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  • 4 Post By twobjshelbys
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Old 08-01-2020, 06:39 PM
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Default Need advice from anyone with an automatic transmission or engine expertise

Hi guys

I test drove a cobra recently with an automatic transmission and felt that the car would literally drive forward on its own more than I wanted it to without me pushing the gas pedal!


I’m currently looking to build my own cobra and have the luxury of picking any engine I like (which I will likely mate to an AODE transmission).

I was told that the reason why the cobra was literally “driving itself“ was that with the amount of horsepower that the car had (about 450hp) coupled with the fact that it was fuel injected would keep the car rolling forward (due to its light weight) because an automatic car is always in gear

In order to minimize this, I was encouraged to go with a carbureted engine as opposed to fuel injected. Would you guys agree?

Also, I would really like to get a 427, but do you think that the higher horsepower would push this car forward much much harder (say when I take my foot of the brake) than something with less horsepower, or if I go with a carbureted engine do you think I’ll be OK with a 427?

I’m currently considering the Roush 427SRX vs 347SR vs 331SRX

Many thanks!
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Old 08-01-2020, 08:12 PM
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Somebody was telling you stories.
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Old 08-01-2020, 10:10 PM
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+1^ on what Tony said but I would go a bit farther and say the individual who told you that either did not know anything about what they were talking about or had some other agenda that wanted you in a carbureted version of the car.

You should know from first hand experience that fuel injected engines do not behave that way. There are lots of high powered daily drivers at or over 500 HP, available from many of the manufacturers, that do not behave like that. If anything they can be controlled as well or better than carbureted engines at any engine speed because of the programmable digital fuel injection and sensors to monitor and correct for actual vs commanded engine operation.

If the car was 'drive(ing) forward on its own'. chances are the idle was set too high and/or the converter had too low a stall speed selected for the converter. You should be able to get a fuel injected automatic transmission equipped car to behave indistinguishably from your daily driver with the one exception the acceleration will be stunningly better.

A properly built and tuned engine with EFI and an AODE will make for a very satisfying driving experience. The operative words here are properly built and tuned. Use a cam with tight lobe centers advance it a couple of degrees and you will get a lumpy surging idle that will not smooth out until you get somewhere between 1500 and 2000 rpm. That will produce an unhappy driving experience. A higher stall converter will help but the real problem is the lumpy low speed operation of the engine. With a 5 speed manual this is not as big a problem because you just drop down a gear to get out of the lumpy rpm range that makes the car buck and fart. With an auto this is not as easy — the higher stall converter will help but it will not eliminate the problem.

If you go to a wider lobe separation angle (LSA) on the cam put it closer to straight up you will get a smoother idle, good vacuum and you will loose the lumpy 60's competition Cobra idle so many people like. What you will get is a stunningly fast car with excellent driving manners — so good your wife or daughter would have little difficulty driving the car.

An item you did not speak to (and most of us ignored when building/getting our cars) is the exhaust. Nothing looks sexier or sounds better than the side pipes. What most first timers do not realize is that exhaust outlet is 24 inches from your left ear — all the time. On anything but short trips (and probably even those) you will need ear plugs to protect your ears. At highway speeds they are mandatory. The fix is under car exhausts. Virtually all the replica manufacturers offer them, all you have to do is ask for them.

Don't get BS'ed by the anti EFI rhetoric of whoever gave you that guidance. It is not real world. FWIW my engine makes a little north of 770 FWHP and 645 RWHP out of 289 cubic inches, so it is in a relatively high state of tune. It idles like a Singer sewing machine at 750 rpm essentially the same as my BMW daily driver. It does not produce the lumpy 60's competition Cobra idle but it is frightenly fast and gets 25 mpg on the highway. You do have to be a bit careful passing. On the highway at 70 mph in fourth gear if you roll into the throttle too quickly, with the traction control turned off, the car will begin to black track with a corresponding increase in vehicle control issues.

Don't worry about drivability with EFI and an automatic. They will go together like peanut butter and jelly — although you might occasionally miss the fun of changing gears manually.


