dmp

My car's missing HP...3000lbs cars with 238hp don't trap at 90mph.

zoom44

http://www.ajdesigner.com/phphorsepo...horsepower.php
http://www.dsm.org/tools/calchp.htm
ive tried several others and they all indicate about 170 at the wheels.

guy321

I don't think there's any doubt we're only getting 170 - 180ish to the wheels??? With a very few getting 200, i'd like to see thier times. Gearing most likely has alot to do with it, since we essentially have only 5 power gears, right?

dmp

My car dyno'd at 186whp....and my best trap was ~91 mph...I weigh 220lbs in clothes...so figure 3300lbs.


figures about right:

weight = 3300 pound = 3300 pound
velocity = 90 mile/hour = 90 mile/hour


--------------------------------------------------------------------------------

Solution:

horsepower (hp) = 187.7560309513 horsepower

or:

weight = 3300 pound = 3300 pound
velocity = 91 mile/hour = 91 mile/hour


--------------------------------------------------------------------------------

Solution:

horsepower (hp) = 194.08436213992 horsepower

zoom44

what was your time and try the second one also

dmp

15.0 with a 2.5 60ft

second one?

zoom44

i edited my post to include a second calc link. the hp calcs indicate you are putting 194 whp on the strip or 8 more than on the dyno of 186.

194whp * 1.20(for 20% drivetrain loss)= 232.8bhp which is just 3% off the advertised 238bhp. whats the isue?

124Spider

Oh, heck, I can answer that for dmp. :D At the same place and morning as he dyno'd his car at 186hp (uncorrected), mine did 219 (uncorrected; 210 corrected), and my car is rated at 240hp. Something's not quite consistent there.

zoom44

that would be the dyno not being consistent.
he said his car cant be putting out advertise dhp based on the reult of his 1/4 mile run - specifically trap speed. however if you actually usea a trap speed to hp calc you get teh numbers about which shows his original remark was incorrect.

dmp

The issue is "online calculators are crap" when it comes to the real world.

186hp or even 190hp is still too low, and your 'drive train loss' percentage is too high.

With Miatas using similar (the same?) transmission, but a non-CF driveline, and sending the power to a similar rear-end, they are losing about 26hp...


I simply cannot buy an RX8's drivetrain consumes TWICE that power.

As an aside- the link I went to was estimating ENGINE HP
...not WHEEL HP...so, even on the run I hit 91mph, that still shows less than 200 'engine' hp...or, about 40 too few.

dmp

and from your 2nd link:

I trust what a dyno showed as mid 180hp for FIVE Rx8s in one day, only hours apart, over what an online-calculator suggests.


For a comparison, I entered the data from my old Probe GT - I went 13.8 at 107 mph...shows:

My dyno was actually 242hp.

But for my Miata, which went 16.5@81mph, it shows:

The car dyno'd at 100.2hp...

Talk about inconsistancies...

:D

124Spider

Warning: Thread hijack in progress....

No, actually the dyno was very consistent. All the RX-8s but one were within a couple of hp of each other.
Does anyone really think that the RX-8 puts out 238hp at the crank, stock?

zoom44

yes but the wording is wrong- why else would the other calc(s)s say at the wheel. "There are two simple horsepower formulas that can estimate the on-the-pavement horsepower for a given car". it cant calculate bhp without knowing the gearing.

second 20% drive train loss is what we have.

check these - all are calulating rear wheel hp and get the same numbers as the first

http://www.amaprostar.com/dynos.asp
http://www.ultimateresourceguides.co...nt/index4.html
http://www.smokemup.com/auto_math/trapspeed_lbs_hp.php

try this one and read it carefully- gives 2 answers 1 whp and 1 bhp by adjusting with 25% drivetrain loss

http://www.gordon-glasgow.org/hpcalc.html

Speed-ER doc

I wish I could donate some horsepower to my underpowered friends.

I've got plenty to spare, and I'm going through back tires too quickly. :p

dmp

There's no way the car loses 20% if a Miata loses 15% +/-...it's hard to use a percent, because it's not really a percentage...it's generally a fixed number AND some percent...

still...same engine as a miata, better drive shaft than a miata, similar rear-end as a miata does not mean that much greater loss.

dmp

...if Mustangs weren't so ugly...

