mark kibort

Just got back from the dyno. As I posted, i noticed that this time at the races, i felt a little flat. i couldnt pull on Scot and ran a 1 second off my best time at Thunderhill. I did countless 60-100mph runs and saw a .5 second slower time, similar to when the cams were not timed correctly and i dynoed 15hp less. after fixing the timing, getting 1 second better lap times and returning to the dyno, the HP was restored (that was 2005) Now, 2 seasons later, I seemed to be in that same HP situation again. However, the cams are in the same place, ive changed sparkplugs, rotors and caps. fuel mixture is close to the same and the lack of HP is from beginning to end, almost equally!

This dyno run: 297rwhp
last time dyno run: 320rwhp

DOWN 20hp!

since mixture is ok and the HP curves vs my last visit to the dyno produced 20hp more, im wondering what could be the cause.

oil sent to the analysis showed nothing unusual, new tune up parts, compression (static) tests showed the same compression as the Holbert engine ever made, so what could it be???? ignition timing? is a knocksensor retarding the timing, but then wouldnt the high HP levels remain in tact??? if it was cam timing changing, wouldnt there be a shift in the curve, not an equal drop from beginning to end?? same thing for ignition timing, if it was a computer change, woldnt it shift the hp curve up or down, but not equally end to end?

One thing that i did notice is that there was a new guy that strapped the car down. he did not really seem to know what he was doing and it seemed it stapped downt the car with much greater force that usual. in fact, when he released the rear straps the car darted forward a few inches with a "snap!" .
maybe the increased rolling friction could be the cause.

any ideas?? OR, is the holbert machine just getting tired after 6 pretty tough full racing seasons (90hours of racing)

Here is the before and after dyno graph.

Also, notice that the last time i was at the dyno, it was winter time, and a 66degree day, with SAE being 320rwhp.(actual was near 330). This time temp was 84degrees, yet SAE was almost no correction factor. actual was within 1hp. (still 297rwhp)

Mk

Randy V

Holbert is rolling over in his grave!

You'll have some 'splainin' to do to big Al in the after-life, Mark.

ErnestSw

Knock sensor, hall sensor, exhaust restriction, etc. try switching MAFs to see whether you're getting out of spec. Try switching brains also.

mark kibort

air fuel looks good, certainly not enough to make a 20-25hp diff. so, would that rule out the MAF?

could the knock sensors be signaling some ignition retard?

exhaust is pretty straight through.

brains??? ill have to try that.

what about hall sensor. how does that effect things. isnt that a go or no-go deal?

thanks for the help!
mk

Lizard928

the graphs have actually moved up slightly, and it looks like you are loosing more in the upper end.

check the knock sensors first and see what you get.

Charley B

Interesting observation on the restrictive tie down. Would an extra 500 lbs laying in back over the rear axels result in a lower HP rating? If so, then that may have been the problem.

Bill Ball

Probably the Amsoil is too thick and is robbing 20HP.

Anyway, the Hall sender is not go-no go (that would be the crank position sensor on the bellhousing). If the Hall sender is not working properly, you lose 6 degrees above certain throttle load. Same happens if either of the knock sensors fails. Check the ignition timing at high RPM.

brutus

You can also unplug either one of the knock sensors and the same retard will happen. It should make a difference.

Bill Ball

Yeah, that would have been a good test on the dyno. Unplug the front knock sensor. If the dyno results did not get worse, then you've got a problem with the knock sensors or Hall sender.

Mark, if you had an 88+ LH, we could use the Spanner or Theo diagnostic program, providing the diagnostic port is there. We could swap in the LH from my 89 and run diagnostics.

