Carburetor Calculator - Second Strike

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Second Strike Page 12 The Newsletter for the Superformance Owners Group January 17, 2008 / September 5, 2011 Vacuum Secondary Springs Main jet extensions should be considered mandatory for hard launches at the drag strip. The hard launch will pull the fuel completely off the jets in the rear float bowl and stall the engine. This has been verified by embarrassing personal experience. Dyno testing has shown that replacing the vacuum secondary spring with the lightest spring available (the white spring) adds about 40 lb-ft of torque in the mid range with no adverse affects. This is one of the tuning tricks that actually works for a high performance street engine. Vacuum vs. Mechanical Secondaries Once you have figured out that rapid opening of the secondaries is good, why not get a mechanical secondary double pumper and control it yourself. I have heard two concerns about mechanical secondaries – low speed bog and poor fuel economy. As to low speed bog, according to the Holley charts, a 427 with a 750 double pumper should not be given full throttle below 1200 rpm. In the real world, this is never going to happen. Not a problem. The main jet extensions require a notched float. The notched float is standard on the Holley HP series. Install main jet extensions front and rear. Cheap and effective. As to fuel economy, the mechanical secondaries have an additional spring. The additional spring can be felt in the throttle petal so you know when you are getting into the secondaries and the second accelerator pump. When economy is the concern, staying out of the secondaries is easy. Again, not a problem unless you want it to be. Whistles and Main Jet Extensions On cars with serious acceleration, braking, and cornering capabilities (like ours), fuel sloshing in the float bowl is a serious problem. Fuel can slosh into the float bowl vents causing an overly rich mixture. Fuel can pull away from the main jets causing an overly lean mixture. Often the problem is severe enough to cause the engine to stall. To keep fuel out of the bowl vent, Holley offers the Fuel Bowl Vent Baffle, also known at the Whistle. Figure 11: Holley Fuel Bowl Vent Baffle – Part 26-40 Cheap. Install on both primary and secondary float bowls. To keep the fuel from sloshing away from the main jets, Holley offers main jet extensions. Figure 12: Main Jet Extensions and Notched Float Part 116-10 Air Bleeds (Not!) Racers like to change the air bleeds to get more punch coming out of corners. In my experience, this modification makes the car hard to drive on the street with a 500 rpm or better increase in minimum cruising rpm The Holley HP series offers replaceable air bleeds. However the HP documentation says, “Do not change the air bleeds. Serious injury or death may result.” Pretty clear. Leave them alone. Personal Experiences In my position within the Superformance family, I talk to many owners, engine builders, dealers and installers. The biggest disappointment that many owners have is that they have overdone it – too much engine, too much cam, too much carburetor – and the car is no fun to drive. The car sits in the garage until its premature departure to a new owner. I am certainly not immune to the “bigger is better” syndrome. When I replace the original 357 CID 351W in SP 218, I went all out. My goal was to make the car quicker than any original Cobra including competition Cobras and even Shelby’s personal twin supercharged car. The first iteration of the engine exceeded the target horsepower, but it was a beast to drive. It would not cruise comfortably below 2000 rpm. We motored down the Blue Ridge Parkway in second gear. Not good.
Second Strike Page 13 The Newsletter for the Superformance Owners Group January 17, 2008 / September 5, 2011 Making horsepower is the easy part. Here in the heart of Dyno runs indicated that the stock main jets produced a full NASCAR country, there are any number of engine builders throttle air/fuel ratio in the 12.5 to 13 range all the way up the who can make all the power you want. Making it drivable is rpm range, so that was fine. another story. While building the engine for SP 218, I talked to a number of aftermarket performance parts tech hot line. The engine launches hard with no hesitation and pulls strongly They all knew how to make power. No one knew the impact and cleanly to the 6500 rpm redline in every gear. On the on drivability. other end, it is happy cruising along at 1500 rpm in fifth gear. It took a lot of engineering work to reach the drivability goals while still making the performance goals. Changes were made to the cam, ignition, exhaust and carburetion to improve the situation. These changes are detailed in Second Strike Volume 7 Number 1, available on www.SecondStrike.com under Newsletters as a PDF download. Also see Example 1. The results are very satisfying. The performance goals were exceeded. The 129 mph 11.2 second quarter mile (on 15” street tires) far exceeds even the supercharged Cobras (116 at 11.9 according to the Feb 1968 Road & Track road test). And from time to time I manage to lap the field at track events in the street tire run groups. Just as important, the street drivability goals were also achieved. SP 218 has over 40,000 miles on it including over 3,000 track miles. It has been on vacations as far away as New England and Alabama and day trips through our beautiful Blue Ridge Mountains. It is well suited to brisk motoring on back roads. The goal of balanced performance was achieved. In testing on SP 218’s 427 Windsor, we ran the Holley 3310, Street Avenger and HP carburetors on the street, track, and dyno. The HP is the best and worth the extra money. I settled on the HP 750 using charts base on 100% volumetric efficiency. If I had the Calculator back then, I would have picked the HP 650. The Holley HP 750 made about 10 more horsepower than the Street Avenger 770. So the HP 650 would make about the same power as the 770 Street Avenger. And it would run better at low end. Going “better” can be better than going “bigger”. I purchased my HP 750 double pumper used. The original tuner had tricked it out by blocking off the power valves, changing the air bleeds, and changing the main jets. The owner had given up on it because it was undrivable. The first step was to put everything back to stock specifications as a starting point. I then set up the idle to get the correct transfer slot exposure. The holes were already drilled in the throttle plates and they seemed to work out. My first trip to the drag strip was humiliating. It stalled dead off the line every time. The main jet extensions and whistles fixed that. The idle mixture is a little on the lean side on purpose. I get 18 mpg cruising at 65 mph; dropping to 16 mpg at 80 mph. Overall fuel consumption is 14 mpg. Not bad for a large high performance engine in such a blunt car. For cold starts without a choke, it runs a little roughly for about a minute and then settles in. The minimum rpm for full throttle of 1270 rpm is not a problem since I never go there. I am very pleased with the Holley HP 750 and the way it works across the full spectrum of driving. References 1. Holley website http://www.holley.com/division/Holley.asp 2. Holley 4150 & 60 carburetor handbook, Mike Urich, HP Books, Tucson AZ, 1980 3. Holley carburetors and manifolds, Mike Urich & Bill Fischer, HP Books, Tucson AZ, 1976 4. Road & Track on Mustang 1664-1977, Mustang Trans-Am Track Test, Road & Track, Jan 1971, Brookland Books, Cobham, Surrey, England, 1986. Updates 11/18/2007 Add estimated airflow at maximum rpm 12/22/2007 Add Example 2, additional tips 12/28/2007 Documentation upgrade 1/4/2008 Add minimum rpm 1/17/2008 Specify rather than calculate maximum rpm Add support for two-barrel carburetors. Add airflow and volumetric efficiency to output table. Move the VE curve intercept from the maximum power rpm to the maximum power rpm + 500 which has the effect of increasing the airflow at maximum rpm. Add two more examples to documentation. 9/5/2011 Add High Performance Street/Track Engine Type VE = 0.90 at peak torque, 0.85 at peak power Move the VE curve intercept back to maximum power rpm. Calculate maximum rpm as peak power rpm plus 500. Documentation upgrade.
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Carburetor Calculator - Second Strike

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