ebaker

Mods that increase flow such as better flowing heads, intakes, exhausts, will usually increase the bmep. I wonder if Porsche's figures are measured with the engine connected to a turbo and stock exhaust. Also engines with a high bmep usually have a narrow torque band.

Fred R. C4S

That is true for most naturally aspirated engines, but it does not apply for forced induction engines. The latter can have a torque curve as flat as Kansas. Newer high ouput NA engines often use complex intake tuning such as variable length intake runners, variable valve timing etc. to produce a wider torque band by providing a bit of forced induction caused by the cylinder filling, firing, and exhaust pulses.

Cheers,

Joe Weinstein

Hi, thanks Jean,
All I care about from my shop's dyno is that it's consistent. If Porsche was
around the corner and would dyno my engine or yours, we might come to
a more accurate number, but I believe there are unavoidable differences
even between two stock motors that would have them making slightly
different power.
Your info *is* valuable if someone is 'graph-racing', claiming a better car
than someone else's based on a number from a dyno run. Sorry if I sounded
too poopoo about it. My position is that I would be just as happy if my shop's
dyno printed numberless graphs. All I'd need was the grid to see that change
X gained Y% at a given RPM (under identical circumstances). For me, a dyno
just needs to be consistent, and for what any of are doing, or could feel in the
street, I'll bet that almost all dynos are capable of being operated in a way that
would suit.
We agree, I think (once I explain myself). You say to question the numbers
if they exceed the charted maximums. I would say to question the numbers
even if they are well within the realm of the possible. Inaccuracy does not
require improbability. In fact, I say to ignore the numbers, except to the extent
that your changes make them change, and then to trust only the proportional
difference. Eg: I would tend to believe if a vendor says "product X will increase
power by 3-4% at 6000rpm on a stock car" which could be adequately determined
(in my opinion) on any competently run dyno. What I trust less is a claim like
"Product X adds Y hp, or car A delivers XYZ hp"
Joe

Jean

Thanks for the interesting input Fred.

ebaker, the numbers are calculated numbers per BMEP formula, from the Porsche officially published HP and torque numbers, therefore in stock running condition on their engine dyno with all components (including electrical) per DIN standards.

Joe, I understand where your interest lies and I think the same way.

However the problem is that even if your stock setup shows you the same as factory numbers on a dyno, the "after tuned" result is not neccesarily correct by default. It would be, if the dyno was able to load the run as car factories do, since it is the sustainable HP (all IMO)

Net net, we have seen many dyno charts out there regardless of which tuner they are coming from, with numbers that beat the logic of the components used and state of tune, i.e. flowing much more than what the turbos would allow for, or simply showing torque numbers that are not sustained by the physics involved etc..

When one thinks that the extensive engine modifications done on the new 997 GT3 vs. the stock 997 Carrera, basically, an entirely redesigned engine with higher compression, better heads, intake , cam drive etc..only yields a 10% increase in BMEP, it is food for thought maybe.

This would simply let you know where that threshold of nonsense is approximately..

Thanks.
Jean

wross996tt

Good points made by all....the reality is there is measurement uncertainty (error) associated with any dyno. To my knowledge, there has been no study of dynos to get such an estimate (I speak of precision not accuracy)...therefore all of the data is questionable...to some extent I must even be careful assessing changes made to my engine with the same dyno when the precision of the dyno is not known. BTW, this is true of all of the measurement systems used including others used to estimate power, hp, torque...the good news is you can use this lack of knowledge in the measurement system to make any claims you want....

Oak

torque is over rated.

JEC_31

Excellent data and presentation, Jean - thank you for sharing.

The main point I'm impressed by is how much the Turbo engines have to be compromised so that they can be boosted. The low chart readings at zero boost for these motors clearly indicate the penalty/cost of the design-for-boost.

It reminds me of how old-school hot-rodding firmly believes it is necessary to drop static compression to extremely inefficient levels before putting any boost into a motor. This is of course a legacy of the early days of non-intercooled and poor-efficiency Roots blowers boosting poorly-A/F-controlled V8s. They need huge safety margins to stay below detonation threshold.

