Can a factory Holley 4150 be made to work in autocross?

[j-rho]

Hello,
In the planning/contemplative stage of a potential future build. Car would be a '67 Z28 clone, put together to run SCCA Solo (autocross) events but also P-T type stuff.

Car would be built under a very restrictive ruleset. Motor would be a stock 302 from carb to oil pan (though baffling is allowed). I can put headers on and replace the distributor but have to run the stock carburetor.

In this case, my research shows the '67 302 came with a Holley 4150 carburetor with vacuum-actuated secondaries, no accelerator pump on the secondaries. I talked to a knowledgeable carb tuner (in drag racing circles anyway), and he states there really isn't any way to get the vacuum-secondary carb to work right in something like an autocross, where we are on and off the throttle often. He believes there is too much lag with the slow-opening vacuum secondaries, that the car will stumble and run lean for too long a time when the throttle is quickly opened under load.

I was wondering what sort of experience folks here have with this type of carb, or others of its type, in an autocross or track situation. I know about the stumbling/stalling under braking, which I can live with, but can't live with horrific throttle response, as good response is needed to balance the car properly in our busy autocross situations.

I know most people end up throwing out stuff that doesn't work and bolting on something better, but that would not be an option in this case. If the factory-built 4150 carb could not be made to work as it came on the car in 1967, what would be the minimum modifications you would need to make to the carb to make it work okay? If I can't make it work under the current rules, maybe I can get some new allowances created to help things. The smaller the change, the better the chances of it happening.

Some people may think - "just cheat and do XYZ" - if I were just going to cheat, I wouldn't be asking for the new allowances in the first place. I also can't run the crossram or any of the dealer-installed "trunk kit" stuff that was homologated for the cars in those days, has to be plain-jane factory.

Thanks in advance for your insight everyone!

[Gitter Dun]

I honestly dont see a reason why a vaccuum secondary would not work for one. Secondly it is going to depend on the size of carb and camshaft power range. And third, you would have to be running pretty darn lean to do any damage auto crossing. It has been a long while since I have worked with vacuum secondaries but I believe Holley makes kits that can solve any lag issues. I'll do some research for you and see what I could find.

[Gitter Dun]

I honestly dont see a reason why a vaccuum secondary would not work for one. Secondly it is going to depend on the size of carb and camshaft power range. And third, you would have to be running pretty darn lean to do any damage auto crossing. It has been a long while since I have worked with vacuum secondaries but I believe Holley makes kits that can solve any lag issues. I'll do some research for you and see what I could find.

I found my Holley Carburetor handbook by HP Books. Its very handy and has helped me plenty, definitely a must have for old school guys like me. After doing some research I am confident you can make this work. Especially if you are going to run a 3.73 or 4.11 final gear. Your rpm range will probly start at 3500 rpm. The key is to adjust the carb so that the secondaries actuate as early as possible for max engine output but not so early as to lose the signal and main system flow in the primary venturi with a result of sag or bog.

The way to do this is to play with the spring rates in your diaphram operated secondaries.

The first thing I would do is pick up a Holley quick change secondary spring housing kit. Holley part no. is HLY-20-59. For $14.95 its a no brainer and will aid in the removal and installation of secondary diaphram springs. The second item to pick up is the secondary spring assortment kit. It comes with 7 different springs and spring rates. This part no. is HLY-20-13. I have a kit somewhere in my garage and if I remember correctly each rated spring is color coded.

Hope you continue to move forward with this project!

[Vegas69]

I don't think it would be ideal personally.I run a 4150 850 with mechanical secondaries and accel pump. You need throttle response and now in autocross.

[j-rho]

Thanks for the replies gents.

Engine would be a stock 302, so it'd have the "30-30" cam, 11:1 compression. The stock carb is 780cfm. We can run fuel up to 100 octane. Final drive would be 3:73, and I would spend 95% of the time in first gear, with engine speeds between 3500 and 7500rpm.

