Novice q - How exactly do ITBs work and what do they do? I've seen the youtube videos and they are cool. Are they legal for streets in the US?
It's really a whole topic unto itself.
If you are looking for the most technical, formal definition wiki has a page on ITB's or 'velocity stacks'
here. As Shawn mentioned some previous OE applications such as the E30 series M3 had them stock in the most minimal definition using a plenum and air box. The valves required more frequent adjustment- but all in all BMW invested the time into drive-ability and it proved fine.
There is a decent overview of OE implementations here:
http://www.autospeed.com/cms/A_3049/article.html
For raw performance- the basic principle is the same as with any NA tune... the quest for higher efficiency and output from any given design. A more efficient induction and fuel delivery system on the order of even a few percent... can and does yield some degree of gain on most any engine. Moving to an individual throttle body per cylinder also lends itself to better throttle response when tuned properly.
In an aftermarket tuning context they seem to be a point of confusion whenever technical forum discussions on them come up... mainly because very few people have real-world hands on experience installing or tuning them in the aftermarket. They are more frequently seen at the pro level. Their are a handful of NSX/2K options-
Toda of course designed their parts for the factory backed teams and is expensive as it gets.
Revolution as posted has a good reputation in Japan with community racers on builds but is probably out of reach on support for most of us. As I recall the owner of Advance (can't recall his name) had a set.
TWM is perhaps the most well known US vendor having been around forever. They offer their 2000 series kit for the F20's (+14hp BTW) and Miata's in the import world; but mainly their bread and butter remains the competition Chevy big block guys. My impression is that in the past they didn't deal with street guys more than they had to outside of maybe Porsche, but that is starting to change. I received some interest from them in an NSX kit awhile back during off-season down-time.
Lance Hayward's ITB offerings are well received in the S2000 & K20 circles.
Here is a customer video. I am unsure if they would be willing to explore an NSX offering but he is usually very receptive and I'd start there.
You could also of course try Wilson, as he once stated he had a pet project going on for awhile with one on his street car using a beater block. Either builders with some degree of hands on could probably give one a glimmer of what you realistically might come to expect with such a setup and help with misc. integration questions specific to our platform.
None are going to be technically emissions legal due to visual inspection and EMS, although I've heard of one instance using Hondata on another application with ODB2 got passed. In general if that is an issue than I'd go another route.
Based on the sound, responsiveness and overall cool factor, I think this should be a worthy alternative to anyone considering a Supercharger (10k+)
There is enough HP gain to make it significant enough, and given the increase in throttle response, should be more valuable around the track and the twistys than *comparable* HP ...
AEM tuned and other mods (maybe increased compression..) could do 400 NA HP, no?
Paul... 'Cool factor' is the absolute worst way to make expensive engine/tuning decisions.
I'd say the primary mis-conception is that due to the racy look, that velocity stacks some-how show huge power gains unto themselves or are somehow comparable to another unrelated FI solution and that is just not true. I'll state up-front that if a member thinks they are going to see a +130hp gain street reliable NA build on pump gas in an NSX by bolting one on or do some build up GT500 spec engine with a few JDM tuner mods and a set they are dreaming in la la land. Touting them as a comparable upgrade approach to a CTSC is not viable either- as I can't imagine an informed customer comparing the two upgrade paths as being some-how in any way similar after reviewing the facts practically with a pro engine builder. Check the 2K dyno charts from TWM. From stock, I'd guess more like +25hp for about 15 grand by the time you are done with a good EMS, rails, bells, tuning, etc...
To get best results, adding individual throttle bodies is the last step in an extensive (and in the NSX relatively expensive) full NA tune. Your engine needs to already be well prepped (open up the head / port match, cams, re-designed fuel system, a stand-alone aftermarket ECU, delete emissions systems, ideally higher compression, headers/exhaust/valves, etc...) to take the best advantage of the added air flow and minimal efficiency bump in the fuel/intake charge these custom induction systems provide. Otherwise, it is akin to adding titanium lightweight lug nuts to stock wheels.
Due to their tuning characteristics- all in all they really best lend themselves to
serious motor sports competitors whereas every last bit of power output counts- as technical rules have them running a restrictor plate and teams need every last technical advantage to qualify let alone put on a good showing for their sponsors. Since outside of the spec'd plate this area is often 'free' it is easier to justify more engine R&D into the design & tuning to the degree of pressure waves, variable runners, computer design, etc... by
backing manufacturers with really deep pockets.
In a club street/track context (for what most of us do) the issue with any full NA tune is going to come down to tuning/drive-ability issues and of course reliability/cost/service balanced against basic practicality/application. It is important to be more critical of your actual usage patterns, as club requirements/usage/goals are generally very different.
I've seen that the Miata's guys can put on ITB's with an AEM for about $3500, and when the motor blows after a few events or a summer it really isn't a big deal to do a DIY tear down. I know a SCCA national guy that at any one time is probably working with a customer on one in his shop. On the flip side the F-car guys with factory ITB's on their V12's I'd like to think may be able to occasionally afford those service bills... but the reality is usually they just don't get driven instead. Same with many of the historic race cars, engines tuned like that are why so many tend to be rollers.
For the Honda's it won't scale in the pocket up to an NSX... I'd say based on a few threads on Honda Tech $7,000
might buy you a nice NA tuned ITB setup on a B18 but to do this and do it well on an NSX C30A/C32B think a lot, lot more. You need to see an expensive bell/rail/linkage JDM piece like that as one small piece in a much larger build picture- and have some genuine method to the madness.
You're joking, right? I don't think an airbox will have any impact on that setup. In fact, I'm sure of it.
Any restriction has an impact. For me, it helped to understand the history.
A side effect of moving an unfiltered charge at a high CFM is a lot more dirt makes it into the engine which causes very rapid internal engine wear. Back in the beginning builders ran ITB's open for maximum efficiency. However, race tracks are inherently dirty environments and it was soon discovered not long after those days that brake dust, tail pipe emissions, sand, etc... getting sucked in were the primary cause for this rapid wear. Later the Lotus 7 guys used to just run moist socks over them which was popular for a short time.
Now most all pro guys, even open wheel/LMP prototypes run with a sealed air box usually with a huge K&N for maximum flow. Even still, engine life-time between re-builds is best measured in track operating hours due to their overall engine design. The piston to valve tolerances are kept ridiculously tight, and as a whole the engine internals only need to be strong enough to last a weekend- so the added airflow wear gets pushed to the bottom of the priority stack.
On some of the cars at WOT you can literally see the pull of the induction at the inlet. The engines fuel maps are typically tailored to only operate well at significant course speeds. Personally I think that all is fine when you have three more engines Lamborgini sent you on pallets in the truck rig but- the reality more often than not is that your own personal results idling at starbucks drive-through window may vary. :biggrin:
In conclusion- more likely than not for the vast majority of street or even street/track import enthusiasts... gains are going to be minimal on a stock block and the drive-ability/reliability/maintenance compromises relatively significant unless you are willing to invest in the tuning and R&D yourself. FI and other paths will look more appealing. Even for more serious track day and club cars it's difficult to justify those types of radical setup decisions due to the annual hit on the bottom line to field the car.
Personally, I'd invest in other areas of NSX setup first, and go after the good HP on a more moderate/conservative NA tune. Other applications will fair better in the pocket if someone really wants to pursue ITB's.
