Hello,

Assuming a S4 NA (of course) bone stock needs a rebuild. Is it a good idea to replace the S4 Rotors with S5 rotors and housings to gain compression ?
What about the S5 housings, are they stronger than the S4 ones ?


Facts I found:

• S4 comp: 9.4 @ 4553 gramms
• S5 comp: 9.7 @ 4328 gramms

Looks like a WIN WIN situation to me, any downsides ?

 

You don't really need the s5 housings unless your s4 housings were already too worn. If you go to the s5 rotors you will need to make sure the rotating assembly is matched so the balance is correct.

 

Thanks,

yes getting the matching assembly was obvious to me, I just forgot to write it down.
Balancing the whole assembly will be considered as a must.

I was more interested if it would bring the desired effect of more compression when using S5 rotors.
Are the S5 housings stronger than their S4 counterparts ? I believe I read it somewhere but I would like to have it confirmed. What exactly is stonger about them if so ?
I am still not sure If it will be a 100% fit. I mean will they bolt together without trouble ? Same specs ?


Many thanks
Steven

 

S5 housings use a different spark plug hole location than S4, the S4 is "preferred" because it is the same as orientation/location/dimension as the race rotary engines that mazda builds. They are equally as strong just find a set that has least wear ideally.

they will bolt together just dont mix s4 with s5 rotor housings, make sure that your rotor housings are paired together in condition (close as possible) and engine series.

 

since he has a european car, the plug location might already be in the S5 spot.

 

The most important thing to note is that if you change from S4 series rotors to S5, you must also change to an S5 front counterweight and an S5 flywheel.

 

^E shaft as well? Or just rotors, counter weight, and flywheel?

 

I think the S4 eshaft can be used with S5 rotors without problem.
S5 rotors has more compression different spark location the rest should be the same. (E-shaft wise)
But I would check endplay regardless to be on the safe side.

The only thing is when using an RX-8 shaft the stationary gears needs to be changed or modified.
Mazdatrix has a good description of it.

Steven

 

Not worth the bother, a large part of the performance difference has to do with the vdi intake on the s5.

Make it easier & simpler and just get a lighter flywheel, atkins aux sleeves and invest in a really good exhaust.

 

I agree. I've considered gathering S5 parts (rotors, counterweight and VDI intake) when it comes time to rebuild my S4 motor. If my S4 parts are reuseable, I'll use what I got, rather than spending hundreds of dollars and months seaching for hard to find parts. If I want a lighter rotating mass, I'll just replace the flywheel. The VDI adds power at the extreme end of the RPM band, but I don't hang out there too long.

(Now I'm even questioning if a lighter flywheel is going to change my driving style too much and force me to shift like a race car driver while I'm going through the city.)

Regardless, you can't go wrong with a good exhaust header.

 

A lighter flywheel doesn't change how you shift, it changes how you press the gas pedal. Jerky throttle movements are going to translate into jerky driving because you've got less mass keeping the engine from revving freely.

 

My mistake.
I thought with less rotating mass, that stepping on the clutch would cause the rev's to drop quicker. Matching rev's when you shift would demand quicker shifting, or blipping the throttle to keep the RPMs up.
Normally after the shift, I either release the clutch and mat the gas pedal or get up to speed and set the cruise. I can't see how driving would be jerky due to a lighter flywheel.

I've heard the difficually is to get rolling from a dead stop without reving too much and spinning or reving too low and stalling.

I don't know. I don't want to drive like a jerk.

 

The engine response might be better/directer, meaning revving a bit faster then with stock parts.
Remember to get a scattershield for the trans once messing around with flywheels, nothing funny about a flywheel/clutch assembly going off like a grenade at redline.

Personally I use heeltoe all the time, I can imagin with a lighter flywheel it might be a small bit trickier to keep it from stalling while clutching in in 1st.
Can be fixed with better sense in the right foot...
A good heeltoe is nothing to laugh at, people still look confused, but it sounds just right.
Sounds like a sequential tiptronic race gearbox

 

The s5 manifolds have better area under the torque curve than the s4. It's not just for top end. When developing the s5 VDI system, Mazda experimented with different runner lengths:



During lower rpm operation, the s5 intake manifold runners are 400mm longer than the s4, resulting in torque curve "d." After the changeover point, the runners become 200mm shorter than the s4 manifold runners, resulting in torque curve "a." The s4 manifolds were a compromise design because the lengths were fixed. The s5 manifolds have two fixed lengths, while the R26B engine found in the 787B 4 rotor race car had continuously variable runner lengths:

 

A lighter flywheel has less inertia, so yes the revs do drop more when you shift and left off easy (like in normal daily driving). They also drop faster when you free rev the engine when you are stopped. They drop faster as you let the clutch out coming off a stop sign, which requires careful modulation of the gas and clutch pedal.

