Raising the (RPM) roof - Ford Mustang Forum
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post #1 of 48 (permalink) Old 01-21-2011 Thread Starter
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Raising the (RPM) roof

Hey guys,

I see a lot of threads here which mention raising the rev limiter for the car from ~whatever it is now, 6.5k? (Could someone answer this for me? My car is far away getting headers designed) to 7.3k / 7.5k with a tune. Not that I particularly care about my warranty, but I do care a little about the engine and its longevity. My car's only flashed red (on the tach) once AFAIK, but wouldn't pushing it 500-1000rpm past its designed point be one of the worst things you can do to the engine?

Thoughts?

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post #2 of 48 (permalink) Old 01-21-2011
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It is pretty common for tunes to raise the rev limiter by a few hundred RPMs. I have done it on my past 3 cars with no problems ever. I think Ford is pretty conservative w/ the rev limit, especially w/ these 4v engines. I wouldn't think you'd want to push it another 1000 rpms or anything, but 300 or 400 should be ok.


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post #3 of 48 (permalink) Old 01-21-2011
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The factory rev limiter is set at 7,000 rpms. In regards to raising the limiter, I would only do it if the factory-set limit was preventing me from shifting at the peak HP rpm. That's why the guys with tunes raise the limiter, their tunes increase the rpm at which they are at their peak HP, so in order to shift at peak HP they have to raise the rev limiter.

In regards to longevity, there isn't much out there for concrete info as the engine is a brand new design. I would think it would depend on how the car is driven, normal (and some spirited) street driving with some track time shouldn't be an issue, but shifting at 7,500 rpms at every gear change may cause issues. This is just a hunch though due to how new the design is and all the variables that factor in to it.

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post #4 of 48 (permalink) Old 01-21-2011 Thread Starter
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Ah, so it's only 7->7.3 or something. I thought it was a larger jump. So probably not a particularly big deal. Thanks!
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post #5 of 48 (permalink) Old 01-21-2011
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Torque causes acceleration. Horsepower is necessary to sustain speed. The higher the horsepower, the higher the speed can be, but its torque that gets you there!

So in drag racing, the rev limiter should be based on achieving the highest range of the torque curve.

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post #6 of 48 (permalink) Old 01-21-2011
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Quote:
Originally Posted by Bobby B View Post
Torque causes acceleration. Horsepower is necessary to sustain speed. The higher the horsepower, the higher the speed can be, but its torque that gets you there!

So in drag racing, the rev limiter should be based on achieving the highest range of the torque curve.
With all due respect, I am sorry Boddy but that is just not true. I would suggest that you do more reading on this particular subject matter.

torque vs hp - Google Search

In the end, you can always multiply torque by changing the gear ratio but you cannot multiply horsepower.

For the application that we are interested in, more low end torque also translates to more low end horsepower. This implies a wider power band, hence quick acceleration.
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post #7 of 48 (permalink) Old 01-21-2011
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I spoke with Bama's tuners (by phone) about this. You can specify the soft and hard limits when you order a tune. I think their racing tune defaults to 7500 hard - which seems a little high to me.

I think stock is 6800 soft and 7000 hard (where "soft" does some magic to try and slow the RPM increase rate, and "hard" is full cutoff - fuel and ignition I'd guess - anybody know?).

Remember too that dyno's ONLY measure Torque! HP is a mathematical abstract from the TQ reading (TQ*RPM/5250 if I remember right). So, at 5250 RPM the HP & TQ curves ALWAYS cross. If you raise the RPM limit a bit AND TQ doesn't drop much, you'll get and increase in PEAK HP, which is what all the tuners like to brag about. For normal DD this is pretty much irrelevent! HP wars amongst tuners = BOGUS. What matters more (again for DD) is the total AREA under the TQ (or HP) curve. If TQ gets bumped a little across the whole RPM range => real perf. improvement. A little bump in peak HP at 7500 RPM isn't gonna help you...

If I decide to get a tune, I'd spec. a 7300 RPM hard limit to be conservative...

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post #8 of 48 (permalink) Old 01-21-2011
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Quote:
Originally Posted by icckart View Post

Remember too that dyno's ONLY measure Torque! HP is a mathematical abstract from the TQ reading (TQ*RPM/5250 if I remember right). So, at 5250 RPM the HP & TQ curves ALWAYS cross. If you raise the RPM limit a bit AND TQ doesn't drop much, you'll get and increase in PEAK HP, which is what all the tuners like to brag about. For normal DD this is pretty much irrelevent! HP wars amongst tuners = BOGUS. What matters more (again for DD) is the total AREA under the TQ (or HP) curve. If TQ gets bumped a little across the whole RPM range => real perf. improvement. A little bump in peak HP at 7500 RPM isn't gonna help you...
+1 Awesome Post. I don't think it could be said better...

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Last edited by wobriant; 01-21-2011 at 09:42 PM. Reason: clarification
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post #9 of 48 (permalink) Old 01-21-2011
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So let me ask (an admittedly newbie) question: if the hard RPM limit on, lets say the Bama race tune, is 7500 rpm, and they program the automatic transmission to shift at 7000 rpm on WOT, would requesting them to change the hard limit to 7300 ever really make a difference? In the sense that the automatic is only ever going to shift max at 7000.
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Quote:
Originally Posted by orgchem View Post
So let me ask (an admittedly newbie) question: if the hard RPM limit on, lets say the Bama race tune, is 7500 rpm, and they program the automatic transmission to shift at 7000 rpm on WOT, would requesting them to change the hard limit to 7300 ever really make a difference? In the sense that the automatic is only ever going to shift max at 7000.
Id stick with the 7500 RPM. Although it may not make a huge difference compared to 7300RPM, keep this in mind: If the transmission is set to shift at 7000RPM, that shift takes time and may not complete until like 7200RPM, dependant on the transmissions ability to shift quickly (time) and how well the engine is desinged/capable to Rev easily. The way you choose shift points is by choosing one so that the RPM sweep through your gears covers the geatest area under the curve (you "low-end torque" mongerers need to think about what your definition of "low-end torque" is and think about how long you are actually in an RPM below 2500RPM [not an attack, just think about it]). An old racers rule of thumb used to be shifting at an RPM 10% above peak HP. Also, another thing to keep in mind, with TIVCT, the torque curves for engines with this technology are pretty damn flat across a large RPM range compared to static cam timing engines.

