I got a 1969 Firebird with a 400ci motor. When I picked it up earlier this year, the previous owner told me that the engine was completely rebuilt and has a mild cam installed, Edelbrock spread bore intake, along with HEI distributor. I purchased headers for it which I plan on installing soon as well.
The carb is a Holley and I was able to narrow the cfm down by the # stamped on it which is 8879. Looks like its a Holley 650cfm, model 4175 w/ vacuum secondaries. I am pretty good at tuning Holleys so I would like to stay with them.
My question is, with the setup I have, can I get away with running a Holley 750cfm? Do you guys recommend another Holley model or should I just rebuild the current one I have...
750 is too much for that, 650 should be fine...if you have too big a carb , gas could partially go down in engine, wear bearings...a smaller, right size carb will give betrer performance and gas mileage....bigger isnt always better
Looks like that may have been made in August 1972. They used them on some chev pick ups and a few others. Holley has it listed as a replacement carb for a few buicks chevs and 67-69 Pontiacs with a 350 engine. Seems they are Holleys replacement for the spread bore. You said you are good at tuning Holleys so you can make it work. But if you want a spread bore carburetor I think a Quadrajet is a better choice. Depending on your cars weight, gearing and transmission type a Holley 4150 style carburetor may be a better choice. But you already have that carburetor so I agree with the others, tune it up and see how she runs. If you're happy leave it if you're not then change to another carb.
Thanks for the info! I would assume the car"s weight is standard for a 69 but the trans is a Muncie 21 4 speed manual. Not sure of the gear ratio but I can tell you at highway speed, I am a lil over 3k rpms...
Just thinking about your choices if (IF) this one doesn't work out for you. A heavy automatic car with a gear ratio in the twos or low threes may be a candidate for a vacuum secondary while a lighter car with a four speed manual with a gear of say 3.4 and up may be better suited to a mechanical.
Thanks Bluebird for the info! Does Holley manufacture a mechanical secondary for the spreadbore intake? I only found a model with vacuum secondaries that will fit this kind of intake...
Thanks Bluebird for the info! Does Holley manufacture a mechanical secondary for the spreadbore intake? I only found a model with vacuum secondaries that will fit this kind of intake...
I believe they do. However, although a mechanical secondary carb, is more suited for use on a manual trans with higher numerically ratio rear gearing than auto, that still doesn't mean it's the ideal choice. Mechanical secondaries are more difficult to tune properly, and are better suited for track use where most of the driving is WOT. Other than bragging rights ("I have a double-pumper,") it won't provide any benefits for street use, and the vacuum secondary will actually provide better throttle response and can be tuned (because of the differences in circuits) for better fuel economy as well.
If there's nothing wrong with the carburetor you have, and you know how to tune it, don't waste your money. Unless you stroke the motor, or add significantly higher flowing heads and a camshaft, what you have will do nicely.
Firebird Mike has some good points. The main one being, give the carburetor you have a chance before trying something else. The carburetor you have now may work out just fine, if not then look for something else. Just because you have a standard, low weight car doesn't mean a vacuum secondary is not suited. For normal street driving with a stock type engine the secondaries are open based on engine demand. When you stomp on it or have an uphill type load the vacuum will drop and the secondaries will open to meet the air/fuel demand. Another advantage of the vacuum secondaries is if you've chosen a carburetor that may be too large for your engine, the vacuum secondaries will somewhat compensate due to opening only when the demand is there. For the average driving practices and build it may be the best choice. But doesn't mean mechanical secondaries are not suited, it depends on the type of driving and how they are tuned. I've seen a lot of people having trouble tuning their vacuum secondaries while tuning the mechanical with no problems. Personally, if I had a stock 400 Firebird with no carburetor I'd find a Quadrajet. Not that the Holley spread bore isn't a good carburetor. I just think for a spread bore Rochester is a better choice.
Holley has the Street Demon 625 and 750 mechanical secondary spread bore, but that is their version of the Carter AFB carburetor. It's a mechanical secondary but has the air valve that opens depending on air flow, the butterflies open but the air valve above opens dependent on demand. Just like an Edelbrock which is a Carter AFB made by Weber USA.
As far as the intake, what intake do you have on your engine? I've had the Edelbrock Performer and the Performer RPM on mine and used square bore carbs on both. I've run a 4150 style carburetor on those with no problems, a thin (1/8") plate was used between the carburetor and manifold.
I hope I haven't muddied the waters with my opinions, sometimes too many choices.
Although the 4150 style double pumper is usually associated or thought of as a race carburetor, where speed not economy is the main goal, I don't really agree with the idea that the vacuum secondary 4160 style can provide better throttle response and be tuned for better fuel economy than the 4150 style mechanical secondary. With the secondaries closed in both types what you have is basically a two barrel carburetor. Unless you have the 4150's secondaries set to a one to one ratio with the primaries, the secondaries are staying closed until you go past the 40,60 80 or whatever % the linkage is set to. On the street leaving a red light or just driving around town the primaries are tuned the same and act the same. When a strong launch is wanted the 4150 secondaries are opened when the pedal is floored. The 4150 secondary accelerator pump is also activated to prevent bog associated with the sudden opening of the throttle plates. I've seen a lot of vacuum secondary carburetors bog like heck when the secondaries open too soon without the benefit of secondary accelerator pumps. Once the correct secondary spring is installed for the type of driving the bog is usually eliminated. Different styles of driving and different conditions may require a change of spring strength. One also must have the proper spring for the weight of the car. Holley even sells a quick change vacuum pod cap that allows for a change of springs without dismantling the vacuum advance.