Ed
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Last edited by eschaider; 08-01-2020 at 10:13 PM.. Reason: Spelling & Grammar
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Old 08-01-2020, 10:49 PM
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It was driving forward because the idle rpm was too high for the stall speed of the torque converter, period.

idle was most likely set because the cam shaft chosen requires it.

They did not match the engine requirements to the torque converter or visa versa depending on which way you look at it.

EFI bad, carb good, comment is total BS. EFI will help get the idle rpm down on a big cam, not hurt it.

Unless you have a physical handicap it's silly to put an automatic in a Cobra, unless you have a mental handicap.

You really need to back up several steps here and explain what you want to do with your Cobra.

Also there are better options for less money on your engine choice. Big names usually don't give the best customer service to the little guy.
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Old 08-02-2020, 01:45 AM
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The only time I had as problem was when parking on gravel and the back brakes were not working properly - there was enough drive from the torque converter at idle to push the car forward with locked front brakes sliding over the gravel!!!! Needed some quick reactions to knock it into neutral and stop before hitting the wall in front of me as I was close to it.
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Old 08-02-2020, 01:57 AM
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My Cobra is the only automatic I have ever owned in fifty years of car ownership and I must say that i find it a really nice combination that suits the whole torque profile and low speed low rev cruising ambiance of the Cobra. And even on the race track I can stick it in second and comfortably do 60% of our local race track without a gearchange or dropping below 3000 RPM which suits the torque curve of my engine. (A Chevy 383 with TH400)
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Old 08-02-2020, 02:11 AM
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Thanks for all the advice gentleman

This car will only be used for street driving, and will never be raced.

In regards to the automatic transmission, I have a little bit of a knee issue but I also want to cruise in the car without the need for manual shifting.

In regards to carbureted versus EFI, I was initially going to go for EFI (because I wanted less smell in my garage) until I was told the difference in smell between the two is nearly negligible nowadays and the carbureted engines are just as reliable without the need for tinkering (I would not be changing altitudes and the weather in Los Angeles never gets too hot or too cold).

The engine would be professionally installed and tuned, so I just wanted to see if you guys thought that if I would be better off with carbureted vs. EFI or whether you think the end result in drivability would be the same
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Old 08-02-2020, 02:17 AM
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Agree with all comments from the lads above.

Idle speed (engine torque) versus torque converter stall (low or stock stall), is why it "drives forward".

You would find it will hold on certain hills, and then when they don't, which is complaint I get a lot of with Autos.

Loosen the torque convertor to 2500 stall rpm or so, and stay with EFI if funds allow. You will achieve a much better package.
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Old 08-02-2020, 05:53 AM
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Quote:
Originally Posted by mdbound2005 View Post
Thanks for all the advice gentleman

This car will only be used for street driving, and will never be raced.

In regards to the automatic transmission, I have a little bit of a knee issue but I also want to cruise in the car without the need for manual shifting.

In regards to carbureted versus EFI, I was initially going to go for EFI (because I wanted less smell in my garage) until I was told the difference in smell between the two is nearly negligible nowadays and the carbureted engines are just as reliable without the need for tinkering (I would not be changing altitudes and the weather in Los Angeles never gets too hot or too cold).

The engine would be professionally installed and tuned, so I just wanted to see if you guys thought that if I would be better off with carbureted vs. EFI or whether you think the end result in drivability would be the same
I can only guess the same person that told you about the car having so much horsepower is the same person that told you a carburated engine is the same as a fuel injected engine?
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Old 08-02-2020, 05:56 AM
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You need to reach out to "Saltshaker" here on the forum as he is in the process of converting his latest build to an automatic with overdrive transmission behind his very stout FE. He has done all of the research you are in need of.

