Big Throttle bodies.. WHY?!?!?!?

[Careless]

" This engine was breathing enough air to produce this 1100+ HP and was doing so through only TWO 2†ID throttle valves, EXACTLY like the 2†ID OE L-28 EFI throttle valves with full diameter throttle shafts etc."

 

The Nissan VG30 in the #75 IMSA car run in the 90's (Z32) ran smallish throttle bodies, but the really amazing thig was they made 1100 HP, without restrictions. The IMSA rules required them to run 26mm restrictions on the turbos, and with those the engine made 'only' 750, but talking with Steve Millen, he said the Torque was really helped by this restriction, making the car much more driveable, and much more tractable coming out of corners.

 

Where you have the restriction is as important as the size some times.

 

Larger throttle area magnifies tip-in response, and on humongo bodies really requires non-linear response to make it driveable at all. After a certian point, it's just minimizing inlet restriction, but as has been noted velocity and manifold design will dictate how the dynamics work in any system.

 

The Barrel Throttle Bodies available to replace 'Weber DCOE' style throttle bodies show some very interesting design features. The 45mm Barrel Throttles will have the ultimate flow equivalent to 48mm Standard Butterfly setups (thanks to no throttle shaft blocking airflow at WOT), but have tip-in response similar to 40mm Butterflies up to around 15% throttle opening. They taper the opening of the barrels to give tip-in that allows drivability, yet without compromising ultimate airflow.

 

I remember you, specifically, mentioning this information about the IMSA car and the barrel throttles in another thread, and I did some research back then. I found this here:

 

http://www.autocar-electrical.com/roller.pdf

 

Looks like a really good system. Almost like a coates roller-barrel cam.

 

But I have a question regarding the actual airflow dynamics. With throttle plates having their shaft in the way, and with barrels having a top side that would create a high-pressure zone, or slow down air-flow (looks like a slight restriction, considering the size of the air horns), why hasn't anyone come up with an aperture blade or iris type throttle body setup?

 

I was toying with the idea of using a system similar to minolta camera blades, in that it has a spring that when pulled on, moves a bearing ring around which causes a pin holding onto the back of each blade to move further away from the pivot point. I think with about 7 to 9 blades, it would work exceptionally well.

 

Infact, I think using the flap design from a jet engine woudl work even better because it wouldnt be a flat set of blades... they'd actually be angled into the trumpet, and it can make full benefit of the bell-mouth and air horn/runner ID size, no?

[Tony D]

Flat Slide Lucas Mechanical Injection EFI conversions are out there as well.

 

An iris would be pretty complex, and hard to seal tightly...as well as being very prone to fouling. Normally for competition, the easier and more robust the mechanisim, the better.

 

But a simple sliding blade (Flatslide) setup does exist, and offers the 'no restriction barrel' at WOT.

[olie05]

I was looking through one of my last megasquirt datalogs, and I found that at 60% throttle, I get full ambient pressure in the manifold. So that last 40% is doing nothing. This is with a 60mm 240sx throttle body and throttle body flange ported to match.

 

On my 42mm ITB's i'm planning on attaching the 944 throttle cam that everyone gets rid of. It should help with tip-in and low speed driveability.

 

[Careless]

Olie, your car is an N/A setup, right?

[HowlerMonkey]

Funny how that stock 944 throttle cam has become desirable again.

 

I remember getting the linear one from Al Collins of autothority in 1986.

 

Fitting the 944 cam was the first thing I though of when I saw the thread title but it seems someone else beat me to it.

[olie05]

Olie, your car is an N/A setup, right?

 

Yes it is

[speeder]

I dug up this thread to vent about the difficulty I'm having tuning low speed running on my 2jz-powered Lexus. I have a TEC3 and 720cc injectors - not TOO big...and a 90mm throttle body. No, I didn't choose/install it.

 

At low rpm/loads, my bigass TB has such a small opening that the TPS was near the idle setting, intermittantly triggering the decel fuel cutoff with small thottle position changes.

 

I have almost tuned around this problem, working with the decell enrichment settings, TPS/Map blend, along with a smaller idle opening and using timing to set/smooth the idle RPM - but even so that big TB makes tuning so critical that it's a MAJOR PITA.

 

I will probably go to a 75 mm TB sometime in the near future (My intercooler tubes are 3", so any bigger TB is gaining me ZERO).

 

From one who has been there -Resist the manly urge to go big.

[Evan Purple240zt]

Rick has the most badass lexus I have ever seen up close and personal.

 

Random thought.

[HowlerMonkey]

Careful when adjusting toe on the IS300 or the crossmember suffers.

[Midnitz]

regarding TB sizing, bigger doesn't always mean more airflow. Intake air velocity will have more of an impact with volume of air than shear size opening will. The larger the TB assy on an L28 (even turbos) the less off idle response you will get. This moves the torque curve up the rpm ladder.

 

I purchassed a big throat to augment extensive head and intake work. Camming the butterfly shaft will be in the works.

 

I have to finish the ET upgrade first.

[iBang]

I won't pretend to understand the detailed implications of throttle body size but I would be interested in what a smaller/larger throttle body will get me if I use one in terms of better/more linear throttle response, more power at certain power levels, etc.

 

I've been told that my engine setup (RB25/greddy intake manifold/3076R/supporting mods) will do better with a Q45 throttle body on there. In fact the intake minfold requres an adapter to use the stock throttle body. Now I do know that my engine made 400whp with the stock throttle body and a much smaller HKS turbo so I know it can hit my power goals with that small TB, but with everyone telling me I ought to go for the bigger one it makes me curious.