The revs can also increase faster. When you free rev the engine (vehicle is stopped) it will generally increase faster. If you pop the clutch in to rev match, the revs will go up faster. This doesn't apply here as much but on high horsepower turbo engines, when you shift at the top of a gear the revs will actually increase before they drop down.

I guess it depends on how you define "jerky"

It definitely requires more effort from a stop, especially on engines like a rotary which have limited low-end torque.

 

I like light flywheels because it lets the engine rev more freely and increases the effect of engine braking. In my opinion, having the throttle react so much more quickly (in both increasing and decreasing rpms) is a huge benefit. The downside is without that mass, the engine will be more likely to surge or stall (flywheel mass helps keep the idle steady). On my previous car, I went from a stock flywheel of around 17 lbs to a 7.5-lb Fidanza, and I absolutely loved it. It makes the car feel so much faster when it's that quick to respond to throttle movements.

I had a bit of a problem adjusting to a 6-puck clutch with such a light flywheel. I stalled the car a couple times (the clutch wouldn't slip). Honestly though, that's something that never actually bothered me. I liked the fact that nobody wanted to drive my car because it was "so hard to drive".

 

keep the flysheel stock, get a better clutch, streetport the endplates and get atkins sleeves, job done and budget friendly.

or, source all the s5 stuff, figure out the conversion (could get complicated) and you will squeeze a little more out of the motor. For that money though, i would do a TII swap.

 

FWIW I switched from 84-85 13b rotors 11.5lbs 9.2 compression to the S5 n/a rotors and the difference was very noticeable. Revved so much easier and the extra compression can be felt. Just lifting the rotor was easier during assembly. If you're looking for that extra punch use the S5 rotors. Is it worth the cost and effort? That's up to you.

 

i personally i think the stock flywheels are too heavy! i think the RB 17lbs flywheel is what the car should have some with stock.

with the stock flywheel, the revs DON'T drop, and you either have to wait, or get the rubber band thing where the car is upset during a shift.

i also don't like puck clutches, they just aren't for street use

 

Absolutely agree. Even sprung, they are a bear to drive. The grip was awesome, though. I could dump the clutch at 7500 rpm and spin all 4 wheels.

 

What is the length of the runner for the "0" value, where no "extra length" is added? I have a pport and would like to compare the length versus my manifold. Is it measured from the opening in the motor itself (the inside of the rotor housing), or from the face of the housing?
And curiously, what length is the header primaries (and size) of the 787? Or any mazda pport engine? Mine has 21 inch primaries.

 

^I've searched high and low for 787B header length and came up with nothing. I've skimmed every SAE paper I could find and still nothing. There are several publications on 13B/12A header lengths, but only long primary systems for the most part.

 

Well here's the problem. The telescoping intake system has a stationary section and a telescoping section, and we don't know the length of the stationary pipe. That would correspond to the "0" value I think.

One paper does discuss some major experiments they did on a small single rotor (like half of a 10A engine) p-port engine research engine. Varying the intake runner length predictably changed the engine brake mean effective pressure curve.



The exhaust length had crazy resonance effects and seems pretty unpredictable without lab-grade equipment and expensive/proprietary modeling. They did seem to like 300mm though, as it was the fixed exhaust length used in some of their intake piping tests.



There's a lot of stuff here about port timing as well, and a comparison of bridge port + peripheral + semi pp. PM me and I can send you the whole thing.

 

i think the fact that they don't mention the exhaust AT ALL, means its just pipes....

having seen the 787B a TON of times, its a 4 into 1. there are 4 "S" shaped pipes, which make a 4 into 1 header. its all slip fit, with spring clips. it collects into 3", goes into a 12-14" muffler, and then theres a crappy pie cut section to turn out in front of the rear wheel.

its really short and simple.

the 787 (non B!) has the exact same thing, except its got a mandrel bent piece to exit the car