I'm curious as to if there are any actual negatives to having an overly large Throttle Body on an EFI application. Taking the TPI/LT1 style Throttle bodies for example, going as high ~1000cfm in a 58mm, and ~12-1300cfm in a Monoblade, both being relatively "acceptable" in a healthy larger Cube Small Block.

How about a LARGER TB yet. Something in the 2000cfm range.

I understand that the larger blades will effect Tip-In throttle, since at any given % of throttle, you will have much more air being able to pass. Could something like this lead to SERIOUS problems in tuning and driveability.

How about power production. Obviously we would have relived any/all restriction the TB could present, but would there be any ill effects (Lack of velocity, etc).

Thanx!

Kurtis Tamez

 

Car Craft did a test on a 1989 Formula 350 back in 1999. They took a stock TPI 350 car, added Accel large tube runners, and dropped 3/10's in the 1/4 mile. They then swapped on a 58mm Accel TB, and gained 1.5/10's back. Solid evidence that you can go too large.

 

every person Who has tried bigger TBs has found a gain in HP. On a EFI motor you need no signal save for Map that goes to 100 anyways.

I gained 25RWHP going from a ported stock to a 58mm. I know people who have went to the biggest TB they could (mono) and gain even more hp over a 58mm.

You are not going to drive your car on the street and tip in can be tuned Depending on how you do it you could run a HUGE progressive four barrel system or Oval Two barrel system also progressive to give the tip in you want and still have the huge CFM

It is all about pumping losses. Build the right ports and throw all the air at it she will take

 

<~~~still running the stock 48mm tb!!! i bet i could gain a sheetload if i went bigger.
jeremy

 

On my GTP, I went from the inefficient stock 70mm, to a modified LS1 TB. I had a few things done to the LS1 TB to make it work on my car and to enhance flow, but the butterfly plate is the stock diameter. I dropped a good 2 tenths on my car. But since I have a supercharger, anything I can do to free up the intake side of the blower helps. I suppose on a NA car, you may need to make sure that you don't kill the air velocity going into the intake manifold.

DEE

 

Losing power is a joke. There has to be an additional parameter that caused it. On the intake side there is absolutely NO way one can go too large with a throttle body. One CAN go too large on the MAF, however.

On the intake side it's all about reducing restriction!

SC

 

Actualy the MAF doe snothing more than monitor the amount of airflow coming to the TB. The PCM will Adapt and reset to a ported MAF after time. If you have a monoblade on a basicly stock motor the air coming through the TB has no velocity b/c there is no kind of restriction. Its like having air forced into your lungs while taking the biggest breath you can. You would just gag and cough. Without having a powerful enough motor for the need of a bigger TB its a downfall. you need to have a need for that much air for it to be a worth while mod.

 

I don't see that as a correct comparison though. The air isn't being forced in the engine for one. Using your lungs analogy, how much CFM does YOUR mouth flow.

On the Intake stroke, "X" amount of air *can* be pulled in. If you present a restriction to the cylinder pulling the air in, it will pull less air.

Now your point of velocity is what we're after. Does anyone actually have evidence that velocity changes and/or is necessary when it comes to the TB, or is it simply a matter of reducing the restriction.

Kurtis

 

I see your point.... i believe this is only a factor if the motor is starving for more air. on a mostly stock motor i want to see dyno sheets of anyone gaining 25rwhp. maybe 5-10rwhp. 10 at the most. I also feel that porting the stock manifold is more work than what its worth, the stock 54mm is plenty of space

 

well if you do not port your intake to match the 58mm there is always that "Throttle blades stuck open" problem

You guys need to take a few more stepps away from your carb's

I will be gettin my intake flowed in a month or so. i will get back to you then

 

Here you are:

http://www.ls1.com/forums/showthread...hreadid=158866

 

I don't see where velocity at the TB would affect a FI car. Of course, you need velocity/vacuum signal for a carb to work right, but that's so it can meter fuel, which is taken care of by the computer on FI anyways.
Where the velocity matters is going to be in the intake runners and in the chamber, and this is controlled by intake runner, and valve size.

