Chevy 350 intake vacuum waveform

Hello guys,

Blue trace is primary ignition coil in a distributor type and red trace is intake vacuum.

This engine has modified camshatf (a little overlaping according to customer)

Vacuum idle (cold) is about 6.5 in/Hg and (warm) about 9.5 in/Hg.

My question is how modified camshaft could affect vacuum waveform, since I am watching just one valley every two ignition pulses instead of two valleys (all cylinders sucking separatedly).

Any comment would be apreciated.

with a normal aka stock camshaft the lobe separation angle is a bit more separated so the exhaust valve closes before intake valve opens. Now on a modified camshaft aka high performance drag strip cam that separation angle is moved as power is the primary desire (not engine vacuum or idle quality) which is why the drag cars and high compression engines have the lope to it when its started. with this move of the separation angle your power band with the camshaft is moved to the upper rpm ranges or vise versa depending on which way you go.

as for the waveform did you take it with the engine running or just cranking?

Thanks hardtopdr2 for your prompt response.

Capture was taken at idle.

Another picture a took before, was secondary ignition on cylinder #1 (blue) and vacuum (red) at idle (see attached file). I could notice cylinders 1, 4, 6 and 7 are the ones which generate that valleys. On the other hand 8, 3, 5 and 2 match the peaks (which are the ones i would expect to generate another valleys)

Due to not all the valleys or peaks are being generated on the same bank, and also the fact that every peak is followed by a valley (so symetrical waveform)  i would discard a mechanical issue)

I am definitely not familiarized with modified camshaft, but my common sense doesn't explain why i am not seeing the intake job (valleys) for every cylinder (since camshaf overlaping is aplying to all cylinders and intake stroke is 90 degress separated one from another)

Do you have an intake vacuum waveform while cranking?

juergen.scholl.

Just give me a couple of hours and i'll post it.

Thanks in advance.

wrote:

Another picture a took before, was secondary ignition on cylinder #1 (blue) and vacuum (red) at idle (see attached file). I could notice cylinders 1, 4, 6 and 7 are the ones which generate that valleys. On the other hand 8, 3, 5 and 2 match the peaks (which are the ones i would expect to generate another valleys)

Due to not all the valleys or peaks are being generated on the same bank, and also the fact that every peak is followed by a valley (so symetrical waveform)  i would discard a mechanical issue)
 

There's a good chance 1, 4, 6 & 7 are tied together as one half of a dual plane intake. You might have your transducer hooked to a port that only sees one of the intake planes. Or there could be a restriction in one of the planes.

Ok guys.

Here is a picture of vacuum while cranking.

Map voltage (red trace) is 3.9v (which is barometric pressure here (82 kpa)). As you can see there is no vacuum pressure.

Matt T. All readings were taken at throttle body ( 400 hp 4 barrel fi tech) system.

Intake manifold is eldelbrock 2101 (i know nothing about this intake manifold but i'm afraid it doesn't have any test port)

wrote:

Matt T. All readings were taken at throttle body ( 400 hp 4 barrel fi tech) system.

Intake manifold is eldelbrock 2101 (i know nothing about this intake manifold but i'm afraid it doesn't have any test port)
 

It is a dual plane intake. And it sure looks like the vacuum ports on that throttle body could isolated from one plane somewhat. One thing you could try is moving the transducer to another port to see if that shows the other four intake pulls.

 

Matt T mate you are genious!!!

I confirm by watching through throttle body blades that intake has two separated paths. One for cylinders 1,4,6,7 and the other for 8,3,5,2

You saved me lots of hours. That must be the reason for that waveform.