OK, so I'm not as horsepower savvy as most of you on this board, but I'm here to learn, so could someone answer these questions for me?

1. Simply, why are members' cars dyno'ing at 400RWHP when MB advertises 469 on the W211?

2. What is the difference between the two HP numbers and how do you get manufacturer's numbers when you do mods? For example, the SL600 RENNTECH has over 711RWHP on the DynoJet, would that mean he's close to 950HP, by Mercedes standards?

I'm just a little confused as to where all this power is

Thanks guys!

 

Mercedes, as well as all manufacturers and most tuners, advertise crank numbers (Brake horsepower), while a dynojet will yield wheel HP numbers. IMNTBHO, rear wheel numbers are a better real world measurement, while bhp numbers are used to sell products to people that like to see big numbers.

Some people use a driveline loss percentage to make up for the HP/TQ lost through the tranny, driveshaft, differential, and wheels, but all it will give is an estimate as the only way to get a real brake horsepower number is with the engine out of the car and on an engine dyno.

I haven't heard of a renntech car with 711RWHP (I think you might mean rear wheel torque, but then again, I could be wrong).

 

its easy cars are rated at there crank hp with nothing hooked up on the engine.No ac,power steering,brake booster,drive line ect.

rwhp is what the car is acually putting to the ground.theres usually a 15-20% driveline loss depending what type of tranny the car has

 

I agree with everything said except the 15 to 20% driveline loss.

This is a hotly debated topic, my OPINION is that the newer lockup auto trans are not nearly as bad as everyone claims that they are.

The issue is that it is quite easy for tuners to slap a huge driveline loss percentage on their RWHP numbers to claim huge HP gains.

In the real world, the only reasonable was to compare is RWHP to RWHP. Show us the before and after dyno pulls done on the same dyno and you should have a shot at a reasonable comparisson. You still have temp and humidity to deal with but you should be close.

As mentioned, the only real way to compare is to run the engine on an engine dyno, then put it in the car and run it again on the chassis dyno.

For the life of me I don't understand why someone like Renntech has not done this to put the debate to rest around the 5 speed and 7 speed trans loss.

My GUESS is that you would find out that it is closer to 10 to 13% than it is 15 to 20%

If that is the case all the tuners cars don't make nearly the HP they claim.

Just my .02

 

I would say the 15-20% is pretty accurate of course a lot of variables could come into play and in my experience forced indution cars dont fair as well in the SAE correction avlues as I think they should. My C55 before the mods dynoed 305rwhp it is rated at 367bhp. The majority of C55s I have seen have been from 290-315rwhp. That is pretty close to a 20% loss.

I always do a before and after dyno when buying and installing parts. Of course I own a race team and a dyno so its pretty easy for me and as one would suspect some mods do not deliver as advertised. The only thing I am concerned with is what power my modifications have put to the ground, advertised numbers for the most part are just that advertisements.

 

I would agree on the 20% loss for the auto tranny. Whatever is gained in the lockup converter is partially lost because of the heavier drivetrain components needed to handle high torque. This is a decent write-up:

Why does an Automatic take more power than a Manual transmission?

So, what about the 15%-manual and 20% automatic? Well, they are good places to start. Some transmissions are more efficient than others and some rear ends are more efficient than others. In the end, none of this matters as wheel horsepower is what is actually used. You will find the heavier the transmission parts (gears, shafts, etc.) the more power they will take. I have seen engine dyno comparisons to rear wheel and you typically see 15-17% for manual transmissions and 20-25% for automatics. How much does your transmission take? Take the basic values 15 and 20 percent and consider the following.

HP loss in auto vs stick is mostly related to the converter slip (there is always some slip in a normal converter). However, even with a mechanically locked up converter, in the planetary gear system used in autos, there are more gear meshes occurring, which increases HP losses since each gear mesh results in a HP loss (relates back to statement 1). And don't forgot, in an auto, you have direct pumping losses from the oil pump (you don't have this in a manual trans). And the higher the line pressure, or fluid flow rate, the greater the pumping losses.

