Aerocharger turbo installed (& distributor question)

[b930]

About 2 months back I started the installation of the Aerocharger kit on my 88 930.

Center-Mount Variable Turbo Conversions are here...

So after breaking off a few exhaust studs, realizing I had a broken #1 lower head stud and getting pissed off with a few pesky oil leaks I decided to tear the heads off, re-ring, re-seal, etc. Typical 'while I was in there' story really.

I did the build keeping the block in the car, pulling heads, cylinders and pistons out and rebuilding etc in situ with the motor lowered and resting on a stand. Went well - got the heads all refurb'd with time certs on all exhaust side, new guides, new performance springs, 3-angle seat job etc. Cylinders were treated to scotch brite and new Goetz rings. While I was in I threw in SC cams, timed to 1.55 mm (Left) and 1.57 (right). I had to juggle relative chain/gear position a few times but happy with how close they both timed in the end.

With the Aerocharger having a self contained lube system, I also dumped the scavenge pump and lines and also ditched the smog pump and injection system (heads were plugged up during the head work). I used a block off plate on the back of the left cam tower to seal that off. Also, I used a blind aeroquip banjo fitting at the pressure switch so I could ditch the turbo supply line. All this greatly cleaned up the engine bay.

The Aerocharger is a superbly made kit with comprehensive instructions and all parts tagged etc. The only major complaint was my kit had heat and they obviously designed it around a case who's left side flange did not extend beyond on the lower rear end like it does on mine so it needed modifying. I had a shop weld in the clothes iron shaped recess you can see in the picture and it just fits now. It was also a pain to fit and needed quite a bit of force on #3 and #4 header pipes to align them and then put the studs through. No way I could get it over pre-installed studs, but then again it is a one piece design. Here the mods I had to do to it:


The performance: It just feels massively different and so responsive. Boost just rolls in at part throttle in 1st and 2nd driving around town. At anything over 2,500 rpm its got a huge amount of throttle response, the boost roll on and recovery is so fast its a little freaky. It appears to work very well with the SC cams. I ran a chipped 996TT with K16's for a few years and this is more responsive.

The separation of the oil system makes the car run cooler, there is no worrying about heat soak either on shut down. I'm still chasing a few boost leaks that there there before the rebuild and I only have 100 miles on it so far. Have been rolling into the boost to seat the rings, and looks like they are quite happy already - no smoke, or other misgivings yet, no oil consumption to date. Coming months will be interesting as I'll be running 1 bar when its broken in. I have an Andial I/C on at the moment also.

The sound is unique also, the small opening of the vanes makes it quite nice and mellow off boost, still very angry sounding though, on boost it screams nicely. I like the sound a lot.
I'll report back when i get it all broken in and crank the boost up, but so far its impressive.

Off topic question for you all: Another reason I have been laying off the boost is that my distributor diaphragm is not holding vacuum on one side, the side that goes directly to the throttle body. Its a 930/66 with the diaphragm that has two ports, one red line that goes from the front port to the TB via the thermoswtich and one blue line that goes straight to the TB from the rear port. The red side holds a vacuum with a mityvac but the blue (back) side does not. I am worried about not getting the retard under boost and blowing the motor up. Anyone got ideas on troubleshooting this, are the diaphragms rebuildable ? It does add mechanical advance when it revs so the weights are probably ok.

Cheers

[vas930]

AWESOME!
Been waiting for someone to do this.
Look forward to seeing what you think of it after some tme.

Sorry cant help with the dizy.

[spuggy]

Originally Posted by b930

About 2 months back I started the installation of the Aerocharger kit on my 88 930.

Cool!

Originally Posted by b930

Off topic question for you all: Another reason I have been laying off the boost is that my distributor diaphragm is not holding vacuum on one side, the side that goes directly to the throttle body. Its a 930/66 with the diaphragm that has two ports, one red line that goes from the front port to the TB via the thermoswtich and one blue line that goes straight to the TB from the rear port. The red side holds a vacuum with a mityvac but the blue (back) side does not. I am worried about not getting the retard under boost and blowing the motor up.

The red side sounds like the emissions-thingy that manipulates the timing off-idle (once the car is warmed up) as soon as the RPMs rise on the 930/66 and 930/68 motor types (and the early CA-only cars).

Which would make the blue one the same as the single-hose on the non-CA/Japan early (pre-1986) 3.3 cars.

There is no boost retard. Not on any 930.

The blue hose imparts around 10 degrees of extra advance (I never measured mine, but a guy on here years ago put his on a dizzy dyno with a MityVac) when you have vacuum in the manifold, which (semi-obviously) bleeds off as boost pressure rises in the manifold (and thus the advance pot). There is no change to the timing as the pot "sees" boost.

So at 0 PSI relative in the manifold (aka 14.7 manifold absolute pressure - or atmospheric) there is no vacuum advance left, only the mechanical advance. Which, as there is no retard from positive pressure in the pot, is the timing you set with the vacuum hose disconnected @4,000 RPM - and this is the timing you keep from 0 PSI to full boost.

