Author Topic: Holley carb: accelerator pump cam tuning  (Read 1682 times)

Offline English

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Holley carb: accelerator pump cam tuning
« on: November 28, 2015, 11:54:04 AM »
I've finally got round to getting an accelerator pump cam kit.

Any tips on where to start? Google searches bring up "just try them all till you find the one you like best", but I'd rather have a vague idea what I'm doing before trying out eight cams, with two-three positions each...

It's a Moroso kit, and comes with a tuning guide, but I don't know how to apply this to my requirements. GM350 with Comp Cams 12-300-4, in a K10 with a manual gearbox. Carb is a Holley 570. Is there any relation between the torque/hp curves of the cam to the graphs shown on the tuning guide? As in, what's my ideal rate of throttle blade opening to suit the camshaft?


Offline English

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Re: Holley carb: accelerator pump cam tuning
« Reply #1 on: November 28, 2015, 11:58:00 AM »
« Last Edit: November 28, 2015, 12:11:47 PM by English »

Offline rich weyand

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Re: Holley carb: accelerator pump cam tuning
« Reply #2 on: November 28, 2015, 01:50:57 PM »
Hi there, English:

With the 12-300-4, you are pulling a ton of vacuum, probably 18" to 20" of vacuum, which means when you open the throttle plate, air is going to rush in there like the thing was supercharged.

So you need a big shot from the accelerator pump early.  Much richer than the standard curve on a stock carb.

Also, that cam has a ton of torque at low rpm.  Torque, like horsepower, comes from how much air you are moving through the engine at a given rpm.  So you are moving a lot of air at low rpm compared to most engines, so the shot has to be long, until the carb can catch up with the air flow.

I would start with at least two steps up from the stock cam, and set it on the richer of the three positions, and see what you get.
Rich

"Working Girl": 1978 K-10 RCSB 350/TH350/NP203 +2/+3 Tuff Country lift

Offline English

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Re: Holley carb: accelerator pump cam tuning
« Reply #3 on: December 02, 2015, 09:20:31 AM »
D'oh! I totally forgot I had a David Vizard book on Holleys. Says the same thing, high vacuum engines need big pump shots. Thank you!