No carb is ever really ideal, just trying to see if it's going to be workable or not. I know there are some really fast CP cars with carbs, but those systems I'm sure are a long way from anything ever delivered from the factory. This element is sort of the last piece of the puzzle, to making this car possible.

Thanks again!

[John Wright]

Are you allowed to mess with the vac secondaries....lighter springs? accel pump cam changes, accel pump sizes? Can work be done to the metering?

[Vegas69]

It's not hard to try out. Take it out to a parking lot and go like hell in 180 degree turns and slalom and see what you think. I've run a bunch of autocrosses with mine now and a couple road races. The only negatives I've found are bowl spillage(Down the carb throat) coming to a fast stop and high g corners off the throttle. I have jet extensions in the secondary bowl and it still kills the engine if I come to a fast stop. You can watch some of my videos at http://www.youtube.com/user/1969Cama...k?feature=mhw5

[j-rho]

Car sounds great Todd!

Unfortunately at this time, I don't yet have a Camaro. I'm still researching them, to see if I can make it nationally competitive in the SCCA autocross class I'm interested in. I know, most people would think such a thing is crazy, but I think I have enough allowances to sort out the chassis, suspension, and handling, but am very restricted on engine/drivetrain modifications. As you know, the ability to balance the car with the throttle is very important in autocross, so this carb is the last hurdle.

I'm not too worried about what happens coming to a fast stop, as we never stop completely while the clock is running, though we do brake hard from time to time.

John, at the moment there aren't really any specific allowances for carbureted cars. Most of the rules were written for fuel injected cars, which are allowed to change their fuel programming. I am definitely allowed to make any adjustments available to the stock carb, but changing out hard parts will likely require me to campaign for a rules change. Do you think there is enough adjustability in the components you listed to get things working well?

I apologize for my lack of knowledge or clarity here. I have limited experience with carburetors, to a stock '79 RX7, and a '68 Datsun 510 with dual sidedraft Webers. I found the responsiveness of both cars acceptable, even when autocrossing, though their carbs were likely quite a bit different than the Holley 4150.

Here's a run from yesterday in my big fat pig of stock-class car:
http://www.youtube.com/watch?v=Eine5yYwcpU
As you can see in the video, there are several times where I use the throttle balance the car - if it does not respond consistently to those throttle inputs, it makes it much harder to drive quickly. I know a carb will not be as good as EFI in that regard, I'm just wondering if it can be made workable, which I understand is a really vague target.

Thanks again!

[rrunner68]

Mary runs a Demon carb, with no issues. I run an Edelbrock, and I'm having some starvation issues that I hope to have sorted out soon. I can't seem to keep enough fuel in the bowls. I'm starting to think its the junk Edelbrock dual feed chrome stuff.

[Randy67]

Just curious, what autocross class are you running in? Street Prepared?

Carbs can work quite well when they are setup for autocross. We have a few Cobras running carbs with no problems and I was running a crossram setup on my El Camino with stock Holley street HP carbs. Road race floats help quite a bit to keep the bowls from running out of fuel and there are other things that can help as well.

[j-rho]

Hi Randy,
I'd be running one step under Street Prepared, in the "Street Touring" category. Street Touring allows all the suspension modifications of Street Prepared, but is much more limited in fuel/induction. We also run on street tires on wheels of limited width, instead of the big crazy R-tires of SP.

[John Wright]

Do you think there is enough adjustability in the components you listed to get things working well?

Yes....you can minimize the stubbling/starving issues normally associated with vac secondaries. Lighter springs in the vac can, along with a timed accel pump squirt to cover the tip in of the secondaries can help the transition between the primaries and secondaries.
Testing with an assortment of accel pump cams, vac secondary can springs, and accel pumps will be required to get your carb setup. I'm sure there are problably some other adjustments that are escaping my memory at the moment that can help the part throttle to WOT and the rapid on/off throttle transitions.

[66SuperSport]

Our Chevelle has a 496" with a modified 750 mechanical secondary carb from AED. It flows a little over 800 cfm now. I told AED what I was going to do with the car and I have never had a problem. I autocross this thing hard and it has never stumbled once. The car runs perfectly no matter what I throw at it. Would highly recommend AED to anyone running a carbureted engine.

