Rev Limiter Crystal

StanTheMan · Postby **StanTheMan** » Thu May 10, 2007 2:33 pm

not sure where you get them from but they are only a few dollars.

Just noticed yo have the 96. so make sure you do get the right crystal. There was 1 year where they went from 4mhz to 8 Mhz ......so make surre yuou solder in the correct one.

Postby **Boyracer** » Thu May 10, 2007 4:23 pm

Don't bother...power drops off at about where the rev. limit is...

Matty · Postby **Matty** » Thu May 10, 2007 4:48 pm

Boyracer wrote:Don't bother...power drops off at about where the rev. limit is...

It's not about power, it's about being able to hang onto a lower gear without upshifting between corners.

But that said, in the 1-2 shift, the car still makes more power at the OEM redline in 1st than at the same speed in second, so there should be a small benefit. It will also lean out the (too rich OEM) mixture slightly, which should pay some dividends.

The standard xtal is 4MHz (or 8MHz in some cases), so depending on what redline you want, you scale up the clock speed accordingly (for example, a 4.4MHz xtal will give a 10% higher rev limit, 7200x4.4/4.0 = 7920 - that's about as high as you'd want to go before hardware issues start)

Electronics suppliers should have them - Jaycar, Farnell, RG, etc...

Matty · Postby **Matty** » Thu May 10, 2007 4:59 pm

Prinju wrote:Thanks for that Matty.

So if my ECU has a 8mhz, i should get a 8.8mhz xtal...right

yeah, but I'd suggest either geting both and returning whichever one isn't right, or pulling the cover on the ECU and checking first...

Of course, the gains will be pretty small. There's some dyno results on miataforum if you search.

Note that everything scales up by the clock speed change: idle speed, ignition timing curve, etc.

Personally I'd stick to about a 5% clock speed change (that's what I'd planned, but the xtal - 4.19MHz - was hard to find and I ended up giving it to Stantheman so ask him about the results)

StanTheMan · Postby **StanTheMan** » Thu May 10, 2007 9:49 pm

well....its goes something like this.

I mucked around with the AFM a few years back & tightened the spring by 5 clicks from stock.

Then I installed the 4.19 crystal Matty gave me. so I was running extreemly lean. very very peaky. I couln't tell the difference at first because when i first drove it the temp in outside weather had dropped significanly overnight. I hadn't dríven the car for a few days either so it seemed a lot more powerfull. because of the colder conditions mainly.

so I drove it around for a little while & then decided to play with the spring a little more. I released the spring by 2 notches & found it even idled a little smoother. less peaky but much smoother.
I chaged the spring about 5 or 6 times to get to where it felt best.

extreemly scintiffic that is. So overall i'd say its a vry marginal difference. its a bit like the timing bump. But a little less. if I could compare it to anything.

I've hit the rev limiter twice. Thats when testing it.....so see how it worked.

I still change gear around 6500 when going hard. But the 7550 is there if i need it.
not extreemly scintiffic ....no Dyno chats on my behalf.....just gut feeling.

yes it does go a little harder.......but its hardly noticeable.

would love to know what they say on the Dyno testing.

Happy to make my car avail. if anyone wants to compare Standard & polarised ECU.

zoomzoom · Postby **zoomzoom** » Fri May 11, 2007 12:52 am

All the dyno charts I have seen the power drops right off after a peak at 5800rpm and goes down quick. The oem redline is 7000rpm but by 6500 when you change gear you drop back into about the same power before the peak at 5800 so I just don't see the point of raising the rev limiter from 7200 when you really shoudn't need to use it IMO.

As for the car being rich as I have heard numerous times, my afr reads 14.7 most of the time, the richest I have seen it go is 13.8 under heavy load over 6000rpm, which is rich but again IMO not worth leaning out the rest of the rev range for, epecially when you are running the 14degree timing advance an the car has no knock sensor! Sounds like a recipe for a holed piston to me?

Just my opion, I am sure there is someone out there who actually knows there is advantages not just going by feel, (no disrespect meant) and can correct me.

Tim

StanTheMan · Postby **StanTheMan** » Fri May 11, 2007 8:37 am

no offence taken zoom zoom......lets face it. bum in seat sort of feeling is always doddgy.

but here is a little ineteresting reading.

http://forum.miata.net/vb/showthread.ph ... 2A+crystal

when he talks about bipes.......Personally I'm not sure what he's saying. could that be some kind of knock sensor?

