Anyone play with changing the secondary spring in their 4bbl for our 300 cid motors?

The chart that comes with the spring kit says that in a 350 cid engine, the stock spring (plain) has the secondaries starting to open at 2240 rpm and fully opening at 8160 rpm. So with this spring, in a 300 motor the secondaries would never fully open. They might not fully open with even the lightest spring which on the 350 would fully open at 5680 rpm.

But I'm told that the real issue is preventing the secondaries from opening too early.

Anyone played around with this?

 

Yes, quite a bit and what I found on my engine there was not much difference between the lightest spring and the heaviest. Generally, the secondaries won't begin to open until your engine hits between 2500-3000 RPM.

 

Arizona, What size Holley are you using? As Harte says, they may never fully open on a 300, even with as small as a 465 cfm. With a 600 cfm you may have so much cfm available that is in your best interest to keep the secondaries closed or mostly closed throughout the WOT operating range.

 

I'm using the 600 cfm. I'm going to disconnect the secondaries to see how she runs on just the primaries and then play with spring rate.

 

If you're running vacuum secondaries on the Holley, they're controlled by engine vacuum at a given (Read assumed, ballpark) RPM...different for each engine and it's health....

If I'm not reading this right, please disregard.

 

If you're accelerating hard, you should be able to hear an audible pitch change when the secondaries open. The tone of the engine will noticeably deepen.

If, during this change, there is a hesitation or a bog, your spring is adjusted incorrectly. If it's a smooth transition, you're good to go. If it hesitates and you feel a lack of power, it's going lean and your secondaries aren't opening soon enough. If it suddenly bogs and falls on its face, it's going rich and it's opening too soon.

Unlike what a lot of people think, you should not feel the change when they open. There shouldn't be a "kick" in power (even though auto engineers used to purposefully tune carbs wrong to do this because customers expected it so much).


Either way though, from my experience, the stock spring is usually pretty close.

 

Another old hot rodder trick is to put a bleed (leak) past the vacuum diaphragm check ball. Early on we did it by putting a phillips screwdriver down the port and staking the seat with a hammer tap. Later somebody published an article - forgot who or where - that showed how to drill a more precise bypass hole in the casting. That brings on the signal to the 2daries sooner.

I like your idea of disconnecting the secondaries. I once talked a fellow drag racer into disconnecting the secondaries of his 250 Chevy / 750 Holley powered altered roadster and making a run. The result? - same e.t.
You'll need a long straight piece of road to do this.

 

You are right. But Holley sells a kit with springs that change when the vacuum diaphragm moves and the secondaries start to open and the rate at which they open. That's what I am asking about.

 

Having them vacuum operated is a great way to have them open based on your engine's needs. But, (with that kit), you still need to adjust where in the vacuum range they start to open, since every engine's different.

My main gripe with those springs is how easily the paint comes off. I have a full set of them and none of them have any color left, so I don't know which one's which anymore.

 

Sounds great when the secondaries open. Really just playing around and learning. Disconnecting the secondaries will provide a baseline against which I can tell what the secondaries are actually doing.

 

I've done that on a few of my carbs, it's a good way to get a feel for the carb.

Keep in mind that your primaries can still support the engine well above where the secondaries will open. You just won't make as much power. Mathematically, the 300cfm of the primaries should be all you need until around 3500 RPMs.

After that, if you have a vacuum gauge, when you fully open up the throttle, your vacuum will drop to (or pretty near to) zero. That means that your cylinders are getting the maximum amount of air. Each 50 cubic inch cylinder is pulling in around 50 cubic inches of air and fuel. If your secondaries are wired shut, you'll notice that around 3500 RPMs, your vacuum will start to increase since now your primaries are a restriction. So, you maybe only able to pull 40 - 45 cu/in into each cylinder. That's where the secondaries come in. They open up the carb to allow you to continue to get a full cylinder fill well past where the primaries would fizzle out.

The secondaries also have another perk to them in that you can run them richer than the primaries. At full throttle, you may have around a 13:1 air/fuel ratio with just the primaries. But the secondaries can richen that up to a much more powerful 11:1 - 12:1 AFR.

It's the little things.

 

My neighbor who has been helping me with this built and races a drag car. He wants me to remove the check ball altogether and go with the stiffest spring. We are going to try that.

 

Understood and agree.

Just sayin' what everyone else is about the tricks and tips....and about checking when the secondaries will start to open....as every engine is different.

Good way to start with removing the check ball and using the stiffest spring. Keep an trained ear on the engine revs and/if/or laboring.

Good luck

 

Thanks. Just found out that my carb doesn't have the check ball. It has a pressed in restriction instead. So no check ball removal!