Joe Dyer

The bracket is an early Delta-P.

Fastest928

Euro S4 engine used an air pump to feed the ports in the heads...thus mantaining a balanced laod.

heinrich

What does the port injection do?

bd0nalds0n

How tight do SC belts need to be to eliminate slip? I gather resistance increases with RPMs?

Jim bailey - 928 International

Good catch Marc you are correct about the air injection for the Euro 1987, I was misinformed.

shaaark89

belt load, and consequently belt tension will be determined by the drive power required to generate a given amount of boost and airflow divided by the belt velocity.

drive power=boost x airflow / 229
one must then factor in belt efficiency and adiabatic efficiency (typically 97% and 60%)
this will give a drive power in horsepower. multiply by 550 to get ft-lb per sec

divide this by belt velocity (crank pulley diameter x crank rpm / 229) to give the belt load in lbs.

for my 89 at 10.5 psi boost and airflow of 741 cfm the load calculates to 169 lbs.

that's my math for the year

p.s. that's for a centrifugal blower.

Tim Murphy

Wow, lots of reading (an opinions)!

So far, we have not seen any visual wear at all on the main bearings with the sc'd cars. The "26 psi red car" will be torn down this winter and inspected in many ways. If we find any significant wear (I doubt it at this point) you will all hear about it because we will be talking about what we are going to do to fix it. As of now, there has been no evidence of wear on the main bearing in the direction of the sc and is not even on our list of things we might have to do. As a matter of fact, we have reused the main bearings in every engine that we have had apart because they were in such good shape.

Fastest928

Tim, pls post: SC miles before teardown, static belt tension and boost.

This will be very useful info.

Carl Fausett

Maybe this will help. I send my engine oil in for a gas chromatograph exam for particulate matter. Costs $20.

They will tell you if you have any copper, zinc, steel, iron , etc in your oil - evidence of bearing wear, piston ring scuffing, cylinder wall damage, etc. These tests are very common in aircraft and heavy machinery. The oil tests are adjusted for the engine hours on the motor. The fewer the engine hours, the lower the limits on particulate matter.

This one is the last one I have - from last season - 1500 miles on the oil. Racing and street driving with our supercharger on it.

The scan is not the best - here is what the columns say if you cannot read them:

Test Date: 09/15/03
Oil Weight Within Limits? = Y
Water in Oil = N
Fuel in Oil = N
Anti-Freeze in Oil = N
Chromium = A (Within acceptable limits)
Copper = A
Iron = A
Lead = C
Tin = A
Aluminum = A
Silicone (Dirt) = B

You will note all is normal - no bearing wear can be found. The prescence of copper and tin would identify bearing wear if there were any.

If you read the notes at the bottom, they did find that I had a high amount of lead in the oil... and said it was from running racing fuel. They were right, last event I did was an 'enduro" style deal and I filled up with racing fuel at the track. Normally I do not use the stuff.

Also in the note to me at the bottom "Dirt entered the sample bottle when the oil sample was taken" - accounting for the silicone (dirt) particles.

Carl Fausett

Bill Ball - Cogged belts are known to set up more vibration than v-belts. And, ans Andy pointed out - vibration can prematurely wear bearings.

Cogged belts have to be tensioned like a ****** too - they do not get to slack off or they jump teeth.

Example - Porsche 928 belt tensioner for the 16v drive train.

Why, then, do you ever use a cogged belt? When ANY amount of slippage is not tolerable - like a valve train on an interference motor. You can use timing gears, a timing chain, or cogged belts and guarantee proper indexing of the camshaft pulleys to the crank.

On the superchargers we do not need indexing - if the belt slips a tad at 6,000 rpm its no big deal, just a little loss of power at that moment - no harm done. But you get a quieter, smoother engine as the benefit.

If the belt tension has to be excessive to prevent belt slippage, then the driven pulley is too small. Usually this is because the drive (crankshaft) pulley is too small. When a kit builder makes the crank pulley too small in diameter, they have to use a way little driven pulley on the blower to get the right ratios they want to spin the blower at the right speed.

Now, because the driven pulley is so small, there is very little contact area with the belt and inadequate friction. The belt slips. Think "wider tires provide better dry traction" here.

The solution is not tightening the bejeezus out of the belt - it is resizing the crank up an inch or so, so you can resize the supercharger pulley by the same ratio, get the belt contact patch you need, and the slippage goes away at a much lower belt tension.

