MSefi.com read only forum (Jan.15,2018)

Your theory of infinitely increasing fueling for an infinitely increasing volume of air is sound, for a jet turbine.

For an internal combusion engine which is naturally aspirated, each revolution will consume a given amount of air at ambient pressure. No more air will be ingested in a given rotation so the only way to increase the air into the engine in a given time period is to increase the revolutions in that time period... what I'm trying to say is that regardless of the TB size, your engine will only gulp as much air as it can.

You can put an 80mm throttle body on a 1-cylinder engine and at wide open throttle, it would be identical to a 40mm one. (You just reach ambient pressure much faster.)

The effects of an oversized throttle body are poor off-idle response and a very large un-useable throttle range. You will reach an effective WOT position at some position well below true 90° throttle plate movement.

For instance, a properly sized TB on that 4.2 at 5000 RPM won't reach ambient pressure untill you reach at or very close to 90° of throttle movement. A much larger (oversized) TB will reach ambient at that same RPM earlier, say 75% open. Also, coming off idle can be tricky as metering the air is more difficult for a given RPM.

-Brian

Unless you have a huge cam and lots of head work, the 62mm will be fine. We are using a 62mm on our racer and it pulls fine up through redline.

We cleaned up the ports in the head. We also did a lot of work on matching the ports between the intake and head. Like you we are running the 1999-later intake on an earlier head. The port match from the factory is horrible.

We do run a spacer, but it didn't make a difference that we could feel. When we get out and do some tuning, I may do some data logging with and without the spacer to see if there are any differences.

The biggest mistake people make with these engines is to install a cam that is way too big. These are only 240 cubes or so, and they only spin up to 5000 rpm, and I see people installing these monster cams trying to gain upper rpm hp.

sort of a thread steal...

i am looking to mega squirt my 88 4.0. doing my reading so appreciate your asking and the answers!

I've been reading a number of discussions about TB size and what 78Spit1500Fed sais is true: in the end there is only so much the engine can gulp. There are also TB size calculators but each engine is different. In the end, the best way to know is to measuremvacuum at full throttle around the red line. There was a value which was not exactly athmospheric pressure but I don't remember it. Does anyone know it?