what would be a good length for a 1/2 bridge? I was thinkin 20cm was good for a bridge. or if any body is willing to give me the formula i can figure it out. please and thank you everybody.

 

I request that we sticky rotarygod' post "Here's how to figure out intake runner length"

 

Oh man don't bring that one back! I did that thread a long long time ago. Back then I thought I had it right but I made a mistake. I know the correct way to figure it now but the end result is always the same when tech information is put out on forums. You end up with one big argument. I either need to fix the info on that thread or we just need to let it die. Most of it is correct but an error in one place changes the end result. Keep in mind that formulas like that make some assumptions. They need to since a formula assigns one value to each variable in the formula yet in the real world it is dynamic and constantly changing a little bit. Formulas are guidelines, not absolutes.

 

Exactly! I did tons of reading when I was going to build my 4 runner intake for my ITB build. Almost all the information was conflicting to a certain extent and there was no formula that gave me an exact way of determining how to build my intake.

The big questions I had were where are the measurement points, i.e. face of housing to ..... Throttle plate, bell opening or airbox opening and the effects of diameter on intake lengths. I finally read enough that it gave me a general direction on total length of the intake from housing to bell opening. I built it to meet that length, then approximated the i.d. of a stock S4 intake on the runners and built a manifold for my 4 port 1/2 bridge.

It works very well for me as my car is a race car that will never see street use and pulls hard from about 3,500 rpm to well over 9,500 rpm (a lot stronger over 7,000 rpm). I was targeting a peak of 8,500 rpm and I think it came pretty close. It would be way too short for a street car, but it works and looks good for my application.

Eric

 

+2 i've read a lot of the Mazda SAE papers, and the length and diameter chosen really depend on the port, and when you want the peak power.



picture is intake pipe length vs VE on a P port engine. they are looking for peak power @9,000rpm, so they choose a 370mm length, but it sure looks like the 450mm length would be a HUGE FAT power band. actually they eventually went to the variable length so they got both.

the diameter of those runners is way different because its a P port and not a side port, and this effects the length. the stock 84-88 6 port intake is 500mm and it makes peak power at a much different rpm than the P port!

 

My half-bridge liked the S4 N/A intake manifold a lot more than the S4 T2 manifold. The shape, runner length, and plenum volume are all different, but it made more torque down low, in the middle, and the powerband extended 1500rpm, so it was a win everywhere. Nice having a powerband that goes from 2000rpm to 9500+.

Racer23 (i think) posted info on his intake setup. His is very nice, independent throttles stuck on runners that are nearly a straight shot into the engine. Looks well worth copying.

 

For stock and streetports I like the S5 na manifolds. Everything still working on them of course.

I had an S4 na manifold that I hollowed out the plenum on to one large chamber. It did some neat things but I didn't prefer it on a street car.

 

Just some details on the intake, the secondary runners are 14.75 (approx.) long for the aggressive bridgeports I run and the primaries are almost 15 inches long with a 2" inlet tapering down to a 1.5" runner just as it goes into the keg. The primaries are a Judge Ito street port. Works very well, fits into the inner fender and uses easy to access parts. I am using 50 mm Hayabusa bodies, Orient Express (I think) bells and trumpet socks for filters. I am cleaning up the welds on the intake right now and should have it all back together to run some dyno in December.

It makes strong power and is pretty flexible.
https://www.rx7club.com/naturally-aspirated-performance-forum-220/my-na-car-968585/


Eric

 

nice 23racer I like the build iv read it before. I just finished putting my keg back together 10 min ago. and rotarygod I see and understand but theres still a lot of useful info. I was thinkin about shortning a s4 intake and flipin the dynamic chamber the other way so its where the tb is and the tb is where it is (I know complicated but read it again). I came up with the length 14.5 for it (1/2 bridge 6port). the s4 intake is 17.2 right? sound good to everyone?

 

and that was tuned for 7300 rpm for my sec and aux and then 16.5 for the pri. thanks

 

I'm telling you don't use that formula. This is coming from the guy who wrote it! Yes there is some good info. However trying to get your intake length by using that formula isn't it. There is a mistake in it.

 

Might put my 2cents worth in here. My mate is running 4 x 60mm butterflyed injection, the same as my turbo manifold. We are both on Alcohol but he is running large bridgeport. On the dyno would only rev to 7500rpm and then flat line. The problem ended up being stalled air flow. The fix was to shorten the ram tubes. To know what to build, pick a butterfly you would be happy with, then head to the dyno with ram tubes that you can length or shorten easley and have the engine dyno man tell you the lenght of your over all manifold.

 

cool thanks rotory26
rotarygod I see is there some thing that will get me close? this is just for my current dd, so just wanna make it more driveable and since im doing my header too id like to do my intake too. tip advise? thanks

 

You know what, that may explain a conversation I had with a certain R. Ambrose at the PRI Show a number of years ago. He told me to build the inlet as short as possible and then do all your tuning with the ram tubes.

Thats what I did, as the measurements I gave out above are length to the end of the ram tubes. Even at a length far shorter than what most people run, the car seems to be very flexible. I do have some spit back out of the primaries at 4500 rpm that goes away above it, hmmmmmm. Also, I wonder what the effects of alcohol versus gas have on the runner lengths. All great things to think about.

Eric

 

A runner that works well at one length will also work well at lengths longer that equal the harmonics of that frequency. One thing that was correct in the intake formula was accounting for which harmonic you wanted to tune to. With a short runner you may have been tuned to a 4th order harmonic.

Fuel type should have no effect on runner length.

 

I see but wouldnt the r2 and r3 be to low in the rpm range?