cagotzmann

Most factory Holley jetting has bigger jets for the secondary's vs the primary.

Does this have an effect afr for the back 4 cylinders vs the front 4 cylinders

Would it not be best to have equal jets primary vs secondary

This is what most Efi throttle body systems would do.

There must be a reason holley uses this from the factory.

Would there be a difference vacuum secondary vs mechanical

Shark Racer

EFI throttle body systems don't have jetting at all, and the throttle bodies open pretty much completely linearly, not a progressive setup like a 4-bbl carburetor.

I believe the idea is as demand (load) increases you'll want more enrichment from the secondary circuit for maximum power.

cagotzmann

I wonder how well the air fuel mix is to each cylinder in a carb vs throttle body EFI. It seams the carb with secondary's may not provide equal amounts of fuel to each cylinders therefor to be safe you need to run richer vs EFI to prevent 1 cylinder from running lean for (WOT).

When I looked at my plugs cylinder 1 & 2 show more light color grey / tan vs 7&8. current carb setup is 69 primary 74 Secondary.

gerry72

Whether a vacuum secondary Holley carburetor is jetted "square" (same main jets all around) or biased depends upon how it is built and what the engine likes. The primary (front two venturi) always uses a power valve(PV), though, race applications may plug the PV and just go on main jets. A power valve allows the car to run on the main and subordinate systems and engage fuel enrichment from the PV only under heavy throttle application. This helps with both fuel economy and driveability.

The vacuum secondary rarely uses a PV and has to accommodate the fuel enrichment needs of an engine under heavy load by using a larger main jet. This is okay, though, since you aren't going to open the secondary unless you're pouring the coals to the engine. The benefits of fuel economy and driveability from the primary side PV aren't apparent on the secondary. I've not seen every aftermarket Holley ever made, but the only time I've seen a PV on the secondary with a vacuum secondary carburetor is with Fords.

To your question on biasing jetting, there are instances, primarily with open plenum intakes, where you may have a different jet on each corner. It depends upon what the spark plugs are telling the person tuning the car. You just do what ya' gotta' do to get the best power out of the engine.

lars

Carbs are jetted with bigger secondary jets in order to achieve equal air/fuel distribution on both ends of the carb at wide open throttle (WOT). Most carbs have a power valve on the primary side only. When the power valve opens (under power), it is equivalent to an 8-jet increase on the primary side. In order to achieve equal air/fuel ratio on both ends of the carb at WOT, the secondary side must, therefore, be jetted about 8 sizes larger than the primary side. Vacuum or mechanical secondary makes no difference.

If you "square jet" a carb with only a primary power valve, your air/fuel mixtures will be way out of wack from one end of the carb to the other.

On carbs with secondary power valves, the carbs are jetted "square" (or very close).

cagotzmann

Thanks that makes sense. I forgot about the power valve adding fuel to the primary side.

I will continue to increase the jetting on the secondary unit I don't get better performance. With AFR @ 12.4 - 12.8 @ WOT I may already be at the sweet spot with 69-74 jetting. Factory for the carb was 72-75.

Bhus

I had my car on the dyno a few days ago.
I have a Holley 670 Street avenger, and ended up with the same size jets on the primary and secondary. .68 in both of them. With bigger jets on the secodary the car was too rich on WOT.

cagotzmann

That's interesting. Where in the world are you ?

I am in Canada 2200 ft alt. (local track)

Do you have your dyno sheet with AFR Readings. ?

Mine is a Stock ZZ383 with a 770 Street avenger.

I also have a Zeitronix wideband data logger so I can see / log all afr readings.

When I tuned my 770 I did this in 5 separate steps.

After each major step I also checked the color of the plugs to confirm my AFR Readings. (Plugs don't lie)

1 Primary
2. Power Valve
3. Secondary
4. vacuum spring
5. discharge nozzle / accelerator pump cam


Each had a different driving condition to test and set.

1. Primary looked at low rpm too high rpm with high vacuum AFR ( Light load) steady state driving and slow throttle changes. Aimed for 14 too 14.4 AFR.