A 4150's accelerator pump can be adjusted for the duration of shot, for the amount of shot and for the timing of the shot. The secondary idle, transition and power circuits can all be adjusted to give an economic fuel consumption and still give all that is needed when power, not fuel economy, is the choice. Most tuning advice I've seen given by Holley and others concentrate on changing main jets and power valves. I've found it best to start with the amount the butterflies are open at idle in both the primaries and secondaries., idle feed ristrictors, idle air bleeds, emulsion air ristrictors and bleeds first, then make changes like jets and power valves later. I have my 4150 idling at 14:1 air fuel, cruise at 14.5-15:1 and wot at 12.5:1. It occasionally drops to the 11 mark and depending on conditions will wander in the 13s at cruise but I get pretty good mileage for a 463 cubic inch engine with a 236/245@50 camshaft. I also have good throttle response and acceleration.
Not all 4160 vacuum secondary equipped cars are limited to the street, many drag cars use the 4160 with great results.
It's all a matter of driving style and tuning for that style. The point I was trying to make when I asked about the car's weight and gearing was generally a double pumper is not really suited to a heavy automatic low geared car and a light, high gear ratio car with a large camshaft may be better suited to a mechanical secondary. Just a generality, nothing is written in stone.
I'll refer to the mechanical secondaries as "DP" and the vacuum as "VS" for descriptive purposes. The 4150/4160 designation is not related to the secondary actuation method. It's whether it has a metering jet versus metering plate on the secondary side. Although not common, they do sell a 4160 configured with mechanical secondaries. And there are a number of 4150 variants with vacuum secondaries.
The street advantage of the VS is that it acts like a load-sensing compensation for secondary opening. When transitioning throttle position, having the plates open only as much as needed to meet that engine demand allows high velocity past the plates, and best cylinder filling (volumetric efficiency) for that particular load (not just WOT). This can help fuel economy in normal stop and go traffic, as well as throttle response. It's similar to a torque-based fuel injection with drive-by-wire strategy. Systems setup like that use the TPS at the gas pedal, which then is translated by the ECU what the driver is commanding for torque, and then it opens the throttle (which also has a TPS feedback) just enough to achieve said torque demand for highest efficiency. So for stop and go, even though the lambda readings (or air/fuel ratios) can be tuned the same for both types of carbs, the actual engine efficiency (thus mpg), can be different. It's true, the opening rate on a VS may still need to be tuned based on vehicle weight, engine size, and other factors... and doing it incorrectly could result in a bog. But that's really because too much opening, not because it lacks the an accelerator pump on the secondary side. Whereas the DP really needs to have the accelerator pump on the secondary side to compensate for the quick opening during throttle transitions at medium loads. Which is why throttle response may not feel like a dramatic difference (plus if you have a large displacement in a light vehicle, it's going to be even less noticeable). Also depends on how well the carburetor was sized for the application. In steady state conditions, the engine is mostly driving on the main jets, so mpg during those conditions should be about the same for both carbs.
When tuning for large camshafts, I also prefer adjusting the secondary butterflies (much better than having to drill holes in the main throttle plates). A small adjustment, plus ensuring sufficient ignition advance can go a long way to getting a decent idle. But for those following along, be careful if adjusting butterflies on the secondary side because going too far can uncover the secondary transfer slot, which will also cause a bog.
So from my side, it's not to say DPs have no a place on the street. And they can be tuned for decent drivability. But for the average street build, the VS more often than not, tends to be a better fit.
My apologies to Yockstar for getting off track. Getting back to his original question, keep the carburetor you now have. The 4175 is built as a direct replacement for the Quadrajet. You already have it on your engine. Essentially it's a spread bore 4160, it has a primary metering block a secondary metering plate and vacuum secondaries but with side hung floats.. It should work well on your engine.
Also apologize for for classifying all 4150's as double pumpers, there are 4150's with vacuum secondaries. Some of them still have the casting for the secondary accelerator pump with no pump installed. Using the terms DP and VS as Firebirdmike suggests would have been less confusing. I've never seen a 4160 with mechanical secondaries though, but then again I'm not an expert. Some guys change out the secondary metering plate of a 4160 and install a metering block with the removable main jets, essentially turning it into a 4150 VS. The bowls can also be changed to make them dual feed. Fine tuning 4160 and 4175 secondaries can be done by drilling out the metering plate jet orifices or buying a plate with different size holes. On the average car that shouldn't be a requirement. Other than the spread bore throttle plate, the similarities between Yockstar's 4175 and the 4165 to the 4160 and 4150DP are evedent. The 4175 and the 4160 are both vacuum secondaries with a fuel transfer tube and a secondary metering plate while the 4165 and the 4150DP have two metering blocks and a secondary accelerator pump..
I do agree with Firebirdmike for the average build and the average tuner the VS may be the best choice, not to say the DPs have no place on the street. A lot of folks don't have the time, experience or inclination to start modifying their carburetors, a more forgiving VS may be for them. I'm the same way with fuel injection, anything computer controlled is VOODOO to me Just reading the part about drive by wire, TPS and ECUs made my eyes glaze over. I must be old!
