Brentis

For whatever reason I have been obsessed with intake efficiency often finding myself weighing the benefits of cool intake temperatures versus efficiency. I’ve done a reasonable amount of research for a non-engineer sans calculations/etc.

Based on my understanding, there are several factors that contribute to optimal intake efficiency. The equivalent of to having an rx7 turbo intake receiving positive pressure from outside the engine bay.

I guess I am writing this to convey my rudimentary understanding of intake efficiency, drive a more in depth discussion of factors impacting performance, and figure out if there are any improvements which we can devise. For purposes of this conversation, intake materials and capabilities are not discussed.

The key variables to creating the ultimate intake are:

- intake temperature
- intake opening structure
- intake diameter/ choking
- piping efficiency
- pressure charge
- piping friction

The standard rx7 has limiting factors that have to be reasonably adhered to in creating the perfect intake. With that said, idealistic values for the above variables cannot be accomplished as they conflict with one another in execution.

- intake temperature – perfect world 0 kelvin – not happening within a mile of an rx7
- intake opening structure – zero turbulence
- intake diameter/choking - infinite
- piping insulation – 100%
- pressure charge / ram effect – 1000+ psi
- piping friction - 0

Optimization of these variables could lead to the perfect intake. I am convinced that a 3-5% improvement can be achieved through optimizing these variables. In my small world, this could be a 20 horsepower improvement on a 400 hp engine (at the flywheel). Keep in mind, this is not measuring the flow efficiency of a particular filter element.)

If everyone theoretically agrees that having a turbo charger outside the car receiving a positive air charge sans piping is a good thing I’ll go on.

Next from my perspective, weighing the effect of the above variables is critical. Does the intake charge temperature matter more or less than the ram effect, does the insulation of the piping matter less than the size of the intake diameter, etc., etc.


Restricting factors on intake for rx7.

CFM – 400 rx7 hp engine with a 11.7:1 a/f ratio requires 600 cfm -- fuzzy area as can the compressor wheel overcome the inefficiencies through work? Also if this is reasonably accurate, 600 cfm would only be required part of the time at peak horsepower. So a flow of 400 cfm of cold air would be excellent.

But what areas under the hood in the car can support such a volume of air giving a chilly temperature? Furthermore, can I achieve this ducting in an efficient manner that doesn’t kill the temperature benefits (additional piping could effectively heat up the cold air and coupled with a longer intake pipe further degrade the benefits of the cold air.

Originally I had the belief that a “cold air intake” would be better. Without thinking of the ramifications of this approach and trying to save a buck, I did the modified stock intake with a K&N filter. I cut a 12 square inch hole in the bottom of the intake and inserted a partition between the radiator and the frame to flow cool air up into the cutout. The improvement was significant, but with my new Greddy SMIC, I felt that the heat from the large plastic intake box was heat soaking the passenger side of the IC as it wouldn’t cool after it warmed up.

The next step was to find something else that would reduce heat soak to my IC. Knowing that I knew I would be going with an open intake of sorts as I didn’t want to spend $500 for an M2 intake (IMO is the only attractive “cold air”). I started thinking of how I could drop the under hood temperature without some sort of ducting rig. While researching, I found that the aforementioned variables contribute to air intake efficiency – I knew most contributed, but I didn’t realize that the intake structure mattered (or was of significance). Further reading showed that the intake temperature became a constant after a given CFM. I think with the rx7 this is further evident as the small available orifices require that a cold air intake scavenge warmer air when demanded.

I believe cold air intakes that mandate that only cold air is available is worse than providing an overflow mechanism that will provide the additional flow regardless of warm or not. From what I learned with my mkiv TT, the TT folks always said an intake was critical to prevent turbo failure. The failure occurred from the added strain of trying to draw additional air at higher boost. Also, somewhere I read that the strain required to draw in “just cold air” through a vacuum increases the engine temperature beyond what an open element would.

This diagram shows, from my understanding, that depending on engine air demands, that the frictional loss from the intake design and piping and the “choke” or intake opening size have a significantly greater impact 5x at higher flow.




The entire document can be found here http://www.gmrc.org/gmrc/techpaperspdf/AirFlow.pdf


So today I installed my Apexi Intake. The difference was immediate. While peak horsepower may be minimal, the turbo’s (especially the primary) spooled much more quickly – to the point that I hit the highest boost on my pfc ever without trying. Furthermore my engine was running 4 degrees cooler following sitting for a few minutes and my intercooler was also running cooler – and the night was warmer and more humid than the past few weeks have been with my old intake.

This is not to say I think that cool air wouldn’t be nice. I do, just that given the turbulence of the stock intake presents offsets any benefit of a cold air approach. On the flip side, I think the Apexi has the best flow.

Next steps to further improve the cold component of the intake include ceramic coated hard intake pipes, fabrication of some sort of shrowd to redirect the radiator heat as well as possibly some sort of fan assembly to suck up the cold air between the radiator and frame. Two 4” fans could deliver 160 cfm of air from this area. Based on the graph above, I think the smooth hard pipes would improve flow, primarily boost response and mid through high HP easily by 5-10 horsepower. – I think the flow difference from the rubber intake hoses to smooth metal is equivalent as the subtle change in the efini pipe, which many people say they see a 10 horsepower improvement.

