holley 390

Steve W · Post by **Steve W** » Thu Mar 22, 2012 1:03 pm

Hi, I've got a 3.5 rv8 with holley 390. what would happen if you linked up secondary butterflies to open along with the primaries??? instead of with vacuum..

kiwicar · Post by **kiwicar** » Thu Mar 22, 2012 1:23 pm

You would have made a mechanical seconday holley 390!

You would probably want some sort of progressive linkage to open the secondarys after the primarys are about half open.
Not sure if you can get kits for this carb from Holley but I am pretty sure you can for the larger carbs in the range.
Best regards
Mike

Steve W · Post by **Steve W** » Thu Mar 22, 2012 2:30 pm

Just had a look at some youtube video from holley about this very model and they say that you should not try to convert a vacuum secondary to a mechanical one. I just wondered if any has done it and what they've done to compensate for the extra airflow...

Daniel Jones · Post by **Daniel Jones** » Thu Mar 22, 2012 8:31 pm

Vacuum secondary Holleys have an accelerator pump on the primary side
only. Mechanical secondary Holleys have a second accelerator pump (double
pumpers) to inject additional fuel into the secondaries during transient
throttle openings. When you open the throttle plates, fuel lags behind
the airflow causing the mixture to go lean. The extra fuel from the
accelerator pump compensates to keep the mixture in the proper range,
preventing a lean stumble.

The opening rate of a vacuum secondary system is controlled by the diaphragm
spring located in the vacuum secondary diaphragm housing. A lighter spring will
allow the secondary throttle plates to open more quickly. Likewise, a stiffer
spring will delay the secondary opening. Holley makes a spring assortment kit
(P/N 20-13) for tuning purposes. The color-coded springs run from light
tension to heavy:

White - Weakest
Yellow (Short Spring)
Yellow
Purple
Plain (Steel grey)
Brown
Black - Stiffest

If there is a bog or hesitation when the secondaries open, the spring tension
is too light, go to the next heavier spring. To make spring changes easy,
Holley makes an inexpensive quick change kit (P/N 20-59). It consists of a
two-piece secondary diaphragm housing cover which allows spring replacement
without housing disassembly. Holley, also makes an adjustable thumbscrew
operated diaphragm cover (P/N 20-99) that limits how far the secondary throttle
plate can open by limiting the travel of the diaphragm. It makes secondary
throttle opening adjustable. If you are a bracket racer and are using a carb
with vacuum secondaries, you can adjust the throttle plates for changes in
weather and track conditions or for changes to your "dial in".

If you're tuning at a drag strip, pick the secondary diaphragm spring that
gives the quickest 60 foot time (without bog or hesitation). Sometimes, it
requires a lot of fuel to cover up the lean pump shot on the primaries. The
spring will also affect mileage, lighter the spring, the more performance and
lower mileage you will get. When you have the mixture right, the secondary
spring can be very light. If you're changing springs for the first time,
install the stiffest and then try lightest spring to get a feel for the effect.

Note that you can't easily verify secondary operation when the engine is
unloaded because the vacuum level won't drop. Sometimes you can fool the
secondaries by revving the motor and stuffing a rag over primaries forcing the
engine to pull from secondaries. Be careful though, you don't want to suck the
rag down the motor.

Dan Jones

mgbv8 · Post by **mgbv8** » Thu Mar 22, 2012 10:42 pm

Steve W wrote:Just had a look at some youtube video from holley about this very model and they say that you should not try to convert a vacuum secondary to a mechanical one. I just wondered if any has done it and what they've done to compensate for the extra airflow...

There wont be any extra air flow!
The engine will suck only what it wants at any specific rpm. Thats why the secondaries rely on vacuum. If the engine aint sucking the secondaries wont open. If you open the secondaries mechanically on this carb I guess you run the risk of knocking out the inlent manifold vac signal which may reduce the amount of fuel being drawn from the primary side. ie, the engine could lean out heavily.

I've only got experience of my 600 and 850 cfm DP carbs though.

Perry