I dunno if it's the lack of humidity or allergies but every time I go to the Midwest (Kansas, Nebraska) my nose starts bleeding from the dry air. I won't be there long enough to find out what it'll do to my baby-smooth skin! I'll take hurricanes over dust bowls anytime.

But the reason I asked is because I looked up the weather in the 949 area and morning humidity on Saturday was 85% going down to the low 40's in the afternoon. So I understand humidity changes throughout the day but 6% and 8% seemed rather low. And since I've seen the dyno I go to read similarly and there is no way we got down to that dryness. So I *assumed* there might be something wrong which could lead to questionable results since the dyno charts are being presented with corrected factors and the data collection will be spread out over some weeks.

Then again, I have no idea how much humidity affects an engine or the correction factor so maybe I am over thinking it.

In any case, this mani being 7 lbs lighter and quite a bit more adaptable, IMO is a great alternative to the square top. Personally I have itb's and have no plans on getting one so I have no dog in this fight but I do enjoy reading up on new parts being developed for the Miata platform.

 

Sorry, I should have directed my comment directly at your dyno chart.

 

As the plenum volume approaches infinity you get less to no restriction in flow but an increase in throttle response time. Think if the plenum was 1 million liters - the engine would behave like there's no throttle plate and would run wide open until it put a vacuum on the intake manifold. Step on the gas and the same thing - it would take a while for atmospheric pressure to get into the manifold. Opposite of ITBs with closer to 0 plenum and instant throttle response. But under the hood, adding a liter or two to the plenum, the difference in throttle response is probably closer to milliseconds.

The runners are curved only for packaging I believe. The runner length is matched to the camshaft along with the cross section of the whole runner and valves to give a tuned rpm for the engine. This runner length is not really for a 'high revving' 4 cylinder, I'm not sure how good the BP is at that, but it's still big enough to have about the same power down low and open up at 7k+ rpm. Turbo BP doesn't really need to go much over 7k rpm to make ~300-400 hp.

When you add a turbo the dynamics change sure but not necessarily all that different. With boost you can compensate or find your weak link, and argue it doesn't really matter so much if your manifolds and runners have great flow because you're at the limit of the pistons or rods or whatever anyway. The flow mods work just as well for efficiency either way but without boost those extra few % might be more valuable, or not as cost effective with boost. Even a naturally aspirated engine still has its manifold 'pressurized' by the atmosphere. So with a turbo you just have 2-3 bar absolute pressure instead of 1.

You can calculate it on paper and probably get close like x, y, z, runner length, plenum volume, runner cross section, but in reality many if not all things have way more variables than the books and mathematical models account for. The other side is you just play with/try out what you have and different things and see what works best.

 

The BP isnt really designed to be a high revving motor since its got a pretty terrible rod/stroke ratio. It has very high piston velocity and at high rpms the side loading on the pistons is pretty substantial. I think 8k is the most that a BP should be revved personally as I dont see it making much power north of that or lasting very long. Thats why I really want to see how this manifold does with boost. Im hoping the extra volume of air from boost will show some nice gains past 4500 rpm where the typical BP usually peaks in VE. I would like to see the s2 manifold holding the torque curve flatter for longer. Still its nice to see that it did make some gains on the N/A build past 6500 rpm.

 

Yeah it may have sounded that way but I think it's probably not the best pursuit trying to make the BP rev much higher than that. Like you said, since the engine is reciprocating, rpm causes the acceleration forces to rise exponentially* hence rpm will kill a motor much easier/faster than torque or boost. Yes raising your torque rpm will make more power just look at the horsepower equation, but that rabbit hole is much more expensive than keeping rev limit near stock and adding boost.

