GregBBRD

Try running W5s in the #2 and #6 positions and a W6s in the #3 and #7 positions. Run the W7s in the outer corners. W5's are pretty cold for the corners and will require more timing to make them "happy".

"Knocks" come about by increased combustion chamber temperatures....so anything you can do to "equalize" those temperatures in the engine will allow the timing in individual cylinders to be similar.

In the end, engine performance is exactly the same, if it has 25 degrees of timing in a cylinder with a hot spark plug or if it has 28 degrees of timing with a cold spark plug.......as long as the combustion chamber temperatures are the same. After all, it's all about the rate of travel of the flame front and the resulting pressure.

PorKen

Spacers...hmmm. I could not get the rail to seat on all of the injectors properly as I could not push straight down over the o-rings. No way to reinstall the front hold-down nut. After the first spray of fuel over the engine I said nay.

There is plenty of room for the 2 and 3 as they are in the center (duh, this makes more sense if you don't look straight into the plenum). If anything they might need a shield so that air does not travel directly from the TB into the runner, forcing it to go a longer route like the right plenum.

Anyhu, back to the original program. Timing is everything!

Lizard928

Ken,
Pull the studs out, and use Allen key bolts to hold the rail down.

PorKen

Ah. That makes sense. Would fix both the o-ring problem and the hold-down.

I may have talked myself out of using it for my chip, though.

FredR

Greg,

Thanks for the input/suggestions- may well explain why I had to pull the timing back to about 24 degrees to keep 6 & 2 happy -I do not use the WOT mapping but overlay onto the cruise maps- I found the WOT switch to be erratic.

Rgds

Fred

FredR

Ken

Not advocating it but I reluctantly had to leave that retaining nut off- figured the rear bolt plus the clips were enough to retain such in place and tested carefully [initially]. Figured that with the foam covers bolted down and everything else holding the rails it was not going to go anywhere but to be clear- I did not like doing this.

Luck or judgement- I have not had a problem to date. Must take a look to see if Colin's suggestion would work on my installation. Did not appreciate that stud was removable.

Rgdds

Fred

PorKen

Oh no! I have found the cylinder ID variable. (Pretty much staring me in the face. Bit of a forest-for-trees/byte-for-bits problem.)

Since I don't think individual cylinder maps would help that much more as the rpm resolution would have to be reduced, I think I will group the same plenum side cylinders by cylinder bank, IE. short/long, straight/noodle.

There goes another 20+ gallons of high-test...

ptuomov

Not following, please explain. Very interested in this topic.

PorKen

Up to this point I did not have a way of telling which cylinder was which from the code. I had only understood the flags for front/rear (knock sensor) and right/left (distributor), so I could only group the four corners. Now I can individually identify a cylinder so I could make maps for each but I would have to halve the number of rpm columns which might cut off a peak or a valley. Instead, I am reworking the code to group the 'short' and 'long' cylinders, aka the inner and outer cylinders.

There is a fairly large difference in the right plenum 6/1 and 4/7 groups that could either be from 6 vs. 7 OR the 6/7 (long) vs. 1/4 (short).


Any change requires a whole bunch of new WOT runs to 'pre-chew' the base maps for my chips, since I don't want the learning to take too long or have to correct too much.

UpFixenDerPorsche

Gosh that's a familiar looking sight. I've seen far far too much of mine in the last few weeks. Grrrr.

Lizard928

Ken, you need to pull your manifold, and have it powdercoated.

One fellow on the board here had serious damage to his engine when the coating inside the runners broke off and went in through the engine...... If memory serves me it was the fellow who owned the Saudi car.

ptuomov

Is the flaking factory coating that dangerous? I know some aftermarket powdercoatings can do real damage if they flake off, but I didn't know that the stock coating would be that harmful. Obviously you'd rather not feed paint chips to your engine or your child, just trying to figure out the difference between the "latexy feeling" factory coating and typical "ceramicy feeling" powercoating.

Lizard928

Tuomo,

I remember having one fellow who posted on RL that this happened to his 928. If my memory serves me it was the fellow who had the arab/saudi car who it happened to. But it has been a number of years and my searches have not been able to pull it up.

ptuomov

We've discussed the intake manifold runner length and its impact on ignition timing in great detail (i.e., until the cows come home). One thing that hasn't come up is the exhaust side. Since I've got a turbo car, the exhaust and its complications are somewhat familiar to me.

If the exhaust gasses are not evacuated from the combustion chamber during the overlap period, the cylinder is going to knock very easily. Actually, there are two offsetting effects. First, the inert exhaust gas mixed in will reduce the knock probability. Second, the exhaust gas is hot (unlike in EGR systems that cool it first) which will increase knock probability. The second effect dominates and better evacuation leads to lower knock probability.

If the intake runner is in tune in terms of intake valve closed resonance, there's high pressure in the intake port when the intake valve opens and this helps push exhaust gas out. So on the one hand, better filling runners that are in tune at certain rpm will lead to better trapped unburned mixture, which requires less advance. On the other hand, better filling intake runners empty the combustion chamber better, which allows for more advance. My guess is that with at least our engines the former effect dominates.

Now, here's the nail in the coffin of the one-size-fits-all ignition timing. The firing order of our engine is 1-3-7-2-6-5-4-8. This means that at low to medium rpms, the exhaust blowdown of some cylinders coincides with the overlap of other cylinder in the same bank. Cylinder 2 fires 180 degrees after cylinder 3, which at low to medium rpms means that the exhaust blowdown pulse from cylinder 2 stuffs the cylinder 3 full of exhaust gas. The same for cylinder 1 stuffing cylinder 4, cylinder 6 stuffing cylinder 7, and cylinder 8 stuffing cylinder 5. This is what's happening under 3000 rpm. You've got four bad cylinders right there from the exhaust perspective at low rpms.

At medium to high rpms, the 180 degree separated pulse doesn't quite have the time to make it to the other cylinders, even with the stock "shorty" exhaust manifold. But at those rpms, the big problems are in the cylinders that fire at 90 degree spacing. That is, cylinder 3 is now stuffing cylinder 1 and cylinder 5 is stuffing cylinder 6 with hot exhaust gas at the overlap. My guess is that this effect is taking place at some rpms above 4000. Now you've got two bad cylinders from the exhaust perspective at low rpms.

I am browsing some data from Ford/Jaguar test engine (twin turbo V8) and they find that one percentage point of more exhaust gas remaining in the cylinder requires retarding ignition by about three additional degrees at 2000 rpm. The variation across cylinders in their data is about 3.9 percentage points between their bad cylinder 6 (our cylinder 5 given the 928 having a different firing order) at 7.7% and their good cylinder 7 (our cylinder 8) at 3.8%, so this matters at least at low rpms.

At both low and high rpms, different cylinders are going to want different ignition timing. Furthermore, it's not the same cylinders at low and high rpms that can't take much advance.

Now, I didn't mean to write another dissertation. (It just happened.) To summarize the above minutia: Given we have a cross-plane V8, unequal intake runner lengths, and short-runner exhaust manifold, a cylinder-specific ignition timing is going to be very valuable.

FredR

Tuomo,

I think you have solved Ken's dilemma- individual cylinder maps all round!

Not sure of the complications here but given that most of the work in the stock maps is done in relatively few high load cells some judicious adjustment of the load values might do the trick. No idea if it is possible but some kind of hybrid mapping [stock like at lower load values for cruise] and more focussed maps for high load per cylinder for say 80% plus [?] load values. I get the impression that individual cylinder high load range mapping is probably more critical than a higher number of rpm based cells across the full spectrum.

Should keep Ken busy for a day or two!

Rgds

Fred