Throttle body porting

[Randy Blaylock]

Curious if anyone has any information and/or experience with throttle body porting. My Marelli throttle bodies have a venturi shape on the intake side that necks down smaller than the butterfly plate, and the exit side has a rather abrupt bevel shape that tapers to 52mm. The opening on my intake manifold is 66mm.

I'm curious if there is a critical rationale for the shape or contour of the interior of the throttle body that would make match porting the throttle body a bad idea, PCM issues etc.... This is for a 100% race car, so small/part throttle opening smoothness isn't really an issue.

 

[Torquemonster]

Some say 70mm TB's make no more power - but I've seen 2 x 100mm TB's work to the tune of 802hp at 5600rpm.

The trick is increasing cfm everywhere. If the stock TB's flow as much as the intake, heads, cam require - bigger ones will not help. But if you run bigger cams, heads etc - then bigger TB's and a custom intake can make a huge difference.

 

[Fred(DrivingSchool)Kinder]

CDI built a motor with 600 with cats. Stock throttle bodies were evaluated with larger ones while on CDI's engine dyno and larger T-bodies resulted with a 1 hp loss. I consider that no change as the dyno likely can not repeat within 1 hp at 600.

Heads were ported and polished, intake extrude honed, more aggressive cam.

 

[Torquemonster]

that CDI 600 is interesting. I suspect the intake is the blockage.

no way stock TB's will flow 800 NA hp, but it makes a point for 95% of hot street motors.

 

[Randy Blaylock]

Appreciate the reference points. Understand the necessity of tuning the system as a whole. My car was once a Speedvision World Challenge race car, and I think that there were intake restrictors installed. This is an assumption based on the smallish 52mm I.D. of the rings, and the fact that they were pressed into the exit of the throttle body. Even after removing the ring, the I.D. of the throttle body exit is still slightly smaller than the intake manifold opening. The airbox is a custom fabricated aluminum piece that houses twin conical filters and seals to the hood at the intake naca opening, that is also fed air through some segregated channels from the fascia opening. I only acquired the car this summer, so I'm still discovering things, and going down the list collecting data. I pulled the pan last week to have a look at things and was pleased to see some nice Oliver rods and forged pistons along with a well thought out trap door system for the pan. Considering the fact that there may in fact have been intake restrictors installed, and that the World Challenge rules at the time specified a O.E. cam grind, I'm kind of curious what cam is currently installed. There is limited documentation for the car, although the engine is said to have been built/serviced by Caldwell at the time. FWIW, the car made 480rwhp/520rwtq on the last dyno run. I'm curious to see if there is any benefit to removing the throttle body restriction, I should be ready for a dyno run in a few days after prepping for an upcoming race and changing gears.

 

[Randy Blaylock]

Just got back from the dyno, looks like opening up the throttle bodies was a good idea. The last run netted 480rwhp/520rwtq, and today it put down 502rwhp/531rwtq.

 

[mad0953]

Do the Marelli TB's come stock on the Viper motor?

 

[Randy Blaylock]

Heck if I know. I would be inclined to think they were at some point given the fact that the linkage and what not looks O.E..

 

[Fred(DrivingSchool)Kinder]

The intake was extrude honed. On the engine dyno with the GTSR firing control the motor made 650hp. In the car with cats it dropped to 600hp. The larger throttle bodies were evaluated on the engine dyno by CDI. With more tuning the motor may make a little more hp but the throttle bodies are not the limiting factor.