Max RPM

[Swiss Toni]

Just wondering what the consensus is on maximum RPM is on a fast road engine?

Of course there are many permutations but what is the limiting factor.

I'm on triple webers and the thing pulls like a train and feels like it will keep going until something pops.

Also, do people run rev limiters?

 

[andrea]

Hi Tony
I also have experienced the inclination of my engine AH 2640cc full balanced with Fast road cam to pull over my attended limits- at the moment 2 SU HD are on but 3xWEBER are ready for next year- I have more than 4500 Miles on my 100/6 Longbridge
https://picasaweb.google.com/112770819864514987162/AHWEBERCarburatoriMOD#6139126896958079618
https://picasaweb.google.com/112770819864514987162/AHWEBERCarburatoriMOD#6139128304093964882
https://picasaweb.google.com/112770...MayJune2011endMayJune2013#5713882661113046258 the WEBERS TUNING
https://picasaweb.google.com/112770...LEY10061958BeginningMayJune2011endMayJune2013
at the moment I haven't any issue but I am very careful haven't mounted the dumper - engine is perfectly balanced with no vibrations -but I have maintained the original fly-wheel NOT lightened-I am anxious thinking to this heavy mass running at more than 6500rpm-probably a rev limiter can be the right choice for a long engine life
my engine oil is SHELL Racing 10W-60 full Synthetic, with oil cooler thermostatically controlled- oil temp never go up 85°C- Oil pressure cool 60ps then 40-50 - water radiator have a TEXAS fan, and a 14" push electric fan -normally not exceeding 90°C

 

[BoyRacer]

The main limiting factor, assuming you have an engine that can rev that high, is the inherent torsional vibration that kicks in around 5900 RPM.
Having said that, many racers greatly exceed that limit with the appropriate modifications.

 

[BoyRacer]

One more point, rev limiters are useless in my opinion. If you keep your foot in it for too long you will float the valves, assuming you can rev it that high. The usual damage from over-reving is because of the down shifts.... and a rev limiter will not help you there.

 

[dougie]

The main limiting factor, assuming you have an engine that can rev that high, is the inherent torsional vibration that kicks in around 5900 RPM.
Having said that, many racers greatly exceed that limit with the appropriate modifications.

Here's a couple big revs Boy Racer might recognize. They're well past the factory redline....

 

[Swiss Toni]

Interesting, so with the stock damper keeping it under 5,900 all should be fine?

With this in mind is this Super Damper a wise investment?

https://www.bighealey.co.uk/performance/austin-healey/engines/3000-bottom-end/super-damper-ceng643

Are there other options?

Is this an engine IN job?

With a good damper in place, what's the next limit?

 

[Randy Forbes]

Though not purchased to get the rev limiter per se, one (1) of the functions of the MSD 6AL-2 digital controller is that it has that function built in.

Specification of my 29D engine pretty much mirrors that of Andrea's; balanced, Isky cam/valve springs, SuperPro damper, lightened flywheel, temperature-regulated oil cooler, Mallory distributor (single point trigger), MSD and a trio of 45 DCOEs.

The engine seems willing to wind itself higher, but the rev limiter is set at about 5900/6000 (set and forget...). Since the limit on this model ignition controller is set with dip-switches, I started out low (4500 or so), to test its capability, and then walked it up in 100 RPM increments. A very satisfactory controller (and if it ever craps out, relocating a couple of *key* wires, and I can drive it home) and absolutely cured the issue of loading up the plugs in stop & go traffic__the only running scenario I didn't seem to be able to tune out of the Webers (specially ordered with 36mm primary chokes, versus the standard 38mm variety).

Though I did install a fresh set of plugs last October (prior to an All British show up near Tampa, I probably haven't actually "tuned" the car in about five (<5) years (the full, valves to carbs tune-up). I did increase the idle-mixture volume after moving to Florida, apparently that was enough of a difference in altitude, but cracking six (6) screws CCW 1-8th to 1/16th of a turn hardly qualifies as much of an effort...

The damper can be installed with the engine in situ, but it does need to be raised up a few inches, and well worth the effort to gain more room by removing the radiator. There are many posts here in the forum (several by me) about the damper installation, and I recommend that you read all of them before an attempt is made. You don't want to face a damper stuck half-on as it cools down!

