RE: Chickens?? AKA TB sizing

[ederdelyi]

Ain't no feathers sprouting out of  my butt :>)
I know of four methods for calculating TB/Carb sizes ...

Near Equal Velocity Method:
The idea is that the TB should have a slightly higher velocity than the intake ports.
   
Maximum Velocity Method:
The smallest size to keep velocity to a maximum of 54.9 meters/sec (180 feet/sec) at 85% open throttle.
   
Area Constant Method:
0.026774l/s/mm^2 for carb and 0.03415 l/s/mm^2 for injection.
   
Capacity Index Method:
Uses air quantity, air density, throttle plate drop and is the most accurate. Assumes 1 psig or 6.89kPa pressure drop across the throttle plate.

The Capacity Index Method is the one I'm most familiar with and the one most commonly found on website TB calculators. And that's all I'm gonna say about that! Everyone is welcome to research any or all of these methods on their own. There are a couple of others that I have seen/heard of but I honestly don't remember what they are and know nothing about them.

EDIT: Link to a TB size calculator that uses a variation of the Capacity Index Method.

http://www.revtronix.com/techinfo/calculators/throttlebody.php

With the right input data this one matches up quite well with other tools I have and real world test data.

[uglyDougly]

  Where does a properly designed plenum play a part in those equations?

  Doug

[ederdelyi]

Good question. Most assume that the flow through the TB is based solely on the diameter and length of the TB and engine capacity/VE. Some tools/calculators make "standard" assumptions for single plane, dual plane and plenum manifolds. I've found that the plenum size on a standard "Y" HD type manifold has very little effect on sizing the carb or TB, but it will have an effect on flow and throttle response, the latter most no ticable with carb setups.

The biggest error I see folks making when calculating TB or injector sizes when VE is an input parameter is over estimating what the VE of their engine really is. 100% VE sounds really good, but getting an actual 100% VE is not that easy to achieve. I'm sure that many would surprised to find that at full song their street driven hot rod SBC or HD is maybe getting VE's in the mid 90's if they have done a good job. The air turbine setups on engine dynos can be real rude in the data you will get from them!

EDIT: This equation may prove to be useful for some folks. I have found it to be pretty close to reality if the BSFC figures are reasonable.

REQUIRED VE% = (( 9411 x HP x BSFC ) / (DISPLACEMENT x RPM)) x 100

Displacement in cubic inches. It can be used for airflow/TB calcs and as a "sanity" check when anticipating power output from a build or the reasonableness of a dyno chart or claimed power output from a build.


Perhaps the best way to look at what the various methods are attempting to do is calculate the flow through a piece of "pipe" that will either flow the amount of air needed with a specified max pressure drop or velocity. What happens after it leaves that piece of "pipe" doesn't really matter unless it alters the amount of air that the engine can actually ingest ... in which case one could say that the input data was bogus to start with! The good news with TB's ....


wait for it ....



Unless you grossly overestimate the air requirements and the TB will flow all the air the engine can use while only using 15 -25% of the available throttle opening it won't be much more than a tuning exercise. Yes, Virginia, you can have too large of a TB, But, IMO/IME it's gotta be grossly oversized for it to be a major problem for driveability.

Guess some data might help.

Test Mule: 107 CI
10.5:1 CR
Cam IVO 17 IVC 47 Duration 246
2.04 intake valve, 1.71 intake runner HTCC CNC heads
SuperTrapp SuperMeg 2:1 exhaust (modified)
Peak HP point 6000 RPM
Measured max VE on engine dyno 95% (53 mm TB)
Dynojet 250i Wheel Power: SAE TQ 125 lbs-ft, SAE Hp 121, 4th gear 3.15 final CF 1.01

TB sizes calculated by:
Near Equal: 55 mm (54.6)
Max Velocity: 48 (47.95)
Area Constant: 56
Capacity Index: 52

Revtronix 52mm TB, 40 C. inlet air temp, 95% VE calculated a 0.655 Kpa drop and a 0.9 Hp loss.

What was interesting to me was the size calculated by the constant velocity method ... no way that motor is gonna make it's peak power at 6k with that 48 mm TB and I didn't bother to try it. The others would all work just fine, IMO and not present any major tune/driveability problems. Larger TB's were tried with no gains in peak power worth noting, none were large enough to have an impact on tune/driveability ... couldn't find one big enough :>)