In an Automatic transmission you will find several factors that determine power requirements.

Weight: a comparable C4 take less power than a C6 - primarily because of the weight difference of the moving parts. A turboglide would take even less and an AOD would require around the same as a C6 (just using weight)

Lockup/non-lockup: Many modern Automatics use a lockup feature in drive and overdrive. This will increase horsepower available at the rear wheels because you don't have any slip in the torque converter. The slip in the torque converter generates heat which is an indicator that power is being lost. Even at over the stall point of a torque converter where you have complete hydraulic lock up you will experience some power loss through the hydraulic coupling process. Having said this I still prefer non-lockup converters as they are easier on the drivetrain (less harsh shifts) and allow the engine to run in the proper RPM power range.

Stall speed: The purpose of installing a high stall torque converter is to allow the engine to spin up to a desired RPM range quickly. Once this stall speed is reached you have a hydraulic lockup and the transmission is locked to the same RPM as the engine. Measuring horsepower below the stall speed of a torque converter is completely meaningless. The slip will give bad readings. Normally you can see the "flash" or hydraulic lockup point on the dyno sheet (I will post an example later). Having said all of this a high speed stall converter will normally take less power than a stock type converter. The reason is a high speed stall converter is typically smaller and lighter - that is the only reason.

Torque converter design: I do not know enough about this to go into detail however some designs are more efficient than others. With torque converters expect to spend between $500-$1000 for a good quality converter - if you try to go cheap here you will only hurt yourself later. In my AOD I run a Lentech 9" 2,800 RPM converter that flashes (hydraulic lockup) at 3,400 RPM (higher torque output will push your flash point further in the RPM band).

Valve Body: Again, I don't know enough to tell you exactly why but some are better than others. Some valve bodies will route hydraulic fluid more efficiently and activate clutch packs more effectively. For my AOD I use a Lentech Strip Terminator valve body and I love it.

With a manual transmission your primary factor in power loss is weight. The heavier the gears, clutch, flywheel, etc. the more power required to turn it.

 

Nice write up but I still do not agree. I would think these number would be true of TH350 and TH400 of the past. I would hope that the newer generation of differentials and trans help to reduce the loss. Again, could be totally wrong.

My CLS with 469 Dyno's at 424. My SL65 with 604 dyno's at 527 my E55 with 469 Dyno's at 420.

All on the same dyno.

Again, just my experience with MB products on the dyno.

 

MB is 'sandbagging' the HP of these engines. No way with a 469 HP engine would you dyno 424. The true crankshaft HP of these engines is around 500 HP - I think that's pretty well acknowledged. Your stock dyno results still seem just a bit on the high side though.

 

This might be a slightly off-topic question, but why doesn't Mercedes just look into some manual trannies? Even if its just for one car. For the SLK55 or something. A lot fewer power loss at those wheels.

 

MB and every other German manufacturer AND tuner has to rate their engines or aftermarket perfomance acessories in HP (DIN or metric) according to the standards set by the German TUV. I think it is called DIN 70200 (look it up) and it does not allow a manufacturer to over or under rate performance. All engines are rated in full dress uniform. In other words, every accessory like AC, PS, alternator, air pumps, intakes and exhaust are installed and functioning for the test. This HP rating is taken at the crankshaft and is sometimes called PS which is German for horsepower.

Any car sold in the USA since 1971 must rate their engines in HP (SAE) according to standard J1349 or sometimes referred to as "net" HP. It also requires full dress uniform and measured at the crank like the TUV method and the numbers are practically identical when converted from DIN to SAE. The old standard sometimes called "gross" HP allowed a manufacturer to strip off the accessories and add open intakes and headers when measuring HP. Also, there was no rule in effect that said a manufacturer had to advertise "peak" HP. That is why muscle cars like the 1960's L88 Corvettes were listed at 430HP in the books but in reality put out closer to 600HP. The former and lower number was taken several hundred RPM lower than the true HP peak. It was not a lie, but was not the whole truth either. There is a newer more stringent standard forthcoming but it is still voluntary at the moment.

