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Topic: 75MM TB on a STOCK H.O.? (Read 5441 times) previous topic - next topic

75MM TB on a STOCK H.O.?

Reply #15 –
Not talking about S.O. or carb. I am talking multiport EFI with a stock intake, or any aftermarket EFI intake. Actually if you read the links that Paul posted the next to last post, he put a 80MM TB on a SD engine with absolutely no problems
http://sbftech.com/index.php?PHPSESSID=8c3450ee5f4bb72343de25286d224756&topic=5732.msg55243#msg55243

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Here's a good example......you know everybody is very "careful" in doing any upgrades to speed density systems....right?.....I installed a 80mm TB in a '86 SD Mustang with an AOD, the rest of the engine was 100% stock.....throttle response + the famous "torque down low" did improve (via the SOTP meter, driveability was perfect .......and the most important thing of them all........a happy customer.

Yes.... go big, leave the air bottlenecking problem to the rest of the intake.


The exhaust has obviously got to be upgraded. How does a larger TB hurt velocity.(Don't mean to be a dick but I really would like a logical explanation or some sort of proof, I just cant see in my mind how it would hurt the low end. I do have an associate degree in automotive technology and from what i know about EFI, I cant make the connection between a big TB and loss of low end torque. I don't want to start an argument it is just the way my mind works) The numbers are right there in the article they didn't lose any low end torque or power.

75MM TB on a STOCK H.O.?

Reply #16 –
Perfect example is 351 clevland 4bbl heads,the ports are huge(almost fist size)and it is dyno proven that down low (unless you have a blower to fill them up) they hurt low end torque big time,ie too much air for too little velocity and compression,the engine cannot suck in enough of the air to need that much flow.
Too warrant a 75mm tb you need alot more intake,head,camshaft and compression.
The engine has no need for that much airflow coming in.
Same applies to exaust going out.
If you stick dual 5" exaust pipes(like a 75mmTB) on a stock 5.0L you will hurt low end torque alot(not enough back pressure).
Your engine has the need to only take in what it can through the duct work it has,too a certain point yes you are right in saying the engine is just a big air pump but it does not just take in more and more air for the more fuel you give it,the other components must match.
If this was the case why not just put a 90mm TB on it and say well if i add 42lb injectors i should make 500hp(more air=more fuel right,wrong),not the way it works.
But hey if ya want to waist your money on a 75mm tb when you don't have the engine to support it be my guest,however you will notice a small loss in low end grunt.
If you want a logical explanation either go back to school or go back to planet vulcan lol,not trying to be a dick.

75MM TB on a STOCK H.O.?

Reply #17 –
Not talking about ports on the heads. The heads have nothing to do with this question. Well i may have to go back to school for the explanation cause you certainly arent giving it. Unfortunatley the school i went to didn't offer performance classes it was all about what was required to become and ASE certified Technician. Not one thing you have said explains how it loses low end torque.
The question was: How does a bigger TB cause a loss of low end torque?
I haven't seen(or heard) anybody say i put a 75MM TB(or 70MM TB for that matter) on and it killed my low end torque. All i have seen is people say: That is too big you will lose low end torque, with no real evidence that I have found. Honestly I would appreciate it if someone could point me in the direction of the evidence that a large TB will cause a loss of torque.

75MM TB on a STOCK H.O.?

Reply #18 –
Explain to me how air velocity at the TB has any effect on the air velocity at the lower intake and head intake runners if you are feeding the same cfm in?

75MM TB on a STOCK H.O.?

Reply #19 –
Quote from: 88sportcoupe;149785
Honestly I would appreciate it if someone could point me in the direction of the evidence that a large TB will cause a loss of torque.


from personal experience, me and a buddy use to race almost everynight after work in our stangs, both had the typical cheap bolt ons, (headers, exhaust, pullies CAI, gears, etc.)  we use to be side by side with everyshift, the only way either one would win is if the other missed a shift.  He put a 70mm TB on and thought he would rock me with it.  Didn't work.  Taking off I got a car length on him, and pulled on him with everyshift.  He would regain the ground at the top end but you could tell he lost something with it.

That was all the proof I needed. and like dominator said, would you put a 5" exhaust on a stock 5.0L?  doubt it.  And I know you weren't talking about carbs.  Fine you want an example of too much air hurting torque?  grab yourself a Box style upper plenum and slap that on your stock 5.0L, watch what happens to your power.  a 70MM throttle body will not do anything for a stock motor.  MAYBE add 2hp on the top end but it will lose on the bottom.  Most of the dyno sheets I have seen in the magazines have shown a loss (even if it is very slight) in the bottom of the power band after a big TB swap on a near stock motor, for a small gain in the top end.

75MM TB on a STOCK H.O.?

Reply #20 –
1wldbrd:
Just curious did your friend upgrade CAI when he swapped the TB?

As I said, I have no plans of putting a 75MM TB on a stock H.O. I am just trying to learn how the TB would affect the air velocity to cause a loss of low end. It seems that Anderson Ford Motorsports agree that that there is no effect, though they were planning upgrading the heads,intake,and cam in the near future.

75MM TB on a STOCK H.O.?

Reply #21 –
Velocity has everything to do with how a carburetor works and nothing to do with how fuel injection works.

The Venturi (named after a smart old Italian) in the carburetor uses the Bernoulli effect (another smart Italian) to make a carb work.
 Pressure changes created by fluids in motion. Air is a fluid in this context.

