PDF

Is there such a rule? As soon as I get an engine dyno I'll experiment with this but for now I just need a starting point. I have built an injected 13B PP with renisis internals for the upcoming drag race season and need the do the intake and exhaust system. Is there a formula that says exhaust primary length must be so many times longer than intake length or something along those lines?

j9fd3s

i was thinking about this too. the factory pp motor (well the early 80's one, like in the comp prep book) have a short intake, and a long exhaust (230mm, port to collector), long exhausts make a broader power band, vs short.



seems like with the short factory intake it would want a short header exhaust?

Stock Josh

230mm is only about 9 inches. I dont refer to 9 inch primaries as "long", was 230mm a typo?

j9fd3s

whoops, i always mess up some mundane detail, should be 230CM

Travis R

I think it has more to do with the port timing of each. So their length may not be related at all.

j9fd3s

ran across this on the "evil" forum, we're watching videos of the mazdatrix ep car...





Yes, we are running the E/Production engine to 10k -

Do NOT try this at home, experienced (not PRO, since it's amateur racing) driver on closed course.



The SCCA rules dictate "street port" 6-port engine (meaning not bridge), so the ports are VERY large.



With VERY large ports, and absurd port timing, you get an engine that ONLY works at VERY high revs. That is why the exhaust is single, and large. -- but the dyno said it still wanted fairly small primary pipes.

Sooo - everything had to be done to attempt keeping it together that high.

Off the top of my head:

VERY lightened rotors (STARTING with lightest ones)

side cut rotors

ceramic apex seals

hardened gears

competition rotor bearings

3-window rear bearing

93 style front bearing

oil shooters in the eccentric modified

oil passages "flowed"

oil pump massaged

3 lb flywheel (yes, three)

lightened counter weights

5 1/2" double disc clutch

MSD 7AL's

Mallory coils

VERY balanced assembly

competition thrust bearings

2 oil coolers

HUGE radiator

52 mm IDA carb

very short manifold

it liked a VERY long exhaust

smallest eccentric pulley

largest water pump pulley and alternator pulley

restrictor in water hose



+ other stuff?



AND it STILL scrares the crap out of me everytime I see 10k on the tach !!



Dave Lemon

http://www.mazdatrix.com/faq/epconv.htm



http://www.rx7club.com/showthread.php?t=167063

Stock Josh

Boy, you road race guys have to do so much just to get those things to live. In drag racing, you only need about a 10th of that stuff. The rev limiter on my 12abp is 9900 and I only have a couple things off that list. Ive torn it down twice and the bearings look fine.

j9fd3s

yeah i know, street car is the same as the drag car

PDF

Ok so the factory used a 230cm collected ex system on their 13B PP, does anyone know the inlet manifold length and the overall height of the throttle body incl velocity stacks? Port timing in my engine is very similar to the MFR 13B PP so their inlet and exhaust lenths would probably be a good place to start.

Lynn E. Hanover

It depends on how close to the front you can afford to be.



The highest HP output is well over 10,000 RPM. If you want fast, just get out the check book.



Things to think about.



The high winding race engine will have (as Dave Lemon points out) a narrow power band. This requires as many gears as the rules allow. If you are looking for a 10 second car, then you cannot be pissing away much over 1/2 second for all of the shifts added up.



The best trans although very heavy would be a Lenco, with gear changes done by stopping a planetary outer race with a band brake. For far less money and only 4 speeds, the Jerico co has a neat drag trans based on the T-10 looking case. They have an adaptor to mate that to a third gen bellhousing. The dogs are two piece and face only the drive direction. The coast side is just a ramp. Without the clutch being used, you just jam

the next gear as the tach reads 10,700 or whatever. The trans stays in the lower gear until the next gear engages and beginse to drive the mainshaft. When the mainshaft speed goes higher the ramps on the lower gear dog push appart without any effort at all. So the shift speed can be actually faster than the lever is being moved. That's real quick. It is required that you shim the pressure plate so that the clutch can slip just a bit

as the next higher gear engages, lest a driveline part breaks.



If there is little money, a stock trans has wide spaced gears. More torque over a wider power band is required. Peak power will be down. A better bottom end for the power band means smaller runners, better runner velocity.



So lower RPM means longer intake runners, and longer, smaller diameter, primary headers. This does not mean you cannot have monster power up high, but it must have power down low too. The wider gear space will mean you will drop 1,700 to 2,200 RPM on a shift rather than the close ratio 1,000 RPM drop.



So if you shift at 10,500 you will end up at around 8,500. Right on the money for a Pport. If you shift at 9,500 the RPM drops to 7,500 RPM and the car falls on its face for a while as the revs come back into the power band.



The rotary has very little torque. So, to generate high HP, it must be reved up a bit. Look at the formula

HP=Torque X RPM/5252. So more HP from any amount of torque means more revs. Easy.



Same thing for the primary pipes. First is that both must be exactly the same length. For a high winding big Pport, 2" ID 8'-10" primaries into a 2 1/2" then a 20" megaphone to 4" medium run out under the rear end of the passenger door. For example, a National champion ran such a system with a stainless steelpack muffler the length of the door in 3 1/2" from a 12A Pport.



For high RPM (about) multipals of 8-9 maybe 10" for primaries. The 2 inche ID is good, but I would pick an ID to match the ID of the port and just hold that to the collector. So 10" or 20" or 30" to the collector. in 2" ID tubes. Depending on the space available.



Lower RPM means longer primaries, so, maybe 12" 24" or 36" to the collector. For lower RPM stick closer to the port ID, 2" or 1.78". to 2 1/2" with a longer megaphone to 4" say, 24".



The racing beat cataloge has a short primary length of 22" to 25" The long system is 120" to 125"

for bridgeports or street ports with a stock or weber carb.



15" to 18" for short or 113" to 118" for the long system with 600 to 650 CFM Holley on a bridge port.



For Pport short system 10" to 18", and 89" to 94" for the long system.







The intake runner length operates over a much smaller range, and just 1/4" spacers will allow a complete range for tuning of peak torque. Like the road race car the power is way up high, along with the parts cost.



More gears is way better. A special drag trans is way better.



You will have to go to the track and find what weight flywheel people are running in similar weight cars. You need just enough to get a great launch. After that the flywheel needs to be as light as is possible and still launch well.



In any case the Pport willhave enough power to scare the crap out of you for several weekends before you start getting a handle on where you need more of this or that. Mixture, timing, a rev limiter a scatter shield.

Lots to do.



Lynn E. Hanover