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.

Jon Redder (61-63) might know what he put in my max wedge carbs he built for me?Never been touched.Tom

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.

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

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.

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

Jon a Carb Shop in Missouri might have help.

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.

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.

Thank You 61-63.

Tom V.

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.

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

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.

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.

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.

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.

FYI,Steve at Westec uses EGTs way more than O2s.His opinion is the 02s are too slow compared to EGTs.FWIW,Tom

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

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.

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.

Thanks for the kind words supporting my previous experiences on a dyno and in vehicle testing, Mike.

Tom V.

Will be a cool deal to see how it goes.

Some of the older Wide bands used a sensor that had to be fresh air calibrated. There are some newer ones that do not, I have one of the newer ones on the data logger on the IA Camaro.

Steve Morris used the Sensors that had to be fresh air calibrated before each run.
Not sure of the brand, many years ago.

A Lot better parts (if they come from a good source) vs the old days.

Tom V.

I have an even older K&N O2 monitor that used a narrow band sensor and tried to extrapolate from there.

I have a few preliminary results from a second dyno session with the replica bathtub intake and a pair of 750 AFB carbs. Total dyno time, about 9 hours. 20 gallons of what is called around here, recreational gasoline. 90 octane, zero alcohol. I am not going to post dyno sheets because the printer is dead and may need to be replaced on the dyno. I will take some screen shots and post next week. Bottom line, that intake with my engine combo at least wants TONS and TONS of fuel!. It's just crazy. The more fuel you throw at it, the more power it makes and I don't have jets or metering rods to make it "too rich". The largest jets for the AFB style carbs are 120's. I had some 116's and it was still pretty lean. Like 14.8-15:1 AFR in spots. So what's a caveman to do? Drill them out of course. So I ran a 1/8" bit through the secondary jets, to see what would happen. It loved it. Picked up 30 HP with that change. But still averaging 13.2-13.7 AFR. It's much smoother now through the pulls but still not rich enough. Also has one very lean spot between primary to secondary transition, when the velocity valve in the AFB is opening and secondary fuel begins to flow. Has a couple hundred RPM spread with high 15:1-low 16:1 AFR. So still some tuning to do. But it's getting close. Picked-up 100 HP yesterday and there is probably 10-15 HP more still out there. But that's just about it IMO. Will play some more after the Norwalk event.

I know people want to know HP and torque numbers so here the peaks so far. 452 Ft. lbs.@ 4800 RPM. 441 HP @ 5900 RPM. I think it's going to finish at about 460 HP.

Two things are really hurting it that were part of the Street engine compromise. Compression is low. Static: 10.1 with aluminum heads. Running on readily available pump gas was a must. The stock iron long branch exhaust manifolds are too small. I can just tell they are really bottled-up over 5200 rpm's or so. You can hear it on the dyno.

Overall, it idles nice @ 1000 RPM, is very snappy and sharp on the primaries with direct linkage, zero leaks, and it looks cool.

Mike,
If these are "SFD" files. If you email them to me I can print them to PDFs and posts the PDFs.

Stan Weiss <srweiss1@comcast.net>

Stan

And that's why chicks dig him.

(forgive the glass reflection)

The next step is clear.....take a drill bit to the primary jets!!

Eric

Thank you for the offer and I may take you up on this. I have to shift gears and get ready for Norwalk. But when I get back, I will either get a new printer, fix the old one or get the files to you. I don't have ready access to them at home.

I spent considerable time at the Norwalk event talking to people I thought might be able to offer a little assistance tuning my engine with the bathtub intake. Spoke at length with both David and Rodney Butler, Arnie Beswick, Dimitrie Toth, and others. All have experience with the various Bathtub replicas and original Bathtubs and AFB carburetors. Everyone i spoke to seemed to feel a 4-hole spacer was a must to make this set-up work better . So I ordered a couple spacers today. Was trying to avoid using any spacers due to limited hood clearance. But if needed, I will run them. Also, everyone agreed the iron long branch exhaust manifolds are not adequate for this displacement and cam, heads and induction system. So I borrowed a pair of Hooker competition 4 tube headers for dyno comparison purposes only. Plan to run again Wednesday or Thursday. I will share results. This will be my last dyno session as the engine will have to be removed for school to start next week. Hope to pick-up another 20 HP or so with the spacers, jetting and timing. Maybe another 20-25 HP with a good set of headers. We will see.

I'd be curious how the headers effect the jetting.

Mike,what about just a couple 4 hole gaskets stacked to save you some room?Tom

That's a great question Tom. I will start with the 1/2" spacers, since many were run that way. Then maybe try 1/4" worth of gaskets. I really wish I had more time to try all kinds of fun stuff. But unfortunately, one more dyno session is all I am going to be able to do. Want to get the most bang for the buck. I should mention, it is running very well now but of course I want to get the most I can out of it. Just feel like it's down on power based on the hardware in the engine. In my car, on the street, it's probably fine. But we always want more. Also, the better it runs, the more efficient it is.

