mark kibort

I forgot to mention, the dyno run was with 24psi tire presure. Think that matters that much??

mk

Larry928GTS

Is that with the results for the cylinders in this order?
Passenger Side:
#1 - 222
#2 - 215
#3 - 220
#4 - 220

driver side
#5 - 220
#6 - 217
#7 - 220
#8 - 220

Louie928

Thanks Mark. The trip South has been squashed for next weekend.

I have found that tires and tire pressure do make a difference on rear wheel dyno output. My first runs after tuning with the GT when I got 580+ RWHP (STD corrected) was with Hoosier slicks at about 35 psi. They were on the wheels I got from Don Hanson. When I began driving it on the street, I put on some Dunlop SP8000 tires. I dynoed it again a couple months later and was down about 25hp. I thought I had some big problem, but couldn't find a reason for the loss. Pumping the Dunlops up to about 40 psi got back about 5 hp. Tire pressure does make a difference, but it isn't huge. I now have Goodyears, and they dyno better than the Dunlops, but not as good as the Hoosiers.

GregBBRD

Seems that, on a Dynojet (or any dyno), that any slippage of the tires would be a loss of power that the dyno would not be able to measure. On engines with big torque (like 928 engines), it would seem that the stickier the tire, the better off you would be. I think Louis' experience is an example of this. Of course, how hard the car is strapped down is going to make a difference, also When I went over and watched Mark Anderson's car get dynoed, last week, all the runs we made were within 2-3 horsepower, which is very repeatable.

At the lower torque levels, tire slippage will not be as big an issue and you should get more repeatable results.

hacker-pschorr

Interesting.
Might be nothing, but with the shark tuner the first cylinders we see knock during testing are #2 and #6. This has been very consistent with every car.

You will be running some race gas correct?

Fabio421

And those are the same cylinders that see the rod bearing failure. A knock will cause increased load on the bearing. Coincidence? Probably not.

hacker-pschorr

Bingo......

mark kibort

Now its correct. I forgot to fix this post that i made on the cam timing thread.

do i have an issue with #2 and #6?????????????????

mk

hacker-pschorr

IMO it's going overboard to get worked up over 5 or so psi (the difference between mine are 15)

But..... you are running a mostly a stock tune on a very not stock engine.
We also know from many stock 928's that #2 and #6 are the most knock prone (using the factory knock sensors and the Shark Tuner to monitor these events).

I would pull those plugs, using a magnifying glass inspect them very closly for pits and / or what looks like pepper marks (I've done this more times then I can count, it's the only detonation protection I have on my 81).
And while you are at it, put the colder plugs in and race gas mix for the track.

Who knows, maybe your engine was knocking on those cylinders before and there is less carbon buildup with those combustion chambers lowering the compression

I'm not sure what to think but the tune would be the culprit IF something was going on.
This is what I was worried about. Not enough time before the first race to properly tune this motor.

Thanks to Larry for pointing this out.

Lizard928

does this apply to the 85/86 LH cars too?

hacker-pschorr

No knock sensors, so I have no idea. I'm sure Porken will find out one way or the other with his knock sensing equipment.

One of these days I want to sit down with each intake and study them closer, see how extreme the runner length distance is per cylinder compared to the S4. Who knows, maybe the "cheap" upgrade for a track 928 S4 is the 85/86 intake. Basically having the flappy stuck open might not be ideal though.

I'm sure someone has a "spare" 85/86 intake lying around (shane, you out there...). Maybe if Mark gets bored he can do some testing

mark kibort

I will be running race gas and the colder plugs. I have the plugs marked and in order and see if i can see anything with a double magnifying glass.

Might be nothing, but sure looks like it could be more than coincidental!

mk

Lizard928

be interesting to see if by swapping the S4 to a 85/86 intake lowered the #2/#6 cyl from knocking as much.

Fabio421

Excuse my ignorance if I'm missing something obvious here. ( I am pre coffee )But please explain for the masses. How does the #2 & #6 cyls having slightly less PSI during a compression test relate to them also being knock prone and prone to bearing failure? I can see why the knocking and bearing failure could be related, but what about the commpression test? This engine is only a couple of hundred miles old, do you think it may have already cracked a piston ring due to detonation? If not, what am I missing? This is a compression test so knock or running lean really doesn't have anything to do with it.

Lizard928

Fabio,
Eric has stated that he cyl #2 and #6 are more prone to knocking on all sharktuned cars.
Once seeing this comment mark looked at his compression numbers and saw that his cyl #2 and #6 were the lowest of the numbers. So he slightly paniced :P

What Eric and myself are wondering is if the S4 intake system is partially to blaim for cyl #2 and #6 being more prone to knock due to an imbalance in the airflow causing those 2 cyl to run leaner than the rest.