Holley Air/Fuel curve nonliniarity

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

I calculate that i need about 780 CFM at high end. But this carb is going lean in the middle . it's good at the lower and higher ends. i'm not sure i understand why this would point to a carb that's too large?

[Jack H.]

Re: Holley Air/Fuel curve nonliniarity

Have you ever done business with Walt/Chip at EPS in Evergreen? Yep, they're dedicated FORD guys, active in RMVR, but the last time I checked they do have a chassis dyno at their shop with spark and exhaust monitoring AND they've got a full line of Holley carb parts to be able to 'tweak' this/that aspect of the system to race optimize a given car...

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

Good suggestion Jack. i know Walt and i've met Chip. nice folks. I may need to haul the race car up the mountain. Wish we had some holley experts on this technical board.

[Duke W.]

Re: Holley Air/Fuel curve nonliniarity

I calculate that i need about 780 CFM at high end. But this carb is going lean in the middle . it's good at the lower and higher ends. i'm not sure i understand why this would point to a carb that's too large?

To yield good low end throttle response, the carb for a road engine should be sized to provide the engine's maximum air demand at no more than about 1.5" Hg depression, which is the depression that four-barrel carbs are flow rated. For a road racing engine I would size at about 1.0 -1.2" Hg, and keep in mind that depression increases with the square of flow.

A "327 LT-1" (massaged heads, LT-1 cam) has about 85 percent VE at 7000, so the carb size should be in the range of (327/1728)(0.85)3500 = 562, so the OE Holley and AFB are okay.

The same calculation for your engine, assuming 95 percent VE at 7500, is 674, and the carb size to provide this flow at 1" Hg would be 674 (SQRT 1.5/1.0) = 825, so your carb is as big as it needs to be, and should provide acceptable WOT metering from the mid-range up to peak revs.

The maximum VE for the above "327 LT-1" is in the low 90 percent range. A road racing engine with a well designed and fabricated header/open exhaust system, should be in the range of 105-110 percent, and peak VE will always occur at or somewhat above peak torque. It's at this point of highest wave dynamics interaction that the carb leans out slightly.

You did not provide any actual A/F data, but my previous post provided what is acceptable. You will never be able to get a carbuetor to provide a constant/ideal A/F across the entire rev range. The best you can do is get it into an acceptable range and ensure that is does not lean out excessively (A/F greater than about 13.5:1) anywhere in the useable rev range, particularly at the top end of the rev range.

For typical road engines, the WOT A/F tends to lean out at the top end, so one must accept an overrich mixture at low revs to preclude excess lean-out at the top end.

Duke

[Joe C.]

Re: Holley Air/Fuel curve nonliniarity

Hi Clem, my dual plain manifold divider has been modified as shown in the Chevy Power service manual. Were they modified for additional boost? What I wish this carb had are adjustable air bleeds. Jerry

Jerry,

The Power Manual shows the plenum divider removed. This makes the dual plane manifold act more like a single plane manifold..........steals some low end torque and pays back with additional top end power. The LT1/Z28 intake should have the plenum divider lowered about 1/4" below the carb flange. This enables some sharing at the high rev range, although deepening the opening to about 1/2" is a better compromise.

[Timothy B.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

If you decide to go at it yourself think about enlarging the power valve channel restriction. This will provide a richer WOT A/F ratio but not effect the main system when there are times the power valve is closed. What's the approx. idle vacuum?

The pic shows the various holley metering block plumbing and helps some in understanding.

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

If the carburetor does NOT have a secondary power valve, the secondary jets are approx 12 sizes larger than primary.

Do you think the primary power valve could be opening normal in the 3 to 4500RPM range and closing because of high vacuum (high RPM) causing lean condition. You may want to consider a higher # power valve so it stays open when vacuum increases with RPM. All trial and error but just do one thing at a time.

WOT air fuel ratio is determined by jet size area and power valve channel restriction area (primary) and jet size (secondary). Timing of power enrichment (power valve opening point) is determined by vacuum opening point of P/V.

Most of the racing is either WOT or closed, not a lot of modulation, some in corners. But on the dyno where this test was done it's WOT all the time. It's a race motor so it really doesn't idle below 1500 RPM, lot's of overlap. I really don't know what kind of strategy to use to sort out which power valve to use. I only have one power valve in the front. I'd appreciate your thoughts.

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

If you decide to go at it yourself think about enlarging the power valve channel restriction. This will provide a richer WOT A/F ratio but not effect the main system when there are times the power valve is closed. What's the approx. idle vacuum?

The pic shows the various holley metering block plumbing and helps some in understanding.

very helpful. Thank you.

[Timothy B.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

Again, I am no tuning expert but I think power valve is going to be trial and error. You need to make sure the valve does not close in the high RPM range, I believe the vacuum will increase there also. If you get a numerically low # valve 2.5-3.5 etc, the idle will be OK but could be a lean issue at WOT if the motor makes more vacuum than that at WOT.

If what Duke say's holds true then the V/E at WOT is the reason your going lean. Try three jet sizes on the secondary and lets assume the power valve is open throughout the high RPM range. Sounds like the power valve # you posted is 7.5" vacuum to open so it may be OK.

[Michael H.]

Re: Holley Air/Fuel curve nonliniarity

I think power valve is going to be trial and error. You need to make sure the valve does not close in the high RPM range, I believe the vacuum will increase there also.

Tim,

I don't think the power valve is an issue. Also, they work in reverse of what most think. The valve is designed to open at or below the HG number stamped on it. With a 6.5 valve, for example, it takes 6.5" of manifold vacuum to close the valve. As more throttle is applied, the vacuum drops below the value required to keep the valve closed.

As Jerry mentioned, in road racing, manifold vacuum is mostly either zero (full throttle) or high (closed throttle) and what is in between isn't important because the engine isn't required to produce high HP at that time.

[Duke W.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

If what Duke say's holds true then the V/E at WOT is the reason your going lean.

HUH!?.........................