I have been going to a local track for some time now, but after last night something caught my eye and figured it would be worth my time asking somone with more knowledge than myself.

In short we are running a F20b/hx35/E85 combo on 205/55-16 street tires and making around 325-350whp @ 9psi. Race weight is 2700lbs (me/car/full tank of fuel)

We have been doing shake down runs over the last few weekends before we throw slicks on, so again street tires.

Last night my best pass was 8.8 @ 90.10 mph. My data log (AEM EMS Series 2) shows I hit rev limiter in 3rd gear .05 before I let off throttle.

M2b4 - 25" tires = 104mph @ 8100 rpms.

So according to my calculations I should have been crossing the Trap lights around 7800-8000 rpms. This is still around 100mph.

Why does my slip say 90mph when mechanically I should have been closer to 100mph?

I also have 3 other passes from 88mph to 90mph that all agree..

Is my AEM off or is the track off?

Sorry its long and confusing but I am confused myself.

Thanks guys

Jerod

Also spinning stopped after 2nd gear (around 250-300ft down track)

 

Your trap speed is an average speed between the last 60ft at the end of the drag strip. So you may be going 100mph at the end of the 1/8th, but unless you enter the last 60ft @ 100mph its not going to be that high.

Not saying that is your issue, but something to keep in mind.

 

If I am actually crossing the second line @ 100mph, to trap 90 as an average I would have to enter the first line @ 80mph.
@ 80mph I'm traveling at 117.33ft/second. meaning I had apx 1/2 of a second to gain 20mph.

So in other words according to the traps I would enter first at apx 88-89mph and exit @ 91-92mph.

This is still quite a bit off vs my datalogs.

I guess I don't know of anyway of actually checking the accuracy of their numbers?

 

Is it possible that your vss is off?

 

Put a gps in the car that records peak mph and that should be relatively accurate

 

Not even recording vehicle speed, revs and conveted via mechanical equation to speed.

Gps is great but how do i cross reference my actual line crossing to its data recorded?

Basically according to my data I went from 6000rpms (78mph) to 7000 rpms (89mph) in apx 1.30 seconds and from 7000 rpms (89mph) to 8100 rpms (104mph) in .83 seconds.

I'm digging for the slip for ET at the first line to ET at the second line as they do not give you a mph at the first line just the 60' average i assume.

 

Never mind speed is not calculated as an average on the system they use, speed is calculated @ 594 feet or 66' prior to the 1/8th mile line.

Which would explain why I let off at apx 100mph, or .5 seconds after I crossed the first line.

So I am picking up apx 7-10mph in the last 66'.

according to TSI's website it is not an average, there is only one speed marker.

Which also makes alot more sense as far as cross referencing power produced vs power needed to accelerate to that mph.

The 1/4 mile we go to gives both 66' and 1320 marker, guess I just need to go there.

 

yeah i would figure it goes by how fast the 2 sets of wheels pass the beam. and it knows what the distance should be between axles based off of the distance calculated when you first launch. Ive always thought it got the trap from how long it took to trip the 1254' and the 1320' beam.

 

Tachometer reading off?? or are you reading RPM from datalogs?

 

I think it is that way with almost all systems, but the system the local track uses is pretty old. I'm going to ask about it next time we go to get a better understanding.

RPM is read via datalog on AEM EMS Series 2. Both from Laptop log and from ECU log.

The tach itself is off, but not as bad as everyone claims, and its nearly impossible to pay attention to anyway in the first 3 gears. Just use the CEL light as a shift indicator at 8000 rpms and go.