Fighting vacuum on throttle opening / Mustang TB

sberry · Post by **sberry** » Thu Jun 15, 2006 4:49 am

Hi everyone. Longtime squirter - been a while since I've posted.

Question:

I've bolted a supercharger on to my volvo B20 and I'm using megasquirt to fuel it. It's working great. This is a general question about the throttlebody setup I'm using.

With the supercharger I used a 55mm OEM throttlebody from a 90's ford mustang with the IAC housing flange blocked off. Pictured here:
http://pics.1800philes.com/supercharger ... C_0004.JPG

Since the ford idle controller stuff doesn't fit (I tried!) I just use the gas pedal to do a fast idle for the first 20 seconds and then the motor warms up and runs fine. I adjust the motor's idle by cracking the throttle open a little bit with the adjustment stop.

When the engine is off everything works great - no binding of the throttle cable, spring tension is perfect - pedal feels really good. When I'm driving , however, I am always fighting to modulate the initial throttle opening. As I go to depress the pedal I am initially fighting the engine vacuum to crack the throttle plate - and then it reaches a tipping point where it opens quickly and easily. The net result is a jerky throttle that makes smooth street really difficult.

I'm scratching my head as to how to smooth this out. The OEM setup was a similar diameter throttle plate and this was never an issue. Anyone run into this before? I feel like I am overlooking something obvious.

steve berry
http://1800philes.com

greggearhead · Post by **greggearhead** » Thu Jun 15, 2006 7:39 pm

I have run into similar situations, and a stronger return spring usually did the trick. You might look at the combination of a stronger spring and a higher leverage pull or a cammed initial pull like the old 944 Porsches, etc.

shnitzel · Post by **shnitzel** » Fri Jun 16, 2006 7:07 am

i had this problem with my turbo pressurized weber... under high boost the airflow would hold my throttle blades open... very exciting!!!!

i just stuck a huge spring on it and problem fixed.

sberry · Post by **sberry** » Fri Jun 16, 2006 10:28 am

I'll try the spring. I was hyperfocused on trying to figure out how to make the opening easier - not how make the rest of the travel equally difficult. I think an additional return spring will help or solve it.

Pays to ask questions. I'll post back here if it works out.

Thanks!

steve berry
http://1800philes.com

keithmac · Post by **keithmac** » Fri Jun 16, 2006 12:24 pm

Is it a Lysholm Twin Screw charger? A lot of VW boys run the TB before the charger (acts as a silencer) and I`ve never heard of any problems on throttle opening?

sberry · Post by **sberry** » Sat Jun 17, 2006 12:16 pm

Yup, it's an autorotor. Details of kit here: http://v-performance.com/about.html

It made my car go a lot faster. Tried it out at autocross today - did worse in the pack than I normally do(!). It's all about smooth transitions - a jerky throttle doesn't sent me all over the course. Trying a stiffere return spring setup tomorrow.

steve

TPI 85 Blazer · Post by **TPI 85 Blazer** » Sat Jun 24, 2006 9:19 am

Could there be some play in the bushings that allows the throttle blade to contact the throttle body under vacuum? I suppose the position of the pivot point determines whether the engine tends to suck the throttle open or suck it shut.

PSIG · Post by **PSIG** » Sat Jun 24, 2006 11:08 am

It is likely mechanical geometry or TPI 85 Blazer's binding idea. First - is the original throttle return spring intact and in the correct preloaded position?

You can check for throttle linkage geometry problems by directly operating the throttle body by hand while idling. If it operates as easily as when not running it's a throttle linkage geometry or binding issue. If it's harder to operate directly by hand at idle with the linkage disconnected, then I would consider the internal binding as TPI 85 Blazer suggested.

You may be able to identify internal binding by pressing the throttle shaft (not blade) firmly inward (simulating a max vacuum load on the plate of around 20kg/45lb) to see and feel any changes.

HTH,
David

sberry · Post by **sberry** » Sat Jun 24, 2006 12:49 pm

The problem was not a problem with the linkage or throttle itself. Rigging up a return spring largely alleviated the 'on/off' action and made the car 100% more driveable although the pedal is stiffer than I would like. Will try changing leverage on the throttle but I'm not sure how to go approach it.

Car Dyno'd at 155whp today with a big usable 151 lbs of toque on 10psi. Pretty stoked.
http://1800philes.com/supercharged_volvo.html