Jean

WARNING: Only read if genuinely interested in the truth about your numbers, otherwise , please jump to the next thread

Where do we draw the line, day after day Hp and torque numbers thrown around left and right are becoming a real joke. So how does the customer establish whether he got what he paid for, or he was made to believe so by some dyno mumbo jumbo? Read on....

Engines have a specific efficiency that basically is the limit of the torque output that the engine is able to produce.

The simplest way to measure this efficiency is through something called BMEP (Click me) , which is widely used by engine builders, from factory engineers to F1 engineers and is used to benchmark against other engines, or against different modifications performed in-house.

Other than hardware and succesful programming, major drivers of BMEP are boost and compression, however at a fixed boost and compression, the best engineers in the world would struggle to improve same engine BMEP by 5% + on any engine that is already very performant, without involving expensive (in $$$ millions) R&D and re-engineering. Typical heads/valves, air intake, cams, exhaust, turbos, IC, etc.. might improve by 5% or less that BMEP at the same boost levels.

For reference, an outstanding BMEP on a race engine is considered to be anything above 200, with the top F1 engines standing at around 230. This is at atmospheric pressure (no boost).

So here is some BMEP data based on Porsche factory numbers and let's see if the practice supports the theory, all numbers are adjusted for atmospheric pressure:



It looks like it does…N/A engines are more efficient than turbo engines, race engines are more efficient than street engines, newer engines are more efficient than older engines. You can work out the percentages from above.

Obviously open exhausts, straight through intakes, valves, aggressive cams and mainly, higher compression on N/A engines is key to driving BMEP upwards.

Therefore considering Porsche data, a 997RSR engine improved by 3% vs. the Cup, about 8% vs. the 997 GT3, and 11% vs. the CGT. No wonder you can extract 130HP/Ltr from its atmospheric engine, this puts it very close to F1 territory at same RPMs.

Now to the heart of the matter, how do I know if my engine has the torque I have been told it has? Quite simple indeed..

How much maximum torque can a 996TT engine have at 1.2 Bar of boost?

Max. TQ= (BMEP X displacement)/150.8 x Absolute pressure
Max. TQ= (167 X 220)/150.8 x 2.2 = 535 lb.ft of torque <---This is the maximum REAL TQ you can see on your engine at 1.2 Bar

Here is a chart that will help you situate graphically where your engine should stand, you decide how accurate are your numbers afterwards...



Morale of the story, if you are seeing at 1.2 Bar more than these numbers with bolt-ons-- or over, say, 5-7%, with internal extensive mods (including heads, valves, turbos, itake , exhaust etc..) you might want to question the data provided to you.

Hopefully this will be taken as an eye opener to RL users here, rather than a confrontation to any tuner.

fly2low

That is another data point to be considered. Thanks for your effort

tkerrmd

interesting, also however how much change in torque is required for a regular human being to notice the difference??

Jean

Tom, I don't have that formula

It is certainly relative , not absolute! If your car has 400 lbs.ft and you get another 30-40lbs.ft, I doubt you will ever feel it. If you get those 30-40 on a 200 lbs.ft engine, you might certainly do. I know I cannot feel 50HP in my car.

Joe Weinstein

I kinda think this begs the question and actually muddies the waters a little,
because it depends on Porsche's quotes on it's BMEP figures, which may
or may not be accurate, might be conservative, and which are likely determined
by running some motors on a dyno anyway. Change the dyno and/or the motor
and it might be different?
I think the most accurate info any owner has, is what they get from a
dyno run of their own car. This data may not be reproducible with other
environmental conditions or with a different dyno, but it's close to what
you can compare with, at least with other cars on the same dyno.
You get what you get *at the wheels*. In fact, any one number is
pointless. If someone says they get 500hp/500ft/lbs, they get that
maybe for 1 tenth of a second during a quarter mile run. Most of the
time the motor is producing much less. If a 3000lb car could put 200 hp
to the ground for an entire quarter mile, it would be a 9-second car.
Also, as I've said before, wheel numbers are good, flywheel numbers
are useless. Whatever percentage you want to take away for AWD or
the transmission *is* taken away, so making up a bigger number is
pointless, like saying how good your wife is if you leave out the mood
swings and the jewelry requirements.
Joe

Geoffrey

Can I just say that you have a misspelling in your x-axis label on your chart.

tkerrmd

Well Jean cool info but I am surprised how close the numbers of various cars are ie 997TT and NA 993 are the same?
I guess I should have paid more attention in physics in college!