The Turbos are not that bad but there's still an unavoidable penalty. It's more than static compression, lots of other design parameters hamper unboosted MEP. Major are the cams and variable timing system which are crucial to the dynamic compression - a critical aspect of managing the boosted air pump's operation.

Modern cutting-edge boosted motors are creeping up the effiency scale with more technological tricks. The 3.6 is going to be replaced eventually, and knowing Porsche - and the current auto business climate - they're working on something quite trick indeed.

- Josh

Joe Weinstein

Oak, you forgot the smiley! (I hope)
Torque is everything. Especially as
measured at the wheels. It determines
the power you have at any given
RPM. The *max* torque is overrated,
especially in peaky motors, but so is
the HP listing. I have a 351 ci Mustang
motor built for autocross, which needs
instant power at any time, often from
low-mid RPMs. It only max's out to 330hp
to the wheels at 4600rpm, but it produces
no less than 400 ft/lbs of torque from
1700rpm, so it *jumps* at any time, and
you can find yourself backward on an onramp
if you're not careful in low-traction
circumstances. Some bus motors are only rated
as 250hp, but they make tons of torque just off
idle, so they can pull a bus all day, but if you
swapped in a Mitsu Evo '300hp' motor, it'd burn
up on the first hill the bus came to.
Joe

JEC_31

Torque is work. Horespower is how quickly we can work.


Joe, funny you should mention a torquey Mustang on an onramp, a friend of mine totalled his old '86 that way. He had installed a shift kit for the auto tranny but let the tires get a little worn. One night he was out for a cruise and the tranny unexpectedly kicked down hard while on a local narrow one-lane "concrete canyon" on-ramp - in a split second he nosed it then tailed it. Bountiful torque with a few bolt-ons - then he and his passenger are suddenly parked sideways, stuck in an onramp, in a smashed car, looking at each other like WTF just happened?

Sorry for the off-topic. I love funny stories (nobody got hurt).

~~~~~~~~~~~~~~


What's the BMEP of a CGT?

Jean

Wross,

I agree with the measurement uncertainty, just as I would say that this theory is not 100% accurate either, it is, just as many other things, a reality check rather, and has to be used smartly to be useful. A few percentage points variance here and there are obviously within reason..

JEC_31
Thanks. Good points. As you rightly pointed out, turbocharged engines have a few restrictions that hamper their BMEP without boost vs. N/A engines, which are compensated through forced induction, such as smaller ports, less efficient intake manifolds, smaller static compression.

More importantly, BMEP calculations for turbo engines are obviously not 100% accurate when extrapolating to atmospheric conditions since when you introduce forced induction the air volume and density, change as a result of the pressure (Boyle).. thermal stress on components is different (efficiency drops), more friction, etc..But when comparing same types of engines to each other, variances are reasonable.

If anything, BMEP will not increase proportionally with boost, meaning incremental torque delivered will be less porportionally, the higher boost you add.

I am not really worried about torque vs HP in this discussion, BMEP can be measured from both anyway, but we would add another variable which is RPMs.

Thanks
Jean

BuddyG

So Jean how much torque would the Evoms car have that just ran 9.6 in the 1/4 or Markski's car which ran 10.0?

Jean

BuddyG

Re-read my first post please. Thanks.
Jean

BuddyG

Jean those numbers make my head hurt! If the EVO and Marski cars are running 1.5 bar and I know Markski's car has head work what would their max torque be using your information.

I am not being a smart alec I am asking a legit question and don't like math!

Thanks!

Jean

Buddy

I would not like this thread to become a battlefield, some people are very sensitive about their numbers so I will pass. Torque alone is not what makes those cars fast in the quartermile, so it does not really apply to this, there are weight, gearing, suspension and other considerations involved.

Sorry if I did not answer your question, I will in private if you would like.

Jean

BuddyG

Yes, please pm I am just curious what you think the torque of those two cars would be using your formula.

Thanks

Buddy