03-25-2012, 02:57 PM
Join Date: Oct 2010
Location: Saskatoon SK Canada
Total Cats: 4
From what I understand, the stock NA ECM's measured RPM limiter kicks in around 100-200 lower than spec but you see 7200/7000 on the OEM Tach when fuel cutoff kicks in.
This from research into early ECM mods in Japan (and via Google translate) where before the ECM was well understood, they would alter the stock ECM by replacing the crystal oscillator with a different one; in essence overclocking the ECM.
This fools the ECM into believing the engine is revving lower than it actually is, which has two effects ... it delays the advent of the rev limiter, and it affects the fuel/timing maps since the ECM will use a map for a lower RPM than actual over the entire RPM band.
The effect will be the ratio of the new xtal vs the stock xtal clock frequency.
Take it any way you want, but they seemed to think 7400~7600 was a safe RPM limit for a stock engine in good operating condition. Not that I recommend it; just reporting what I was able to figure out via the Japanese-English Google translator.
The standard practice to determine safe RPM limit is to calculate piston speed by punching in the engine working dimensions. A rough guide can be found by using stroke alone, but you really want to include rod ratios, piston and rod mass, etc as well if you can.
All assuming an engine in good working condition; if it's broken or weak all bets are off.
3500 ft/min = long lived engine with conservative limits
4000 ft/min = stressed engine but possible with adequate preparation
4500 ft/min = race territory with preparation required and reduced engine life expected
Most OEM factory stock engines are in the 3500~4000 range, depending on the intended performance level.
The 1.6 has a 83.6 mm stroke, the 1.8 is 85 mm.
Stock redline on the 1.6 is 200 RPM higher than the 1.8 (7200 vs 7000)
1.6's 83.6mm stroke (by itself, no other inputs) shows stock redline of 7200 to be 3950 ft/min.
6400 is 3500 ft/min
6850 is 3750 ft/min
7100 is 3900 ft/min (same as 1.8 ft/min @ factory redline)
7300 is at 4,000 ft/min
7750 is at 4250 ft/min.
A 1.8's 85mm stroke (by itself, no other inputs) shows stock redline of 7000 to be just over 3900 ft/min.
6300 is 3500 ft/min.
6750 is 3750 ft/min
7100 is 3950 ft/min (same as 1.6 ft/min @ factory redline)
7625 is 4250 ft/min.
(1.8's stroke used in the following, just for perspective)
8000 is about 4500 ft/min. 8800 is at 4900 ft/m which is getting into NASCAR and F1 territory. Engine life is measured in hours.
10,600 is at 6000 ft/m which is Top Fuel Dragster stuff with full engine teardowns expected every half dozen runs or so. Might be worth mentioning that the typical TF engine turns about 600 or fewer engine rotations in a 1/4 mile pass, so engine life at WOT is measured in the very low minutes, possibly less than one minute.
We know the Miata engine is capable of high RPM performance so the stock numbers of around 4000-ish are probably quite safe limits. Were it me I'd keep it under 4250 and at that point every 50 or 100 fpm matters, so going a bit lower is probably worth considering.
Mazda lightened the crank and the flywheel to get a bit more RPM in the B6 1.6 for Miata duty, so there's an indication of how rotating mass plays a role. They added back some flywheel weight in the 1.8, so maybe the 1.6 has some untapped rev potential to exploit. Your valvetrain also has to be up to snuff.
I rounded off the numbers slightly.
Last edited by Johnny2Bad; 03-25-2012 at 03:16 PM.