H2

Except for the wasted time passing over the jabs that people take at each other (probably not meanspirited), I find this discussion to be thought stimulating. I certainly will pay more attention to tensioning ALL my belts from now on...and double check other items when I have it apart. Then again, I'm self-diagnosed ****. As a 928 owner if you're not **** about its upkeep...you probably should be (there...I got a jab in too). Buying a used car where the PO didn't have a clue about what he/she/it was doing when they attempted to DIY is what causes some people considerable anguish after the purchase.

Harvey

Harvey

PorKen

Post cylinder hydrocarbon/NOx reduction.

Would the silicon content in the block metal show up as this?

Doug Hillary

Hi,
IMHO both Jim and Marc are correct and in these circumstances (external add ons) a used oil analysis programme can tell you a lot relating to excessive wear.
In this case the previous eight oil analysis results I published on here are very worthwhile as base line results (taken over 2 years and about 35k kms (20k miles)

The readings in parts per million (the International measuring standard) were;

Aluminium - operating limit 10 to 30ppm
highest 6ppm
average 4ppm
median 4ppm
uptake per km 0.0003ppm

Copper - operating limit 10 to 50ppm
highest 8ppm
average 4ppm
median 3ppm
uptake per km 0.0002ppm

Lead - operating limit 10 to 50ppm
highest 11ppm
average 6ppm
median 6ppm
uptake pr km 0.0006ppm

The analysis interpretation should concentrate on "spikes" or readings beyond the upper operating limits and viscosity increase. Readings above the median should be tracked over time as well

In Quick Carl's oil analysis the results are "interpretive" rather than provided to a measurable standard - they are still valuable. The problem rests with the ability of the Technician who has done the interpretation and no ppm readings are provided for reference purposes
Using the Internationally accepted ppm standard protocols are VERY important when comparing samples and most Labs routinely report to these. All Oil Co Labs do as well

As well it is very important to use the correct engine lubricant. Any 928 engine lubricant should have these as very basic standards;

Rated as a Group 3 synthetic (preferably Group 4 or Group 5)
ACEA A3-96>/B3-96> test conformity (API SJ>/CF-4> as an alternative)
HTHS viscosity 3.5cSt minimum (preferably over 4cSt) (measured via cycle terst protocol @150C)
Porsche Approval (this is a special anti foaming protocol)

It is unlikely that an oil viscosity below 0w-40 (eg 10w-30) will have a suitable minimum HTHS viscosity. Oils with an HTHS viscosity above 5.5cSt are counterproductive in some engines. These can cause excessive journal bearing wear and etc.
The poor shear strength of oils with a HTHS below the Porsche minimum (3.5cSt) will lead to excessive bearing/engine wear

NOTE: New test protocols will eventually replace the HTHS test and some exotic low viscosity Group 5 ester based synthetic oils (such as FUCHS, Motul and Mobil 1 R) are probably suitable in a 928 when used with a Used Oil Analysis programme

I would prefer it if Heinrich would not comment on my posts!

Regards
Doug

Doug Hillary

Hi,
Porken - there is NO evidence that the silicon content of the engine's structure will figure (skew results) in the Used Oil Analysis result. The eight sample taken from my car were all below Industry averages for all engine types
Excessive silicate in oil samples is usually from the inlet tract and most new oil starts with a silicate level of around 2-8ppm

Quick Carl - the "C" critical Lead reading may also be as a result of the low viscosity oil you are using - do you know what the ppm figure is for the Lead. I suggest that this is more likely than the type of fuel used
What brand and specification of oil was tested?

Regards
Doug

Carl Fausett

Doug - in the upper-right hand corner of the report, it syas "Mobil 1 10w-30"
That's what I was running when we tested the oil.

You are right about these results being interpritive - I used to send my samples to Caterpillar and got a nice PPM readout that I liked much bette. Thought I';d try these guys, "Titan" instead cause of the price. I do not like them interpreting it for me - I, like you - would rather have imperical data.

My point about the belt tensioning is this - if you all have not picked up on it - I have seen a LOT of posts about slipping belts on certain kits (not ours) and how tight you must need to tighten that SC belt to prevent slippage. I am trying to point out that, if you want the SC to not slip, and you do not want the belt tensioned like a piano string; go back to the beginning and redesign your crank pulley in a larger diameter, thereby allowing also a larger driven pulley on the blower, and eliminate the slippage while running far less tension on the belt.

We do not run any more tension on our SC belt than say, an AC belt, and we do not have slippage.