So don't worry about having lost "boost retard" - you never had any. If the timing is set correctly/per the book, your full boost timing figures won't be affected or changed in any way.

What I think you should worry about is, if the vacuum pot doesn't advance the timing plate, you've lost 10 degrees or so of vacuum advance before boost builds or RPMs rise enough to engage the mechanical advance.

This would show up in low to mid-range throttle response - like from 2,000 RPM or so when you open the throttle, but without enough load/RPM to get boost.

I might have a spare two-port pot on the dizzy that decided to jam the mechanical advance wide open...

[b930]

Great info, thanks. I'll see if I can get my head around it. I always thought boost retarded the mechanical advance so it would fail dangerous if you know what I mean. Since they run around 18degrees on boost and timing is set at 29degrees at 4000rpm with no hoses, i figured there must be something boost related pulling out the 10 odd degrees. The fact it does not hold pressure though probably means it's fk'd though anyway right?

[rsscotty]

But there is boost retard if everything is working right. I've seen it live on the dyno.
Read this link for more info.

Ultimate 930 Distributor, advance, retard, timing, Turbo lag, MSD, mod, thread.

[spuggy]

Originally Posted by rsscotty

But there is boost retard if everything is working right. I've seen it live on the dyno.

That thread was started 3 years after I bought a 930/68 distributor for my 930/60 under the impression that the two-port pot provided boost retard.

I now don't believe that is true.

My 930/60 distributor (which certainly doesn't provide boost retard according to Brian (RarlyL8) in post #39 in that thread) works great set to 32 degrees @ 4000 with vac disconnected (e.g. mech advance only), using the boost retard feature on a J&S to dial it back to 29 degrees at full boost.

The 930/68 distributor I recently removed would probably have behaved differently on the dyno/track if it pulled any timing on boost. Other than the softer advance curve on the US springs below 4,000 RPM, they seem to behave identically.

[mark houghton]

Kids, I don't know that I have the energy to read that entire thread on timing again....having followed it back when we first attempted to hash this out. But I did peruse just enough to find one comment by Speed Squirrel that I felt pretty much describes what's happening timing-wise. To-wit:

From the manual, the 1978 California cars have the initial timing set at 31 degrees and 4000 rpm. From the charts in the manual, at 4000 rpm the centrifugal advance is 16. The vacuum advance is at 10, so the total from the distributor is 26. 31 - 26 = 5, which is the static timing. At 5000 rpm and full throttle, the charts indicate a centrifugal advance of 16, and a pressure retard of 6. 16 + 5 - 6 = 15 degrees advance at full throttle.

For Euro cars, the initial timing set at 0 degrees and 1000 rpm. From the charts in the manual, at 1000 rpm the centrifugal advance is 0. I think that the vacuum advance is also 0 because the vacuum port is above the throttle plate at 1000 rpm. So the static timing is 0 for Euro cars.

At full throttle the charts indicate 11 degrees of centrifugal advance, and 0 vacuum advance, so 11 + 0 - 0 = 11 degrees advance at full throttle.

Now, you gotta read this and re-read it more than once to plant in your mind what's going on in our cars. The term "pressure retard" is the issue and if for sake of discussion you disregard the mechanical advance then it would appear that timing (independent of mechanical) swings from 10 degrees advanced due to vacuum, to 6 degrees retarded (in other words, a 16 degree swing) when the vacuum is gone and pressure takes it's place. That would lead one to believe that (1) when vacuum is absent, you lose 10 degrees of advance and (2) when boost pressure is present you lose an additional 6 degrees....ending up with something in the neighborhood of 15 to 16 degrees BTDC at full boost.

Dependent on your car, even though the manuals state somewhere between 26 to 29 degrees BTDC at 4000 rpm, that does not mean that all that timing will be present when on full boost. Of course, that's just my interpretation and I could be wetter than a hunting dog retrieving a dead duck...but I think we would be seeing a lot of blown engines with that kind of timing on-boost. The only way to answer this to everyone's confidence is to see a dyno chart that shows timing in response to boost and not just rpms.

So, we've done gone and hijacked another fine thread! B930, thanks for sharing your experience with the Aerocharger. You've got a lot of listeners at this stage.

[jcc911]

Man, I wish some one draw this up on a chalkboard for me, there's something I missing that I am not picking up of this.

[b930]

Thanks all. I should have probably started another thread. I too am struggling to understand this. Where I struggle is if my car is set to 29degrees at 4000rpm with no hoses, the only way to dizzy will know there is a boost situation is a pressure reference. So to run less timing at 4000 under boost there must be some pressure related retard happening. I can not recreate this with a pressure source connected to either side of the diaphragm at idle so how do I know it works under load (without obviously putting on a dyno).

Think I'll just lock the dizzy down and use an MSD 6AL box as I will be more confortable having no doubts about knowing how it works.

[jcc911]

I keep getting the advance vs retard terminology mixed up.
Why do you want to retard or take out timing when on boost?