Hope this is helpful and reasonably accurate... If you have any insight, I'd like to hear it...

Marshall

had a nice long reply but I lost it. Changing the inlet track from ribbed ducting to smooth pipes helps more than most would think. Check out anderson ford motorsport, Vortech guys have been replacing their "dryer ducts" with solid pipes and picking up 2-3 psi of boost. Due to the closed loop nature of a turbo system (active wastegate rather than fixed pullies), you probably won't see any more boost -but you will help the efficiency. I have the new greddy smooth tubes intake, btw. Works great!

Brentis

I agree that the smooth pipes are going to be helpfu. Also, I think the smooth pipes won't give you more boost if your wasteget is set for a specific range, but they likely will give you the ability to create a little more boost *portential* and faster spoolup.

pp13bnos

iTrader: (1)

This is a great post, and I wish there where more like it. But anyways, now you guys are having me go out and buy a set of hard pipes! This forum is going to wreck my marriage. All in the search for better gas milage. "Yeah honey, it'll get anouther 3-4mpg no problem" Her "Yeah, kinda like the mid pipe would make the car quiter!"

CJ (I'm just joking around.)

HeatTreated

I cut an old polished intake (off my honda) to use for my intake piping and I dont think it helped with gas mileage (I still get 17 city/26 hwy) - I got 233HP/222TQ to the wheels with just intake and exhaust.

Vintage Red Touring

Great thread, and I look forward to reading more responses.

I also have the Apexi intake, and have been thinking about replacing the accordion hoses with hard pipes. Is there a hard pipe kit in existence that will work with this intake? If not, is anyone working on fabbing a kit?

Brentis, on a side note, was that you I saw a week or so ago on Alma in North Plano? I was in my 93 VR Touring FD. If so, your car looks great with the 99 wing!

David

Brentis

That was me - thanks! Your's is very clean as well. There are polished pipes available from Pettit but they are $250 which is ridiculous in my book for 18" of bent pipe. I'm looking at getting some aluminum ones fabricated and ceramic coated for hopefully half of that. I've heard good things about ceramic coating on the exhaust side and if we can keep the intake pipes from getting heat soaked by using AL and the coating we could improve upon the design. + the ceramic coating will look pretty nice. from what I've seen you can get an anodized black satin finish or a polished like finish in the ceramic.

let me know if you're interested.

7racer

Brentis!

Nice write up! I have always been curious if those vented hoods with the direct feed into a airbox would be the best??

I wish you would have told me about the aluminum pipes. I just ordered (and recieved) the rx7fashion box and hard pipes. Not too bad with a 25% discount but still expensive for a aluminum box. I wanted the m2 but they are too hard to get...plus with my koyo and the lower fit of the rx7fashion hopefully it won't be a prob.

Now your getting wheels...what else is on your list of things to get?!?!?!?

Brentis

- after the wheel purchase the other stuff I want is fairly small pricewise.

That rx7fashion intake is very pricey!! Any chance you want to let me take a look at the pipes before you put them on? I figure I can get yours ceramicoated with the lot I eventually make..

FDjunkie

After reading this twice and studying the grmc reference I think I'm beginning to grasp your theory. I hand drew a graph to help my understanding, but everytime I attempt to attach it here my browser comes back with an error stating it cannot find the server. Anyway, let me attempt to restate what I think I understand:

You suggest that turbulence in the flowing air with in the intake passages between the air filter and turbo compressor has a much greater impact on boost behavior than previously understood, and that once air filter restriction is addressed to some acceptable limit, reducing this turbulence will have a greater impact on performance than will the achievable minor changes in intake air temperature possible through current cold air intake approaches. Further, your testing seems to support that the positive effects are most noticeable in the lower boost ranges because other losses become more significant at higher boost and RPM levels.

My limited knowledge of turbochargers leads me to believe that you can/have improved the compressor efficiency by reducing inlet turbulence. I think this would (at least help) explain both the faster rise in initial boost levels and the simultaneous lowering of intake charge temperatures. The lower charge temperature should also provide or least allow some increase in peak torque and horsepower, along with an always-beneficial reduction in the tendency to knock.

Wouldn’t some simple data logging of boost vs RPM at WOT help quantify what you've found? It would seem that in particular a comparison of boost behavior before transition would the most telling. I don’t know if the intake air sensor would react fast enough to measure, but some other types of thermal probe would. Do you have data logging capabilities?

Shinobi-X

Wow man, awesome thread.

Brentis

FDJunkie - you are mostly right in your interpretation. In short, I think the cold air has a constant benefit, however, given the relatively small difference in air CFM required at say 50 or 100 degrees (see http://www.stealth316.com/2-air-fuel-flow.htm)
I think the variables associated with air delivery are more important. The graph , I believe tends to support it. Furthermore, by not providing adequate flow, the turbos have to work that much arder and probably heat the air to the pre-cooled temperature anyway.

Another, albeit sort of weak anecdote regarding the piping, have you ever realized how much more difficult it is to suck water out of those straws that have the flex area vs the straight ones.
I'd say almost twice the effort is required..

