I don't know what the stock 944 filter and airbox are like, but it could be that a K&N made more power on there. The stock 928 filter is big with a lot of pleats, giving it a large surface area. You may have seen those K&N filter demonstration units that they have in some stores. The paper filter they use for comparison to the K&N is very small and has few pleats. I doubt that they chose that particular paper filter for the comparison by coincidence.

 

Okay, I placed a brand new Mahle air filter on there and I don't feel the difference at WOT. I do know that the shifts between gears are a lot smoother. I have an automatic.

Andrew
84 928S

 

Mike Schmitt wrote: [quote]...I know that there's some literature from Porsche that says the venturis do not cause any loss. Look at the date that information was written. That was from the years of the early cars, when even the Euro cars were making only 229hp...[/QB]<hr></blockquote>

Just a side note on that: the 1980 Euro S produces 300 HP. The S4, produced in 1987 is 317 HP - and was using "venturied" air intake tubes.

For now, for me, Louis Ott's answer will do: venturi or not won't make any difference 'cept in sound... insulation will make a difference, maybe 1% at speeds above 20 MPH, will be most pronounced at speeds of 60 MPH and above... but still, only 1 or 2%.

 

I didn't state some things too well before. The intake tubes were designed when the cars were making less power than they did closer towards the end of the 928's production. The venturi intake tubes will flow less air than a straight, smooth, intake tube. On at least most of the cars, the venturi tubes will not reduce the power produced though, because the engine can't take in more air than those tubes provide anyway. As the power is increased, the venturi intake tubes get closer and closer to becoming a restriction. Porsche did switch to straight tubes on the GTS. Louie did see some power increase on the dyno using straight tubes compared to the stock ones, but it wasn't anything significant. I don't know if he had modified his airbox at that time yet or not though, so the restriction of the stock airbox may have been a factor. If the airbox was a significant restriction, any potentially greater airflow that the straight tubes might have allowed wouldn't have showed up then. The straight tubes that he used weren't that smooth inside either, so that could also have made a difference. He does have straight tubes on his car now.

The insulation on Louie's intake tubes, airbox, and radiator top did make a difference in the airbox temperatures. With the insulation, the temperatures were 10-12 degrees lower than without it, at each of the speeds tested at, and also for each of the time periods that the car was idled. The real jump in airbox temperatures over ambient temperature occurred while idling or going below about 20 mph though. The temperature difference between 20 mph and 65 mph was less than 5 degrees. That difference of less than 5 degrees between those driving speeds was the same with or without the insulation. The insulated temperatures were all lower, but the amount of temperature difference at the different driving speeds remained the same. So if you were driving 65 mph with the insulation, the airbox temperature would only be 15 degrees lower than if you were driving 20 mph without the insulation. The biggest power killer comes after you've been idling for a few minutes without the insulation. The airbox temperature goes to 50-60 degrees above ambient temperature. The insulation brings that down to about 40-50 degrees above ambient. That's a little better, but that would still be a 4-5% power reduction compared to what you'd have with ambient temperature air getting into the engine.

Driving with the car's radiator air flaps locked in the fully open position showed a decrease of about 10 degrees, compared to having them operating normally. The temperatures Louie measured were all taken with the flaps locked open.