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Old 08-02-2020, 06:18 AM
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Look into lift-out engine/trans/wiring packages from salvage people-get a modern complete assembly from a very new, low mile performance type vehicle-------you/ll have horse power, driveability, fuel economy, alternator/ac/water pump, etc, exhaust manilods for under car exhaust----------just search for engine drawings to see what will fit in your chassis---------
well, that will eliminate the overhead cams, hemis, ------????????? really leaves the aluminum LS/LT versions---------and probably best pick will be Camaro because the Corvette has rear mounted trans
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Old 08-02-2020, 08:13 AM
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It's all about stall speed on the torque converter. If you're fighting the brakes, then you didn't go high enough. Mine is rated at 3,200-3,400, and it works well. But I think I could have gone a little higher, say 3,400-3,600.

Also, remember that with a higher stall, you'll generate more heat at low speeds. Make sure your cooler is up to the task.
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Old 08-02-2020, 08:52 AM
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Back in the days the factory sold carbs, which is nearly 40 years ago. Transmissions were 3 speeds with no lock up toque converter. I believe by 1988 the EFI computer was controlling the shift points of the transmission. The old transmissions had complex valve bodies with a throttle position mechanically connected and vacuum connected. All this was to figure out when to shift. Then hydraulic pressure powered the band brakes to change the gears.

Today's transmissions do not have all the mechanical crap. The ECU tells the transmission when to shift and when to lock up the torque converter. Ford is putting 10 speed automatics in everything I just looked at, while Lincoln is still using an 8 speed in the Nautilus, I just bought. The previous Ford transmission was a 6 speed that was used for about a decade.

Then there are bolt patterns that different engines have. Not all transmissions will bolt to all engines within a manufacturer. In fact the 4.6 ltr that came in the front wheel drive Continental has a different bolt pattern than the rear wheel drive cars with the same engine. No rear wheel drive transmission will bolt to the front wheel drive block. There are way to make adapters, but this is a big hairy can of worms.

Now I'm sure that someone has figured out a way to control modern transmissions without the ECU and a way to connect up an old transmission to a newer engine. Be aware that this type of stuff is usually figured out by the racing world and they only care about winning races. Drive-ability for a street car is not even a consideration.

You need to do a lot of learning to head down this road.

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Old 08-02-2020, 09:04 AM
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Quote:
Originally Posted by Jerry Clayton View Post
Look into lift-out engine/trans/wiring packages from salvage people-get a modern complete assembly from a very new, low mile performance type vehicle-------you/ll have horse power, driveability, fuel economy, alternator/ac/water pump, etc, exhaust manilods for under car exhaust----------just search for engine drawings to see what will fit in your chassis---------
well, that will eliminate the overhead cams, hemis, ------????????? really leaves the aluminum LS/LT versions---------and probably best pick will be Camaro because the Corvette has rear mounted trans
I agree up to the point of only leaving the Chevy LS engines. Backdraft has been putting over head cam modular and coyote engines in for decades now. I have seen them in FFR as well. I think Superformance also advertises this.

If you care anything about the racing history of these cars, at least stay with a Ford engine.
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Old 08-02-2020, 09:49 AM
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I just wanted to see if you guys thought that if I would be better off with carbureted vs. EFI or whether you think the end result in drivability would be the same
Carbs pros:
can make a lot of power.
are cheaper. (single carb setup)
they were original equipment in Cobras.

Carbs Cons:
Fuel may boil over when shut off.
Vapor lock.
Choke dump to much fuel on start up, contaminating oil.
Engine flooding.
Lean condition can backfire flames up the carb.
Cobra acceleration can uncover the jets and stall engine. (common)
Tuning happens at one condition.
They develop leaks.
Fewer and fewer expert tuners are still alive today.
Generally use with a mechanical distributor (vacuum and centrifugal advance)
Many certified mechanics today have never worked on a Carb.

EFI pros:
Controls both fuel and timing (shift points too).
Infinitely tune-able
compensates for temp, altitude, and sensor calibration drift.
Lower emissions
Better fuel mileage.
Consistent perfect fuel on start up (no flooding).
Timing curve can be shaped to fit the engine needs.
Can retard timing and save engine with knock sensors.
Easily handle boost.
Will rev limit the engine.
Uses timing to rapidly respond to idle rpm changes - shifting auto into gear.
Mechanic everywhere understand EFI.