The acronym BHP means Brake Horse Power which comes from the use of a brake dyno to measure HP. It is an indication of true HP and not theoretical or calculated HP at either the crank or at the wheels.

RWHP does indeed mean that the measurement came from a chassis dyno and includes friction losses from the entire drivetrain including the tires. Watch out for hub dynos that bolt up in place of the tires. They usually give a higher reading since the tires account for some HP losses.

The reason why you see a lot of RWHP is since it is very easy to measure changes to the car at any time. Measuring at the crank requires the engine and all the intake and exhaust plumbing to be pulled every time you need a measurement.

HTH,
Greg

 

I wish all manufactures would just rate their cars with RWHP/TQ...save me the first baseline dyno perhaps. I hear the M5 came out per one mag at 471 rwhp...that is not 507 crank.

 

Most MB buyers don't want to be bothered with stirring the cogs. To offer a manual to such a small group would be prohibitive. It also goes against the MB image of luxury.

 

I have personally seen multiple E55's all run around 420 RWHP at our local Dynojet. The day we ran my E55 we also ran my CLS55 and anothe E55. All of them were within 4 or 5 hp.

I am glad to see someone else bring up the TUV. It would be cool to see what the variance is that they will allow. I think the days of wild swings in HP are gone. The ramifications of bogus numbers would be huge for MB. I just don't believe that AMG would risk this.

I do not subscribe to the conspiracy therory that the E55 puts out 500 HP and the only reason that they print it at 469 is because they don't want to tick off SL55 owners.

Again, these are just my opinions.....

 

I agree with you on the E55 and CLS numbers on the dynojet. I have also seen dyno numbers for S55,CL55 and SL55 on the same dynojet with all the results being from 394 to 430 rwhp.

In my opinion MB over estimated the power difference from the differences in exhaust and intake. The posted number of 469 is to conservative imo.

 

Schiz,

If your CLS55 is putting down 424 at the rear wheels then the 469hp rating can NOT be true. There is no other way to explain it. That's a 10% loss at best. Smells waaaaaay too fishy to me. These are 500hp engines

 

...yet others only put down 395 to the rear wheels or a 16% loss. Go figure?!

 

We all know that there are manufacturing tolerances in production engine building. Take for instance the guy who dynos his E55 or CLS and only gets 394rwhp. Taking common losses for the automatic trans that equates to roughly 470-475hp. That lives up to the claimed 469hp. I have a sneeky suspicion they are giving us a factory rating for the engine at the lowest range of actual hp that could come out of production engines. My feeling is that the guys that get more than that are just on the high side of the tolerance window.

Anyone have any thoughts on this?

 

The e55 is def underated from the factory specs.

I dont think its a bad thing its just marketing.I still dont get why BMW gave the m5 a 507 rating tho.Some say it was because that the 507 model bmw was one of there very best back in the day.

from memory the mag that got 471 was using a hub type dyno so id subtrack at least 15-20whp enless the software the dyno was using would adjust to that.

I saw a dyno sheet of a bone stock m3 on a hub type dyno and it put down 203 whp and that is what there supposed to be putting down.

 

I've never been able to find the exact TUV requirements but I've heard that they must be within 5% which would allow for production tolerence diffs. I also heard that this is the reason why the orginal HP number for the 55K engine in the SL went up several HP when it was actually realeased.

 

It could very well be that marketing influences have pegged the E55 HP numbers on the low end of the scale. I guess the real question has to be then what is that scale? If 5% as has been rumored is correct, then it barely fits.

 

Yep, hub dyno's will read a little high because they don't have the inertia of the wheel/tire to spin. Any rotating weight removed from the drivetrain will have a significant effect on HP. 15 - 20 HP is probably a good guess for wheel/tire combo's that weigh about 50 lbs each.