I was reading the other day about premix and someone said that after adding MMO (marvel mystery oil) to their gas in a 4oz/10gal mix actually raised their vac in/hg back to 19, which got my little gears going. I haven't ever noticed anything wrong, or lack or power, but I'm only running 16 in/hg at idle, while cruising at 2700rpm or so I have roughly 19-21 in/hg. Just wondering if I'm in an acceptable range or I need to be concerned?

 

Most people talk about idle vacuum. This is vacuum at 750-850 RPM idle.

16" is decent. The higher the number, the tighter the engine. Saw a car with 20" at idle, that's mighty tight.

Your motor may not be a new spring chicken, but there's nothing wrong with it.

Dale

 

Thank you sir. It has roughly 40K miles on the new motor, so I wasn't expecting the top notch numbers, but to know I'm in the right range, allows me to breathe a huge sigh of relief.

 

can you do a math for me ? 1150-1250 idle -430mmhg thats at night time with 40~60f

day time 55-70 1000-1200 at -440-470mmhg..

mild street port

 

19 is normal. Mine pulls a solid 19-20, even though I don't have a perfect idle lol.

But when the idle does find its place, my vacuum is 18. But normally my idle is 900-1000. Then I see 19.

 

I just build mine and its pulling 11" at 950rpm

 

11" is pretty low. hopefully you have a big streetport or your seals are still embedding

 

Yeah it is a large streetport, I'm also 6000 above sea level.

 

Dale is right, it rather annoys me when people say my idle vacuum is 19"

later you find out that their idle is 1200RPMs.

vacuum will increase as RPMs increase and dynamic sealing characteristics come into play.

idle vacuum will decrease the higher you go above seal level, but 11" still sounds pretty low, unless it is a bridge port..


there really is no target number aside from anything above 15" with the engine idling properly below 1000 RPMs, ignition timing will also have an effect on how much vacuum it pulls. the more radical the porting is, the lower the vacuum will be as well.

so with all that, vacuum doesn't necessarily mean a whole lot it just might give a hint that the internals are worn or not.

 

All my seals are new, housings had less than 10k on them. Starts right up when warm. I had an rx8 that was pulling 15

 

wasn't meaning to say it had an issue, just that vacuum readings can vary quite a bit and aren't always relevant to an issue being present. just knowing that there are a number of things that can equate to differing figures.

 

Porting is a big change for idle vacuum. On stock ports and 41k on the clock my 7 pulls 18" at 750 rpm.

 

430 is about 16.9
470 is about 18.9

I have a medium port and pull about 470 cold however it was correctly stated that there are quite a few of affecting factors (timing,afr, throttle body setup, exhaust system, etc, etc)

 

there are 25.4 millimeters in an inch...

vacuum is directly related to engine sealing but port timing, throttle valve opening angle, and ignition timing affect it a lot.

Just a little FYI... A lot of new gasoline engines make almost 0 vacuum under normal driving due to using other methods of controlling torque output besides the throttle valve. Valve timing and lift can control engine output, and so can fuel quantity (like a diesel) depending on the design of the engine. Any upcoming rotary engine may very well make 0 vacuum most of the time.

 

when they won't start, sure.

 

Research how to steam clean you're engine and post before and after results if you are up to it. I think that would be interesting but perhaps someone already knows and can tell us.