 

[andrea]

Tanks Randy for reply -WEBER (45) was tuned ad jets re calibrated- for accurate testing I have added two Lambda probe on the exhaust-- I have some perplexity to add a damper -
my mechanic have done a great work-crank shaft (with fly wheel- if I remember correctly)was dynamically balanced wit extreme care and all pistons and connecting rods weighted at the same weight (one acquired used for substitution, required more than 100 gr take away)-engine can run at medium high rpm- with a glass of water on it, without any problem-
adding a damper in front I solve probably the hide torsional problem of my crank shaft-but I can also modify this perfect balance
I agree with BoyRacer a lot depends from drive style- I must reduce my heavy foot, in some occasion, but especial in the down shifting - not easy, we have a lot of downhill with hard cornering in rapid succession on the Alps
I remember again my wrong downshifting on my 911S --with all my valves to rubbish

 

[DerekJ]

Why do you want to rev the engine so high anyway? Unless you are in competition I can't see any need to thrash the car through every gear change. My engine has triple Webers, a rally cam, tufrided, balanced original crank, DWR damper, forged pistons etc. It puts out 215bhp but maximum power comes in about 5500. Its downhill after that.

 

[Randy Forbes]

Andrea, in years past, I have taken a damper that needed to be replaced, and its replacement to a machine shop that did balancing. The readings of the old damper were duplicated to the new one. End result, was the engine stayed balanced.

I'm not sure I comprehend the entire scope of engineering parameters, but parts can be individually "zero-balanced" and then the reciprocating/rotating assembly balanced as a whole, with the addition/removal of metal on the counterweights. Talk to your machinist/balancing expert, but I believe that you have options to replace a single component, without upsetting the work already carried out.

 

[andrea]

Hi Derek
I agree with you, I am surprised from the capacity of the engine (one old design and heavy unit) to go at hing rpm without any difficulty and valves troubles - probably my rev counter need a re calibration (is the original on car) -
but sounds are the well know....high rpm - I am convinced that a rev limiter can help to maintain a max of 5700-5900 rpm on High way, but as commented by BoyRacer NOT solve the problem of occasional down shift
Haven't a Dyno station in the nearest of my home (more than 100 km far)-effective power and other engine parameters of the car are not tested

Randy
I will investigate with a experienced machinist the possibility to add a damper and if it have a effective function on my engine-( long crank shaft on few supports, torsion forces )
I have changed 3 oldest connecting roods ( little defect on planarity test)and I was very surprised from the difference of weight find on all "new" 3 pieces - the pieces are used and probably from different engines but I haven't never find so great weight difference on same engine type- also the crank shaft have required a lot of work for the balance, damper have the function to partially solve this rough production problems ???

 

[Michael Oritt]

It seems rather silly to install a rev limiter on a street car. Unless one is engaging in some kind of competitive activity--road racing, autocross, etc.--where one operates at repeated high revs on a regular basis I cannot see the need. And unless you are running a modified cam and have performed accompanying induction/carburetion mods there is probably nothing up at top end except for a lot of noise and vibration as you will have reached peak power and torque several hundred rpm's earlier. A good general rule to remember is to never pass up the opportunity to upshift.

Even on my race cars, where I cannot constantly watch a tach and have frequent reason to operate at or near redline, I do not install rev limiters. I DO install a shift light mounted within my field of vision and operate off of that and "ear". Another reason I do not install rev limiters is that they close off the option of over-revving and occasionally there are times when I choose to go over by a couple of hundred rpm's for a short burst, as in racing to a corner and choosing to avoid an upshift, etc. I cannot think of a time where I found myself unwittingly exceeding my rev limits.

BTW Raptor Performance makes a great solid state rpm-activated switch that is small, weatherproof and easily adjusted with thumbwheels. They also make what are probably great and highly visible shift lights but I use a simple bright warning light and dash-mount it where I like.

 

[DerekJ]

Andrea,

I'm running a 3.9 diff and on the Lavant straight at Goodwood I reach 115 mph at 5500rpm. I don't think you will ever need to get near 5700 revs on the highway in any gear. The only time you might do that is with a mistake on the down shifting.

I have revved my car to 6000 on a rolling road but the reality is there is just no need to go that high and with a standard crank you are pretty much at the limit around 6000, after that there could be trouble.

 

[Swiss Toni]

Just to close the loop, I went for a OMEX "Speed System" which is a rev limiter and shift light comb. I've used their products in the past and had been pleased with them.

You can program the hard limiter in which a soft limiter comes in 200 rpm before and the shift light point.

It installed neatly out of the way under the dash.

It has an LED shift light, now normally I wouldn't cut a hole in the dash but there was a hole already from a redundant cold enrichment cable. It's fairly discrete, yet visible.

Just a little insurance to stop an over rev (under load) for about £110.

 

[andrea]

Thanks TONI -One easy and cheap solution to over rev problem

 

[Swiss Toni]

It's also easy to fit, with 3 wires.

One to the + and one to the - of the coil and a chassis ground.