Vintage Red Touring

I am definitely interested in ceramic coated hard pipes for my intake.

I also have a local Dallas connection that can handle the ceramic coating, so please keep me posted.

Thanks,

David

Marshall

Exactly, for this the straw analogy actually works. On a similar note, I think with a custom exhaust for the stock turbo outlets -one that didn't slam the flow into the opposite turbine and instead had maybe 2 90 degree pipes off each turbo coming together with a v-band to attach it to a dp (kinda hard to decribe), you could get smaller single turbo type numbers (380-430 rw) when coupled with a good low restriction intake. Great thread btw, I love it when people actually think about stuff.

Brentis

I've actually seen some japaneese mfg (don't remember who had a header like assembly) a nice big combiner assembly with twin 2" or 2.5" pipes in about down to the midpipe rather than a seperate exhaust manifold then downpipe.

mmaragos

Not too sure how many of you have seen a bellmouth inlet, but that is what many drag racers use at the turbo inlet. No air filter, no piping, etc. Any time you have air traveling through a pipe, there are losses, the more bends, the more loss. The bellmouth inlet actually conditions the flow and "smooths" it out, essentially minimizes the turbulence.

As velocity increases, loss due to changes in the intake (bends, diameter change, acccordians, etc.), become larger.

Since air density decreases as temperature increases you need more velocity to equal the same mass flow.

Higher air temp = less air density = more velocity (for an equal mass flow) = higher loss due to intake restrictions.

BUT...It is more important to reduce the intake restriction to a minimum, then concern yourself with intake temps. Do your best to minimize intake temps, just not at the cost of intake restriction. Use the intercooler to lower temps.

Nathan Kwok

Don't forget one of the primary reasons many people restrict themselves to cold air intakes is because the air pump needs a cool air source. Hot air intakes have shown over the years to lead to premature bearing failure on the airpump. Of course, if you don't have an airpump... Also note that the stock turbos have a hard time "outflowing" even a fairly restrictive intake (although the stock intake is insanely restrictive) at 10psi. That is to say, freeing up the intake path is often at the point of diminishing returns at 10psi. Slap on a big single running 15psi and then you see why all big singles run huge (or no) air filters with as little restriction as possible.

Brentis

It was interesting in reading I saw the coefficient of turbulence or something of the like for intakes with the rounded bell being the best. From memory I think


(___

-----
(

Best

|________

|-------------


ok


-------------
-------------

worse

I'll try and find the diagram as I am obviously not doing it justice.

7racer

Brentis,

let me know when you want to look at the pipes...

I was going to install the cai and pipes today...but will hold off.

I was wondering about how much thermal insulation ceramic coating will do?? 5- 10 degrees?? More?

In the real world of our cars with the dp sitting there (some with the oven of the precat)...it seems like you are fighting a loosing battle with heat and that ceramic coating would make little difference?? I dunno...I got to go back to work.

ps...email me if you want to see the pipes I can swing up to your place if you want

Brentis

Don't worry about it - I certainly understand wanting to install new toys.

Its hard to tell how much the inuslation would help - but keep in mind that the continued airflow also has a cooling effect to the pipe - you just need something to help in its favor.

I've heard stories of folks who've had engine parts that previously burned their hands and when they touched the ceramic coated ones they were cool to the touch... Some sites also show a 100 degree reduction in exhaust pipe temp as well after the coating...

http://www.thinair-usa.com/Pictures/BlackSatinGraph.jpg

2of3running

From a person who just wants to keep the car going, no mods.

The 1000 ninja used (it may still) a box on the front in the faring that gathered air and compressed it by shrinking the size of the box as it went to the intake. It worked like a turbo, the faster you rode the more hp you got.

178mph off the showroom floor. Kawaski USA was not happy, figure people would splatter themselves.

Hope this is along the lines of the thread

Brentis

I think true -ram air has a benefit, but it certainly has to maintain a positive pressure at mid range speeds as well and also the plumbing must support the velocity at high speeds.

ZeroBanger

I own the rx7fasion cold air box. for those that don't know what it is, its just like the m2, except it sits lower so you can use all aftermarket hoods with it. Anyway, I wanted the best of both worlds, cold air AND the most flow. I have extensive 1/4 mile track times to back up my claims. In a nutshell I gained 4-5 MPH on my trap from routing a hose under the car to ram the air into the box. I will be updating my modification to put a hose where the R1 oil cooler is (I have the pep with out the oil cooler) that hose will point to my secondary turbo, while the main hose under the nose will point to the primary turbo. I expect to gain another 2-3 MPH just from that mod.

Anyway, my intake temps in northern cali are 23-30 when I arrive at wo rk after about 30 minutes of driving (remember, its 40-50 in the mornings here), on my way home (50-70 degrees) my intake temps are 32-34. Here is a link if anyone wants to read about it, its long and drawn out.

Nice write-up on your observations, Keep us updated.

https://www.rx7club.com/forum/showth...highlight=trap

RonKMiller

I think you guys are full of it. Here's the ultimate in cold air intakes:

mmaragos

Is it imaginary? Or is the "ultimate" nothing?