(sigh).

:p

zoom44

how else do we verify the accuracy of his statement?

dmp said there is no way his car traps at 90(1) while providing the advertised hp.what math formula should we use? these calcs are using the math that everyone used before to show that the cars werent making 247. the statement on the one calc site is so you dont sue them. the math is the math.

really dynos are consistent? then why was one car's results so drastically different? dmp says "talk about inconsistencies" but his reults show a car that put down significantly less HP on the dyno then the calc says it should have (problem with car on the dyno see article about bimmers and dynos) and the miata being very close on the dyno to what the calc says it should have- math isnt inconsistent so its got to be the cars onthe dyno that are.
it doesnt matter what anyone "thinks" its what tehy can show/prove- the math shows that at least dmp's car puts out within the legal 5% window of advertised power

dmp

side note: Zoom - on your last link, use:

3300lbs, 15.34 and 91mph

My 15.0 run had a 90mph trap.

187hp - so about the same as my dyno day, no? I still hotly debate the car would lose 25% of it's engine power thru the drivetrain. What did FD's lose? I think stocker FD's dyno around 225whp....Much less than 20% loss there, imo.

zoom44

im going to split this out of this thread.

dmp

But the math has nothing to do with real-world, hence all the disclaimers on those sites.

Actually, those calculators tend to support what my dyno findings were - however measured HP is more reliable than math-conversion online.

Except it does not. (shrug)...and 80% of RX8 owners are in the same boat i am.

[edit - 90%? 95%?]

zoom44

really ? math has nothing to do with real world? thats ridiculous.
so when the car came out at 247 hp and people used the formula for deriving whp from trap speed and ET to show the car was not making the advertised power. you cannot now say that the formulas are wrong because you dont like the out come

dmp

I'm saying read the sites you linked to. Every one says "Don't take this stuff as gospel".

Here are some facts:

Racing Beat got 216hp with a Renesis on an Engine Dyno.
My car dyno'd at 186whp (~15% drivetrain loss is more reasonable)
A very similiar car loses nearly ALWAYS 25-26HP from crank to engine, verified again and again. (miata).
My car hit a trap speed of 91mph, on my fastest (but not quickest in terms of ET) run
My car weigh, with me in it: car wieghs 3040, in race trim (3/4 tank gas, two 10" subs in large enclosure) + me at 215lbs, clothed for a total of 3255lbs..

You're implying drivetrain losses worse than most any car i've known...I think it's easier to believe Mazda doesn't really know how much power the RX8 makes; or knows, but isnt concerned enough about it do to anything.

You think the car loses 20% of it's power from it's flywheel to the rear tires. I say 15%. Let's meet in the middle, and say 17%.

186whp (even confirmed by what i consider inaccurate online calculators, and what you say is an inaccurate dyno) / .83 = 224hp

zoom44

they say dont take this as gospel because their legal people told them to put a disclaimer on the site. but the formula has been accepted as correct since the 50s or early 60s.

5% off of 238 is 226 and since Mazda is firm about it now being within 5% your drivetrain percentage must be wrong.

i cannot account for racing beats dyno but then i have never seen it or the plot from that dyno or any subsequent runs they may have made. but 15% off of 216 is 183 hp. so clearly Mazda is using more of a percentage than you are.

dmp

Good reading: http://forum.miata.net/cgi-bin/ultim...c;f=1;t=013972


I don't know any scientists. I know racers. I know 'car guys' and 'drivers'. Those neat caculators are to give ball-park figures - as I showed, they can vary wildly in their accuracy.

...and since my car made 186hp, as measured on a dyno (less if I use the 'SAE number), that number by RB of 216 looks about right - and lends to significantly less than 20% drivetrain loss.

zoom44

car guys and racers are the ones that say this formula is accurate!!

another thought- your trap speed. others have hit 94 or 95 mph in stock 8s and road and track tested it at http://www.roadandtrack.com/article....&page_number=1

so maybe its your trap speed that is the problem