It's been a good few days. I've got a couple of nice Pontiac AFB's pieced together from a pile of parts. 600 CFM I think. Right now I set them up with the stiffest springs in the pack, 101 primary jets, 98 secondary jets and 7047 metering rods. Kind of a shot in the dark but you've got to start somwhere. I found a pair of secondary air valves with the heavy weights so they won't open to fast with the blower vacuum. I do have the linkage setup progressive so the primary of the second carb starts opening a bit before the secondary of the first carb does. Anybody run AFB's on a blower? How does this setup sound so far? I still need to figure how I'm going to run my fuel lines. Probably something very similar to what I have now. A stub from each carb going to an aluminum pipe. This time I'm going to have a pressure gauge at the back of the pipe.
I am running two 600 cfm Edelbrocks on my blown 409 (out to 474 c.i.). I am NOT running a progressive linkage as I was having a problem of running too lean when I orginally started up the motor some 8 years ago. Here is my setting which I have used for the last 8 years... Slightly rich at idle... RODS 73 X 42, Pri. Jet 104, Sec. Jet 101, Needle & Seat 110 Fuel lines: Used a Morso "Y" block, polished it, screwed a liquid filled fuel pressure guage into the side of the block and made a bracket to hold the block on the bottom/center of the blower where it mounts to the intake. Two fuel lines up to each carb from the top of the "Y" block, fuel supplied to the bottom of the block using a 90 degree fitting, running fuel line to firewall..... Good Luck....
Awesome. That's the kind of info I'm looking for. Considering I'm only feeding 365 CID I should be plenty fat enough to start. Did you make any changes to the accel pump or squirter? What springs are you running?
hey Kerry - Pat Ganahl's Street Supercharging Book has a couple pages on tuning AFBs for blowers. I read it but couldn't regergitate it in any way that would benefit you. I'd read that for sure. The guy that gave him the information for the book may also be usefull to speak with. Those look sweet - good luck.
Yeah, I read the stuff in Ganahl's book. Good tuning info but it didn't help me figure out a base for my tuneup since they started with a compe***ion carb. I've got the carter book that has helped. I also have an odd ball Chevy racing magazine with some serious details in setting up a pair of 750 comp AFB's. Just kind of looking for some real world feedback from folks running them. Your six 94's look a lot wilder! What are you going to do with the power valves? Can you run boost referenced power valves in a pair of the carbs and block the rest?
Kerry, a friend of mine ran two 700 Edelbrocks on a blown 355ci Chevy motor and ran both of them all the time. The back barrels were set up progressive, but he was running on the fronts on both carbs. He never had a problem with running rich. This was on a street driven car. I can not help you with the jetting, as I am not sure what he was running.
ah... the outer carbs are set up as dumpers and no idle circuit so that should take care of them. I think I'll have to play with the center ones if there is a stumble. I think those boost referenced PVs will be necessary - otherwise the power valve would be open all the time and it'll would be real rich- I have that right - right??? Ganahl's AFB guy said you need those heavier wing weights to tune the secondaries opeing to fix stumble on those carbs - did the other articles you have say anything similar? I have a set of those AFBs too - I'll be curious to hear how you come out.
I'm not an experienced power valve guy but that sounds right to me. From just looking at them, the 4bbl Holley power valves seem to be identical to the 94's so the boost referrenced PV's should be a bolt on. Inquiring minds want to know... how did you disable the idle circuits? I scrounged a pair of very nasty older Chrysler AFB's that had the heavier secondary weights so I'm all fixed up. Other than that these carbs are mostly Pontiac. As for the progressive linkage, it was easy to do and now I don't have to tinker with the linkage to get both carbs to shut at the same time for idle. We'll see how it works.
- It was easy - I sent ****ster27 some cores and he sent me back beautifull dumpers. He changed the ****erflies out with the ****erflies that are chamfered to fit the bore and seal (so as not to lose vacuum at idle) - but still open without sticking, then I think he blocked off the idle circuit holes (maybe not I can't remeber it was a while ago) and then the accelerator pump rods were disconnected. That's about it - **** - correct me if I left something out or mispoke
me too - but I wanted these to work! My carb rebuild history isn't so stellar and my cores were rough. I needed an expert here!
