Myth Busters - Does a Larger Throttle Body Add Power to a 3.2?

[GH85Carrera]

Bottom line: The engineers at Porsche using 1970s and very early 1980s designs and engineering really did not not leave much on the design table for power.

How much does it cost in real dollars to have an enlarged throttle body replacement and maybe gain a couple of horsepower. Like spending thousands and losing the heater to gain a few HP and having an exhaust that annoys the neighbors.

[Steve W]

I think one factor you are skipping here is rise time to atmospheric. Although full throttle power may or may not be similar because at that point the pressure in the intake manifold with either TB is similar, it's the time it takes to go from vacuum to atmospheric through the venturi of the TB. That response time makes all the difference when you're driving at part throttle whether it's at 1500 rpm or 4000 rpm, on the street, or at an autocross when you're trying to throttle steer the car with immediacy and precision. You probably don't notice it on your car because as you mentioned you've replace your system with Motec and my guess is your part throttle maps are not tuned and optimized that well so any such change such as a larger TB is not noticable. I can tell you that on my stock 3.2, and for many owners that put the larger TB on their 3.2s and 964s, the difference is very noticable, just like turning on the sport button in a modern car. I noticed it dramatically at 2000 rpm, the additional response and punch - and no it's not like just stepping down on the gas pedal all the way down.


Theories? Well besides the larger bore, the 1mm step around the circumference of the bore does not help. With the butterfly at low angle openings, the step ridge causes Eddy currents that swirl in a circle around the perimeter of the bore, which effectively disrupts flow and reduces the diameter of the aperture, slowing down the airflow.

[mepstein]

Originally Posted by GH85Carrera

Bottom line: The engineers at Porsche using 1970s and very early 1980s designs and engineering really did not not leave much on the design table for power.

How much does it cost in real dollars to have an enlarged throttle body replacement and maybe gain a couple of horsepower. Like spending thousands and losing the heater to gain a few HP and having an exhaust that annoys the neighbors.

I don’t really buy that. Porsche still had to design for a lot of objectives besides power and performance. While it does cost money to make more power from an engine, we’ve been able to increase the power by 20-50% (depending on use) on a lot of our performance built engines we built. Of course there are trade-offs but the idea that Porsche was getting everything they could out of their production engines isn’t true. They might have been getting what they wanted from them but they for sure left a lot on the table. Even the basic 2.0T with 110hp was built up to 180hp on the S model. The RS took a low hp 2.7 (mid 150’s) to over 200hp. So there was obviously a lot left on the table on their more basic engines, even back then.

[dkirk]

Consider this as continuation of my original post:

http://forums.pelicanparts.com/porsche-911-technical-forum/1138475-oversize-throttle-body-3-2l-engine.html

Please note that I’m still running the original Bosch airflow meter with a custom 911Chips map. Other engine mods have been performed and stated in my original post. First thing I noticed after making this modification was greater part throttle sensitivity. Downshift throttle blips are very much improved. Steady state cruising requires noticeably less throttle opening. Cruise control operation is unchanged. In my case, measuring acceleration and top-end power is just by “feel” as I have no equipment or a dynamometer to determine these parameters. I can certainly report that the modification has in no way hindered the performance, just by feel. The general running quality of the engine throughout the operational envelope is unchanged.

I did examine the analytical effect of this modification by studying how the geometry of the throttle bore increase could benefit the induction airflow at max power conditions. A 3.0 mm increase in the throttle bore diameter results in a 9.75% increase in circular cross-section area, but considering the presence of the throttle shaft, the actual air flow area increase is 10.94%.
Assuming 6800 rpm for a 3.2L 4-stroke engine, a volumetric efficiency of 100% and inlet air temperature of 60 deg F, the original steady state flow velocity through the stock throttle body was .387M (that is, .387 of the local speed of sound). The modified throttle body shows a flow velocity of .349M, or a 9.88% reduction in flow velocity. As increased velocity through a restricted area can result in a flow loss, certainly this modification is going in the right direction. A restrictive air filter or air filter housing could nullify any benefits but in my case, these have been addressed. However, to quantify any improvements in airflow and ultimately greater top-end power, a dynamometer is really the only measurement instrument to use.

So, to conclude, I’m very pleased with this modification and feel it was well worth the money spent.

[Peter M]

Originally Posted by Steve W

I think one factor you are skipping here is rise time to atmospheric. Although full throttle power may or may not be similar because at that point the pressure in the intake manifold with either TB is similar, it's the time it takes to go from vacuum to atmospheric through the venturi of the TB. That response time makes all the difference when you're driving at part throttle whether it's at 1500 rpm or 4000 rpm, on the street, or at an autocross when you're trying to throttle steer the car with immediacy and precision. You probably don't notice it on your car because as you mentioned you've replace your system with Motec and my guess is your part throttle maps are not tuned and optimized that well so any such change such as a larger TB is not noticable. I can tell you that on my stock 3.2, and for many owners that put the larger TB on their 3.2s and 964s, the difference is very noticable, just like turning on the sport button in a modern car. I noticed it dramatically at 2000 rpm, the additional response and punch - and no it's not like just stepping down on the gas pedal all the way down.

Steve,
Thanks for swinging by and making comment. I have certainly appreciated your opinion over the last decade that I have been messing with 3.2's and your acknowledgement that larger TB's don't automatically provide more WOT power.

I agree that a larger TB will change the "feel" of part throttle. However I didn't experience a dramatic change. Maybe that's a result of my already very responsive tune or having an agile right foot in the first place!

I can confirm my "part throttle maps" are competently tuned on a dyno. Actually in my case my load modelling is based on Manifold Absolute Pressure rather than throttle angle so changing TB's does not automatically invalidate my results.