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MG MGB GT V8 Factory Originals Technical - Carb settings with a hood scoop
|What results have you had with mixture ratios and jet/rod combos when using 'thru the hood' open scoop. My question arises with a MGA, 215 Buick having a D&D down jetted Edelbrock 500 carb. I have about 2" max for a filter and thought an open scoop would give me a large filter and ambient air into the engine. I am concerned with WOT at speed giving a lean mixture due to the supercharging effect. So far I have heard 1)No problem and 2)Need to decrease rod diameter off idle. Any experiences?|
|I've got a Rover 3.5 in my MGB running the same carb with a hood scoop. I've played around with the needles and jets trying to get the engine to run leaner. I'm currently using .063 x .047 needles (#1443); .083 main jets (#1422) and .092 sec jets (#1425).|
Up to now the car runs good but certainly doesn't need any choke to start and will produce a bit of black smoke on hard acceleration. The plugs indicate a rich mixture around the threads but the electrode is a tan colour.
Scott, what needles/jets did D&D use in your carb? For an air filter, I'm using a K&N 2" filter with the "Extreme" 14" top.
Something of interest in the Edelbrock carb owner's manual: "DO NOT allow the vehicle air-stream to blow across the top of the carburetor(s) such as on an open-bodied car or full-bodied vehicle with a tunnel-ram manifold. The flow of air across the carb will result in an upset to the fuel metering that cannot be accommodated by recalibration since the change to the A/F Ratio will be different for every vehicle speed".
I haven't noticed any difference running a solid top filter and the K&N Extreme but as I mentioned, the car still runs a bit rich. I'm putting the car on a dyno tomorrow with a mechanic checking it over so may have some more info then.
If Dan reads this thread, perhaps he will have some answers.
Dan put .062 x .052 rods (#1441) and .080 jets (#1421) in the primaries. I haven't had it running yet. This seems to be the standard setup for a stock 4V 215.
I am leaning away from this since I thought some metering problems may happen. How do you think the flow changes the A/F ratio?
Come on guys. I know alot of you have considered this.
|I've got the same numbers for needles and jets from RPI in the UK.|
Good time to have this discussion as I have the results from the dyno run.
We ended up installing the stock needles and jets; #1460 .065x.052; #1422 .083. In the secondaries, I've got .089 #1424 but the mechanic suggested .086 #1423.
The numbers from the dyno: max power 142.7 @ 4750 rpm; max torque 187.9 @ 2500. Base run: power 140.2 @ 4750 rpm and torque 182.2 @ 3250. This is at the wheels. I asked the guys if there's a formula to work back to get flywheel HP and they said the wheels are typically 20-25% less. Based on this, I'm guessing 170-178 at the flywheel but no real basis (other than wishful thinking) for this. The 170 is in line with a stock SD1 3.5 engine though.
The guys also suggested upgrading the ignition to an MSD system for more efficient burning. I've got a pertronix in there now and it works okay. Once I change the ignition and the sec. jets, I'll take it back to dyno it again.
I asked them about the note in the Edelbrock manual about the air flow across the top. He said this was more to do with open-top carbs (ie. race cars) as the air flow across the top works like a venturi and will create a low-pressure area on top of the carb thus affecting the mixture. With the K&N assembly, it won't be a problem with a scoop.
Interesting thing I did find out; with the hood open, I gain about 4 extra HP. Closed, it robs power due to lack of airflow. A/F ratio changed quite a bit with the hood closed. Go figure....and I thought I was doing good with the scoop. Unfortunatley, the scoop I'm using isn't quite tall enough to capture the air flow as intended. Looks good but it would have to be about 2" above the hood and situated further forward to be really effective. All I need now is access to a wind-tunnel! I didn't ask about cowl-induction scoops.
If you can get your car onto a dyno when it's on the road, it's a great way to tune it. Even a degree or two on the timing changes things quite a bit. My engine is set to 3 BTDC at 700 rpm.
|"I asked the guys if there's a formula to work back to get flywheel HP and they said the wheels are typically 20-25% less."|
Are you using an automatic trans? I was told 15-16% loss with a straight drive.
3 degrees BTDC seems a modest amount of advance, from what i've heard most Rovers like between 8-13 degrees of advance, but also to consider is the fact that the timing marks can be very inaccurate.Also the power loss at the wheels depends on the cars actual tranmission system, tyre size and pressures and any other frictional losses also make a difference.On a Rover V8 with LT77 and 3.07 diff I am told that 30-35BHP is about right.ie a 200BHP engine will show about 170 rear wheel HP and a 300 BHP engine will probably show approx 265 rear wheel HP allowing for addtional frictional losses created by the higher power engine, assuming everything else is equal,
|Scott, don't worry about the hood scoop messing things up. You would have to exceed 100-110mph before it becomes an issue, uness the opening on your scoop faces the windshield. The low pressure area in front of the windshield would consistently draw more air into the carb than the forced induction of a forward-facing scoop. |
This information is taken from a muscle-car show I saw on the History channel a while back. Engineers at the 3 big American auto manufacturers agreed that while scoops sold cars, they aren't usually functional, with the exception listed above.
|Transmission is the Rover 5-speed that came with the engine. I like your numbers even better, Carl.|
The timing was set while the car was on the dyno and it was interesting to see the changes when a degree or two was added or taken away. You're probably right about the marks, Kevin. 6° BTDC threw everything out the window!
I'm using the drive-train you mentioned with 205/60-15's so those numbers would appear to be about right.
|Jeff, I'm sure you meant the "high" pressure at the base of the windshield, AKA "cowl induction". I am an MG midget and Barracuda owner. My 1969 big-block Barracuda has a 4.5" high by 27" wide, forward facing scoop that sits on the top of the bonnet & seals (with a foam rubber gasket), to the base of the aircleaner. This setup is to allow cool air to enter the carb but not introduce positive air pressure to the engine compartment itself. So as to not create resistant back-pressure to cooling air trying to pass thru the radiator and eventually help carry away heat from around the engine proper. Later Mopars employed "above boundary-layer" scoops or "snorkles" whose openings were located several inches above the hood line to get into the laminar airflow but, they compromise some of the driver's visibility. I'm keeping my "Super Stock" scoop because it is period correct and looks scarey to the Cheby and Phord racer boys. (Small birds beware)! M|
|Marc Judson (2- 1978's)|
|In order to get a ram effect, yhour scoop would need to be all the way to the front. The faster you go the higher the air stream will be. Experiment with cotton teders and place them in front of the aair cleaner as it is now. At low speeds they will react to the turbulant air increase your speed and they will not move. Max air ram effect with a well design scoop can be from 3 psi to 6 psi. This can be achived by having a cool air box plus other things.|
Look at picures of Salt flat cars and see where the scoop is.
This thread was discussed between 26/09/2005 and 08/10/2005
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