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

[BRAAP]

(This is a cut and paste, with some editing, from another thread. The topic of big throttle valves came up again and my gut reaction was to go public with my point of view…)

 

Ok, I’ve been reading a lot lately on this forum about the desire to install "huge by large" throttle bodies on little engines and have been biting tongue every time. I have to know as my curiosity is killing me here. Why is there SO much interest in these huge by large throttle bodies lately? For the L28 it use to be the 60mm, which, I can’t argue with. Then it was the 70 MM. Now it seems to be the HUGE by LARGE 90MM, (3.54”) from the Q-45?! Please don’t take this offensively, I am trying to keep an open mind about this though I am struggling with my knee jerk reaction to view those huge throttle valves on an L-series, (or any engine under 6 liters of displacement) as “rice”, on the same playing field as the coffee can exhaust tips on a 4 cylinder fad. I feel that pretty much anything larger than 60mm on an L-6, N/A, or even pushing CRAZY radical boost levels, isn’t going to free up any more power, but only make it harder to modulate what power it does produce, as full power will be in by 25% throttle opening... (Of course the throttle actuation could be “cammed” to offset that issue…)

 

In the L-6 crowd, there has always been an interest in installing a bigger throttle valve on the OE EFI intake manifold. The stock EFI L-28 intake manifold port runners have only 53% the area of the L-28 intake valve! Being as the port runner wont breathe what the valve is capable of breathing, why are we spending money, time, and effort on bigger throttle valves that are not restricting the air flow like the individual runners are? My philosophy in regards the EFI L-28 is to fix the runner size issue, THEN address the throttle valve.

 

Why do I feel this way towards the huge throttle bodies in genreal, you guys are thinking? Here is one boosted example that we built back in the late ‘90’s. It is a Twin Turbo MPFI Big Block 540 CID Chev with 10:1 compression, drinking 100LL AV gas, for an endurance white water Jet boat, for Justin Boice of Boice Jet. The engine was running an Autronic MPFI system on a tunnel ram modified to accept fuel injectors, a water to air intercooler, custom turbos built by Garret, (Garret was a sponsor and donated these custom Turbos which at the time utilized Indy car exhaust housings. That was all the info they would give us about the Turbos and made it very clear to us that we were not to dissemble the turbos under any circumstances, and that the Turbos were to be sent back to Garret if they needed ANYTHING at all). We ran the engine on the dyno at Sunset Engine Development, (jgkurz is familiar with this shop). The engine produced 1167 corrected HP, (1134 HP measured), at 5800 RPM with only 12 lbs. of boost! Now get this guys. 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. That is over 575 HP worth of air flow through just ONE stock L-28 EFI throttle valve with only 12 PSI of boost helping to shove that much air through that small throttle valve! More boost under similar conditions is even more air in terms of density, which is more power, (most of you are running considerably more boost than that). Could this particular 540 CID TT BBC engine be more efficient with larger throttle valves? Maybe. Having machined and built this engine myself, ran it on the dyno, and spent many trips on the rogue River watching this boat run 125+ MPH UP the rapids, I’m a firm believer that a super large Throttle body isn’t going to gain anything other than questions from tuners as to why it was installed. So as you can see, I don’t see what all the hype is about installing a throttle valve that the engine will never make use of, (at least that I can see from my personal experience), N/A or boosted, just like those huge 5” exhaust tips. I can see it on a show car, but for performance reasons, I just don’t get it…. If I am missing something here guys, please enlighten me. I am all about wanting to make big power... Maybe I'm just suffering from Throttle Valve envy?..

 

Here are some pics of the engine just prior to dyno tuning and while on the dyno as it was producing the numbers shown on the dyno sheet below, and the boat that it powered on the Rogue River in Southern Oregon…

 

Note how well the water to air intercooler worked by looking at the inlet air temp and air temp after the I/C..

 

 

 

Edited October 11, 2009 by BRAAP

[1FASTKINGCAB]

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.

[JMortensen]

Technically 6 44mm barrels on a set of Mikunis flow WAAAAY more air than an L28 can. Why they work so much better than 40's I can't really say, but they do. Why 50mm ITBs work so much better than 44 carbs is also something of a mystery to me. Absolute flow capacity of a given TB isn't the a good measure of what works the best, that much I know for sure.

[blueovalz]

My understanding is that ITBs require larger openings due the lack of a plenum to draw from during the intake charge. This may explain why increasing the plenum size on a typical V8 intake helps. How this relates to lengthy runners prior to that plenum is another question.

