87base RX-7

I seem to have seen this mentioned a few places but nothing real specific is said? my question is how much have other people shortened the stock s4 intake manifold? Im curious because that way I could also get in there to do some polishing/porting on the center intake manifold (in the one area i cant quite reach and eliminate that inward bump in the manifold) but figured why not shorten it while in there. The motor this would be going on is a 6 port side housings, T2 center, intakes have a pretty decently sized street port, the exhaust is street ported with opened up na sleeves, 6 ports have been completely removed, a angle has been added where they used to be so the air doesn't hit a brick wall, ported intake/throttle body, s4 rotating assembly (wish it was s5 but all bearings and apex/side seals are good and I cant justify the cost plus if I ever want to go to the dark side with some boost...), some oil mods, premix, custom 2 1/4 header that will probably meet back by the diff (engine is out in the process of rebuild so haven't finished the exhaust yet. Yes I know this is mostly a top end set up but I may make my own intake manifold in the future and have some small barrel throttle bodies to control the 6 ports to help the bottom end. I'm planning to take it to 7500/maybe 8000 occasionally? back to the original question... how short do you think it should be made? I was figuring on taking around 1.5-2 inches out? and the main aim is top end power.

fidelity101

iTrader: (11)

if you shorten it you just shift your torque curve, the length of it is to add low end torque.

87base RX-7

yes but it should help the top end.

j9fd3s

iTrader: (3)

there is an SAE paper about the s5 engines. they have a graph of runner length vs torque

the 88 length makes a broad peak about 5000, LONGER by 200mm makes MORE tq and peaks at 4500ish

200mm SHORTER is less tq everywhere, its so low it "peaks" earlier, but its a really flat curve. it does make about 10lbs ft more @7500.

the S5's low rpm runners are 400m longer than the s4's and the short runners (vdi) are 200mm shorter. so thats where the length numbers come from

fidelity101

iTrader: (11)

see

mario1386

so for in example if i add a trottle body spacer, torque will be shift earlier?

nillahcaz

iTrader: (2)

as long as the stock runners are a TB spacer will not do a lot. this is also ignoring the fact that any thing before the plenum has little to no effect on runner length tuning.
but just for fun lets look at this
Runner length tech O_o so cutting 1.25" out of the stock S4 runners gives you a 500 rpm boost. and going to 7250 rpm is about the two inch the OP was asking about.
also keep in mind that welding cast aluminum is not easy for a weekend welder.

So.... if the stock intake is made for 6500 RPM why is it so long?
FC intake systems used 2nd wave tuning where most intakes are 3rd,4th, ect.
If the intake was set to 4th wave its down to 8.24" from the stock 17.1" long

rotarygod

iTrader: (1)

Don't use that thread! I need to update it. I was on the right track but made one fundamental mistake. I'm smarter now! I need to get on that or delete that thread or something. I also need to split it off into intake tech for both ir and plenum based systems as well as exhaust tuning. That article can get MUCH longer.

j9fd3s

iTrader: (3)

yeah people actually do write big thick books on subjects like that one

Mazdanowski

Are you saying we should not use the calculations in the "Here's how to figure out intake runner length"?

nillahcaz

iTrader: (2)

Intake design is still voodoo magic as there are a lot of ways to find length that are right, the "big thick books" look like a webster dictionary and cover alot of ways of tuning a intake. The method Rotarygod posted is a legit way "I've seen others use it in the past unless I'm missing something" but looks like a plenum less formula. all NA FCs have a plenum. I like the Helmholtz method but can be a little scary if your not good with numbers.
this is what i use.
L = [ [(720 - EPD) * 0.25 * V * 2] / [rpm * RV] ] - (0.5 * D)

RV = reflected value
D = diameter of inlet pipe in inches
V = speed of sound
EPD = effective port duration
reflected value = the wave you wish to use ie 1st, 2nd, 3rd.

Or

642 x c x [ �√ (S/[L x V] ) ] x [ �√ { (CR-1)/ (CR+1) } ] = F
F = Torque RPM tuning
c = Speed of sound
S = runner area
L = runner length
V = displacement of one Cylinder
CR = compression ratio

rotarygod

iTrader: (1)

What I posted in that thread is CLOSE but it's not correct. Don't use it. I have also found one variable that is listed in books that is a bit different in rotaries due to our hotter exhaust temps. When I get a chance, I'll update that but do not use the formulas in that thread. At the time someone pointed out that my numbers equalling the published length numbers for the S4 runners was purely coincidental. That's true. They were! I know the correct way to do it but please stop using that thread. Let it die.

nillahcaz

iTrader: (2)

Errr.... Im an R-tard.
A piston engine takes 720º to go from cyl 1 TDC to cyl 1 TDC
A rotary takes 1080º to get from rotor 1 face 1 TDC to rotor 1 face 1 TDC and I forgot to update the formula to match, so the proper formula is
L = [ [(1080 - EPD) * 0.25 * V * 2] / [rpm * RV] ] - (0.5 * D)