[j-rho]

I'd like to pick up a book, to learn more about tuning the Holley 4150. I've seen people recommend "the one from HP Books", a couple turned up on Amazon:

http://www.amazon.com/Holley-Carbure...1187812&sr=1-1

http://www.amazon.com/Holly-Carburet...1187812&sr=1-2

In light of my situation, can anyone recommend one or the other?

Many thanks again for your help gents.

[Pro Stock]

PM sent

[TnBlkC230WZ]

I ran a 750 CFM vacuum secondary 4160 carb and it worked fine for autocrossing. I ran a much lighter secondary spring with good results. The engine is a mildly modified ZZ4 with about 400 HP and 6200 RPM shifts. I'd recommend a quick change vacuum spring kit. It is very easy to figure out which spring you need. If the engine doesn't bog down when you floor it, put a lighter spring in.

[Gitter Dun]

I found my Holley Carburetor handbook by HP Books. Its very handy and has helped me plenty, definitely a must have for old school guys like me. After doing some research I am confident you can make this work. Especially if you are going to run a 3.73 or 4.11 final gear. Your rpm range will probly start at 3500 rpm. The key is to adjust the carb so that the secondaries actuate as early as possible for max engine output but not so early as to lose the signal and main system flow in the primary venturi with a result of sag or bog.

The way to do this is to play with the spring rates in your diaphram operated secondaries.

The first thing I would do is pick up a Holley quick change secondary spring housing kit. Holley part no. is HLY-20-59. For $14.95 its a no brainer and will aid in the removal and installation of secondary diaphram springs. The second item to pick up is the secondary spring assortment kit. It comes with 7 different springs and spring rates. This part no. is HLY-20-13. I have a kit somewhere in my garage and if I remember correctly each rated spring is color coded.

Hope you continue to move forward with this project!

Here are the kits, should work fine.

[Roadrage David]

Our Chevelle has a 496" with a modified 750 mechanical secondary carb from AED. It flows a little over 800 cfm now. I told AED what I was going to do with the car and I have never had a problem. I autocross this thing hard and it has never stumbled once. The car runs perfectly no matter what I throw at it. Would highly recommend AED to anyone running a carbureted engine.

I second that!!.
i have a Holly 850 ho AED carb its as quik as a fuel injection(when in the right gear) als for the stumbeling and fading on hard breaking and tight slow corners. it means your boils are to full and trows fuel out of the vents. and the lack of ecseleration is your simple running the wrong gear. this is a 2 $$ solution that wil cure the stumbeling and fading on hard breaking and tight cornering. it works perfect for me.

[Vegas69]

David, that's a hell of a good idea. My float bowls are set right and I run jet extensions. I'd call it a gurgle when tipping in during a high g corner. Coming to a fast stop will kill the engine. At RTTC I was stopping from 103mph in under 5 seconds. I'm not sure your trick wouldn't even solve it. I'm going to try it though.

[Roadrage David]

`Vegas69 when the stalling happens at hard braking ore tight cornering your vents trow gasoline direcly into your carb drouwning your engine at low speeds.
Useing this trick gives the gasoline more room to rize withouit falling direckly into your carb saving it from stalling , and it gives the gasoline a place to go and runn to the forward boil. . make sure the 4 holles combined are as big as the 2 holles from your vents!!. and your problem will be solved.
Try to get as high as posible direcly under your air filter..
This is a old transam racing trick that is also commen used in Marine engines!!!. and belive me it wil work!!!!!! Also AED sells road race floats!! for there carbs. i dont even need them but i wil get them anyway...ps doing 140mph here and stopping in no time no problem pps im the green car lol.
http://www.youtube.com/watch?v=q5h-kI_t1oc http://www.youtube.com/watch?v=Opza1XD_Y9M http://www.youtube.com/watch?v=QSWpn...layer_embedded never mind the smoke i was leaking radiator fluid...