Help deciding between 750 mech and vac on my 454

[americanmusc1e]

Originally Posted by burnin oil

If you really need the 750CFMs go mechanical. I say this for one giant reason. Vac seconds are rarely fully open and when they are it is at a high RPM. One of the vacuum springs in the tuning kit barely lets the secondaries start to open up until red line. I have a 750 vac on a 400sb and ended up putting a bolt in the secondary linkage to force to be a mech secondary. This is the wrong way to do it and causes a nice lean spot when the secondaries come in. I make more power and drivability went up while milage stayed the same. Funny how that worked out. I need a new carb but that is a $500 I don't have at the moment.

have to done any testing to confirm that putting a bolt in the secondary linkage is actually an improvement?

The vacuum secondaries open on demand from airflow. If they aren't opening then you might not have enough airflow to really need them. Holley's secondary spring kit is more what you need. I'd bet that it's faster with that bolt taken out.

https://www.holley.com/products/fuel...ts/parts/20-13