Now the problem is I don't know if i should desing axu ports with or with out secondary timing
should the aux ports be 758º or 745º?

fidelity101

iTrader: (11)

your wording of it seems odd but I know what your trying to say so yes, because by increasing the runner length you are taking your previous torque curve (which had peaked higher) and making it peak sooner at a lower RPM. The power will probably be the same its just going to change when peak torque is produced.

nillahcaz

iTrader: (2)

As i posted before the TB is after the plenum so a TB spacer will not change the length of the runners. Any power band change will be due to a change in the plenum volume.

fidelity101

iTrader: (11)

but in order of air entering the engine you have

airbox/filter - maf - intake piping - TB elbow - TB - TB gasket/"spacer" - UIM - dynamic chamber - LIM - rotor housing.

so how is the TB after the plenum if thats what it bolts to essentially?

rotarygod

iTrader: (1)

nillahcaz is correct. Your intake runners are what is doing most of the tuning. Once the runners hit a larger chamber (plenum) their length ends. The plenum size does not change this nor does the location of the throttlebody. If the tb was open all the way, it is effectively not there anyways so where it is becomes completely irrelevant. Where the tb is in relation to the plenum itself does affect plenum volume. There are pros and cons to all sizes of plenums but runmner length has nothing to do with it.

FWIW: If you were to build an exhaust that had 2 pipes of a known length that dumped into a chamber rather than a collector, what you do after the chamber would have no effect on tuning whatsoever as the only thing that would matter at that point would be flow and nothing else. That means your exhaust would effectively end at the chamber even though the pipes travel farther. It's the same basic thing in reverse in the intake.

nillahcaz

iTrader: (2)

I also would like to point out that the intake tuning will never be what it is on paper. Intake tuning has to do with pressure waves moving at the speed of sound, the speed of sound WILL change no ifs ands or buts. Intake air temp will change it as will any other thing that will change the air density like manifold vacuum, fuel type and quantity.
Am i saying that this is all pointless..... Hell no.
im just saying that this should only be taken as a round about and should not be stressed too much.

rotarygod

iTrader: (1)

It's for that reason that formulas are only used to get you close. They are a starting point. Due to heat soak and weather changes, your tuning is constantly changing. This is why race cars see intake and exhaust tweaks at different tracks. Of course altitude plays a role too. Ever wonder why race teams tune around exhaust temps and not afr's? If you hold a steady exhaust temp, you hold a steady tuning point in the exhaust header that took so much time and effort to build. That's one bad thing about a rotary. You don't make the best power with steady exhaust temps. It's all dependent on rpms and load and egt's may be totally different at different spots to obtain max power. You tune around output on rotaries. What makes things even more confusing about rotaries is that there is not any one particular afr that always works best. You make changes to the timing and afrs and onluy concern yourself with the power produces. Seems logical. However when you get done you may find that exhaust temps vary greatly across the powerband and so do afr's.

Just to give a little example, there is a way you can broaden your powerband for free. I never see anyone do this though but it was tried back in WWII on 2 stroke engines. Water injection. However that's not the whole story. It's where the WI is injected that makes it work. In the exhaust! Install 2 nozzles in the exhaust. One for each pipe. You want them as close to the exhaust port as possible. The whole point is to try to cool down the exhaust a little bit. If we can cool down a 1600 degree exhaust temp down to 1500 degrees, we have changed the tuning of our header by 750 rpms! We've moved it down. If you can expel more heat you can lower it further. Obviously there are limits but the potential is there to have an exhaust that can be variable tuned over nearly 2000 rpm but not actually change physical length. Instead of worrying about physical length, worry about effective length. The perfect setup would be a progressive water injector controller that started slowly phasing water into the exhaust at a certain rpm that would have to be determined by testing. Then at a certain point in the rpm range it would need to taper back away. If it didn't your tuning would stay lower and your top end would suffer. This is something I've been quietly working on for a while and it definitely has merit. It's a very unconventional way of thinking about exhaust tuning.

Sorry to get off topic. This was originally an intake thread.

diabolical1

iTrader: (3)

patiently waiting ...

fidelity101

iTrader: (11)

makes sense to me

rotarygod

iTrader: (1)

The goal isn't to make more peak power. That won't happen. The goal is to widen the usable powerband a bit. Of course there's also the beneficial side effect of cooling the exhaust pipes at their hottest point a little bit too. Don't expect huge numbers from it. You won't see them. The important thing is that it does anything useful at all and it does. I need to start a new thread on this topic to keep this one on track.

j9fd3s

iTrader: (3)

i have heard of that for turbo's, as the water changes to steam, it also expands in volume, which the turbo likes