I didn't want to bother cutting my frame with the Greddy or knockoff FMIC so earlier this year I bought a universal intercooler kit from CX racing and had a friend of mine who is adept at aluminum welding fabricate the FMIC setup for me.

Now the thing is, at the time we had decided to go with a 2" pipe, both for fitment reasons, and to decrease the overall volume and improve response. I have since heard from a rotary expert that I should use a slightly thicker pipe between turbo and intercooler, and a much thicker pipe between intercooler and throttle inlet. Something like 2.25" turbo side and 2.75" inlet side. It sort of made sense to me, but I don't know if it is worth the effort. Any opinions?

 

There is a formula out there, I cant remember it. Air can only move so fast through a tube and going too big makes more friction. I know for sure its in the Corky Bell book.

You could just get 2.5 and call it a day.

 

fail


you dont have to cut the frame, the car actualy doesnt have a frame (unibody) and you cut the core support/felderwell sections to run the pipes through. 2" is too small aswell IMO

 

The rule of thumb is to keep the airflow rate below 450 ft/sec using the basic formula of CFM / Tube Cross-Sectional Area. Your fabricator can use the Reynolds number of the material, estimated air temperature, and estimated flow rate to come up with a more accurate sizing guideline. The fact that you are feeding a rotary engine has absolutely no bearing on this calculation other than you may need to help him estimate the flow rate of the engine.

 

All that work because you don't want to slightly trim?

 

Always make sure you dont get restrictions in the intake. You can keep it the same or go bigger along the way, but you must not go smaller..

I built my own intercooler endtanks for my custom v-mount setup.
my BNR stage 4 has an outlet of 2", with a silicone reducer i go directly to 2,5" piping right into the IC and the outlet of the IC is 3". It stays 3" from there to the throttle body.

 

Ok. Even a unibody has a frame, it is just integrated with bodywork. Either way I didn't want to cut a hole in my car or any other bullshit for some overpriced pre-fab kit aimed at a general "tuning" crowd rather than my specific application. I paid about $350 with the custom work and install included = win. A larger tube means a larger volume of air and thus less response. If you are running a BNR 4 or some other such absurdity you are not concerned with response. This does not apply to me.

EvilAviator: thanks a shitton! I don't suppose you happen to know the approximate CFM of the HT18 or T04b off hand do ya?

 

Btw. My fabricator actually said that if anything the thicker pipe should be turbo side since the hotter air will take up more space than the denser air coming out of the intercooler.

 

actually, I built my v-mount setup mostly for response..
and it is sick compared to the old FMIC as i only have about 13inches of piping

 

not going to flame you on how ignorant your piping ideas are.

intake systems are always a compromise between velocity and volume. each lends itself to power in different rpm ranges. the 13bt with an s5 turbo really doesnt start making notable boost untill 2700-2800 rpm and by then 2" piping would already be maxed out as far as intake volume netting a "seal" on the engines power potential beyond said engine speed. i would say 2.5" min for piping size.

 

You are only looking at about 0.1 sec difference between 2" and 3" piping.

If you are concerned about response then you should just stick with the stock intercooler rather than increasing the plumbing and internal volume. If you are intent on a custom intercooler, then you should have your fabricator mount the intercooler horizontally between the engine and the radiator and fabricate end tanks such that the pipes face toward the windshield, install a vented hood and intercooler ducting, and replace the stock AFM with a standalone EMS. Here is an example of a horizontally-mounted intercooler:
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/pics-blue-tiis-awesome-engine-setup-hfmic-145822/

If you keep the stock AFM and run long piping to a large front-mount intercooler then you aren't really going to cut down on lag that much, and you may as well not concern yourself much with pipe diameter. Most people just use the same diameter as the parts (compressor outlet, intercooler inlet and outlet, throttle body elbow inlet).

It doesn't work that way. You need to work backwards from the engine. First you estimate the cfm of the engine at your target rpm and boost level, then you can estimate the lbs/min flow rate. Using the standard volume of air at a given temperature, then you can estimate the cfm flow rate through the intake pipe at a given inside diameter. To find the flow rate of a rotary engine, tell your fabricator to use the standard piston engine formula but remove the "2" from the denominator. The volumetric efficiency depends a lot on the type of porting and target engine rpm, but if he just uses the regular piston engine VE then you should be in the ballpark.
http://www.gnttype.org/techarea/turbo/turboflow.html

 

Furb: sounds cool, pics?

gear_grinder: that is kinda a flame anyway chillax. The stock 13bt used a 2" pipe from turbo to top mount yes? I'd like boost to kick in sooner, but as I am running 8.5 compression with an s5 turbocharger, I may be assed out.

