I thought i would post this here and see if anyone else has an opinion. My 292 6 with 4 bbl has a terrible off idle lag no matter what i do re fuel I cannot fix it. finally have come to the opinion that the unheated aluminum intake gets so cols during idle that it starts to pull fuel out of suspension and then when your on the line and getting ready to go the engine behaves like its lean then riach and bogs and farts till it clears itself and you have the rpms up. then it runs fine. the intake is so amazingly cold that a wet hand can stick to the bottom of it at idle not great in keeping fuel in vapor form. I am going to attempt to wrap the entire intake(or as much as i can) with 3/8 copper line i estimate about 30 ft of it then wrap with insulating tape and run coolant through it to try to get the runners warmer and keep the fuel from puddling. Any ideas or suggestions. I struggle with the IHRA auto start cause i cant get up on the rpm before its time to go.
You pretty much answered your question. You need intake heat. Langdons has an adapter to utilize your coolant as a heat source if you are using headers. http://www.langdonsstovebolt.com/store/#!/Intake-Water-Heat/c/18665977/offset=0&sort=normal Sent from my iPhone using The H.A.M.B. mobile app
I don't seem to have that problem with my 300 Ford 6. Maybe my exhaust pipes are close enough to the manifold to heat it a bit. How does yours look?
Holley carb ? Check the squirters, had somewhat same problem. I ended up with 37`s when I was running single carb. If it has a flat spot when winging it you need bigger , If burbles or loads up you need smaller. The motor, if warm, was cured this way on my car
Had a Carter 400 CFM on unheated aluminum intake on 300 CID Ford. No problems. How large is the carb? May just be calibration. Jon.
i am running a 650 holley dp have spent seasons fiddling and really just decided to do something with it after this summer. Its at idle and during tip in to get on the convertor that it gives problems. it runs like gangbusters once up on the rpm the heat wrapped pipes probably does not help I dont have much problems at my own track we have lots of turbo cars that take time to get up on it so the starter gives everyone time the IHRA autostart is a different maytter i just cant get it to roll on fast. it wings up good in neutral
I've got a Holley 3310 750 cfm with vacuum secondaries on my 300 Ford 6 (pictured above). While I have not achieved the maximum performance out of my dragster yet it appears that it likes the carb. Down the road I'll make a two carb manifold to see if better fuel distribution helps on the straight 6.
What's your water temperature when you're staging and does it do it regardless of engine temp? Maybe unwrap those headers and see if you can get some heat into the intake that way.
engine temp makes not much difference but weather plays a big part cooler humid days are the worst I played with the carb and thought maybe too much unhooked secondary and no difference on the line and ran out of air before the 1/8 still pretty sure heat in the intake is the issue
Coming from a background in aviation, if you are having problems with cooler humid days, there is a strong possibility that your problem is carburetor icing.
Try resetting the idle: (A) idle mixture screws leaner (B) idle throttle positioner screw opening the throttle plates a bit I still think the carb is too large (ON THE PRIMARY SIDE). Because of this, disconnecting the secondary side will not help. The air velocity is insufficient to properly atomize the fuel, resulting in the necessity for an overrich mixture. The overrich mixture then can cause icing. Adding heat will probably help, but may not completely solve the issue. Remember there are three ways to improve atomization of the fuel: AIR VELOCITY, heat, or richer mixture. Very difficult to account for low air velocity; which is what caused the development of the spread-bore four-barrel. If you have access to a 400 CFM carb try it. Rethink the total carburetor CFM as being a sum of primary CFM plus secondary CFM. On a 400 CFM square-bore, the primary side is approximately 200 CFM. On the 650 CFM, primary CFM is approximately 300~325 CFM. With the smaller primary, the venturii air velocity is significantly increased, thereby aiding in fuel atomization. And then contrast the above figures to a 750 CFM spread-bore which is only 150 CFM on the primary side. Again, much better venturii air velocity. Disconnecting your secondary changed nothing on the primary side, but did limit the total airflow, once you got past the low velocity issue. Jon.
