7300 soft, 7400 hard. All stock internals. Balanced rotating mass. 220/201 Hp/Tq at 10 psi

KMag

We must be doing something wrong. If we ran our HPDE and race cars at the rev limit some of the folks in this thread are posting, we would be blowing up stuff constantly. I wonder how many use the stock tach as their reference. I wonder how many shift within 200rpm of the fuel cute every shift during the 5x 20 minute track sessions (including w2w races) every track day.

In a typical weekend, we might have an 30 minutes to an hour spent within 200rpm of the fuel cut at full load.

In my experience, the life of the stock NB long block that might otherwise last 100 race hours is cut roughly in half when the rev limit is set at 7400. Bump to 7600 and it's cut in half again. Bump to 7800 and it's cut in half again. Bump to 7900, half again. Yes, that's about 6 hrs. That's assuming the TB shaft doesn't let go before then, which it will.

I suppose if it's a street car, autocrosser or infrequent HPDE mobile, it might take a few months to accrue 30 minutes within 200rpm of the limiter at full load, the the reports of service life vs rpm far exceeding ours . I guess we are just a bit more conservative.

Thank you for making me feel sane about my "conservative" limits :party:

I noticed you said stock long lockblock? What do you find is the typical wear points for high rpms Emilio? (Other than the throttle body)Just curious.

I'm actually very surprised and now a little worried at how low everyone is setting their rev limiter.

I thought mine was set at a nice conservative 8200 down from 8400. My soft limiter kicks in at 8000 but I hit that at pretty much every gear change. My peak power is around 7800 (a bit over 175 whp) so I need the revs that high to keep the engine in the power band. I also run stock TB.

So far I've run the engine at 4 events, with 12 or so planned this year before a strip down and rebuild at the end of the year. Most my events are time attack, 5-6 min at pace and around 6 sessions per event. (so around 30mins of total engine wear)

I'd too would be very interested to know the sort of failures people see due to revs. I would expect valve related issues, so damaged valve seats, dropping valves etc. TB has already been discussed... any bottom end damage / extra wear?

I will hook my tach back up to the Traqmate before this weekend and do some analysis afterwards. I guess I could also run MS logs, too.

robert

Ran some very quick numbers from a typical 25 minute session at MSR Cresson:

Average RPM: 3959
Over 6500: 173 seconds
Over 6700: 76 seconds
Over 6900: 20 seconds
Over 7100: 11 seconds

And another set from TWS:

Average RPM: 5601
Over 6500: 176 seconds
Over 6700: 37 seconds
Over 6900: 14 seconds
Over 7100: 8 seconds

This is fairly old data (03/2013) with a very basic motor, just I/H/E type stuff done. I didn't know my tach feed has been dead so long. Kind of sad, considering it is just one wire that needs fixing.

robert

This is MT.net. Not TrackHQ.com

Pretty safe to say there are only a handful of folks who wring out their motors anywhere close to what you folks do in your Miatas. Especially those posting in this thread.

This! With a built engine, what RPM limit would you run?

Stock motor with I/H/E is at stock 7200. Frankly I'm a wuss and rarely, if ever see above 6800.

Building a Forged bottom end and a bp4w with +1 Manley valves and Supertech light double springs.

After reading Emilio's response, I may rethink my planned goal of 7600/7800 soft/hard cut. I'd like for this motor to last. Not trying to win anything, just enjoy trackdays. AFAIK the most wear happens in the valve-train when the revs begin to climb, I'm unsure of the amount of wear the bottom end receives from sustained high revs, aside from oil pump issues. We all have BE pumps by now right?

Stock engine, valve sets take a beating if it spends much time past about 7600. The NB1 has less seat pressure than the NB2 so it suffers worse. Main bearings also take a beating at those revs. Engine still runs, just doesn't make any power. So it's clear, I consider the useful life of an engine also defined by still being capable of making the power it's designed to.

Plenty of high mileage HPDE NB's running aroundon their original engine that runs smooth, don't burn any oil and have ~160psi in all cylinders. To me, that is not "still healthy". This is key to interpreting my previous post. Does it still have ~210psi and <5% leakdown?

Past 7600, rod like to stretch which leads to rod bearing wear and eventually failures. Stock BP crank flexes a lot, thus broken oil pump on high power or high rpm builds. It becomes critical at around 8500rpm. Cracked main caps, spun mains, bent rods, broken oil pumps there.

When spun to an appropriate rpm for the build, the engine should still be making within a few % of it's original power 50-100 hrs later. The BP's were not designed for high rpm so getting anything to last for 100hrs when being spun to 8k constantly is nearly impossible. 8krpm/100hrs is a $10~12k long block.

Emilio,

For a stock crank, stock main caps, Forged rods, forged pistons, ACL bearings, Harmonic balancer, oil pump, ARP hardware, and everything you sell for the head to build it, what RPM limit would you say would last 100hrs? Or 50 hours? Can a stock forged crank built engine every last 100hrs at 8K?

RPM Limit: 7400rpm
Built engine: Stock head and bottom end
Engine Specs: MSM BP4W
Track the car: Autocross, HPDE's coming and then more
Avg number of times your car sees redline/day: >25

Assuming light or heavy double springs, O/C that will never let oil go above 250°, cooling that will never let it go above 210°, O/P never below 45psi at full load, OEM width valve seats My guess would be 50hrs at full power. Gradually declining after that. at 100hrs it would still be good enough for HPDE but might have lost enough compression, mostly from valve seat wear, to not be effective as a race engine.

Personally, I have never seen a BP that frequently sees 8000rpm last 100hrs without enough degradation to be deemed ready for rebuild long before that. It's not a high RPM motor. Spin a BP to say, 7600 with all those goodies and 100 or even 150hrs is possible. Drop to about 7300-7400 rpm with all that and I think 200hrs would be a reasonable expectation. Again, that's based on my experience. Others seem to have different results.

Has Andrew chimed in? He does a lot of ail analysis and actually assembles some engines. While he's built fewer engines than our shop, he's far more hands on than I am.

7500 hard limit, I can't recall the soft limit
Built: no
Engine: Stock NB2, 106,000 miles.
Track the car: only autocross, and haven't made it to more than three a year for the past two years.
How often hit rev limit?: very rarely, but I do go above 7k all the time.

Hmm. My stock NB2 only has ~165psi in all the cylinders. Maybe that's why it seems down on power. That thing hits 7k or whatever the stock fuel cut is pretty often. Probably two - four times a day. It has 80k miles i think. Purrs like a kitten, starts every day, and doesn't burn a drop of oil.

Thats how I felt reading that too. When I had my turbo Miata it was 7000 soft 7200 hard.

All you other guys are nuts lol :party:

^^^^^ ditto.
built 1.6 @18psi. W2W 20 min. sessions /hpde/etc. went from 7200 to 7400 and promptly broke throttle shaft. I'm staying at 7000 rpm, thank you very much.
I've got the shift beeper the S2K nuts use, so every shift on the track is at 7000rpm.

Ok so I'm actually kinda surprised at the poll results. 80% of the miata people here who voted (currently 46) are at or below 7,500, with a solid 40% of all voters below 7,200.

I guess my question is, what is the weak link? If higher revs are bad/wearing things out too fast, what specifically is wearing out?

My guess is the bearings wear out from the crank flexing around too much. If this is true, I wonder what could be done to help? Higher oil pressure, better oil, better bearings, different ($$$) crankshaft?

1 month after raising limit from 7000 to 7400, i broke a throttle shaft. I dont believe in coincidence.