How does a faster air velocity increase the torque output of an engine. I hear people say that if you use too big of an exhaust (ie muflex 4") or too big of a throttle body (ie 58 mm) when your engine doesn't require it (like when it is almost stock) you will lose torque.

Can someone please explain to me how the increased air velocity in the intake and exhaust increases torque. Thanks.

CanadianZ

 

"I hear people say that if you use too big of an exhaust (ie muflex 4") or too big of a throttle body (ie 58 mm) when your engine doesn't require it (like when it is almost stock) you will lose torque. "

That is all B.S. The velocity of a throttle body or of a muffler has no effect on TQ. Just like backpressure does not increase TQ.

The first part "How does a faster air velocity increase the torque output of an engine."

That is true. It relates more to Intake Manifold Runner and Intake port velocity than anything else.

Say you have two intake ports in a head. They both flow exactly the same. One is 215cc and one is 240cc. (this is almost a direct example from SBC stuff) They both flow 300cfm @ .600 lift. The smaller port will have higher velocity because it is flowing more volume through a smaller diameter (effectively) hole. Now this will create a higher average runner velocity at a lower rpm than the bigger port. Using the same cam you will have more TQ because the average velocity numbers will be in their sweet spot at a lower rpm.

Now the average velocity numbers that are the "sweet spot" are around 240 -260 ft per sec. You basically use the runner size as a tuning aid to get the HP and TQ where you want them, considering that the head flows enough air.

I hope that is more of an answer than you need.

Bret

 

Thanks, actually all of it was very informative. So I guess running a larger TB on a basically stock engine won't hurt anything right now. I will be getting a cam and P&P the heads in the future. Thanks.

CanadianZ

 

Please explain why a difference in tb runner size doesnt matter in terms of velocity? Its dyno poven that having too large of throttle body kills torque, so if its not from loss of velocity what is it from?

 

I've never seen a dyno test where to big of a TB causes a torque decrease. If it did cause one then it would be because the velocity would be to low at lower RPM's to fill the plenum fast enough. That would be my guess.

We must be talking a grossly oversized throttle body.

Bret

 

Like SStrokerAce, I have never seen a dyno test that "proves" too large a throttle body kills torque on a fuel injected engine. The "velocity" through the TB is pretty much irrelevant, except as it affects cylinder pressure on the intake stroke...... get TB velocity too high, and you get more pressure loss in the TB = lower cylinder pressure = reduced VE. Get velocity low and you reduce pressure loss in the TB = more cylinder pressure = improved VE.

The plenum effectively isolates the TB from the individual runners, causing the air flow through the TB to be relatively steady state. There is nothing to be gained by increasing velocity, to increase inertia in steady state flow. It just dumps into a big open plenum. Any "inertia" is lost. This is of course different than the individual intake runners, where high velocity/inertia can be important, but that is because of the "pulse" flow that occurs in the intake runners.

My 58mm TB provides all the air the 381ci engine can handle at 7,200rpm, with only 77% TPP. It is "oversize" to say the least, but I have never seen any evidence to suggest that I am somehow losing torque, and the engine has been through more than 35 engine and chassis dyno pulls.

 

I have likewise not seen "too large" a TB hurt torque. Well, that's not quite true. If the PROGRAMMING isn't adjusted to work with the larger TB, then you can lose torque, but not for the simple reason that the TB is too big.

Throttle Bodies are NOT like carburetors! They are just an "air door" and do NOT require a minimum velocity through them to meter fuel correctly like a carb does, since they don't meter fuel at all by themselves.

 

I wasnt talking about a 58 mm being oversized, im talking about largely mismatched tb's killing LOW END torque in close to stock motors.

 

can anyone explain why they say too big of an exhuast causes loss of low down torque?

I felt compelled to try this so i went to the track one day-
long tubes catback-
13.2 w/ 2.0 60ft and 13.4 with 2.2 60ft

next week
VERY similar conditions
Took my muffler off
couldn't get below 13.6 in 5 runs with 2.1 60fts

put the muffler on for the last run
13.2

can anyone explain this to me?

is it that on the bottom end the cylander pressure isn't high enough but on the top end it could breath well?

 

?????

Then please post the dyno data or other reference, so we all can learn something.

You might want to read this thread:
Pros/Cons of Oversize TB Thread

 

Guess ive only heard arguments from the other side all my life, alot of the discussions on the 5.0 boards go the other way and advise against going too big. I give up.

 

A smaller throttle body could affect the effective plenum volume of the intake (and as acting as an antireversion device).

It is a possibility but that would only be the case where the design treats the TB as part of the intake in the tuning of helmholz resonance.

Now if you rout out the openings in the LT1Intake then change out the TB, you might have a case.

 

I personally dont care if its anything short of the word of God. Unless you have some data to back it up- I dont listen. I'm with Fred- not that I believe of don't, its just that I won't forumulate my opinions based on other opinions.

Only the facts please

 

I asked for someone to explain what was wrong with the opinions I had heard in the past, I didnt declare I was right Trey. I'll look for some of those dyno sheets I saw, but theyre gonna have aftermarket MAF's that will throw it off in there. I know that efi cars are a whole different story due to the calibrations, but I'll look for some of those dyno sheets, I just wanted to know why reduced velocity into the intake plenum wouldnt affect torque.

 

it was nothing personal-
I just take opinions with a grain of salt. You know how the car world is. look at the 5.85 vs 6'' rod debate.

I just simply say "I dont know, i used 6'' because i got a deal on them" because I stil havn't seen anything difinitive to prove one way or another,,,