Intake Design Exploration

[need RX7]

http://www.rx7club.com/showthread.php?t=199788

Originally Posted by rotarygod

Well everyone always wants to know the mathematical way to figure out intake runner lengths so they can design their own manifolds. There are so many things to understand and the math seems to go on and on forever. Since there are books dealing with the subject in great depth I'll just get to the simplified math so you can figure out the perfect legth for your port style. Using this formula you will also learn why a halfbridge or full bridgeport engine utilizing the stock intake manifold has no top end but great midrange power. Here it is:

L= ( (1080-EPD) X 650 / (RPM X RV)

L= Legth of the runners. This is your answer.
EPD= Effective Port Duration (how long they are open for)
RPM= The spot where you want peak horsepower to be. (If you still have the stock gear ratio transmission and this is a streetcar there is absolutely no point in making this anything other than 6500-7500 rpm.)
RV= This stands for Reflective Value. The pressure wave reflects back and forth several times inside the pipe. For the intake the second wave is best so use the numeric value of 2. For a carburated car use 3 or 4 since the manifold may be too long. If you are figuring out exhaust length use 1. this will give you the proper length for a short primary collected system. If you want a long primary system, take the short length and multiply it times 4. OK lets plug in some numbers to prove this.

Let's look at just the primary ports in an '86-'88 n/a 6 port engine. The ports open at 32* ATDC (after top dead center) and close at 40* ABDC (after bottom dead center). We use 720* as our base point to start figuring out EPD. Since the port opens after TDC, we subtract 32* from 720* to get 688*. Since the port closes 40* after BDC, add 40* to 688* to get a total EPD of 728*. You now have one number to plug in to the above formula! So far the formula is (1080-728) X 650= 228800. Now we need to know what to divide this by. Since the '86-'88 n/a 6 port engine had a power peak of 6500 rpm this is what we will use for this example. Also since I said the second reflected wave is best to use, use the numver 2 for RV. There are the rest of the numbers for you. Take 6500 X 2 = 13000. Now we have 228800/13000. The answer; 17.6" There is one other thing to consider though. The reflection doesn't take place at the very end of the intake runner pipe but rather at a distance 1/2 the diameter of the pipe out away from the end of it. (what the hell did he just say?) Go back and read that slowly. Since the primary intake runner is 1 1/8" in diameter we must subtract half of this value from the length of the intake runner. .56". According to the calculations, the proper primary intake runner length for a stock port '86-'88 n/a 6 port engine is 17.04" The actual length as published by Mazda is 17.1"!!! Holy crap it works!!! The slight difference can be attributed to several small things. First we did not account for how fast the ports open and close. A peripheral port opens faster than a side port. accounting for this would give us shorter runner lengths. We also didn't account for the distance within the rotor housings that the air has to travel. This would add to our length. Basically these numbers almost cancel each other out so I don't worry about it. If you want to know how to figure out down to the last thousandth of an inch it will take some studying. If you look at the above number though we are within .06" of an inch from actual. Close enough. Your altitude where you live will affect it much worse than that.

If you need port timing specs for different year models I do have them as well as some of the Racing Beat port template specs and peripheral port specs. You will see something very neat when you type in the specs for a streetport but retain the factory intake manifold. Your horsepower peak will get lower! For my streetported GSL-SE I actually need a manifold with runners a little over 1" shorter just to retain the stock 6500 rpm peak number. This gets even worse with a half bridge or semi peripheral port. Many people think that since they ported it, their top end power falls off faster because lack of fuel. That may play a small role but they run rich up there anyways so big deal. The real problem is that the stock manifold they are using is too long. BDC if you are reading this it explains why Tony's half bridgeport car has much more midrange power than the streetport but falls off hard on the top end. Before you spend money on a new turbo, build a new intake manifold! Yours is too long. Yes it still matters on forced induction cars too!!!

If we wanted to tune the above engine to have peak power at 7000 instead of 6500 rpm the length would change to 15.75". For 8000 rpm it would be 13.75". Cool huh! Your low end will suffer the higher it is tuned and your power band will get narrower. You may quickly get out of you gear ratio range in which case all your new found power is worthless. If you used a Guru Racing transmission that has a shift point centered at 9500 rpm then you would want to tune for peak power at 9000 which would give a runner length of 12.15". The intake runner diameter may not flow enough air due to size though so this is something else to consider.

Now I suppose you want to figure out proper plenum size. I'll get into that later. I will just say that it gets SMALLER as the rpm limit rises! I'll let you all sleep on that one! If you have any questions about your porting style shoot me a pm and I'll help you out best I can. Have fun!

Cheers! :beerchug:

Fred