 

On to another question: At 12 lb boost this would be 1.8 atmospheres. Would this then require a 2.7" TB in order to pass the same amount of air than the 2" TB (this has 1.8 times as much area) passes at 1 atmosphere?

 

Would the argument about carb size be applicable here? A 600 cfm would work, but an 800 cfm may work better because it will restrict airflow less than the 600. Perhaps at a specific pressure difference (vacuum), the 600 would be enough, but at a lower pressure difference less air will be allowed to flow past the carb restriction, and thus at these lower pressure differences, wouldn't the larger carb allow the additional amount of air into the engine because it is less restrictive? Grumpy sent a link on this a while back, and it was quite interesting.

[eec564]

1. People think bigger is better.

 

2. People think their engines make more power. It's just that they reach full throttle (as far as the motor and intake are concerned) when barely touching the pedal. They just neglect to notise that the second 50% of their pedal travel dosen't do anything any more.

 

3. People do things that don't make sense. I've seen someone take their backseat out to lower the weight of their car and make them go faster. They also had a rather large speaker box with 2 15"s in their trunk.

[BRAAP]

1. People think bigger is better.

 

2. People think their engines make more power. It's just that they reach full throttle (as far as the motor and intake are concerned) when barely touching the pedal. They just neglect to notise that the second 50% of their pedal travel dosen't do anything any more.

 

3. People do things that don't make sense. I've seen someone take their backseat out to lower the weight of their car and make them go faster. They also had a rather large speaker box with 2 15"s in their trunk.

 

AMEN!!

[thehelix112]

Interesting question BRAAP. I must admit, I am running a 65mm throttle body, and won't be going bigger anytime soon as I get 95% power at 50% throttle.

 

I think there are other considerations to be made though. For instance, if the inlet to your plenum is 90mm, a 90mm T/B would allow you to provide minimal disruption to the flow as it enters the plenum, whilst keeping the T/B as close to the plenum as possible (hence improving throttle response).

 

I agree throttle response will suffer with a larger t/b, all other things being equal, assuming you still have linear throttle actuation. I am sure there is an art to tuning t/b actuation, but I'm also sure its possible. If one is prepared to do this, I think the ideal T/B size is one that already exists in your inlet tract, preferably as close to the plenum (in single T/B) or head (ITB) as possible.

 

For example, if you have 3" intercooler piping, and a 4" ID cylindrical plenum, a T/B anywhere between those two, placed as close to the plenum as possible, but not requiring a sharp change in diameter, would provide minimal flow restriction.

 

However, having 2.5" intercooler piping, and a stock L28ET inlet manifold, with a 90mm throttle body, well, yes that falls into the milo-tin exhaust category. I know some fool with 2.5" piping, and a T/B the flares from 3" at its opening down to 2.5" at the exit, feeding into a stock L28E manifold. (Yes, that I DO plan on upgrading).

 

Dave

[SHADY280]

all this being said, what is a good runner size, or how does one calculate it? the valve covers some of the hole, no matter the size of cam etc... no point in making the head runners huge, just to bottle neck down to the valve. especially at the risk of blowing thru the sides.

[eec564]

I think this would be just perfect.

http://forums.hybridz.org/showthread.php?t=117607

[johnc]

I'm not an engine guy so I can only parrot what's been told to me by engine guys. The term I keep hearing from professional race engine builders is: Velocity. Get the intake and exhaust charges moving and keep them moving.

 

Cam specs, combustion chamber shape, valve sizes, intake and exhaust port profiles, exhaust and intake manifold runner shapes, exhaust merge collectors, intake plenums, and throttle bodies are all design to get the gasses moving quickly and continuously.

 

Its actually very rare to have an engine restricted by throttle openings. In ALMS Nissan normally aspirated VG engines routinely made 450hp and had a broad torque band breathing through two 23mm restrictors. Everything was designed around maintinaing velocity in those engines.

[alexideways]

Wow, it's cool to see this thread finally picking up some speed... Keep it goin' guys, it's VERRRRRRRRRRRRRRRRRRRRY interesting!!!!!!!!!

 

Thanks.

[eec564]

In terms of high gas velocity, our small runners on the intake would keep things moving fast. At that point, port missmatch from the intake to the head could be a bigger problem. The exaust on my 82 N/A isn't all that bad, until you hit the cat. It's 3-2-1, then 2" OD piping, with a a crush bent (not good, soon to be mandrel) section leading under the car and into the cat. Everything after that should have less restriction than the cat, so it won't make much a huge difference. I figure 2" OD exaust piping would result in fairly high exaust velocities, and help conduct heat away from the engine.

