Jump to content
HybridZ
BRAAP

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

Recommended Posts

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?

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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 by BLOZ UP

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

I can give an example: partial throttle high speed cruise on drop- throttle with 19" Vacuum under the throttle plate, but 8psi (even for a second)---your compressor stage discharge temperature will quickly spike from the. 5:1+ compression ratio across the stage. Without the modern bypass valves it becomes easy to understand why "wet" blowers have explosions...your outlet temp spikes to 500F+!!!

this is universally most of these kits use bypasses on long term kits...the decrease in parasitic losses is great..but the avoidance of that potential ignition source makes for far less litigation! LOL

Share this post


Link to post
Share on other sites

I'm really impressed with the eaton based magnuson blower I have. I've used it on two different builds with great success. Most of the kits are (dry flow) direct injection and seem to work very well on GenIII & GenIV chevy engines . The 90mm & 92mm TBs are common place, even the 100mm are being used. Another supercharger kit KENNYBELL sells for mustangs utilizes a dual 70mm TB although I am not sure if the TB valves operate in sync. Certainly good tuning is part of the success of these setups. OOoo LOL Lag is not part of any discussion I have read or experienced myself!! Happy Holidays

Share this post


Link to post
Share on other sites

Im no expert in fluid dynamics,however, id like to point out something that might be overlooked. Pressure variants and how air changes density and flow caractoristics when compressed and expanded at multiple stages until final compression in chamber. I assume that as a fluid all parts of the system effect any and all parts form initial intake to final release. Im thinking of headers.. I believe volume stays the same yet compression dose not at points of collectors. (between short and long,,,,,power made early vs later)

 

 

With all that said,, im going to put on the 240sx 60mm tb. I have a chance to buy a adaptec computer and want the reostat on that tb. It would be FANTASTIC to dyno every move,, but alas.. money. I will be putting cam and headers back in after I get it running well. Ill dyno it then. right now only mods are cold intake(self made), advanced timing,, lightened flywheel(balenced),, roaly purple in entire drivetrain. (turbo fly and clutch pack)

 

 

Car is a blast to drive. Ill keep in mind tuning,, understanding i will reach full WOT before floored.

 

I really wish i could dyno every step. Such as puting headers on(dyno),, then cam without headers(dynoed) then togeather(dynoed) TB before anything and with every test before. The math at deepest levels is so damn complex. Its far easier to just trial and error like everything used to be,, then let the academics figured out what we did;) (btw,, we always led collages,, they only followed,,, back in the great times of course;)

 

 

tony,, hope ya still around!! hybridz has your notes;P,,, braap? rock on bro... I missed you guys. warren.. careless...blue...jmort..eveyone

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×