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The post above is very good.

So the only thing I really have to add is that you also don't want to bounce off the rev limiter. Mod motors are known for little RPM spikes, and if we shift at 7300RPM, and the hard limit is at 7500, it keeps a spike from making you bang the limiter while shifting. Same thing with MPH... shifting is based off MPH, so if you lose traction for a split second, etc. you could hit the limiter.

For the 11 v6, it depends on WHAT Bama tune you're running, but we change it to shift from 7000-7300 depending on what gear and Street, Performance or Race.

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The rule of thumb for a modern engine is typically 4,000 FPM, (and 3500 FPM for the old guys).

A purpose built engine designed for some revs can push it beyond this limit safely. I probably would not exceed 4500 FPM at the max on a DD performance street car with stock internals.

RPM x Stroke (in mm) 152.4 = Piston Speed in FPM

7000 RPMs x 92.7mm 152.4 = 4257 FPM Stock...

7300 RPM = 4,440 FPM

7500 RPM = 4,562 FPM

7800 RPM = 4,744 FPM

8000 RPM = 4,866 FPM

Just some figures to consider.

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post #13 of 48 (permalink) Old 01-22-2011 Thread Starter
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Quote:
Originally Posted by BadSector View Post
The rule of thumb for a modern engine is typically 4,000 FPM, (and 3500 FPM for the old guys).

A purpose built engine designed for some revs can push it beyond this limit safely. I probably would not exceed 4500 FPM at the max on a DD performance street car with stock internals.

RPM x Stroke (in mm) 152.4 = Piston Speed in FPM

7000 RPMs x 92.7mm 152.4 = 4257 FPM Stock...

7300 RPM = 4,440 FPM

7500 RPM = 4,562 FPM

7800 RPM = 4,744 FPM

8000 RPM = 4,866 FPM

Just some figures to consider.
Vaguely, the reasoning behind this figure (FPM limit) is the stress put on things like the connecting rods, pistons, etc., during a rotation right?
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Very intersting indeed:

Just for grins, I did the calc. for the 2010 M3 motor. It has a stroke of only 75.2mm for its 92mm bore, whereas the 5.0 is a nearly "square" motor = same bore + stroke.

SO, at 8000 RPM the M3 motor is moving at a "measly" 3950 FPM.

Bottom line - I personally WOULD NOT TRUST the tuners telling you this engine can EASILY handle >7500 RPM. There's a post where Brenspeed says 8000 RPM hard limit is not problem "it'll run all DAY at 8000 RPM".

Well did they mean it'll run for ONE day?!? It's not THEIR engine that is gonna grenade. I'd like to hear from AM/Bama and Brenspeed some data on just HOW MANY engine rebuilds they've done (or other issues) in the course of developing their tunes.

Remember, they're basically DISABLING your knock sensor (fixed octane tune only) AND bumping your rev. limiter. I'm sure they're trying to be fairly conservative, BUT... There IS likely to be some longevity issues with these tunes. The motor is also SO new, there's no long-term reliability data on "mods" yet...

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Quote:
Originally Posted by icckart View Post
Very intersting indeed:

Just for grins, I did the calc. for the 2010 M3 motor. It has a stroke of only 75.2mm for its 92mm bore, whereas the 5.0 is a nearly "square" motor = same bore + stroke.

SO, at 8000 RPM the M3 motor is moving at a "measly" 3950 FPM.

Bottom line - I personally WOULD NOT TRUST the tuners telling you this engine can EASILY handle >7500 RPM. There's a post where Brenspeed says 8000 RPM hard limit is not problem "it'll run all DAY at 8000 RPM".

Well did they mean it'll run for ONE day?!? It's not THEIR engine that is gonna grenade. I'd like to hear from AM/Bama and Brenspeed some data on just HOW MANY engine rebuilds they've done (or other issues) in the course of developing their tunes.

Remember, they're basically DISABLING your knock sensor (fixed octane tune only) AND bumping your rev. limiter. I'm sure they're trying to be fairly conservative, BUT... There IS likely to be some longevity issues with these tunes. The motor is also SO new, there's no long-term reliability data on "mods" yet...
Where in the world do you get your info?

While your post BEFORE this one I quoted was great, this one is way far off base.

We just said the limiter was 7500RPM. Which, based off of your 4500FPM logic, is perfect.

How many engines did we rebuild? None. We've tuned thousands without a failure. We've built high-revving 3vs for years. Even Ford Racings auto calibration (when it's done completely) is going to be set up similarly.

While I don't agree with Brenspeed that this motor "can handle 8000RPM all day" (there's a reason the Boss comes with valves, springs and retainers, and it's not JUST the cam difference), I totally think this motor can handle 7300RPM shift points and a 7500RPM limiter to bounce off of.

Our auto 2011 GT is running strong as can be. The car has probably 200 dyno pulls and a similar number of track runs, all hitting 7300-7500 RPM. That and it's got countless hours of seat time datalogging on the street, hitting the WOT shift points on multiple occasions.

And while I can't speak for Brenspeed, we DO NOT disable knock sensors, so I have no idea where you got that idea. Knock sensors are completely left functional with our 2011 tunes.


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