Arnie indicated that the bathtub + AFB combo took a lot of work to optimize. Mikes experiments will be a great baseline for the new users.

Eric

FWIW the factory inline dual setups mounted on the 542991 and 9770859 intakes had 3/8" spacers on them. I do not know the thickness of any spacers the factory put on bathtubs but found some 1/4" spacers that I've put on bathtubs sets I've restored for people. The next time you have the tops off of your carbs pull the rear boosters out and then the secondary air valves and weigh the valves. The problem may be that you need lighter secondary valves and I may/probably have a lighter set I can send you to try if you wish.

Thanks for the reply, 61-63. I will weigh the air valves after I take the engine off the dyno. Unfortunately, just running out of time . But actually, it is acting like they may be a little too light based on the air/fuel ratio when I first load in the dyno. It goes pretty lean then catches up after a few seconds. I have talked to a couple friends that dyno lots of engines and they say AFB's response is pretty unpredictable on the dyno as there is no way to really duplicate the load of a moving car with the static load a dyno applies. Those air valves probably need to be tuned once back in the car and driving. They said Q-jets can present similar challenges but are soooo easy to tune compared to AFB's. I am planning to try and get a Go-Pro camera mounted above the secondaries to take some video of how the air valves are operating on the dyno. Just to see how they behave. 61-63, I will be in touch after running it again. Thanks.

Years ago, Joe Zajac and Larry Kaufman were trying to get a bit better response out of their 750 cfm AFB carbs with the 'generation 3' intake that Randy Williams did for them for the NSS cars.
The 'generation 3' intake is very similar to the Intakes being produced today.

So they tried the mod I suggested.

Two AFB 4 hole metal plates 1/8th inch thick. Carb throttle bores for the 750 cfm carbs may be different vs the carbs you choose to run.

MATCH the metal plates to your carb throttle bore sizes.

Now for the Modifications:

Measure and cut 8 pieces of steel tubing so that the inside diameter of the tubing matches your throttle bore dimensions.

Bore the AFB 4 hole metal plates so that each machined bore just matches the outside diameter of the 1" long steel tubes.

Weld "in 4 places" the metal tubes to the plates. so now you have two 4 hole "inverse" inserts that do not raise the height of the carbs more than 3/16 of an inch and still provide the "4-Hole" SIGNAL you want the bottom of the carbs to see from the intake plenum.

Worked great on Larry and Joe's intakes and they were very happy with the better throttle response, lower ETs, and slight MPH improvement.

Tom V.

Tom,
Unless I miss understand what you are saying. That sound like what many companies make and sell as "shear plates". I don't know that I have ever seen one made for an AFB.

Stan

Agree Stan, and the plates being sold are made from aluminum and are typically 1/2"
thick.

That is why I posted the info on how we made our plates from steel and for AFB carbs.

Tom V.

And yes the Shear Plates have been around for a good number of years. They work.

We made those plates many years ago FOR AFBs.

Tom V.

That's a nice idea Tom. We have a couple shear plates at the school but for dominator carbs. I like the idea. If I could have anticipated all the issues ahead of time, I could have had things like this ready to test on the dyno. No time now. I have one more shot to get it as good as i can with the parts I can get together very quickly. Engine has to come off after next session. Next chance to dyno test would be June, 2023. School starts next Monday. No faculty projects allowed during the school year, period. It's all about the students and their engines. If I can't get it to a level I am happy with, I could run it on the chassis dyno next spring. That may be the best way to continue this anyway. Thanks for the idea.

As your going to be driving this engine I would be concerned on what the shear plated would do to street manner.As its made it has great street manners.If you have them to be able to be put in a taken out with removing the carbs it might show you something.I DONT drive mine on solid linkage.Tom

As it sits on the dyno, it has 1:1 linkage. It seems very smooth and snappy on the primaries but in the car it could be different of course. The in the car linkage will have the ability to run 1:1 or progressive by changing 1 link. So I can play with it. I am impressed with the stable, idle at 900 RPM with a pretty big camshaft. It's 251, 259 @ .050, solid flat tappet. Idles within a 25 RPM range as long as you want it to.

I ran a 255/260 Solid Flat Tappet for years on the street, Obvious Noticeable Idle but never had any hesitation with the vehicle on the street or the road.

IT WAS NOT A TAKE A TRIP camshaft though.

Drove 150 miles from Kalamazoo Michigan to Grosse Pointe Michigan and almost ran out of gas.
Oh! What fun, running along at 3000+ rpm with Wangers 4.33 rear gear under the car at 55-60 mph.

The NASH TRANS (5 speed) and the proper Ford Gear in the 9" really helped that deal.

So as you know Mike, drive-line match will be worth investigating if you take the car on trips.

Have a great evening!

Tom V.

It has a 3.42:1 gear now and runs 2800 @ around 70 MPH Would like to change to a TKX 5-speed, will have to wait for some more $$$. Also would like to drive the Power Tour long haul route with it possibly next year. Confirmed next dyno session will be Thursday afternoon/evening.