Evil: that top-front mount is an excellent looking setup. I don't think I'm going to end up with something like that tho. Sounds like we have a whole lot of numbers to crunch in order to get the optimum piping diameter. For the record, this car was meant to be used in auto x so there is little tolerance for lag.

 

Oh, and I have a standalone already. Microtech LTX8.

Since the outlet of the compressor is 2" and the throttle body inlet is 2.5" would it just make sense enough to change the pipe from IC to TB inlet to a 2.5" ? (in terms of what most people do)

 

You can install an IC without cutting the hell out of your car. I ran mine thorugh the old AC line holes (made it bigger with a holes saw) on the pass side and just above the oil cooler lines on the drivers side, very little cutting there as well. Im running 2.5 tube and a s5 turbo.

Ive switched back to my top mout on a few occasions and can tell you there is NO DIFFERENCE in lag between the top mount and the front mount.

Ive done calculations for a to4R and a TO4E and I remember them both comming out to be best at ~2.75 pipe. But there is no way your ever going to NOTICE the difference. Absolutely no way. We are talking miliseconds here, just not worth the effort to even burn the calories thinking about it.

Pressureizing the space in the IC and pipe happens so quick its not even worth figuring out on FC's If your worried about lag look to the turbo, not your IC.

If you really cared this much about your IC design youd be spending a few thousand on your IC core. Since your not, just get some 2.5 tube and be done with it. If your really that persnicidy get some 2.75.

 

Also wanted to add that when you do the calculating for pipe size your not doing it to make lag less or more, becuse it is such a small value. You do this to optimize flow.

Quit looking at lag when choosing pipe size.

Also, even on the stock turbo going from 6psi (or whatever stock is) which is what its designed for to 10-12 psi its a lot more flow. Still want 2 inch?

 

Remove the throttlebody butterflies and get you a clutchless transmission setup for the car. No lag and full boost all the time. Don't know what to tell you about slowing down though. Lol

 

Hell... what about starting? It's not a damned sprint car

 

Understood. Given that I already HAVE the 2" intercooler, I'll probably focus on improving response via the turbo first. And move to a different intercooler when time/money allows. (the guys at the dyno are gonna love me )

Though I'm not sure on the actual length, assuming 6 ft of tubing makes the total volume 226 cui for the 2", and 353 cui for the 2.5". A difference of 127 cui is certainly not much space, especially since the tubes are prolly shorter than 6ft. When flow dynamics are taken into consideration I can see how such a diameter might have a choking effect on the charge air.

 

Found that i didnt have any descent pics online of the finished result but my build thread should give you an idea on the setup

https://www.rx7club.com/showthread.p...ht=furb&page=8

 

Interested...

I've been researching into intercooler set ups for a week or so, ready for when i finally upgrade mine. Were you basing the above findings on the stock turbo? I've read that the stock turbo suffers from a fair amount of lag when fitted with a FMIC.

 

S5 Stock turbo on my setup. As I said, no difference.

On RX-7's do not look to IC tube size to increase or decrease lag. Your dealing with less than a second to boost threshold for 2 inch to 3 inch.
Not saying 3 inch is better, its just an example. Same goes for volume of the intercooler.

Your really not increasing the total volume very much at all considering how much the turbo flows in one second.

 

So what your saying is minimal consideration for lag should be taken when considering front mint or vmount set ups. Good to know this as I've been advised lag was increased dude to the extra pipework on a front mount when compared to a vmount and topmount set up. Appreciate your input.

 

Lag is increased by extra pipework and big IC's, that is a fact, its also a fact that is generally less than a seconds worth of difference. And its another fact that the second is more than made up for in other areas of the design. Pressure drop, charge temp, ect ect.

Dont take my word for it, get some books, its also all over the internet but a good book is really the way to go.

 

i did this in reverse on my 20B car. i put the IC in, and then put boost gauges on each side, to reveal a HUGE pressure drop. on that car the pipe size wasn't the critical part, but the bends/routing.

i think 2" pipe should work, IF its pretty straight.

and feel free to check your setup, that's part of building the car.

 

I'd be interested in doing something similar once I get my setup sorted. I currently have a vmic w/ 2" inlet and 2.5" outlet running the stock turbo and response is great.

I plan to upgrade to 2.5" inlet once I finish my hybrid.