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unsure about this dyno readout

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recently had a car retuned. ls1 rx7 stock motor with a cam made 390hp had it out at the track for the 1st time and it run a bearing so rebuilt it to a stroked 383, cnc ported heads, bigger valves etc to get it tuned and make 383 hp they were different dynos so would expect some variation but it just doesnt seem right with the huge torque figures. its insane to drive compared to how it used to be. always thought hp was a calculation from torque... so did an online calculator and it reckons its 590 hp 500 ft lb at 6200 rpm. i am no tuner but i like to know these things, the owner of the car will probably cry if i told him it made less hp after spending nearly 20k on the new motor

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Hi man,

Mainline dynos read much lower than a lot of the ones people in NZ usually use, so that'll be part of it - if it was previously tuned on a Dynapack and made 390hp then made 383hp on a Mainline then it will like be making a reasonably amount more actual power.

You are correct to be puzzled by that, the hp versus torque numbers don't add up - I think that the way the dyno has represented that does not make much sense at all. I am not too familiar with the dyno software but if I were to extrapolate from what I am seeing there I wouldn't be surprised if what has happened is the dyno roller rpm versus engine speed hasn't been configured correctly and the dyno is trying to reverse calculate effective engine torque from what it thinks engine rpm is (it can't know unless it is getting a direct feed from the ECU) based on a calculation from the speed it's rollers are spinning at.

As you has implied, hp can be calculated from torque by "power = (torque * rpm) / 5252". The dyno is likely to be using that calculation, but when it uses rpm it will be going by the dyno roller rpm - not the engine rpm, as it's obviously measuring the roller speed and the torque transferred from them - not the engine speed. In doing so, it will have in its head a power and torque curve based on roller speed - but obviously that's not overly useful to you, so it will rely on the conversion the dyno operator has given to it to create a torque curve using the reverse of the above formula...

... so, the method used to calculate lb/ft from hp and rpm is torque = (hp/5252)*rpm. By nature this means that the torque and power SHOULD intersect at 5252, so if you look at 5252 and use the hp number there the torque number should come back the same - but in this case it clearly doesn't. If I were to make a wild assumption and say this engine itself is actually spinning to 6700rpm in this dyno run and the dyno thinks the engine is only spinning up to 6100rpm then it would make sense the torque numbers are inflated.

That's just a theory, but for what it's worth there are no exceptions to the maths - if the numbers don't add up then the wrong variables are being used :)

I would be expecting a lot more bhp from those torque values too, it could possibly be that the correction factor for calculating bhp is wrong

cheers guys, spot on with the old tune being on a dynapack and the recent numbers being on a mainline. have had a bit of a yarn with tuner and he is adamant that it is just down on power and there is something holding it back, hes saying that there is nothing he can change to alter those read outs other then diff/gear ratios which in theory (Im probably wrong) if went to a shorter ratio will move the curve back in the rpm and will read higher numbers at less rpm but will still equate to the same hp???

Yes that's right, I'd expect it to read the same hp but the rpm numbers will be lower - the trick here is that when it's trying to calculate what torque at the engine it'd be making to make those numbers at those rpm it'd be using the wrong numbers, so miscalculating the torque numbers.... which means they may be inflated, so your power is probably more or less right but the torque not so much.

Well that's what it seems like to me, anyway.

Have a read of this document from Mainline. It explains the situation very clearly :)

http://www.mainlinedyno.com.au/images/downloads/DYNO_TORQUE_FIGURES_-_THE_TRUTH.pdf

thanks Andre that clears it up (I think) so that read out I have is showing roller torque apposed to derived torque but no matter what that hp reading is still right? so it seams to be down on power. now to figure out why haha thanks again

Comparing dyno results from different dynos is always difficult. Depending on brand of dyno, the power reading from the same vehicle can vary wildly. It's pretty well known that Dynapack for example tend to read quite a lot higher than the likes of Dynodynamics or Mainline.

I try not to get to tied up when comparing these results. My recommendation is always to use the same dyno for your tuning or development and that way the results should always be comparable.

yea idealy I would have put it back on the same dyno which is who I normaly use but they are booked out over a month in advance. good for them lol bad for me. it is one of those nightmare cars that has had just about everything go wrong with it but hasnt been driven. the joys of finishing off someone elses project.

I was expecting the result to be lower on the mainline but this seems to be at the point where there is something wrong. even the tuner said that other motors with similar mods went majorly better.

I hate to be that guy moaning about hp figures but im not looking forward to telling the owner of the car because he will think otherwise. all he has done with part with a large wad of cash and the only thing he gets back is a dyno sheet that reads less then the original

I'm just spit ballin' here...but perhaps it will be worth while to check both dyno read outs the see which correction factors were used. But perhaps more importantly, if your first dyno reading calculated crank hp, and the 2nd only kept it a whp, the figures would make sense