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Old 08-06-2022, 02:08 PM
mgarblik mgarblik is offline
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Default Bathtub intake Carb Jetting INFO

I know this type of stuff is often "TOP SECRET" to guys that have spent years working out a great fuel system. But I am having some difficulty getting the AFB carbs on my new Bathtub intake dialed -in. My set-up is 750 AFB's, no carb spacers on the new Bathtub Intake. Fairly big flat tappet solid cam 251-259 @ .050. .550 lift. Edelbrock D port heads, CNC. 468 Cu In.
Would love it if willing to share primary and secondary jet size, step-up rod size and power piston spring color/vacuum amount. My engine makes 13" of vacuum @ 1000 RPM.

Going to install a couple of wide band O2's on the dyno next week to help get this worked out, but a good starting point would be appreciated. Not that familiar with the details of the AFB's. If these were 2 Holleys or a Q-jet, I would have been done 2 days ago!.

Thanks for sharing.

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Old 08-06-2022, 03:08 PM
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Jon Redder (61-63) might know what he put in my max wedge carbs he built for me?Never been touched.Tom

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Old 08-06-2022, 03:10 PM
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Can I assume that the new 2-4 intake was not developed with a good amount of wet flow testing, just air flow testing?

I have only been involved with a dyno test of two Carter 750 AFB 4 bbls on a motor I ported the heads ( Ford 390 FE type ) on 15 years ago.
We where having issues with uneven fuel distribution.
What the dyno operator did as I heard the next day from he owner of the motor was he used the divide and conquer method.

He worked on just the primary’s first with the linkage set for one to one and with the secondary’s locked out.

I may still be able to get more details on this, do I will let you know.

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Old 08-06-2022, 03:23 PM
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The original intake we scanned ran high 9s on Bill Blairs 63 factory SD tempest.He ran 660 center squiters.My carbs are factory 63 421 SD NASCAR Carbs with factory jetting.Forgot Mike,was thinking about the carbs on my Tempest.Tom

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Old 08-06-2022, 06:54 PM
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Talk to Joe Zajac or Larry Kaufman as both guys ran the 3rd generation Randy Williams
(V-Plenum) intakes on their early 60s cars with 750 Carbs.

They know what to do for the calibration for you.

At one time I had both Phone Numbers, Maybe Tom S has their Phone Numbers, now.

Their carbs were clone carbs vs real deal 421 SD carbs and performed as you know very well.

Tom V.

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Old 08-06-2022, 07:36 PM
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Tom,I think what really needs to be established on what Mike needs.Racing and street driving is going to make a big diff.I NEVER run solid linkage on the street so im out for being able to help and I dont think Joe or those guys ever did street driving with those engines.Tom

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Old 08-06-2022, 07:37 PM
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Jon a Carb Shop in Missouri might have help.

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Old 08-06-2022, 07:47 PM
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Quote:
Originally Posted by tom s View Post
The original intake we scanned ran high 9s on Bill Blairs 63 factory SD tempest.He ran 660 center squiters.My carbs are factory 63 421 SD NASCAR Carbs with factory jetting.Forgot Mike,was thinking about the carbs on my Tempest.Tom
So what is the point of a REPOP Race intake that can run high 9s, on a tempest, years ago, if you only want to talk pure street cars. Why do you need 421 SD NASCAR CARBS (Factory Jetting) that is driven 95% of the time at 2000 rpm?

Mike's car is a full size car that will have more load, (Like Kaufmans car and Zajacs car) and will probable be driven at more than 2000 rpm on average vs your vehicle.

Lots of conflicting info here.

Tom V.

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Old 08-06-2022, 07:49 PM
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I once asked Joe Zajac what calibration he used on Rompin' Redskin and he told me .110 jets in warm weather on both the primaries and secondaries and in cold weather .120 primaries and .116 secondaries. Unfortunately I did not get numbers for his metering rods. His engine was 510 cubic inches and as you Tom V say the carbs were 750 cfm afbs. I passed this info on to Mike yesterday.

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Old 08-06-2022, 07:51 PM
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Thank You 61-63.

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Old 08-06-2022, 08:04 PM
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About ten years ago I did a pair of 750 afb sd clones for a fellow named Tom Karpenko in Fargo N.D. who put them on a 496 cubic inch engine with a big roller cam installed in a full size '63 Catalina (I think I'm describing the car correctly?) that he both races and drives on the street. We actually shipped these carbs back and forth to get them right. He dynoed that engine with both a bathtub intake and an 859 intake (so he could compare the results). He decided to use the bathtub because it slightly outperformed the 859. The calibration we ended up with was .112 primary jets, .107 secondary jets, and .046x.070 metering rods. He also ran Edelbrock 750 cfm carbs on the dyno and compared their performance to the afbs and the afbs were slightly better. I've posted this info on here before. I also shared this with Mike yesterday but what he is looking for is other people's experiences with 750 carb calibrations.

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Old 08-06-2022, 08:39 PM
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Tom,Mike put this in the race section where I cant help!I have driven these Pontiac 2-4 intakes on the street since 1963 and I CAN comment on street setups.My 62 Grand Prix Im sure is in the same weight range as Mikes by the way!When are you going to get one of yours on a car and drive or race it where a opinion from YOU can make a diff?Tom

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Old 08-06-2022, 09:09 PM
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Would not be apples to apples with a set of 3 Holley 2 BBLs on a custom top plate made by a McLaren Fabricator I have worked with for many years.

