlets, the exhaust is ceramic coated so not possible. I used 993 inlet stacks because they were 25% the price of 964 replacements! The biggest mod were the camshafts, the torque increase really has surprised me. The low and midrange pull is superb. It simply drives away from a standard engine at any rpm.
I will probably put her on the dyno again to see how the motor performs with a few thousand miles. At that point I will check the difference for filters but the mods I've done do feel like they have had a benficial effect above 5k.

-nick

Stock injectors cannot support above 305-308hp at the flywheel, and even then they're running continuously. You either have an extremely optimistic dyno or those are flywheel corrected numbers. The filter is a slight restriction on a modified engine at peak rpm. This has all been hashed out plenty of times on here.

ras62

Appologies, the list of mods wasn't meant to be exhaustive, the injectors were also upgraded. Motor runs stock rev limit and no cat.

cobalt

Do you have the old intake stacks/manifolds you could measure. I have the stock 993 and 993 RS stacks but would be interested in the differences between the 964 vs 993.

Stock 993




993RS






There appears to be 0.140" increase between the two 993 units.

ras62

From a previous post...Stack inlet diameter is 39.5mm for both 964 and 993, hence the use of the same rubber connectors. Stack outlet diameter are 39.5mm for 964 and 42.5mm for 993.

Molding of body are similar but not identical. Both the injector and the fuel rail mount are about 4mm higher on the 993. This seems to have been done to move the tip of the injector away from the airflow in the stack.