Manifold Vacuum

[rdhunt1]

Does anyone have any manifold vacuum data for a 1700 supersprint crossflow on 40's?  Maybe this isn't even relative since choke size and cam choice will likely put these all over the map.  BUT...

 

With the ever-decreasing temps up here in wisconsin, I decided to try some larger idle jets.  After installing i went about resetting idle mixture and checking balance on the carbs for good measure.  I got pretty darned close with my old unisyn and using my timing light as an engine bay tach.  Feeling like being overly anal I went and borrowed the four gauges you see here.   This first pic is at 1000rpm with balance by the unisyn, and tuning idle mixture by rpm.

 

 

So then i set about playing with the gauges installed.  And although there is some distortion due to the angle the picture was taken, I ended up with pretty identical.

 

 

After all this i wondered if these numbers 30cm or 300mmhg were considered correct, logical, or typical for a crossflow. Vacuum was taken from the small plug right next to the idle mixture and progression hole cap.

Anybody have input?

 

Thanks

Ryan

 

 

[jbcollier]

Looks good to me.  I usually only use vacuum gauges on motorcycles as there is no easy way to get an airflow gauge on them.  Frankly, the first readings were spot on as well.  Small variations like that usually only require a very slight tweak of the adjustment screw and are not particularly significant.

 

Before people ask/comment, motorcycle vacuum gauges are dampened to eliminate/reduce needle bounce.  They give a "steady" reading, not a pinpoint accurate one.

[rdhunt1]

Jb, 

Yessir... correct on the dampening action.   The small collar right before the hose leading to the carb allows damping of the intake pulse.  Played with it a bit, but eventually brought it down to about a needles width of movement and continued tuning from there. 

 

Prompting my original question was seeing some vac gauges that have colored areas that indicate good, bad, etc.  More particularly, one gauge had an area indicated for late ignition timing.  Knowing I have my lucas/aldon with a whopping 12 degrees of advance,  turned up to about 22 degrees base for an all in around 34 and wondering how much of an effect it had on my readings. 

[MV8]

If it is one cylinder reading a bit lower steady vac, it could be that cylinder is not pumping as efficiently as the others. Timing would be suspect if it were a vac reading from a common manifold and not individual manifolds with the same ignition setting across them all. For those without a chart, see attached.

[speedwagon]

MV8; Thank you, have printed it, and will tape it to the hoist. Sometimes with all the modern troubleshooting tools we forget that the "old school" vacuum gauge can still give us a place to start, if not the answer.     "it's engine, not motor, and in fact really more a pump"  john

[jbcollier]

That chart is meant for interpreting a common-manifold reading for a 6 or 8 cylinder engine using a non-dampened gauge.  Multi-cylinder engines with 6 or more cylinders, and common manifolds, give steady readings compared to the same with four or fewer cylinders.  Vacuum fluctuations/flickering is perfectly normal if you are using a non-dampened gauge on an engine with isolated, individual intake manifolds and separate carbs/throttle bodies.  Non-dampened gauges give an accurate reading at the expense of readability.  That's why motorcycle gauge sets are dampened so it makes setting up the carbs/throttle bodies quicker and easier.

[MV8]

For those who don't know, a "dampened" gauge just means there is an adjustable restrictor in the line (like an aquarium valve) to reduce the needle swing. Smaller displacements, fewer cylinders, and cam specs all have an effect on the actual numbers but the nature of the chart is still useful. With separate runners, the lines could be tee'd (i.e. manifold) to a single gauge for an overall indication without restriction/damping instead of four "thumpers".

[rdhunt1]

Love the chart!  Common manifold or not the different positions and respective causes is great info to have.  Thank you!