*I don't know the math for this but balance is obviously important too. http://www.enginebuildermag.com/2013...increase-life/

 

The BP when stock doesn't like to rev but if you are developing a high power N/A engine, just like all twin cams, you need to push the revs higher to get the best power. You do this with cam selection and removing intake restrictions, tuning intake runner length and plenum sizing which is where the Skunk2 comes in. The BP with lightweight rods, forged pistons and all balanced is really happy to push past 9k rpm. You will need to sort out the oiling issues, sufficiently control the valves to stop valve float and manage the bearing wear but that is typical for all high reving engines. The modified Honda Skunk2 manifold on my engine is definitely suited to extreme high revs hitting peak power around 8,800rpm. One question I have is would this Miata Skunk2 with it's slightly longer curved runners bring that down a little into a slightly better power curve. I think that one is up to me to test as I don't think anyone here is willing to push to 9500rpm?

BTW... load on the rods does grow exponentially with rpm but it's not really the rods that are the weak point. It's the rod bolts and bearing wear that are the issue. The balancing helps to reduce the crank movement due to harmonics which makes the bearing surfaces move around and wear faster.

*edit: and yes it is a rabbit hole. You need people like Emilio who are willing to put in the time and $ to test and then release the results otherwise it's down to trial and error to get in the ballpark.

 

Thats pretty impressive that you are revving a BP out to 9000rpm. Did you do anything to improve the rod/stroke like custom rods or offset pistons? I used to own a S2000 and a motor like the F20C with its excellent rod ratio lends to high rpms very well. The BP needs a lot of massaging and still its not the type of motor that is built to rev that high although as you have demonstrated its certainly possible. I bet the longer runners on the S2 manifold would move your torque peak down a little but how much is a mystery for sure.

 

My current engine is stock rod/stroke. I'd like to try lengthening the rods and move the wrist pin higher, there is an engine for sale in Aus with this already done (might have sold now). I was planning on running aluminium rods at some point so changing the length at that point would just mean new custom pistons.

The longer runners on the Miata S2 will definitely move the harmonics lower... but I'm not sure if it will flow as nice as my Honda manifold which is pretty much ideal for airflow with a better bell mouth in the plenum tapering straight down to the short radius of the valve. Maybe next dyno I'll see if I can get the Miata S2 and do a side by side comparison.

 

Yea if you could make a long rod motor I think you would greatly increase the lifespan of the BP while revving it out so high. The FM stroker kit for example improves the rod ratio with a longer rod and a higher wrist pin location. I agree that your Honda manifold is probably superior the internals of the honda manifolds are like you said much nicer for flow at high rpm. It would be SUPER interesting to see a back to back and see how the miata S2 manifold would fare on your motor. I would love to see video for your engine if you have any. Hardly anyone takes the BP to those sort of rpms, must sound wicked up there.

 

The reason for long rods is when you have a head that flows a lot of CFM, which the BP had does not. While bottom end life would improve with longer rods it would only be appropriate for a full race motor with 12 or 13 mm cams. For the rest of the world long rods in the BP are definitely a waste of time.

The FM stroker merely maintais OEM rod ratio with the relocated wrist pin, due to the longer stroke.

 

no sense in polishing a turd building a high rev na bp anymore.
maybe back in the day when all these swaps weren't available

 

It's important if you are running in restricted classes. Under 2000cc production class where you have to retain the block and head of that model of car. If you put in a K20 you are running against 500kg space framed sports sedans and if you turbo your up against 700whp Evos. Some of the classes here require OEM intake manifolds so the guys use the oem NA8 manifolds and get them extrusion honed so that they flow 20% more.

Anyway this getting way off topic. Back onto the Skunk2.

 

Gents,

Lets start a new thread about manifold design and/or rod ratios and whatever.

Doing my best to keep this thread just for actual results, facts about the S2 manifold.

 

This is a good idea.
LMK know which posts you need cleaned up and we'll split them off into another thread.

 

In for boosted results. Huge thanks and many to Emilio for presenting this data and not just hoarding it. I love this community.

 

Sacrifice 3hp in the midrange for 4hp on top. I seriously thought this thing would do better. It will be interesting to see what spacers will do.. but for even a built street engine flat top looks like the way to go.

 

Real data FTW!

 

then we need to see it on a typical boosted miata...

 

No. Let's just keep speculating

 

True, this aint namiata.net. We need boost! Although sjmarcy would be proud of the superior na throttle response posted. I dont think these results apply to fast cars as of yet.