On my Hilborn that I converted to EFI, there are 8 vacuum lines, one connected to each intake runner (see picture). Previously the machinest drilled a small (1/8") hole between thr runners to balance the vacuum signal (See picture). My question is will these holes reduce the vacuum signal or cause it to be erratic. I need the vacuum to us the MAP sensor in the ECU. Thanks for your input. Bruce B
I know absolutely nothing about this subject, but isn't the plenum you built under the manifold supposed to average out the the manifold pressure signal? Even if that hole was large enough to be effective, it would then only balance the adjoining runner. You'd still be doing the majority of your pressure averaging in your plenum. Some of the homebuilt intake setups from the '50s had one big rectangular port that covered both adjacent ports and didn't even have a divider between them. They seemed to work just fine. I don't think what you have will do any harm- or any good. The hole just isn't big enough to equalize the pressures between the ports. You are tring to average a vacuum SIGNAL with your plenum so the MAP sensor gets a fair reading, not equalize the pressures between the ports which that hole would do (if it were large enough). Another comment while I'm blathering- it may not matter, but I think the vacuum lines would be better if they were short and went directly to the plenum. You want the pressure averaging to be done in the plenum, but there may be some air column (resonance) effects in those long tubes. I'd like to do exactly what you are doing some day, so I am following this thread with great interest!
C322348, The new small manifold under the intake has solved my problems concerning a good clean vacuum signal. I did plug the holes between the runners and the injection now works great. I'm going to be removing the Hilborn from my 62 Corvette and putting it on a 57 Corvette g***er I am now building.
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