Jetting sounds ok, you may want to take a look at the Edelbrock tuning site: http://www.edelbrock.com/automotive/eps_intro.html Since a blower - depending on how many psi it's pumping in - makes an engines CID effectively larger you may be able to get some ideas on jetting from the naturally aspirated gang who run 454-455-460 etc. engines. At first glance, running stiff metering rod springs doesn't look right. I don't have any information on vacuum levels under the carb, but it's probably not too much different from a naturally aspirated engine. And since you're opening the secondaries a little slower with the heavier weights, maybe a mid-range metering rod spring would be the way to go. The heavy spring won't hurt, you'll just end up rich at idle and the bottom end. Sometimes I think folks worry too much about the lean/rich deal at low speeds. If the engine isn't under too much load, there shouldn't be any damage from an overly lean mixture other than the possibility of a lean backfire which could damage blower rotors. A couple of things I've heard about blowers - and I envy the experience you're gonna gain running one - are: You shouldn't pump the throttle prior to or on startup because you can initiate a backfire doing so. The other, don't wing the throttle. Sounds cool, but the helical shape of the rotors can have them slamming back and forth in the case due to airflow changes and a little bit of that can create internal damage. Cool air cleaners. Good looking progressive linkage. I'm guessing you made that one yourself?
Hi Jay, It's been a while. Good point about comparing to larger CID. That reminds me of a couple of articles in some old mags that might shed some different light. Wonder if I can find them. I'm starting out with the stiff springs just to keep it richer but I'm going to try lighter ones step by step and see what happens. I'm sure glad for the air fuel gauge! This whole thing is just a starting tuneup. My main concearn is the actual jets. I'll check out the Edelbrock site, thanks. I've heard the part about not pumping it to start but winging the throttle is a new one to me. Makes sense. Vaphead is the air cleaner man! I just did a bit of slicing and dicing. Yep. Made the progressive myself. Thanks for the kind words. It's so easy, I can't believe what some of the cheesy kits go for. The aluminum slide block has a little bearing for the pivot I got out of a dead hard drive.
That's a great article at that link Jay, Thanks! Especially the calibration charts. Exactly what I was looking for. Sounds like my guess at a setup came pretty close to their recomendation. Now I need to get get the blower reworked and get it all together and see how it works. Can you say impatience?
Well, I can't get my 750 AFB carbs(Eldebrocks actually) to run rich enough with my 6-71 blown 455 Buick. I run 116 primary jets and 69/37 primary rods and still the plugs look like I just took them out of the box, even after a like long cruise! It will idle down nicely to about 650 RPMs so I'm ruling out a vacuum leak. I did have lighter primary springs until I realized blowers like to make alot of vacuum at the carbs, then I reinstalled the stock primary springs and response was a lot better, but still the plugs look like new. I also found out about needing heavier secondary weights, but as of yet haven't been able to locate some( anybody have two they can part with?)
Have you thought about putting an oxygen sensor and a guage in? Reading plugs with todays gas is a tough thing to do. You might not be as lean as you think. Have you noticed a bog because your lighter ones are opening to fast? I'll keep my eyes peeled for another set of the heavy secondarys.
Hello Kerry: No I did not make any changes to the carbs other than what I listed. I used the factory setup then began to richen up the carbs as the motor was too lean for my setup. When you initally start up your motor, keep the RPM's up around 2,500 until you develop some heat in the blower case to properly atomize the fuel. You "WILL HAVE TO" pump your carbs before start up to dump a little fuel into the blower case, therefore, into the intake after cranking. I know everyone recommends NOT doing this, but have they started a blower car in cool tempetures, you probably DO NOT have a choke? A blower car does not like to be started and then immediately pull away. Allow the car to sit for a minute to develop some heat in the blower case or you may have a back fire and blow your intake gasket..... I know most blower intakes have a pop off valve, however; this will not always save your intake gasket, but it certainly will save your blower and intake from exploding on a back fire... To prevent most of these problems, pump your throttle 6/7 times before start up, crank over while opening your throttle slightly, hold RPM's at approximately 2,500 for a minute. If the motor falls over/sneezes/coughs, give a little more fuel unless it is going to stall then just shut your iginition off. This works good for me for the last 8 years, running an 871 on top of the 409, stroked to 474 c.i..... I would recommend pulling both carbs at once.....
Yep, been there done that. I just changed the blower gasket because of a cold start backfire(No Choke set-up). I initially had a big time bog when mashing the throttle from idle( actually would stall) then I started reading about secondary tuning the AFB and put the heavier primary rod springs back in and jetted up to the 116s to get more fuel to the engine (the springs work against the vacuum holding the rod down in the jet). It's a lot better than it was, but I'd still like to tune the secondaries. I've considered the O2 set-up as I have experiance with late model diagnostics, but my headers and collectors are lake style and stick outside the frame.
