Need help sizing a throttle body

[Basketcase]

So as a newbie to EFI I am looking for advice on the best throttle body sizing for my 351 windsor. It is a stock early 90"s engine out of an E250. I am running the stock truck lower manifold and injectors and have a BC Bronco adapter plate allowing me to install an upper manifold from a Mustang 5.0. I got the upper manifold with a stock Mustang throttle body which I think is 50 or 55mm in size. I'm confident that this is too small for the 351. Any suggestions.....65..70??

 

[bronco italiano]

Perhaps the upper may not flow enough?

 

[73azbronco]

It's all about the math, and the way it sounds you just want a normal 351 to work right.

If your 302 uses a 55mm throttle body, thats roughly 5.49 mm per ci. So a 351 would need a 63.9 or 65mm to round it up.

You also need to make sure your maf, computer and injectors support the build.

 

[rocknhorse76]

What ecu are you using?

 

[pcf_mark01]

Throttle body diameter is not as critical as it is on carburetor. Too big a carb you can get a huge dead spot in acceleration when the manifold vacuum drops to zero and the carb loses signal. EFI does not have this problem. You cold run 80 mm throttle body and not really notice a difference.

When calculating relative sizes you need to use the AREA of the throttle blade on the diameter. The area increases at the square of the radius (A=pi*r^2). A 50 mm blade has 1963 square millimeters of area while a 55 mm blade has 2368 square mm of area! A 10% change in diameter (5 mm) for a gain of 400 sq millimeters or 21% in area! The bigger the blade the smaller the change required to make a big difference in air flow.

Too small hurts your top end but too big (within reason) does not hurt anywhere except tuning complexity.

 

[Broncobowsher]

The upper intake will be the restriction. Run fine for a stock 351. Idle, low and mid speed will work. Any plans to warm things up and the 5.0 upper intake will be the choke point. Even Ford quit using that intake and made the GT40/Cobra/Explorer intake with the round ports. So much so that the Explorer got it even though it wasn't a HP motor.

The stock mustang throttle body will work pretty good. If doing anything, find a junkyard throttle body off a 5.0 Explorer. It's a little larger.

There is a down side to too large of a throttle body. Poor drivabality. Tip in response. The transition from idle to just off idle sucks. A large throttle body can't regulate low throttle settings. Either it is restricting air (idle), or flows so much (for an engine at idle speed) that it just goes into power. The throttle switch will turn digital, on/off. For a racer, could care less. For something that you want to drive to the store, idle around the parking lot, go through the drive through, pick kids up at school, it sucks. Trying to drive off-road in a technical manor, sucks. Idles, lurches off the line. I've had it several times over the years of trying to go larger and make more power. The 70mm BBK throttle body on the 330" stroked 5.0 was the worst. Couldn't even hold a steady speed on the highway. The slightest tip in and you would accelerate. Back off just a hair, goes to idle, slows down. Fixed it with a junkyard 65mm Explorer. And never felt a loss of power, even at 6000+ RPM (it also has the better flowing explorer intake)

 

[Basketcase]

What ecu are you using?

Well I have a couiple of A9E ECU's, but I can't find anyone to flash the program in them to make it work cleanly, so I am considering using a Holly Terminator X system. This changes control to Speed Density since it eliminates the MAF, but they support the controller and software well. The injectors are the freshened stock 351 injectors.

 

[Basketcase]

The upper intake will be the restriction. Run fine for a stock 351. Idle, low and mid speed will work. Any plans to warm things up and the 5.0 upper intake will be the choke point. Even Ford quit using that intake and made the GT40/Cobra/Explorer intake with the round ports. So much so that the Explorer got it even though it wasn't a HP motor.

The stock mustang throttle body will work pretty good. If doing anything, find a junkyard throttle body off a 5.0 Explorer. It's a little larger.

There is a down side to too large of a throttle body. Poor drivabality. Tip in response. The transition from idle to just off idle sucks. A large throttle body can't regulate low throttle settings. Either it is restricting air (idle), or flows so much (for an engine at idle speed) that it just goes into power. The throttle switch will turn digital, on/off. For a racer, could care less. For something that you want to drive to the store, idle around the parking lot, go through the drive through, pick kids up at school, it sucks. Trying to drive off-road in a technical manor, sucks. Idles, lurches off the line. I've had it several times over the years of trying to go larger and make more power. The 70mm BBK throttle body on the 330" stroked 5.0 was the worst. Couldn't even hold a steady speed on the highway. The slightest tip in and you would accelerate. Back off just a hair, goes to idle, slows down. Fixed it with a junkyard 65mm Explorer. And never felt a loss of power, even at 6000+ RPM (it also has the better flowing explorer intake)

Thanks! I'm leaning toward something in the 65mm range based on feedback. I would love to find a better intake than what I have, but I can't find or afford a Lightening 351 upper and lower manifold. My current upper has oval shaped ports and match the BC Bronco adapter. I'll keep an eye out for an Explorer throttle body.

 

[Basketcase]

What ecu are you using?

I have a couple of A9E ECU's but I can't find anyone to flash the program for me to make it run without trouble codes (EGR etc.) so I am seriously considering running the Holly Trermintor X system which will eliminate the MAF but at least I can get support for their controller.

 

[73azbronco]

I would keep the 55 for now, try it, and unless you run WOT, it will idle and off idle very nice.

 

[Timmy390]

Keep an eye open for a marine version lightning lower. It's cast iron but you can find them for about half the price of a lightning aluminum lower. I run a marine version and 97 explore upper with explore t-body and mass air.

Tim

 

[rocknhorse76]

Throttle body diameter is not as critical as it is on carburetor. Too big a carb you can get a huge dead spot in acceleration when the manifold vacuum drops to zero and the carb loses signal. EFI does not have this problem. You cold run 80 mm throttle body and not really notice a difference.

When calculating relative sizes you need to use the AREA of the throttle blade on the diameter. The area increases at the square of the radius (A=pi*r^2). A 50 mm blade has 1963 square millimeters of area while a 55 mm blade has 2368 square mm of area! A 10% change in diameter (5 mm) for a gain of 400 sq millimeters or 21% in area! The bigger the blade the smaller the change required to make a big difference in air flow.

Too small hurts your top end but too big (within reason) does not hurt anywhere except tuning complexity.

Thanks! I'm leaning toward something in the 65mm range based on feedback. I would love to find a better intake than what I have, but I can't find or afford a Lightening 351 upper and lower manifold. My current upper has oval shaped ports and match the BC Bronco adapter. I'll keep an eye out for an Explorer throttle body.

If you can find a Moates Quarterhorse you can tune that A9L ECU quite easily. I run an a highly ported GT40 (original 1992 FRPP catalog offering) upper and lower intake on my 408 with a 75mm throttle body. Great setup. I used it on my 351 as well but with a 70mm TB. I highly recommend a 65mm Exploder TB for a mostly stock 351 with 19# injectors. I also modified my throttle linkage to make it less tippy off idle with the larger TB.