A bigger hole in the intake will lower the velocity and raise the air pressure in the manifold. Less vacuum. So the air is denser.

75MM TB on a STOCK H.O.?

Reply #22 –
K done with this thread,you guys are  idiots,who here has a 500hp 5.0L....do i here crickets o yea that's right i do and i know what it takes to make hp.
I'm also a liscensed class A mechanic.
Why do you think nobody with real hp except me has posted in this thread,it's because you won't listen.
Velocity is important in any engine,not just carburated,who the hell told you it wasn't and where do you get your info,are you a tech,didn't think so,why is it that ford 5.0L trucks have longer intake runners,don't know the answer well let me give it too you,it's for more VELOCITY which = more torque duh.
Heads,cams,intakes,tb's,exaust,carbs are all relavent to what you ask but being the retard you are you just don't get it.
Seriously man you need to go back to school as you didn't learn anything about how an engine works!
Last time i'm gonna explain it:
To big of a tb,intake,head,cam or exaust WITHOUT the matching components will hurt low end torque and hp due to lack of velocity and being overly lean,the engine doesn't need the extra air so there for can't use it.
Single plain intakes,to big of a tb or carb,to big of an exaust all these hurt torque if not match with the proper components.
Read up on sites like the corral before you go posting stupidity without listening,god people like you piss me off!!!!
P.S. Air is not a fluid in any context unless it is compressed into liquid oxygen so don't use that one with me as it has no relivents here,the only fluid that goes into your compbustion chamber is fuel.

75MM TB on a STOCK H.O.?

Reply #23 –
I am willing to listen.

If anybody else has something to add or something I should read about it I will gladly do so.

75MM TB on a STOCK H.O.?

Reply #24 –
75mm tb is too big for what little that motor has done to it. my buddy who builds the quick 8 cars had a guy bring a 91 stang to him. he had trick flow upper and lower, e-303 cam, GT40's with a little port and polish work, and basically just about everything that's listed for that car they tested with, except he had an 80mm TB.

he was having problems with the car and couldn't get it under 14.9 and it was dead slow off the line. my buddy dennis recommended going to a 65 or 70mm throttle body and now the guy is running mid 12's(he did a little more tinkering, but downsizing the throttle body helped big time).

75MM TB on a STOCK H.O.?

Reply #25 –
Quote from: dominator;149832
P.S. Air is not a fluid in any context unless it is compressed into liquid oxygen so don't use that one with me as it has no relivents here,the only fluid that goes into your compbustion chamber is fuel.


Your ignorance is showing. Look up fluid in your dictionary, assuming you can read a dictionary.

75MM TB on a STOCK H.O.?

Reply #26 –
I found some information.
First is the fact the the TB they used in that article(Professional Products) is known to flow less than other 65MM TB. So that could be a reason that it worked well.
Second i found an article about choosing TB size.

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One other thing to know.  A carburetor requires air speeding over the venturi to draw the gasoline into the mixture.  Using too large a carburetor (high CFM rating) will usually cause derogatory performance in the lower or midrange.  This is because the lower air velocity is inefficient in mixing the gasoline with the air.  In general, and particularly for street use, a slightly smaller carb (less CFM) will give better overall performance. 

 With an EFI system, this is usually not a problem.  The throttle body only controls air flow.  A computer monitors the gasoline supply and the mixing of gasoline and air takes place inside the intake port, and not inside the carburetor.  Using an oversize throttle body is not nearly as detrimental to low and midrange performance as is using an oversize carburetor.

75MM TB on a STOCK H.O.?

Reply #27 –
as I said, it may not effect it as bad with EFI but it STILL LOSES TORQUE.  And explain why we would need to upgrade the CAI when we both had them, re-read the mods I listed for the cars.

do it to your car and tell us what happens.  I know I wont be surprised by your results.

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Your ignorance is showing.


More like YOUR ignorance is showing by thinking velocity has no affect on an EFI motor.  go a head waste your money on a box style upper plenum for a stock 5.0L and TELL ME velocity DOESN"T affect an EFI.  Try digging in to a car and gain some experience with air velocity before you comment like that.

75MM TB on a STOCK H.O.?

Reply #28 –
Quote from: softtouch;149829

A bigger hole in the intake will lower the velocity and raise the air pressure in the manifold. Less vacuum. So the air is denser.



WHAT  !???    It will lower the velocity AND air pressure, NOT raise it.

75MM TB on a STOCK H.O.?

Reply #29 –
Quote from: dominator;149832
To big of a tb,intake,head,cam or exaust WITHOUT the matching components will hurt low end torque and hp due to lack of velocity and being overly lean,the engine doesn't need the extra air so there for can't use it.
Single plain intakes,to big of a tb or carb,to big of an exaust all these hurt torque if not match with the proper components.


Exactly! That's why you dont want to go bolting a RPM 2 or Systemax intake on a stock or mild engine combo. You'll kill your torque. A mild engine with a ported stocker will run all over the big stuff (on the same engine) up to 4K or so RPMs.

I know intake manifolds have a huge effect but I'm not sure how much difference there would be using a 65mm TB as opposed to a 75mm on a stock or GT40 style intake. Probably not much... That being said I think I'll hang onto my 65mm TB for a while.

I haven't read the rest of this thread but I think I might now.
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86 5.0 Turbocoupe (Katrina), 87 5.0 Sport (Rita)