Speedtoys

Dynos have no 'calibration', They can correct for conditions or not. Dynojet can come out (for example) to a shop and say "Yup, its accurate" and certify it, but an input sensor itself would have to be obviously broken to skew the data. You could blow a heat gun on the temp sensor and affect SAE readings by max, 2%..before buring something with the gun...we've done that for when people cry "sensor" or "placement".

The dyno is doing somewhat simple immutable math, "how fast is a fixed measured mass accelerating within a given time". Time is as measured per rev of the drum that is a static size and mass, (or multiple per revolution), torque is modified as a function of how many cyl you tell the software you have, and HP is a math function of that...which is why the two always cross at the same point. It's just math.

Dynojet for example automates the calculation of absolute pressure, air temp, humidity, requiring only a manual addition of elevation to present an SAE measured correction factor, also immutable.

The things that affect any dyno reading, other than a billion things that WILL be different within the car any given day, are tire pressures, tire type, and tire placement on the drum.

Ive been challenged 100s of times at the racetrack on "The dyno is wrong" in a penalty appeal, and a total number of 0 times has anyone been able to determine any mechanical or mathematical error in how the dyno measured their output on that given day.

Dynos as a TOOL have flaws in how they work, and this is why they are somewhat poor as a tuning tool (roads are best) , but as a specified tool they will all measure the same variables the same way. The only variable in the number returned is the car on the dyno, and we try to limit those variable by knowing as much we (and the owners) know about a racecar in a spec class. Some cars will drop 10Hp at a specific coolant temp via different ign/fuel maps, so we have procedures to help, etc. (IE: BMW E30) Other cars _have_ to be moving to cool at all, and are limited in runs (BMW E46).

Mongo..to answer directly, the same family (brand) of dyno will always measure and output the same data as long as input is the same. A mustang EC unit compared to a non EC dynojet will present data differently, because of how they operate, similarly a an eddy current dynojet will let you present data differently than a non EC dynojet. These points go back to why roads create better tunes than dynos.

And tomorrow will NOT present the same dyno curve or peak values as today, even with correction....the largest variables are as power creation and parasitic losses upstream of the drum (or hub) connection to the car.

IcemanG17

I agree that given your mods that the dyno does seem low, but maybe some acceleration runs can give you a baseline....try 3000rpm to redline in a certain gear...i'd pick 2nd to reduce really high speeds...in both directions to eliminate wind/slope variables.... then if there are other 5 speed 928 S4 owners local you can compare times to get an idea where you stand.

My old sharky dynoed very high for a stock automatic at 279whp and it felt it on the road too....that was on a Mustang dyno too which tend to read lower than dynojets.... Another possibility is if one or both knock sensors are bad along with the Hall sensor on the cam.....when sharky had one dead knock sensor and the hall sensor it really hurt the top end power

Speedtoys

Good point, a loss of ONE of the two knock sensors OR the hall sensor, is a dramatic cold blanket on how the car runs...and you wont even know it.

But on a dyno chart you -can- see it...somewhere in RL I have a plot of with all 3 in order, and with the rear KN unplugged..

Speedtoys

Found it:

The difference between the 2 runs is the rear KS was unplugged. The car FWIW is bone stock, and in this chart, COULD have had what I found later was a failing(failed) spark plug boot that was a smog no-pass issue.

I had the flappy hosed up backwards too, my bad, I can gather the same data with the rear KS removed WITH a properly connected flappy if anyone wants it.

Mongo

Thanks for the clarification on dynojets. I'm no expert on them but understand the mechanics of it. I don't think Marti specified if he changed his knock sensors, nor confirmed they are properly plugged in. Surprised there's only around a loss of 5hp with a faulty/unplugged knock sensor. I assume the EZK dials the timing back the same amount of degrees regardless if one or both are bad.

Speedtoys

Yes, it only takes 1 of the three critical items to be bad, It doesn't get worse if 2 or all 3 are bad.

I should get another view of this now that I'm at better system health than I was before.

Marti

I am confident that the hal and knock sensors are good, all are recently renewed and are detecting knocks and pulling timing where required.

During each run I was getting one audible ping from the ST under high load around 3K then all ok, recovery was set at 5.6 seconds which I guess should have given full advance by the time we were reaching max revs.

I have already decided to replace the original exhaust system, it needed done anyway and I have no assurances about what condition it is on the inside. Although it seems ok I have a suspicion about it due to seeing hot "embers" flying out the back of the car on a high speed run a few year back. Made me think everything on the inside might not be well.

Question is twin 2.5" or 2.75"???

drnick

Marti, I wouldn’t get too hung up on the exact number you’ve generated. It’s more useful if you have a before dyno to compare with. I’m sure there are some resident dyno experts here who will correct me but there is often a difference between results recorded in different places for no good reason.

FredR

If your knock sensors were pulling timing then they were working and they were not you would have seen it on the traffic lights in ST2.

One of the best 5 litres performers I have seen was the white GT that Louie built and eventually sold to Gentleman Jim Corenham. As I recall that motor turned out some 340 rwhp on Louie's dyno. I cannot recall the exact spec of the build but it was sharktuned, it had the GT cams, it had a Louie X pipe and a custom exhaust consisting of 2.5 inch pipes, a Bullet muffler where the resonator sits and a RMB at the rear- my exhaust is modelled on this. I think Jim remodelled the cams later on before he had the motor rebuilt by GB and now it is something else.

That you have seen improvement is one thing but the obvious question is whether the starting point was up to snuff to start with. Your dyno might be a conservative [it is Scottish!]. Whether or not the stock exhaust is stifling the gains remains to be seen but for sure it is not going to be helping your cause. The top end torque curve and the fact that the power curve is still climbing at 6500 rpms suggests to me that Colin's cams are doing their job and the improvement you are sensing is probbly down to those gains.

I take it you released the handbrake during the dyno runs!

Rgds

Fred

John Speake

Manual or auto ? The transmission loss factor they used was 27%.

Marti

Auto John

Marti

Thanks Fred

I checked an old email from Colin with a power curve for his cam and it looks consistent with what Colin recorded - ie still pulling at 6.5K

Something is definitely sapping the power over the whole rev range - a choked exhaust might give these symptoms - although there is not significant level of back pressure that I can detect

Strosek Ultra

SwayBar

2.5" is all you need.

Nice exhaust diameter chart!

Lizard928

So long as the Compression numbers are balanced left to right that is most important.
Out of curiosity what is the number that you are seeing for compression?

Has the ignition system been refreshed?

I wouldn't worry about it pulling right up to redline these are large cams and the only restriction left is in the rest of the system.

With the Hall (cam) sensor. Did you replace the plug, or buy a new one?
If you replaced the plug you can reverse 2 of the wires which throw the computer into a fault mode without really showing it......

I agree with recommending doing a couple street pulls from 3k rpm to redline to get a better idea as to how the car is performing.