EFI Cons:
Initial cost.
Self tuning is a steep learning curve.
Radical cam engines are difficult to tune

Note not all EFI systems are the same. This is another huge subject.

EFI engines last longer and can go further between oil changes, because the fuel control is not washing the cylinder walls down with too much fuel on cold starts, as a carb does.
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Old 08-02-2020, 02:00 PM
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OP, as you begin to investigate EFI systems some of the attractive points you will find for the aftermarket systems will be engine protections for low oil pressure, fuel pressure, lean conditions etc and traction control systems. The first protection systems I mentioned will protect your engine the last one protects you. These cars can swap ends so fast you didn't realize what was or did happen. The traction control can save you and your car. It will not turn it into a docile DD but it will make it more difficult but, and this is important, not impossible to loose control of.

An additional consideration is the engine wiring harness. Aftermarket systems usually (almost always) require you to build a wiring harness. The system I will suggest allows you to use an existing wiring harness off an OEM vehicle. The benefit to the OEM harness is you don't have to build it and it is the most durable harness typically available. They can be inexpensively sourced from a salvage yard and here is the best one — they are plug an play with the right EFI system.

The challenge with the aftermarket EFI systems is that the good ones that do all this stuff will end up costing you ~$6K or more by the time you are done. I am going to suggest an aftermarket alternative that is much more price friendly. You should know before you read the recommendation that I am prejudiced (in a positive manner) about the particular system. There are may systems outer there that can do some of the things this system I'll suggest can do and others that can do all of the things this system can do but there are no systems that can do it as inexpensively.

The system is the MegaSquirt3 Pro Plug and Play EFI. Click here => MS3Pro PnP to jump to the product page. It will work with the Ford OEM wiring harness for a 99-04 Ford Mustang 2,3, or 4 valve. If you use a pushrod engine then shut off the OHC stuff in the tune file and tune it as a 5.0 L engine. An extra added attraction is the system uses all Ford OEM sensors so you don't get ripped off for pricey aftermarket sensors that if you knew the brand name of could be obtained for pennies on the dollar.

The included tuning s/w is unusually flexible and will handle virtually any type of ignition, fuel system and injector selection you may want to use. It will support a speed density or MAF based fueling model which will be more meaningful to you as you dig into the project. The $1,350 purchase price includes both the tuning and dataloging s/w. If you use a MAF based system once tuned you can go anywhere with the car. If you use a speed density you can go most places. Significant altitude changes or significant atmospheric changes will require some tuning massage.

This next step is a big deal. If your state requires you to pass emissions with the vehicle it doesn't matter if you are carbureted or injected you have 40 miles of bad road before you. If your state has a smog exemption program for replicas find out the program name and register the car under that program irrespective of which fueling model you pursue — your life will be simpler and much easier.


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Old 08-02-2020, 03:55 PM
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Quote:
Originally Posted by mdbound2005 View Post
Thanks for all the advice gentleman


In regards to carbureted versus EFI, I was initially going to go for EFI (because I wanted less smell in my garage) until I was told the difference in smell between the two is nearly negligible nowadays and the carbureted engines are just as reliable without the need for tinkering (I would not be changing altitudes and the weather in Los Angeles never gets too hot or too cold).

The engine would be professionally installed and tuned, so I just wanted to see if you guys thought that if I would be better off with carbureted vs. EFI or whether you think the end result in drivability would be the same
The gas smell can possibly be captured with a charcoal filter in the fuel tank vent line. Remember to put a loop in it.
If you are going with a big cam, EFI is the way to go to keep the drive ability at lower RPMs. I'd recommend double collectors with O2 bungs in the headers. Some people have issues if the sensor is too far down stream. Visit GP Headers website for a visual.
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Old 08-03-2020, 03:46 AM
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I'd say set the idle to 650 rpm and if it still wants to creep too much kick it into neutral at long lights.
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