An example from a different car - I also own a '92 Chrysler LeBaron - 3.0l V6. Now, this is not a high revving motor, either - stock it's all done by 5K, modified it still doesn't pull over 6K. Still, I know of people using up to 58mm TB's on these cars, and a few looking for a 60mm that will fit.

 

Easy way to check if you could use more air in: check the difference between the MAP reading and manifold vacuum.

At WOT, I pull over 2" vacuum on the 52, I need a 58mm, darn it!! The only reason I haven't done it yet is that with the juice, my MAF readings go down 15%, and I get less than 1/2" vacuum.

 

so basically if ur MAP reading is less than 100 at WOT .. that means that there is still vaccum inside the plenum .. therefore ur engine is sucking more than ur TB is flowing ...

so from my understanding .. if u hit 100 map at WOT u technically dont need a bigger TB ?

i say this because a friend of mine (draco) bought a set of nice heads (Zturbo's AFRs, since ellis knows them) and with a CC305 his MAP readings dont read 100 after installing the heads ..

some correct me if im wrong .. thanx

 

You have to look at MAP at WOT and BAR (barometric pressure) simultaneously - over a full range of flow. You can't just say "if MAP is 100 everything is fine", because a high barometer could be well over 100kPa at sea level, and if you are in Denver, you will NEVER see 100kPa. As long as air is flowing through the intake, MAP can never equal BAR. The difference (BAR - MAP) is the loss in the cumulative obstructions between the outside air and the runners in the intake manifold.... filter, duct wall friction, MAF, TB, and bends in the path, sudden enlargements and contractions, etc. Even the location of the air inlet will play a role, since pressure available at various surfaces on the body can be slightly negative or positive relative to barometric pressure, when the vehicle is moving.

To know where the major obstruction lies, you need to analyze each element in the inlet path. If you can reduce the pressure loss (BAR - MAP) by 1kPa, you increase the amount of air in the cylinder by roughly about 1% (NA). In a 500HP, motor, that's about 5HP. So, if going from a 52->58mm TB nets you 1.0kPa, you will see 5.0HP, while a smaller 0.10kPa reduction in loss would show up as 0.5HP.... a crude approximation, but it helps you understand the magnitude of the problem.... and the finite limits of "noticable" or even measureable gains vs. total available flow area.

When we first set up my engine tuning, it was done in alpha-N.... only throttle position and RPM were used to determine the fuel feed table (with some offset tables for things like MAP and IAT). It was obvious after many engine dyno pulls, that even though the 381ci engine was making 762HP (peak), and could spin to over 7,000rpm, with 275HP of dry nitrous flowing through the TB, the 58mm TB was capable of flowing the total maximum air+nitrous requirement of the engine at 77% blade opening.... in effect, a 58mm TB was quite a bit bigger than the engine needed. Of course the presence of nitrous makes a comparison with an NA motor difficult, because the heat absorption of the nitrous phase change yields a significant reduction in gas (air+nitrous) volume.

I don't see how the velocity of the air flowing through the TB could have a positive effect..... air flow throught the TB is decoupled from the individual pulses of the intake runners by the volume of the plenum. The TB flow is essentially continuous, vs. the pulse flow of the runners, and inertia or pulse wave reflection becomes a non-issue (at least in my limited "theoretical" world... ). Increasing the velocity in a continuous flow situation like a TB only increases pressure loss.

If you think about it, the TB is there to limit flow at part load conditions.... and ideally, it would allow some sort of approximatley linear relationship of flow vs angle over a very wide range of blade rotation, while having a low pressure loss with the blades wide open. An extremely large TB will exhibit low pressure loss at WOT, but would seem to present an extremely non-linear response to blade angle, necessitating some sort of reprogramming on the "enrichment" mode to avoid running lean when small throttle rotations produce large increases in air flow.