mustang289

Well I have a 69 357W stuffed between the towers of my 65 mustang, it most definatly has alot more bite than that sloppy 289 that was replaced, when i built the engine i went with a cam i knew would be to big, but this is more of a cruiser so i wanted that distinct sound. Unfortunatly a by-product of that sound is vacuum, or a lack there of, i was gettin about 8" of HG at idle, waaaay waaaay to low. Therefore I had to install a electric vacuum pump in order for my vacuum assisted power brakes to work, (as well as a classic mustangs can) but anyway, when i installed the pump i installed it directly into the booster, and just by passed the engine, so the brakes are working soley of the electric pump (the brakes work great now btw) however I am wondering if this could have any adverse affects to engine performance (low vacuum that is) i have a MSD Pro-Billet Distributor with mechanical and vacuum advance, right now the distributor is box stock i haven't changed any springs or bushings (yet) and i havethe vacuum line running from the vacuum advance cylinder to the "timed" vacuum port on my edelbrock 600 (1406 i beleive) I was wondering if this could have any adverse affects or if my mechanical advance can override the vacuum advance, right now i have 17* initial and about 32* total, but its not coming in until like 3200RPMS, a little slow for my taste but thats only becuase i haven't put in on a dyno to tune it yet? So basically my question is: Does having a low engine vacuum create any adverse affects on a mechanical/vacuum advance distributor or can the mechanical advance cancel out and override the vacuum leaving me with my optimal timing curve?

67mustang302

Vacuum advance only occurs if there's vacuum. Duh. [8D]It's mainly for cruising down the highway, where you have some rpm, but minimal throttle and light load, generating some manifoldvacuum. It adds another 5-10* or so to the timing to help burn fuel better and provide more economy. When you open the throttle more the vacuum goes away, and the vac. advance with it, and you're left with straight mechanical timing only. Yeah, you need to get your's tuned, and depending on what rpm/gear you cruise with you may not get much benefit from the vacuum advance, but I imagine that unless you have an absolutely ginormous camshaft, that you'll get some vacuum advance at leastwhile driving. The only real down side to having low engine vacuum, is you have low power and vacuum accessories run poorly(like brakes). That's why big cams make poor low rpm power, the low vacuum is a sign of poor engine efficiency. The worst thing you'll get from that low vacuum is bad gas mileage.

mustang289

I have a T5 with 2.95 first gear with a 3.00:1 R&P

The cam has .284 .296 duration

.544 .541 max lift

at .050 its .244 .241

I have about 8inch of HG at idle

Scott H.

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