Boags · Postby **Boags** » Fri May 11, 2007 9:07 am

STM - Bipes is a piggyback which retards timing based on RPM (and I think you can retard for boost too if you have turbo/supercharger).

Zoomzoom, your assumption of A/F being not too rich is based on a narrowband sensor, and I'm not convinced that the Jaycar kit is very accurate anywhere outside of 14.0:1 - 15.5:1. My autotune is set to hit 12.5:1 under boost, and generally, my af guage sits on 13.2/13.3 and stays there under boost. Occasionally, in the right circumstance and when the car is warmer, the af will drop to 12something:1 on my gauge, but not often.

There have been dyno printouts from stock NAs with the bum sniffer wideband which show 11.x:1... AFTER THE CAT. :shock:

Add another 1/1.5 to that for the actual engine afr.

The other thing is, I believe that the drop off of power at 5800rpm is because of this richness. Pulling that fuel frees horsies up high, but actually, I'm with you, other than turn 6 at QR (coming back onto the main straight), I wouldn't use a redline higher than stock.

Boags

Matty · Postby **Matty** » Fri May 11, 2007 4:18 pm

There are a couple of places I know would be good for a higher rev limit: between turns 1 and 2 at Calder, and between 3 and 4 at Sandown. Also in a motorkhana it would be handy not to have to upshift or bounce off the limiter sometimes.

Grumpy · Postby **Grumpy** » Sun May 13, 2007 11:58 am

At what RPM is the stock cutout set at ??
Down the main straight at Phillip Island, it is not worth using 5th, so I am flat out in 4th, 7300rpm @ around 185-190k, and no revlimiter in sight. Since this, a powerchip has been fitted so this will be tested early June at another sprint on the island. Motor is standard apart from the chip and extractors.

Boags · Postby **Boags** » Sun May 13, 2007 1:57 pm

Grumpy wrote:flat out in 4th, 7300rpm @ around 185-190k, and no revlimiter in sight.

The stock revlimiter is at 7200RPM...

Alex · Postby **Alex** » Sun May 13, 2007 2:03 pm

Boags'MX5 wrote:
Grumpy wrote:flat out in 4th, 7300rpm @ around 185-190k, and no revlimiter in sight.

The stock revlimiter is at 7200RPM...

well in the NB8A it is don't ask how I know :oops:

Fatty · Postby **Fatty** » Sun May 13, 2007 2:29 pm

Boags'MX5 wrote:
Grumpy wrote:flat out in 4th, 7300rpm @ around 185-190k, and no revlimiter in sight.

The stock revlimiter is at 7200RPM...

yeah but our tachos' read high apparently. so grumpy was really probably only doing about 7000 to 7200rpm even tho his tacho said 7300rpm.

i was bouncing off the rev limiter all day yesterday. bloody annoying :evil:

that red line approaches a hell of a lot faster these days, that plus being on the drag strip, i just kept hitting it and ruining my times.

Boags · Postby **Boags** » Sun May 13, 2007 2:38 pm

Fatty wrote:
Boags'MX5 wrote:
Grumpy wrote:flat out in 4th, 7300rpm @ around 185-190k, and no revlimiter in sight.

The stock revlimiter is at 7200RPM...

yeah but our tachos' read high apparently. so grumpy was really probably only doing about 7000 to 7200rpm even tho his tacho said 7300rpm.

i was bouncing off the rev limiter all day yesterday. bloody annoying that red line approaches a hell of a lot faster these days, that plus being on the drag strip, i just kept hitting it and ruining my times.

When, where and who with? And why are you just telling us now? What did you end up with?

Boags

Fatty · Postby **Fatty** » Sun May 13, 2007 6:53 pm

vic club had a day at the heathcote drags yesterday mate. it's been in the motorsport forum for weeks now .

started well with a 14.8.. but mainly i was around 15.3, had a 15.1 and a 15 but could not get back down to 14.8 again! :oops:

had a couple of bad runs as well at high 15's low 16's (where i kept hitting the limiter and missing gears, and wheelspinning too much off the line).