 

Thoughts?

[Midnitz]

The final size of the TB assy depends on what the vehicle will be used for. TB size and intake manifold runner shapes play a large role in mitigating turbo lag. After turbos spool up, VE increases like crazy.

 

If you're pushing 400BHP, I would Keep the stock TB assy and focus more on intake port/flow work. Until your engine experiences the boost, it is just another engine drawing air by vacuum.

 

Balancing lag mitigation and top end power will ultimately lead you to a decision.

[Dans toy]

One exception to the rule , as I understand it. Most of the roots style superchargers for V8's prefer larger throttle bodies(90 +). Including the smaller 112 magnuson kits (400 to 550hp).

[BLOZ UP]

Throttle response. I you look down the throat of a 60mm TB and a 85mm TB side by side, and open them about 5% the larger TB will already have significantly more open area.

 

the second thing is overall flow. some intakes, like the new LS7's in the ZO6's and the blown SVT mustangs have such a large open intake manifold on them, that to place a single blade 60 or even an 80mm TB would be a bottle neck even if a single 60mm could flow enough CFM's to make the HP they want.

 

Depends on how you define throttle response. If you want your engine to have on and off throttle response, like a pushbutton instead of a pedal, then I suppose the largest throttle body you can find works. However, I prefer a gradual climb in airflow that is a lot more proportional to throttle valve angle.

 

Any engine of X size is going to have a flow ceiling in the throttle body (TB) area. The bigger TB you have, the higher the ceiling for the throttle body. OEMs more or less match the throttle body to the engine size. If you increase the TB size and keep the same engine, all you do is increase the ceiling, whether or not the engine can actually reach it. But since your engine still flows the same as it did before, you get to the engine's max flow at whatever RPM, at lower throttle valve angles. Perhaps there's a marginal increase in flow overall due to the plate being less of a restriction compared to overall throttle body area, but it's not worth it unless you are really shoving lots of air in the motor.

Edited November 22, 2010 by BLOZ UP

[Tony D]

Feeding a rotary lobe or screw compressor is a different thing than feeding a centrifugal compressor or reciprocating one.

 

An engine is basically a reciprocating compressor.

On lobe style compressors (blowers) the dynamics of lowest inlet restriction come into play and really affect how the boost comes onto the engine off-idle. It also radically effects pressure ratio across the compressor, a few inches of water column restriction in a single stage compressor going from 'atmospheric' to 15psi can, through simple gas law calculations show an increase of alarming porportion. Not good if detonation avoidance and charge density is your goal.

 

It also makes a different in transient boost buildup. The smaller delta-change of a larger body will result in a screw or roots blower going from neutral pressure to full boost in a couple of rotor revolutions. Choke that down and the boost coming on is 1) slower in a transient respect and 2) limited by HEAT production due to the above mentioned phenomenon.

[ap72]

I put a larger throttle body on my otherwise stock (except megasquirt) L28e. I didn't notice any difference other than my off idle throttle control is no a PITA. I have no idea on the size, it was from a Nisan of some sort and had the same bolt pattern and I modified the linkage to work with the stock throttle shaft assembly. I can measure the size if anyone really wants to know.

 

Currently the car has the head pulled and I am porting the intake runners on the head and manifold, I am debating installing my turbo now or later after I have the ported intake, head, and new cam tuning sorted out. When it goes back together I'll try the larger throttle body, butif its still a PITA I'm going to step it down a size.

 

Whatever size I used it would barely fit on the stock manifold, in fact it had a problem on the top with a slight leak due to the opening being so large.

[ap72]

Depends on how you define throttle response. If you want your engine to have on and off throttle response, like a pushbutton instead of a pedal, then I suppose the largest throttle body you can find works. However, I prefer a gradual climb in airflow that is a lot more proportional to throttle valve angle.

 

 

What you are referring to is actual throttle response, which is almost always better with a smaller throttle body.

[Dans toy]

Thought I'd post a bit more information on my project and what I experienced with a change from a 78mm to a 90mm. I have a 6.0L LSX engine with a small Roots type blower (screw compressor). The throttle body change net between 1/4 & 1/2 pound of boost improvement. This is on an engine that supports 473rwhp. The amount of gain at this power level wasn't much," Undetectable by the asso- meter" but there had to have been some considering the boost change. The reason I believe for the full throttle power improvement , at the risk of repeating what Tony said, is the screw compressors prefer a minimal amount of vacuum on the intake side of the blower . Over all I agree with what was stated above for most engine setups ' bigger is not better ', but there are exceptions.

[Tony D]

I could dig up the Quadratic Equation with a general stage factor for efficiency to calculate the temperature decrease across the compressor.

That 1/4 to 1/2 psi can make little difference in pressure ratio across the compressor at WOT but even a little change like that can mean 20-50F charge temp differential.

You will note the Screw/Whipplecharger systems more and more are being packaged with intercoolers. They can make more boost than roots-style.

Curious Dan if you tracked your inlet charge temp Chang before and after your change... That would prove most enlightening!

[Dans toy]

I had the capability to monitor inlet temps but at the time was still getting use to HPTuners. Good point though. The maggie I have has an intercooler/heat exchanger system but the 112 supercharger is a little small for my engine and what I was trying to achieve so it builds heat extremely fast (WOT about 30 degrees in 11 seconds W/ 3.0 pulley building 8 lbs. boost). I would also say that my results and opinion were not proven at the track just yet, but based on the results reported on" LS1Tech" by the users of these blowers minimal inlet restriction does make improvements.