Holley tuning for performance and the AussieSpeed intake manifolds.

This baseline tuning information goes for both Holley 2 barrel and 4 barrel carbs.

The signal strength at the base of the carbie with an AussieSpeed manifold is a lot stronger than most dyno tuners have experienced before. The main reason for this is that all the 6 cylinder manifolds that have come before us have not been flow bench developed for high flow and high air speed. This is a step by step guide to get the best base line from which to work from to then tweak your tune to get that tyre smokin’ power you desire.

This information is applicable for Holley 350 and 500 two barrel carbs, Holley 4 barrel vac secondaries and double pumper carburettors fitted to an AussisSpeed inlet manifold.

STEP 1 ~ Block off the power valve with a proper blank power valve from Holley, part# 26-36. Please note that this is the most important step. In most cases where people are experiencing tuning problems it will be the power valve adding too much enrichment. This will be highlighted by the fact that the fuel mixtures will look ok on the dyno sniffer sensor but you may be fouling a spark plug typically #6 and have an excessively wet floor on the intake manifold runners.

STEP 2 ~ If you are running a 4 barrel carbie disconnect the secondaries so that they do not open.

STEP 3 ~ Run the engine set the idle, and at idle set the accelerator pump shot so there is no stumble when you flick the throttle to half opening. The pump shot will differ from engine to engine depending on engine vacuum at idle, cam size, carby selection and more. The important thing for a start is that it is fairly crisp from idle with no load. That way it is not contributing to an oversupply of fuel.

STEP 4 ~ Now you are set to play around with the main metering jets in the primary fuel bowl without other factors skewing the results. If you are Dyno tuning the exhaust sniffer will tell you if you are too rich or too lean and you can play with jet sizes up or down to fatten up the torque curve.

If you are a home tuner or doing some track tuning you will have to decide to go up or down in jet size, take it for a drive and see if there is an improvement or a drop off in acceleration and power. When there is an improvement keep going that way until it drops off, then go back to where it was best. Always error on the rich side to prevent a lean condition, because a lean running engine can damage internal parts.

STEP 5 ~ 4 barrel carbs hook up the secondaries and do the same as in step 4 to set the secondary jets. Remembering that the rpm that secondaries come in at may need to be delayed [mech link on throttle shaft] and slowed down [stiffer spring for vac sec] for smaller capacity engines because most 4 barrels are sized for a 5 litre V8. Either way this is a trial and error thing. The secondary pump shot will need to be play with also and it may require experimenting with different throttle cam shapes available in a kit from Holley.

STEP 6 ~ Now you can size the power valve, it is a common thing that a lot of race engines running AussieSpeed manifolds do not require a power valve. All the power valve does is provide further enrichment of the fuel mixtures when the engine needs it. The engine may or may not need enrichment, it is not set in stone that a power valve has to be operational or a certain size.

The power valve works at wide open throttle when manifold vacuum is signal is low [remember it is low because the throttle blades are wide open ~ it is low below the carbie base and vacuum is high at the top of the carbie when the throttle is wide open.]

The power valve also operates at just off idle in that transition from the idle jets to the main metering jets when vacuum in the manifold dips. For example as you gently accelerate in a street driven engine the power valve provides some enrichment until the rpm increases and the main metering jets take over and manifold vacuum is high again. In a race engine when you stab the throttle at low rpm the manifold vacuum dips and an incorrect operating power valve of an incorrect size can provide an excessive fuel condition that will cause poor acceleration. This condition will be multiplied when feathering the throttle on low rpm corners, so it is imperative to get the power valve set right.

The power valve will open when the manifold vacuum is at or below it’s opening point. For example a 65 power valve will be open at 6.5 inches of mercury vacuum or lower. A 25 power valve will be open at 2.5 inches of mercury or lower.

If the power valve is open when the engine does not need it it will run like a pig, no matter what rpm it opens at. It is that simple.

If you find you are dropping power off at the top end of rpm then the power valve may need to be added back. Ideally if you can measure the vacuum at where the drop off in power is then that will be power valve size you should try.

As a rule of thumb the power valve should be no more than half of the vacuum figure at idle. For example a stock engine with have a good 15+ inches of mercury at idle so a 75 power valve would be your baseline to try. However a camy engine may only have 10 to 11 inches at idle so it will have to be a 55 or lower power valve. Remember in the summer on a hot day, atmospheric pressure changes and you can see a camy engine may only draw 8 to 9 inches of mercury at idle. This sort of engine can hold a 75 or 65 power valve open a bit or make it pulse at idle hence leading to it idle bad on a hot day.

Typically we have found that a 6 cylinder engine with an AussieSpeed intake manifold will require no power valve or a power valve in the 15, 25 or 35 range.

In some instances we have found the power valve orifices are too big for the six cylinder engines even when you get the power valve coming in at the right vacuum and we have seen or felt the power drop off [becomes too rich]. Now on some of the Barry Grant carburetor metering blocks they have adjustable power valves but we have not experimented with them yet but would be worth looking at. We have had alot of success in restricting the power valve orifice [power valve jet is the hole in the metering block] by using a short, bent, piece of different gauge mig wire that will sit in the orifice of the metering block and restrict it's volume. You make it fit so that when the power valve is screwed in the wire can never fall out of the orifice / hole in the metering block. It has proven to be a cheap and very successful tuning method to leaning out the power valve enrichment charge of fuel, there are some pictures on the AussieSpeed website and it is explained in more detail, follow the link below.

SUMMARY
In summary if you find this does not help you achieve a base line to work from then contact us. If your dyno tuner is dismissing this information and is not tuning by blocking off the power valve and just keeps ranting on then the only solution is to find another tuner. Something to keep in mind is that just becasue that shop has a dyno does not necessarily mean they know how to use it, or tune your application. I have worked with allot of tradesmen over the years and somethings you just don't let some of them to get anywhere near it, your problem is working out who those guys are. Getting your engine dialed in can be one of the most rewarding experiences for us car nuts, even if it the solution hides itself from us for a while, so keep at it and don't give up.