2. Power Valve used the the vacuum reading to see how the carb AFR changed ~ 1/2 idle vacuum. eg if idle vacuum ~ 18 I would transition the load in the vacuum range ~ 9 and see if the afr change was smooth. This was easies to do in 4th gear where the load (vacuum changes and RPM changes more slowly ) again watching afr. Here I tried to keep the afr ~ 12 - 13

3. Secondary this was tested during WOT. AFR 12-13 and to see what RPM it gets to 12. During initial WOT the AFR will drop below 12

4. vacuum spring adjusted during WOT. Adjusted to get the AFR starting a 12 and maintain 12-13 over the widest RPM Range.

5. discharge nozzle quick to aggressive throttle changes and sized to minimized afr changes. usually get a lean spike then rich load until it levels out in the 12-13 range.

This last step is the most difficult since a combination of nozzle vs cam rate (color chart ) too many combinations with different benefits.

All of these steps where done using the data logging of the afr wideband and performance results are based on 1/4 mile MPH results.

1/4 mile MPH is all I looked at ET is a result of driver skill / gears / tires. I started @ 100 MPH from the factory settings to 108.2 MPH with tuning and testing and I am not done yet.

Each visit to the track I would adjust timing to find the sweet spot for the given carb setup.

Changes at the local track included vacuum secondary springs and timing changes.

Timing changes is easy for me since a Have a MSD 6530 programmable controller. This lets me change timing at any given RPM and vacuum reading (MAP Sensor)

Adjusted Timing using this as a guide

https://www.msdperformance.com/suppo...ur_timing/#FAQ

If you are running square with a Street Avenger I believe you have more performance to find yet.

When I was running square 69-69 best a 104 MPH and now running @ 108.2 MPH Also some plugs displayed lighter grey / tan and getting close to white ( too lean and possible engine damage point )

Check your plugs for color variance and re-read lars response on why it should not end up in a square jetting for a carb without a secondary power valve.

cagotzmann

Any 1/4 mile MPH for your car ?

Bhus

I'm in Norway.
The location it was run on the dyno, is about 180 meters above sea level.

My engine is supposed to be a 1970 350 LT-1, here you can see the result.
I hoped for more power, so now I'm in the market for a 383

cagotzmann

Factory for this carb is 65 - 68. and a 6.5 power valve.

What vacuum do you get @ idle ~ 900 RPM

power valve rating usually = 1/2 vacuum at idle.

eg 14" vacuum at idle = 7.0 power valve or next lowest.

You are mainly running rich because the primary jet is too large. or your power valve is opening too early.

I would first set the primary size to provide 14-14.4 afr while cruising.

Then adjust the secondary for max power.

You may find something like 64-69 may give better results with a proper power valve.

Your dyno run still looks rich.

What timing adjustments did they do. Timing will have a bigger effect on power with the AFR Range you are running at.

lars

That's not unusual for the SA carbs. The SA carbs have a C.A.R.B. rating, and the metering block is designed to run the carb very lean on the primary side. During my "Tuning for Beer" tours, we were installing the secondary jets on the primary side on the SA carbs in order to correct the primary lean condition.

Bhus

I have 12" vacuum at idle, the power valve is stock, 6,5, I think.
Timing is 35 degrees at 2500 rpm, at idle 20 degrees (without vacuum)
The car runs great, and it seems like it's close to what I can get out of it.

cagotzmann

With 12" at idle you could drop to a 5.5 PV but if you don't have access to AFR tools it would be difficult to monitor any changes you make without going back to the dyno.

Or you could take it to a local track to get a base line 1/4 MPH. Make some changes and see if you get better MPH.

My Runs great started @ 100 MPH 1/4 mile. I am now @ 108.2 MPH

"Runs Great" Compared to what.

I thought my car always runs great until it runs better.

At least you are on the safe side of AFR (rich) better than lean which will destroy the engine.

For a street only car not sure the effort of fine tuning will increase the enjoyment much.

For me I am looking to improve track day lap times. Every little bit helps.

Enjoy......

resdoggie

cagotzmann, just curious if you have done any tuning with the air bleeds?

cagotzmann

No I never found a reason to look at these.

Bhus

Thank you

I don't have easy access to a dyno, så it will be expensive to take it there for small adjustments.
The car runs great compared to what it did before going to the dyno. I have tried to adjust it for 3 years on my own

Good luck with your adjustments

Jebbysan

I was always in mystery to why I could never make these work....
Thank You.

Jebby

cv67

8mph gain is huge nice job

Like the phrase "ran good til it ran better". True

Give it "what it wants" vs 02 readings and enjoy

PAmotorman

unless the primary and secondary are 2 different diameters.