65mm Throttle body [Archive]

Raven855

03-25-2013, 07:15 AM

You will find at least 2 schools of thought here. You will find several that will tell you that the SVO TB is not a problem at the power levels you will most likely experience. My feeling is that the TB is not a SIGNIFICANT detractor of HP, but some. I have seen dyno results that show a few hp differences. Not allot, but some. I also follow Fords way of thinking. On a 200HP mustang they put a 60mm TB. On a 265-300hp mustang they put an 65mm and so on. Is it worth the effort? That is up to you to decide, but chances are you will not realize much without some way to tune it.

MikeFleming

03-25-2013, 08:56 AM

A few thoughts.

1. The ratio of throttle pedal movement versus airflow changes dramatically.

For example a 65mm throttle opened ten percent flows MUCH more air than a 50mm opened ten percent. Same for 20, 30, 40 % openings. However once the 65mm gets to say 60 or 70%, the throttle is no longer the limiting factor in airflow - it's the engine, ports, valves, turbo, exhaust, etc.

2. The EEC "load" ranges are managed by the TPS values, NOT buy the throttle body opening area. So the difference between PT and WOT values should be updated accordingly in the EEC tune. See #1 above.

3. Back in the IMSA days a 54mm restrictor was good to 650HP in the race engines. So considering the restriction of the throttle shaft in a 50mm, that is still good to at least 500HP.

4. The biggest change you will notice is how the throttle pedal appears to need to be moved only a wee bit to start off or increase power. Makes turbo engines much easier to drive as you don't need to mash it all the way down at every start-off until boost comes in. An old trick used by GM - putting MUCH lighter throttle return springs on their underpowered cars - makes it "appear" more powerful with just a little pedal pressure.

If you have a low-RPM cam and a small turbo this will make hte car more difficult to drive. if you have a high-RPM cam and a large turbo, it will make it easier.

5. You will need to modify the upper intake casting to ~65mm to not have a restriction there - usually by grinding/porting. Some of them will require welding as the material is thin in some areas (on the top, iirc).

6. You may have a difficult time getting the engine down to a low enough idle speed as the amount of air passing the throttle is much more on the 65 than the 50.

Also note the throttle plate position is NOT used to set idle speed. There is a separate allen-head air bypass screw setting for that functiton (usually under the TB body).