Jean

Joe

I was hoping we could leave the disbelief in those crazy German engineers and their dynos at rest for once

However if you don't want to believe the factory numbers, an alternative would be to adjust the base of what you believe the factory numbers should be "in reality" and you take it from there.

Tom, you are comparing a N/A engine to a turbo engine, and yes they are almost the same.

Jean

Oh, thanks Geoffrey

justinmm2

Jean,

Where does the 150.8 figure come from?

Also: How are we seeing these high-lb/ft Turbos on 3.6 or even larger engines? Is it due to the different rods/pistons changing the compression, and therefore the BEMP? Forgive me, I'm anything but a mechanical engineer! I know the HP can be explained by where it comes on and how high the engine revs (oversimplifying, but still), but there are many 996TTs out there that claim to make more than 535 lb-ft.

Interesting to read, thanks for posting it!

Fred R. C4S

Jean,

I am assuming that the 150.8 number is the net number used as a result of the conversion of various units (psi, lb ft, hp, etc.).

Note that the first graph is a plot of BMEP normalized to atmospheric levels. That would be zero boost for a turbo engine. As the boost increases, so does BMEP. If you raise BMEP enough, without any other changes to the engine, you will go KABLAMO! I've seen a few diesels and aircraft engines running insanely high BMEP in tractor pulls. I use to have some photos of an International Harvester diesel that was running such high cylinder pressures that it split the inline 6 block right along the centerline of the camshaft.

From a purely mechanical engineering point of view, all an engine does is produce BMEP, converting it to torque due to the crank throw, which at a that particular speed results in horsepower.

Cheers,

Joe Weinstein

Hi Jean. I don't necessarily disbelieve the German engineers and their dynos,
but I don't believe them any more than what someone gets on a dyno here,
and for a given car, I would work off of what I got on a dyno. I would only
be looking at a dyno if I was aiming to make a major power change, and
wanted to see before and after graphs. I would trust the difference more than
the specific numbers.
Joe

justinmm2

That was my mistake, then. I thought it meant ambient atmospheric pressure was 1 bar, not the boost being at 0. I've looked over it again, and see exactly what you mean. Thanks for clarifying!

Jean

Justin, this is BMEP in Psi. From the BMEP formula in the link above BMEP (Pa) = 4Pi x T(N.M)/V (cu.m); Here is another link: BMEP in PSI

90% of the modified turbos you are seeing do not have compression changes, their BMEP cannot be more than 5% or so above these numbers except for the most extremely modified. Anything above these numbers, give and take a few %, is dyno optimism or engines being run on the dyno at more boost than declared.

535lb-ft is the limit at 1.2 Bar for these engines without internal modifications of heads, valves, compression etc.. If heavily modified, then add another 5% maybe, if you move to 3.8 ltrs, another 2-3%, twin-plug also, etc.... You can have more torque than the 535 lbs but it will be at higher boost levels.

Heat (as boost increases), turbo sizing and other things will in fact drop the this maximum limit potential on the engines.

Fred, the BMEP here has been extrapolated to atmospheric pressure indeed to be comparable to N/A engines, obviously with increased boost, you will increase BMEP, which is why torque increases with boost. However as just mentioned at a certain boost level, this is the maximum you will have.

Joe, don't take it the wrong way, but if you think your tuner's dyno or the chassis dyno in the shop around the corner have more accurate dyno results than the factory with their million dollar dynos, then this will not bring any value to you unfortunately. If Porsche understates their numbers, then Porsche engines are in reality in F1 engine efficiency territory for 1/25th of the price!.

The objective is to bring another KPI for people to check whether their numbers make sense or not. It is here available , feel free to use it or not

Fred R. C4S

are definitely more accurate than any aftermarket "rolling road" chassis dyno. While a manufacturer may understate his number (for whatever reason), he goes to great lengths to get repeatable results. Having run development tests on diesels at the big yellow tractor company in my ealry years I can unequivocaly state that the conditions used are controlled so that one test can be compared with another. Tests are run to specific conditions set forth by SAE (Society of Automotive Engineers), DIN (German Institute for Normality) and others. Under these specs, every effort is used to control input air temp, coolant temp, input air pressure, use of a specific test fuel, etc. All of these variables are controlled so that retesting later with a change to the design of the engine or subsytem can accurately show just what that change produces. The mantra is to control ALL of the variables. Then test one change at a time to measure the effect. I'm absolutley certaint that the cost of one dynometer cell at Porsche costs many multiples of an aftermarket chassis dyno.

Thought you'd like to know..