4 cylinder vacuum spikes

[mark allison]

Guy
I've been researching carburetors and intake manifolds for 4 cylinder motors in a circle track race series. Rules mandate the use of a Holley 5200 2V carburetor. We've had some difficulty acheiving ideal fuel mixture throughout the rpm range. One of the Holley books mentions the common occurence of vacuum spikes in 4 cylinder manifolds. It just so happens we've observed this in both a Toyota 22r and a GM 2.5 engine. Under power, vacuum will be approximately 15", then at a certain rpm it will suddenly jump to over 20". I would expect the vacuum curve to linear. Is this due to pulses in the intake?

[Guy Croft]

Most likely Mark, yes.

G