BLKTOPTRVL

Engineers: Please check my logic and math here… This post is based on my understanding of the information on the Robinette site.

http://www.rx7turboturbo.com/robrobinette/coldair.htm

BTW, I am not an engineer or math major, so these questions may be very simplistic to you.

Statement:

• The 'Cheap Bastard' air intake mod removes the foam insulation next to the fan; there is a shield that is installed to help prevent hot air from mixing with the new cold air passage, and the bottom of the air box is opened (12 square inches or more) to breath in that new air source.
• The turbos breath through two intake pipes connected to the airbox. One is ~2.5" in diameter; the other is ~2.75" diameter.
  
  2.50" diameter = area of 4.91 sq. in.
  2.75" diameter = area of 5.94 sq. in.
  
  Total area = 4.91" + 5.94" = 10.85"
  
• The nose intake of the car is approximately 1.25" x 8". This gives an area of ~10 inches.
• According to the Robinette Site, Mazda found that when cycling through hard acceleration/deceleration, the RX-7 tended to breath hot air into the engine. To fix this, Mazda made two separate tracts for air intake… One tract for the turbos, and one for the intercooler.

Questions:
[list=1][*]These two rough area measurements: at the nose and at the turbos are so close that I have to imagine that this is all the air the car can use. Otherwise wouldn't Mazda have made the nose intake and the turbo hoses larger?[*]If 10.85" of area is all the turbos can breath in a stock setup, won't cutting 10 sq inches out of the bottom of the airbox sufficient to allow the turbos to breath?[*]If we have the turbos breathing from the bottom of the airbox, can't the air passage from the nose be blocked from the modified airbox and used to service the intercooler only?[*]If we open the bottom of the airbox and keep the passage to the nose, can some kind of blocking plate be used in bad weather the keep the car from breathing water in heavy rains (e.g. flooded areas in the road)?[*]If the ten inches from below are added to the 10 inches from the nose of the car, will this both supply good clean air, and also keep the air going through the intercoolers from reversing, or do they need to be two totally seperate tracts?[*]Assuming all above is correct, can I build a duct that goes directly from the bottom of the opened airbox to the new fresh air opening? I would remove the foam next to the radiator and have a custom duct built that runs from the airbox directly to the opening.

The reason I wouldn't want to use a sheet metal blocking plate is that I image a lot of hot air will still get into the area from the other side of the airbox. Using ducting should provide plenty of cold air shouldn't it?[/list=1]

scotty305

iTrader: (2)

1. No, if the engine wasn't capable of using more air than that, then people wouldn't be getting HP gains from aftermarket intakes.

2. See answer to 1. If you end up with an opening that is the same area as it had before, that should be fine. Others claim increased HP from larger area openings.

3. Yes, probably.

4. Yes, probably. It might be a pain fabricating something that blocks water but still allows the car to breathe.

5. It seems like others have had good luck with this method. Two intake holes are better than one?

6. Sure you could. It might work better , and it might not. There's only one way to find out!


-s-

scotty305

iTrader: (2)

PS, I'm an electric engineering student, so you might want to get a second opinion. I've got a decent background in auto-related stuff though, been helping on the FSAE team and learning from some of the senior-level guys.

I can definitely confirm that your calcs for the area of a circular hole are correct.

-s-

mks

You should cut the crossover duct that goes from the IC ducting to the airbox open and measure the cross section there, I bet it's nowhere near 10 sq. in. You can most likely remove the stock ducting safely, and it'll make your IC happier.

BLKTOPTRVL

Good point about the crossover.

The reason I ask so many Q's about this subject is that I am a very cautious person. And I really don't know what I am doing... Just trying to make sense of all the information I find.

Reading more, I see that:

• The stock computer cannot handle changes in airlow rates (cannot compensate with more fuel).
  
• Opening up the intake too much can cause lean-run conditions
  
• the stock maps provide a little room for error by running excessively rich.
  
• Since I have the stock exhaust with the exception of the downpipe, getting colder air into the intake should not put me over the limit.

So, I am going to cut a 5 sq in opening into the bottom of the air box and see if the car responds. I will leave it this way for a while and later, if I am still happy, I may open it further.

Instead of installing a blocking plate (to keep the hot air out), I think I want to use ducting.

adam c

iTrader: (2)

Cutting a hole in the bottom of the box will help, but you will be drawing in warm air. That kind of defeats the purpose of a cold air intake. Ducting can eliminate the warm air, but then you have the problem of a "vacuum cleaner", sucking everything that comes to the nose of the car into your airbox. Water, rocks, bugs, etc..... In addition, the ducting will have to be "squashed" a bit to fit into the space next to the radiator. That will be a bit restrictive.

It seems to me that there was some guy that created a really nice shield to do exactly what you want ................