Tom Langdon has been messing with this same problem and he has modified a 4 bbl into a 3 bbl so very high flow at the lower rpm to help with the air movement
My dad had a tractor we used to move snow that the carb iced and ran like you describe. I took a piece of 1 1/2" or so flexible exhaust tubing attached it to the exhaust manifold and ran it up to the intake like 70s GM cars had to the intake snorkel. Problem solved and this was in freezing weather. You could rig something similar up in about ten minutes to see if icing is actually the problem.
I have a bunch of 300 Ford powered dragsters, one that uses an aftermarket aluminum 4V intake (the others use my home-built 3-2s). On the aluminum intake with a roller cam engine I have experimented with a 650 DP, 600 vac sec, 480 vac sec, 390 vac sec. The 650 was too much flow and not enough velocity on launch, giving me the same headaches as you are experiencing. While I was not entirely satisfied with the throttle response of any of the above carbs I finally tried the dual Autolite 2V carbs shown below. The two carbs I used have an equivalent 4V flow of only 340 cfm (they have 1.01" butterflies). They are of 1961 vintage and I swear to God the only thing I had to do to them after cleaning was adjust the idle speed screws. THEY WORK GREAT! Plus they are dirt cheap to buy. Plus - they look vintage. Runs high 11's @ 2050 lbs w/ 2.47 rear gears.
I may be putting too much thought into this and the math may be wrong but when you think of an internal combustion engine and it's function, it is basically just an air pump. That being said, if you take a 300 cu. in engine and look at it's function it will process 150 cu. in. of air per revolution seeing as it only brings in the charge of air fuel mixture every other cycle on a four stroke engine. That means that at 6,000 rpm it will process 900,000 cu. in. of air/fuel mixture. 1 cu. ft. equals 1728 cu. in. so by simple division 900,000 cu. in. is 520.8 cu. ft. so considering that flow as normally aspirated and not being pressurized with a supercharger it would seem that anything over 520.8 cfm would be too much and with the flow restrictions of most "normal" intakes, more than likely it wouldn't even need that much flow. Nothing really important in these thoughts, just the wandering of my mind as it absorbs the morning coffee. You may now return to your normal scheduled programming.
My Valiant "bogged" then I stopped focusing on base timing and set the total timing at 36°. Every engine is different, but even though my base timing is now advanced at 20°, it runs like a raped ape. I read the plugs before and after and seen no change. I did some minor changes to the Holley with the accelerator pump ramp and the timing of when my secondaries engage. But all in all, my 9.5 to 1 compression, now 232" slant six runs and drives great with zero detonation. My "bogging" off idle is gone... But hey.. My personal experience, everyone else has/have different experiences. I am not proclaiming this is your problem. Just a little idea to ponder if you find any value in it. Good luck! Sent from my XT1585 using The H.A.M.B. mobile app
Hey 64 DODGE 440, I enjoy reading your posts here and elsewhere. Your reasoning and math is sound but you are leaving out one more important aspect of the calculations: VOLUMETRIC EFFICIENCY. That 521 cfm flow is correct assuming that the volumetric efficiency is 100% That is, the actual flow equals the theoretical flow you so astutely pointed out in your post. But in reality the cylinder does not completely fill to atmospheric level; restrictions in the intake tract prevent total filling. A typical six, with a head designed for efficiency and low speed torque only is around 75% efficient at the torque peak. And in the relative RPM stratosphere of 6000 RPM the volumetric efficiency of that six might only be in the 60% - 65% range. So 60% of 521 cfm is only 312 cfm. And 65% is still only around 339 cfm. In my slap-dash, hurry-up way of explaining things I am leaving out some factors like carb flow test depression which may lead you to upsize just a leedle beet from that 339 number but the theoretical outcome will still be nowhere close to the 600 cfm carb size most commonly found around small V8s. A 390 cfm carb is probably a much better choice. My son has one on a '48 Anglia and it works better than a 600 we tried.