[(goldfish)]

What is the desired velocity of the intake charge? Given that you could calculate how big a throttle body was needed to maintain it with a given CFM of an engine @ rpm. otherwise....

 

Didn't you know that it's just like headers and glass packs, more noise= more power

[spork]

One thing I think may have been missed (or maybe I missed it when reading) was turbo compressor size outlet. I typically try to stay with as close to the size of the compressor outlet as possible, as guys start to run bigger and bigger trubos, the compressor outlet gets bigger as well. The turbo that is sitting in the shop awaiting installation on my car has a 3-3.5" inside diameter of the outlet on the compressor side (if I remember correctly) I'll be running intercooler pipes, and TB to match. Will I be running a TB bigger than my cooler pipes...absolutely not. I very much agree in that guys running larger TBs with nothing done to the intake to help flow are porbably going to get little to nothing short of Throttle response increase when going past the traditional 60mm TB. That in itself has some debate about it, but there is no need to go into that "well hashed" subject.

[Matt K]

I don't know...I run a Q45 throttlebody on my SR20DET...can't complain about it. Maybe a smaller one would get me the same power, but I'm not complaining with 522 rwhp...

 

[BRAAP]

I don't know...I run a Q45 throttlebody on my SR20DET...can't complain about it. Maybe a smaller one would get me the same power, but I'm not complaining with 522 rwhp...

 

 

 

 

WOW! Great power numbers from an SR, nice work. Thanks for sharing.

 

There really hasn’t been any claim to overly large throttle bodies being a hindrance to engine output, at least in reference to being too big… not sure your point with this one?

[BRAAP]

2 points really stand out in this thread thus far….

 

 

1) Throttle body to manifold alignment….

I do honestly feel that the size of the throttle valve has an impact if/when the transition from the throttle body to the manifold plenum is offset, i.e. there is a step in the transition. It then becomes critical to size the throttle body “to” the plenum entrance, not so much the overall size of the throttle body itself. I guess what I’m trying to say is, “it’s not the size of the thing, but how it’s installed!”

 

2) Induction velocity….

For an engine that is running and tuned to the ragged edge, velocity should be considered in the design of the entire induction tract from the air filter to the crown of the piston, not just the intake port runners. Good point…

 

 

 

This is good stuff. Keep it coming guys…

[thehelix112]

Just to reiterate(elaborate?) on point number 2 there Paul, the ways to maximise velocity is to keep the inlet tract as follows:

 

1. As short as possible.

2. As straight as possible.

3. As close to a gradually decreasing (slight X? degree taper) ID as possible.

4. Just as with inlet ports, keep the inside of any pipes the air flows through relatively rough. This is to prevent boundary layer build-up leading to separation.

 

 

This will allow you to make best use of whatever pressure differential you have. Be it the vacuum generated by the piston's descent below atmosphere, or the vacuum below the boost pressure generated by a turbo. Either way it spells more air in the chamber, with better mixing, which means more power for you.

 

Dave

[RTz]

I general, I don't sympathize with the current 'bigger is better' craze.

 

However, when properly engineered, there may in fact be some advantages...

 

Fueling the fire....

 

Courtesy Mike Kokima, "Honda/Acura engine performance"

Courtesy Mike Kokima, "Honda/Acura engine performance"

 

These monstrous B&K TB's are mounted to a 2.1 liter Honda that does 10.95's at 125mph... all motor. 260HP@9000rpm to the wheel's, 166 ft'lbs at 7200.

 

Mike Kojima... "Although they are too big for this engine when calculated, their placement helps ensure equal air distribution through the manifold and helps keep the plenum non-resonant".

[eec564]

That's an interesting place to mount injectors on that honda manifold. A design like that would have physics (kinda) similar to tripple TBs on our Zs. For our single TBs, mounted at one end of the intake, I think Paul's #1 point is the one that qualifies.

 

Another point I would add is this:

 

3) Engine Control

An oversized TB resulting in full power applied at 50% throttle can make an engine overly sensitive. Properly sized intake components will allow better modulation of engine power, and far better vehicle control.

 

Exception: I've thought about a design where the engine stays at 17:1 AFR until the manifold is at 0" Hg, around 50% throttle, then increases the fuel to 12:1 afr to increase power. One day I may get around to calculating if this could actually save some fuel even while cruising under a decently high load, such as gentle acceleration going up hill on the highway.