I put one in at the tube going from my left lake style header to the muffler under the car. I used a heated one since it was downstream a ways. You can see it if you go looking but otherwise it's largely unseen. I've got an ****og Westach gauge. Beats the ****y LEDs for sure.
So why is progressive linkage not a good idea on a blown engine? Is it that is not necessary or is it just that everybody uses straight linkage?
Just my opinion, maybe it's because the blower moves so much air, a progressive linkage set-up might cause the engine to be too lean at cruise RPM? Or perhaps the rear cylinders (away from the operating venturies) wouldn't receive enough fuel. I had thought about a progressive set-up ,but want to tune the carbs with the straight linkage set-up I have now first.
I'm guessing it may be due to the lesser 'pump' shot with only one acellerator pump dumping fuel off idle. Even so, seems a well sorted out engine would overcome that and the secondary carb's pump shot would help eliminate bog when you roll the throttle on. It may be a fuel/air mix distribution problem as well, but it seems the blower itself would distribute the mix fairly well before it hit the intake plenum. Maybe you need three 4 bbls. It's been done on a blown engine, but it was a lot bigger than your 331 incher . . . somewhere around 500 inches +. I did a little searching in some of my books yesterday and failed to find any progressive linkage setups. Most of that due to guys running Holleys and their requirement for a transverse throttle linkage. Lots of AFB type carbs though, and none of them that I saw had progressive. Even so, I think you should give it a try. Maybe have a straight linkage setup to swap to so's you could see just what does work. With straight linkage you have a form of progressive in the air controlled secondaries. And for the Holley lovers, seems like a vacuum secondary carb would work great in a street blower setup. Gas mileage should be considerably improved over a pair of double pumpers. I hope you have an easy way to put a vacuum gauge on the engine. Even temporarily. It would be interesting to see just what you get. Along with learning where the vacuum/boost crossover line is at various RPM and throttle settings. Along those same lines, Stewart-Warner used to sell a combo vacuum/pressure gauge that was about the same size as a tach. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You may want to give some thought to having operating chokes. From what I read here, seems it would help. When I ran the dual quads on my 32 I had both mechanical chokes set up to operate from one cable. Easily done, just cut a piece of armor (1" - 2") off the overlong cable, mount that in the front carb bracket. Make the choke wire long enough to reach the front carb. String the choke wire through both carbs choke operating arms and the front carbs short piece of armor, connect then clamp the armor to the rear carb bracket. (Armor being the wound outer wire on a manual choke cable. Not sure - as you can see - what the official name is.) When I did the dual choke bit I tried to find information on whether one choke would do it or not. So I connected both. The car lit off really well with both chokes operating. Since then I've found that factory dual quad installs only had one choke operating - that on the primary carb. On my single 4 bbl installs, I run electric chokes and am pretty sold on them, but I'll retain the manual chokes on the dual quads. If I ever got a blower setup I'd like to give dual electric chokes a try. for sure, it couldn't hurt....
There was a car at the Springfield swap meet blown with three carbs. It did not appeal to me at all. Yuck in fact. You know I like to be different. ;-) We'll see how it runs with the progressive. I had just bought a boost/vacuum gauge on ebay. A 2 5/8" to replace the SW vacuum gauge I was running. I liked the VDO Series 1 gauge better but it was 2 1/16'. Due to where I have this located I like the larger gauge a lot. I do plan on running a vacuum gauge above the blower temporarily, to help figure out what springs I want to run. Do you think it would have any permenant use other than that? The choke on the primary carb will be fully functional using a manual cable. The choke on the secondary carb is clamped open and the fast idle cam has been removed.
I'm not sure. It seems that any vacuum gauge diagnosing type problems - blown head gasket, leaking valve etc. would probably show up on the pressure/vacuum gauge. If the vacuum gauge tapped into the little manifold at the top of the blower isn't in the way, I'd just leave it. You'd get a bit of an education running it perhaps, but the real education will come from the pressure/vacuum gauge. When you get it running and sorted out I'd like to see you do a tech report on pressure/vacuum levels, where they cross over etc. Both under normal drving and performance driving. It would be interesting as well to see what the upper deck vacuum gauge reports . . . I'm guessing it would be very similar to a naturally aspirated engine except it would get closer to zero and maybe even peg the gauge at the bottom under full acceleration. I envy the education you're gonna get....
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