Throttle Plate Hole

[Mayolives]

Why do some throttle bodies have a small hole drilled in the throttle plate and others do not? My newly installed RB type throttle body does not have the hole and the engine doesn't start well. It runs great but starts very hard, cold or hot. The new throttle body replaced one from a BMW and it had the hole in it's plate.

[BRAAP]

Here is the basic theory on that hole. As for it how it applies directly to your application, I only hope this also applies helps… Carburetors also have hat hole as well, for similar reasons.

 

Allows the butterfly to be mostly closed while still allowing air to flow past the butterfly to maintain idle speed.

Ideal scenario as In understand it;

1) Butterfly closed almost to the point it is contact with bore of the Throttle body itself.

2) Idle speed control motor in the middle of it travel at idle, (not sure if that is at warm of cold operation, most likely somewhere in between).

3) Most TB have “ported” vacuum ports that are a set distance in front of the butterfly, “timed”, so as to engage at specific throttle settings/engine load demands for what ever reason or those ports are controlling.

4) TPS is clocked for the desired voltage feedback to the ECU indicating OE specified “closed” throttle.

 

In meeting these goals, if the engine has more radical cam/s, i.e. more valve overlap such as high performance N/A applications, there may be a small hole in the butterfly to allow the extra air required for the engine to idle to get into the engine during idle without having the idle speed motor now operating that much closer to its end of travel or adjusting the butterfly itself “more open” at the closed position thereby altering the point at which the timed vacuum ports kick in, TPS values etc. Granted, most TPS are adjustable for such scenarios, but the timed vacuum ports in the TB are not adjustable, at least easily any how,

 

Not sure if this applies to your specific application but should at least help in explaining why those holes are sometimes there.

 

This picture shows how close the “ported” vacuum ports are to the closed butterfly. EFI Throttle bodies will have ported vacuum ports as well. Some L-28’s have 2 and I recall there being 3 differently placed ported vacuum ports on of those.

 

Courtesy users.sfo.com

[Mayolives]

BRAAP......Good information. After reading your explanation of the hole in the plate and the ports, I doubt my starting problem is caused by the lack of a hole. The engine idles good after it finally starts so apparently it is getting the needed air through the ports. I'll check the adjustment on the TPS with a volt meter and hopefully therein the problem will lie. I suspect it isn't sending the proper signal to the Electromotive brain.

[RB26powered74zcar]

Were you able to get your motor to start and idle right away, using the previous TB? I can not just start mine up with out using the pedal to keep the idle up till it runs for a minute or so. I have the Q45 90mm TB.

 

Reason I ask is we are both using the Tec3r. I'm assuming you don't use a IAC on you manifold, correct?

[Mayolives]

Yes, I am using the IAC. It's installed at the rear of the intake manifold. Your question opens up another trend of thought for me. According to the Tech3r manual, the IAC allows air into the intake, even with the throttle plate closed. So having no hole in the throttle plate should not effect the engine's starting.

[HowlerMonkey]

The hole is there because throttle bodies and plates have a habit of building up deposits between them.

 

If your car uses very slight throttle body opening rather than the hole, then the inevitable buildup will lower your idle over time by closing the gap between body and plate.

 

On OBDII cars, I check out the percentage of idle control in the data list to determine of this has happened so I already know what I will find when I take off the piping to the throttle body.

 

If the percentage is high, I know it is gummed up though I always do a visual before selling the throttle body service.