Good point Frenchtown. I knew that the cylinders didn't fill 100%, but really didn't have any idea of how low the percentage could be. I just remember back in the late '50s talking with a serious hot rodder that my brother knew (I was just in my early teens back then and trying to learn everything I could) and he told me that over carburetion could cause some serious problems. He and his father had built some fast cars so I paid attention.
The 750 Holley works well on my 300 Ford 6 probably because it has vacuum secondaries. This summer I'll put a tattle tail on the secondaries to see how far they open.
I went to a race where another racer had a 250 Chevy in his altered and was running a 750 cfm Holley carb. After a discussion on what size carb to use I said "Disconnect the secondaries and go make a pass." He did. It ran the same e.t.
We ran a stock 4.0 jeep motor in an SDRA car at Tulsa this year. the only changes were a set of after market headers (like the factory ones). A 650 dp on a Clifford manifold. We also tried the 2bbl only thing, no change in Et or mph.
I know "bigger is always better" ; those that have disconnected the secondaries and/or run two-barrels have proved it! However, if one chooses a carburetor with the approximate correct CFM for the engine, the smaller 4-barrel WILL perform better than the big 4-barrel with either vacuum secondaries or secondaries disconnected, or the two barrel because of increased nozzle velocity in each venturi. Easy to lie to oneself, but if one actually listens, one's engine will not lie! Jon.
Here goes something that runs against everything being said. I`m running a 300 cu. in. chevy six and started out with the standard 750 holley. The more cfm I throw at it the faster it goes. I`m now running two afb`s 640 cfm each and into low 11s. Might be the metering plates inherent to the afb, Who knows And yes the motor has compression and everything else I could throw at it
Don't know what AFB's were rated at 640 CFM. Those rated 625 were 275 on the primary. Given the auxiliary airvalve on the secondary side of the AFB, which will not open without sufficient engine demand; it may be that you are actually running (2) 275 CFM two-barrels. Would be interesting to see what the times do if you disconnected the secondary link on both AFB's. Running two carbs on an inline engine should (depending on the design of the intake) improve the cylinder fill consistancy. Jon.
Carbking, The cfm rating I posted is something I got off the interweb researching the carb numbers so absolutely I could be wrong. the reason I went with two carbs is exactly what you said. The plugs weren`t uniform in color until I did this. The reason I used afbs is because of what I called the metering plates on the secondaries which you called you auxiliary air valves. I`m running a Clifford intake Not trying to dispute any of these posts just find this s.... interesting
Old Sparks - Carter only published CFM ratings on a handful of carburetors (mostly aftermarket) before the early-1970's. There are ratings for a few more in the company internal files. They did flow the carburetors (I currently have custody of one set of the flow tests); but on O.E. carburetors, Carter wasn't interested in a CFM number. What Carter, and presumably the O.E. engineers, were interested in was fuel consumption at a specific vacuum. And before anyone asks, there is insufficient information in the tests to come up with a CFM rating. I asked the Carter engineers when I got the tests some 35 years ago. Even if you are only using the primary side, still a pretty good choice of carbs because of the infinate tunability of the primary side of the AFB. Jon.
carbking, the good choice of carbs had nothing to do with wisdom , just maybe luck. most of my experi ence is with constant flow fuel injection running alcohol so the tuning of this combo is mostly close my eyes and throw something at it so talking with someone in the know helps alot
This set up proved to me that fuel distribution on a straight 6 is important. I first tried running the engine on just the front carb(carbs are on a plenum). You know, so I wouldn't have to fiddle with two carbs during break-in. It hardly ran and I could put my hand on the three rear exhaust pipes, they were that cool. I put the carb on the rear and the front three cylinders didn't fire and the pipes were cool.
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