But while you were in California I had many discussions with Joe and Larry and their clone 750 carbs BEFORE you came to Norwalk (when Randy, Joe, Larry, and I were playing with the Randy Williams intakes). The intake may very well get a pass at Bonnevile at some point and I do not think you will ever race there.

So lets stay friends and keep the personal race comments to yourself.

Tom V.

And Smokey told me that the Carter carbs were unworkable (with any boost) on the Pontiac Parts he tested (1961) (WITH BOOST) when he blew one of the AFBs off the engine and thru the roof of his dyno cell.

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Old 08-07-2022, 02:08 AM
mgarblik mgarblik is offline
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Thanks for the replies and keep them coming if you have more info. I have decided to add wide band O2 bungs to the head pipes so I will be able to really get this thing tuned-in next week. To Tom Vaughts point: The o2 sensors will enable me to get the off idle, light and medium cruise mixture better than just making power pulls with the BSFC numbers and air hats. I expect it to take a full day to really make it right. Per Steve, on the first set of little pulls where we had some issues, I did separate the primary from secondary throttles. I am reasonably happy with the primaries. It idles well, but a little rich. With low load, it's snappy and runs good. Heavy load and full throttle needs some work. I will get it. Thanks. again.

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Old 08-07-2022, 08:33 AM
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Wade Congdon, BOP, sold high quality O2 sensors for a period of time to the OEM engine guys in Research in our building.

What O2 sensors did you use in the past, Mike?

Tom V.

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Old 08-07-2022, 10:07 AM
mgarblik mgarblik is offline
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Honestly Tom, I am not 100% sure what we have now. The school bought a 25K major hardware and software update for the dyno a couple years after I officially retired. Basically, we have a Superflow 901 with the 902 updated software and instrumentation. From memory, the sensors themselves are Bosch parts and the software just for them is from Inovate Technologies? Then it is somehow married to the Superflow software. Fortunately, Blaine Heeter, my co-instructor has had all the training direct from Superflow. I am trying to get myself back up to speed. He has been there for all the dyno work and will continue to be until we get this deal optimized. I will share the results once we get it in the ball park and can make some clean pulls. Hoping to have something before Norwalk but depends on when the parts get here. To answer your question directly,

Tom, this will be the first time I have used O2 sensors on the engine dyno. In the past, we had to rely on EGT's, fuel flow meters, and the air hats for tuning. Our chassis dyno has had wide band O2 for 15+ years but i haven't personally tuned a car on it. The young guys LOVE the chassis dyno! I have watched. They spend all day with their laptops playing on that thing. Making 20-30 pulls in a session mapping fuel, spark, boost if applicable.


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Old 08-07-2022, 11:14 AM
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FYI,Steve at Westec uses EGTs way more than O2s.His opinion is the 02s are too slow compared to EGTs.FWIW,Tom

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Old 08-07-2022, 11:51 AM
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I can't speak for O2 sensors. EGT response time is dependent on the thermocouple type. Exposed junctions react fast but aren't as durable. We used those in the Grocery Getter. The funny car had enclosed (shielded) junctions. The abuse of a nitro engine probably would have killed exposed junctions pretty quickly.

FWIW,
Eric

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Old 08-07-2022, 12:42 PM
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Quote:
Originally Posted by tom s View Post
FYI,Steve at Westec uses EGTs way more than O2s.His opinion is the 02s are too slow compared to EGTs.FWIW,Tom
The EGTs do a fine job. I agree the O2s accuracy depends of the model/ manufacturer used.

There are two basic O2 sensors out there: The Bosch Sensor and the NTK sensor.

The Bosch sensor gives a "general" air/fuel ratio.

The NTK sensor reads ACCURATELY from a 6-1 ACTUAL a/f ratio to several numbers higher than stoich (14.7-1) air/fuel ratios.
Maybe not important for drag strip usage but on a Bonneville 5 mile run very important, or a road race vehicle if EFI.
Or a true street vehicle.

Not sure what a SF bench uses.

So if you are trying to tune your STREET Bathtub intake and carb set-up, I would always use a NTK sensor system.

Tom V.

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Old 08-07-2022, 01:59 PM
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Quote:
Originally Posted by Tom Vaught View Post
The EGTs do a fine job. I agree the O2s accuracy depends of the model/ manufacturer used.

There are two basic O2 sensors out there: The Bosch Sensor and the NTK sensor.

The Bosch sensor gives a "general" air/fuel ratio.

The NTK sensor reads ACCURATELY from a 6-1 ACTUAL a/f ratio to several numbers higher than stoich (14.7-1) air/fuel ratios.
Maybe not important for drag strip usage but on a Bonneville 5 mile run very important, or a road race vehicle if EFI.
Or a true street vehicle.

Not sure what a SF bench uses.



Tom V.
Your information and observations lines up with what I observe in stock passenger applications. The Bosch sensors tend to be a little lazy and the NTK's very fast switching when watching or graphing with a lab scope or scan tool. When making car repairs I try my best to use NTK replacement O2 sensors vs Bosch. The Bosch sensors work but switch rich to lean slow enough to set false catalytic efficiency codes on some cars. On the dyno, I will work with whatever we have. I will still have BSFC, an air hat and other data.

So if you are trying to tune your STREET Bathtub intake and carb set-up, I would always use a NTK sensor system.

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