Engine Vacuum vs RPM?
Even the little load from an automatic transmission engagement is a big chunk of the available power at 600 - 700 rpm, if it will even idle that low. But once its off idle, under light load, vacuum goes up quickly.
MAP is never really zero as I'm sure you know. So logging it through WOT can be useful. Obviously if you're running a two barrel or a restricted class the Vac will increase more noticibly than a big carb where it may not be measurable at all.
On the other hand vacuum can drop at the top end if the combustion isn't dialed in either due to fuel distribution or something else. In other words power is going up but the engine is not pulling as hard so vacuum drops. Interesting, huh! Not exactly what I wanted to hear. But it is what it is. I've made some changes since then so maybe I've fixed it. Won't know 'til I get it back on the dyno or have the logger at the track.
For example here's a pair of chassis dyno runs on primary barrels only.
Roller chassis dyno is providing something like max load when the throttle is max. More load can be added but this is close enough for most purposes.
Vacuum is Dark blue.
Its a full automatic so we have to squeeze the throttle to get it to third without having it downshift.
here's the same engine on all 4 barrels.
Looking at the vac drop around 1:10, it appears that something is causing the engine to perform a little off when the secondaries were opening. Whether its the AFR, which is easily correctable, or something more inherant in the engine build, I still can't say without more testing.
Qkcuda's example is good.
Engines set up for racing using higher overlap cams tend to have lower vacuum at idle.
Even the little load from an automatic transmission engagement is a big chunk of the available power at 600 - 700 rpm, if it will even idle that low. But once its off idle, under light load, vacuum goes up quickly.
Even at WOT, there can be a slight difference in MAP depending on which gear your in. I never trust 1st gear for making decisions on jetting or timing. 3rd under full load is what I prefer at the strip or on a dyno. Dyno lets me see the top, but the strip is real.
MAP is never really zero as I'm sure you know. So logging it through WOT can be useful. Obviously if you're running a two barrel or a restricted class the Vac will increase more noticibly than a big carb where it may not be measurable at all.
On the other hand vacuum can drop at the top end if the combustion isn't dialed in either due to fuel distribution or something else. In other words power is going up but the engine is not pulling as hard so vacuum drops. Interesting, huh! Not exactly what I wanted to hear. But it is what it is. I've made some changes since then so maybe I've fixed it. Won't know 'til I get it back on the dyno or have the logger at the track.
For example here's a pair of chassis dyno runs on primary barrels only.
Roller chassis dyno is providing something like max load when the throttle is max. More load can be added but this is close enough for most purposes.
Vacuum is Dark blue.
Its a full automatic so we have to squeeze the throttle to get it to third without having it downshift.
here's the same engine on all 4 barrels.
Looking at the vac drop around 1:10, it appears that something is causing the engine to perform a little off when the secondaries were opening. Whether its the AFR, which is easily correctable, or something more inherant in the engine build, I still can't say without more testing.