Fabio421

Wasn't there just a thread, championed by Mark K., that was arguing that dyno #'s are not gospel? What has changed?

mark kibort

there are some variables on tie down, air pressure, etc. but if all the the main factors are addressed, you usually get very repeatable results. (especially on the same dyno session). Ive been running on this dyno for 10 years, and have had the same runs 2 x per year for 5 out of the 6 years. now, the question is, what has changed?

so, actually, i was behind the dynos being gospils, with a couple of catches.
the SAE stuf is kind of easy to mess up. just take my two runs. ( i usually do a winter and summer run) one was 66 degree temps the other 85 degree temps.
why was the hot run today, corrected by so little, yet it was a 85 degree session? this is the kind of problem that makes the correction numbers a little suspect on occasion.

mk

mark kibort

I think that would be a great idea!

what about a GT computer. i think i may have access to a GT LH box to try. But you point is well taken. if there is a problem, your computer could show it.

which side is the knock sensor. i know there is two of them, there is a connector on the passenger fuel rail that looks like it drops down to the knock sensor, is that it? there is another one on on the other fuel rail. anyone have a shot of what the sensor connector looks like. (thats uncharted territory for me)

what is the hall sensor used for. Is that the sensor on the cam pulley?

by the way, i did take a look at the spark advance and it was around 13 degrees at 1000rpm and 20 degrees at near 3000rpm. at higher rpms, it seemed to get near the 35-8 range, but not near the 45 degree mark. can this change under load? (or is only RPM dependent so i can check this reving the engine)

Mk

Mrmerlin

The easiest knock sensor to check is the front one it is the black 3 pin connector that is just in front and below the driver side fuel rail it should be next to the valve for the flappy actuator, the hall sensor is connected to rear side of the passenger side camshaft, the connector for this usually will have a similar failure mode as the knock sensors do in that the connectors will start to disintegrate, and the wires will short together, also the knock sensors them selves will also disintegrate where they are bolted to the block, as you know you need to remove the intake manifold to replace both of the sensors, also check your harness as it crosses in front of the engine for the leads for the hall sensor see if the wires are routed well and the insulation is not fractured, it should be routed near where the air pump used to be and it will snake up to the rear of the cam pulley housing, goodluck i hope you find the loss , if the compression is the same then it sounds like possibly the ignition may be fouled up, by the way what is your fuel pressure doing?? is the air cleaner new? Stan

mark kibort

thanks stan,

Yes, it looks like the connector that i thought was the rear knock sensor is just that. it connects to the passenger side fuel rail harness. the connector is easy to accesss, and just recently, i noticed that the entire insulation has been melted off , as it dips down towar the throttle bocy, it looks intact. i can barely see it at the connection point and it looks kind of ugly down there.

the front knock sensor comes off the front of the driver side fuel rail harness.
Its connector is buried in the intake, as i can barely see through the openings by the oil filler. can i do a test to see if they are working or causing spark retard by checking and disconnecting the rear one, because i can reach its connector? what is a knock sensor anyway. does it make a connection when the sensor picks up a knock? can you short out the connections up stream to elimniate the knock sensors? If they are two wires, cant you just tie them together to see if they are the issue?

that front cam sensor, i do remember it. where and how do we check that? same thing ? disconnect and see if the timing retards??

thanks for all the help!

The interesting thing about the HP curve is that is just shifted down evenly from end to end. does this say that maybe that everything is ok, but the dyno gorrilla operator just cranked down the car too tight???

mk

michaelathome

Mark,

I know that I am not the first to say it but you are a wierdo. That being said I am a wierdo, we are all wierdo's with 928's. Your 928 may just may be a little different than most.

IIRC air pressure changes in your tires can rob horsepower. Personally I think you might have had an off day. 1 second over a lap on a track is tiny. Did you buy a new pair of shoes or have you gained a lb or two? Did the tires change or did you have a new/old worn bushing.

Sorry to throw it out there but NONE of us can really help you to say that, "Your valves are not timed correctly" or "your car is to heavy or light" till someone pulls it apart.

I love what you have been able to do w/the car but untill the rest of us know what is in there it is a little pointless IMO to ask us what is wrong.



Michael