Recommended Rev limit..
 

I have been working with my Link G3 and MTX-L doing fuel map tuning and learning the various settings etc.

It got me too thinking, WHAT is the accepted rev limit for a 1600 running 6-7 PSI boost (maybe going to 8-10 in the near future). Of all the posts and studying I have been doing on this site, I don't recall seeing any discussion of suggested / recommended rev limits for the various engines.....consequences and trade offs for going high etc..

Can some one advise me?....Engine has 200,000 km on it but seems to be in good shape.

Regular oil changes, service etc..., more like a race car in its attention...

This is for a track day / sunday fang "my toy" type car....not a daily driver that has to last another 200,000 km's ;)

7200 is the stock rev limit. Bad tune and over heating issues are the only reasons to lower it.

Thanks curly, I was wondering about the fors and against's of raising it though....

raising it increases stress on the rods significantly. i think it's nonlinear--i.e. more like exponential. reducing it by just a few hundred rpm to 6800-7000 will reduce engine stress significantly. Mazda did this in the MSM.

one thing to consider also is that the stock intake manifold will make power drop off above 6500 anyway so unless you've replaced that, you're not really gaining anything by raising the rev limit. and if you are going to replace the intake manifold, you'll probably have already installed stronger rods and a valvetrain that can handle more RPM.



tons of dyno plots: http://flyinmiata.com/tech/dyno.php

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.

Background:

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.

Wow, johnny2 bad that is some impressive calculating. THnak you.

That crystal mod was part of what got me thinking about the rev limit.

Anyway, per your post 7200 seems like a logical safe limit in my case.

I wonder what hustler, emilio, savington and the other race guys run at...?

Agreed, nice read!

I would only suggest you exercise caution. Mazda built a road motor not a race motor in the generic Miata.

I've heard that SPEC Miatas running at sustained 7000 RPM show rod deformation and experience occasional rod breakage. These engines must use stock parts by rule.

There is a difference between the occasional foray into High RPM territory and sustained operation. You shouldn't expect an engine to live forever with stock rotating assemblies on the fun side of 7 grand, but a few moments might be fine.

The stock rod does seem to be a limiting factor in sustained high RPM operation and that does bring up the concern that going too far beyond stock redline is not advisable, but for the occasional run through the gears it's probably valvetrain limited. My guess is that's where the 7400~7600 number comes from.

Adding boost makes the limits a bit tighter because you're adding heat to the equation. It also might be worthwhile to mention that the piston/rod assembly approaches and leaves TDC twice in the 4-cycle motor. The stresses are greater during the exhaust / intake period because in the compression / power period the compressed A/F ratio pushes back and limits the forces acting on the rod / piston assembly. Most metals handle being compressed better than they handle being pulled apart. With FI these forces would be higher still; I have no idea whether that is a good thing or a bad thing, but detonation is definitely a bad thing.

I would expect the practical result of those two would be to limit further the amount of time you can expect to sit on or hit extended RPMs over a NA engine.

I personally haven't experienced any ill effects from hitting the rev limiter occasionally while going through the gears in the 1.6 and a high miler at that, so I'm not going to say you "can't do it". But, that was my motor and my wallet on the line. YMMV.

Thanks for this info! This is going to be food for my thought tonight while im setting my AEM up in the Garage. Looks like I will not run at 9k rpm, id like to keep mine around 8k. Good work