Unfortunately, no results to report. Had to cancel due to parts issues. The two strip kits I ordered from Summit that had the rich jets and metering rods in them had been opened. The exact metering rods I needed had been stolen from both kits and then the kits returned. The tamper resistant tape was missing from one and torn on the other. So someone took what they needed, and returned the kits to Summit. Shame on them for just re-boxing them and selling them again! No more kits available in the US. But I have the rods I want to try coming from Memphis. They were shipped today. Have the spacers installed. This thing is fighting me every step of the way. Sorry for the delay.

I have some results to share concerning multiple dyno sessions I have had to squeeze in between classes and all kinds of other problems and distractions over the past couple weeks. Between parts getting lost by USPS for 2 weeks, endless problems with the dyno, printers specifically, and other distractions too numerous to list. I have been able to get 26 power pulls on the engine and get it tuned decently for now. I will consolidate the useful information, try to get it in a format I can post and put it up in the next week. These are my broad takeaways for what it's worth. 1. My overall engine package had too many compromises that were made necessary by not wanting to wait for years for the proper parts to be available. 2. This package needed more compression, round port exhaust and better exhaust manifolds/headers. 3. The bathtub intake wants lots and lots of fuel. 4. AFB carburetors are very, very sensitive to what is happening above the airhorn. Will flat out not run properly with an air hat on the top. I have seen this with some Q-jets as well. 5. Long branch, D-port, iron manifolds are crap on a serious performance engine with large displacement. They are just too small.

Now for some numbers: Best with exhaust manifolds, 464.6 Ft. Lbs @ 4700 rpm Peak HP 442.2 @ 5900 rpm. With borrowed 1 3/4 headers from a 74 GTO NHRA stocker: 18" long 3" collector, 481.6 ft. lbs torque @ 4800 rpm. 20-28 more ft. lbs. of torque everywhere. 448.4 HP @ 5800 rpm. Was not tuned to the headers, so 10-15 HP more on top could be expected. No reason to tune to headers because they are not going to be used.

So I have presented the basic results. Since I have no ego to care about, it is what it is. Overall a frustrating, humbling experience. Sometimes, you just get a reality check when having to make so many compromises for the lousy pump gas, the fitment in the chassis, (exhaust manifiolds), and the super cool looks, bathtub intake and original looking AFB carbs with tiny air filters.
On the plus side, excellent idle quality, starts quick and easy, snappy throttle response, seems like it will have excellent street manners. Anticipate using lots of gas. It runs best at 12.4-12.7 AFR everywhere. Starts to run lousy leaner than 13.2-13.4 AFR. It loves fuel.

Going to put it in the car and drive it around through the fall here in Ohio. We have a chassis dyno at the school, so in the spring, I may try to do some tuning on the chassis dyno. That's more real world and the AFB carburetors can operate in their real world environment. That will be chapter 2. We will see how it runs on the street next.

Good info Mike, thanks for sharing. You might consider the Hooker headers that are are on super sale right now. As far as I know, they “should” fit a ‘62 full size as well as 1st generation Firebirds. At the price, it’s worth a shot. I have a nice set of original IA headers for my ‘63 that fit great.
Looking forward to hearing how it drives in the real world.

https://www.holley.com/products/exha...arts/4107-1HKR

Thanks for the info. Those headers are a smoking deal. I saw you had nice custom headers on your car in another thread. I am married to these boat anchor iron manifolds for now at least. I have a grand in them not counting the rest of the exhaust system. My goal was to have the car be pretty quiet, so I have only a 2 1/2" system all the way back with the largest case performance mufflers I could find. While the F body headers would be a bolt-in on the right side, the big cars have the steering box in quite a different location from the F body. Also clutch linkage would be in the way probably, not sure. Just not willing to go there right now. Also, to really be optimized using my current heads and cam, I think I need 1 7/8 or 2" primary tubes. The Hookers are 1 3/4". What size is your engine and the primary pipes for reference? Also approximate cam specs. of you will share. Thanks for posting.

Early firebirds are rear steer, same as early full size. The long branch fit both cars so I don’t think the headers could be that far off. My headers are 2” with 3 1/2” collectors. Have not measured the length. I don’t want to clutter this thread with other info, just sharing header info. BTW, I committed to a 671 way before this intake was in the works so my intake is for a possible future build.

I don’t know if you folks have ever seen this guide chart for exh header tube size, but I have found it extremely accurate!

The only tempering I add to this is if I need more exh flow then the port is providing in order to get the exh to intake ratio better .

Any exh port I have ever worked with of any make on my flow bench ( and even long ones like Pontiacs ) will pick up 3 to 5 percent more cfm above .350” lift if the header tube that bolts to the exh flange is 1/8” larger in it’s perimeter.

This why especially with a race motor that the exh side is flow tested with at least a 6” long tube in place.