[mark houghton]

Originally Posted by jcc911

I keep getting the advance vs retard terminology mixed up.
Why do you want to retard or take out timing when on boost?

This is where the auto engineers can really shine with an answer (too bad I aint one!)

It's something like this: Even though our engines are really quite low compression around 7.5:1, that's intentional because when you stuff damn near 13 pounds of air in there (or more) with a turbo, you are affectively changing your compression to something like 14:1. At those numbers we're really pushing the envelop to get serious pre-ignition and piston damage. So, you want to back the timing off as a preventative measure when on boost.

That's a really poor explanation. Someone else will chime in.

[jcc911]

That's what I don't get when you say "back off" do you mean the plug will fire closer to tdc or would you want it to fire sooner when on boost?
That's what I get mixed up, all measurements on our cars are btdc, correct? So is that is the case, would advancing the ignition with respect to btdc cause the plug to fire earlier on the compression stroke or is it the opposite?
Like I said, I work better with pics and diagrams...
Thanks.

[jcc911]

Ps sorry asking in this thread.

[slow&rusty]

Congrats..very nice install and looking forward to more feedback as your get your issues ironed out and put more miles on the car.

[b930]

Thankyou. MSD6AL-2 has been ordered. I'll be using a MAP sensor to pull out timing on boost and should be able to create a timing curve to compliement the fast spooling characteristics nicely.

Also have just got the seals for the stock recirculation valve which I think is leaking, and will be putting in the 1 bar spring into the aerocharger vane actuator (not really a wastegate as such; I have no wastegate any more!). Looking forward to shooting some video for you all when its running right.

[b930]

Well I think I got to the bottom of the timing problem. The vacuum can is full of rust scale. The actuator rod to the distributor is completely rusted through leaving one end attached to the pin inside but physically separate from the diaphragm. The distributor pick up coil was just flopping around. Pretty dangerous situation as you could set the timing to 29degree at 4k and that's where it would stay regardless of boost as the vacuum was having no effect at all, and essentially not connected to the dizzy. Makes sense now how sometimes it would run hard and then go all soft (coil flopping from one side to the other, i.e across the range of the vacuum adjustment). It was, essentially running way too much timing on boost also.

Will pin it all up. Just finishing installing a MSD digital 6AL2 which will now assume timing duty. Re-circulation valve is also now rebuilt so should be up and running soon.

[fredmeister]

Originally Posted by mark houghton

This is where the auto engineers can really shine with an answer (too bad I aint one!)

It's something like this: Even though our engines are really quite low compression around 7.5:1, that's intentional because when you stuff damn near 13 pounds of air in there (or more) with a turbo, you are affectively changing your compression to something like 14:1. At those numbers we're really pushing the envelop to get serious pre-ignition and piston damage. So, you want to back the timing off as a preventative measure when on boost.

That's a really poor explanation. Someone else will chime in.

Take some credit for yourself, that explanation is fine.
What you are referring to is the difference between static compression ratio and dynamic compression ratio. Dynamic ratio being the ratio when the engine is running with boost in the manifold. Thats the number we are all concerned with.
The chart that surfaces on the posts here once in a while that Metzger put together for Porsche back in the day shows the effects of various amounts of boost pressure on the dynamic compression ratio when you know the static compression ratio of your particular engine build.

Fred

[jcc911]

At those numbers we're really pushing the envelop to get serious pre-ignition and piston damage. So, you want to back the timing off as a preventative measure when on boost.

And thats the part I do not understand. If the high dynamic CR leads to a higher potential of preignition, why would you want the ignition to retard on boost?

I still think I am missing something because if the higher CR and pre ignition causes combustion earlier on the up stroke wouldnt you want the plug to fire then before the pre ignition occurs? Wouldnt then combustion have already occurred if the ignition has been retarded?
I really am a novice in understanding ignition timing. TIA for the lesson.

[Flieger]

The boosted mixture burns faster, so they retard the timing so that the peak cylinder pressue occurs a few degrees after tdc. I forget what the optimum position is for maximum power over the whole stroke. I think it is about 20 degrees.

[mark houghton]

Originally Posted by Flieger

The boosted mixture burns faster, so they retard the timing so that the peak cylinder pressue occurs a few degrees after tdc. I forget what the optimum position is for maximum power over the whole stroke. I think it is about 20 degrees.

You sure wouldn't want your peak pressure to occur before TDC (firing that charge too soon with too much advance would be like fighting an uphill battle with your piston still trying to go up while the combustion pressure trys to force it down).

Great description of the relationship of ignition retarding vs. when the peak cylinder pressure occurs. Peak pressure may indeed occur after the piston is on it's downward post-TDC stroke, and some of the ignition timing advance is removed to cause the peak fuel burning force to occur after TDC, but the actual ignition timing is still somewhere before TDC...though reduced (retarded) somewhat at boost from where it was prior to boost.

I'm a visual kind of person. Visualize a piston going up toward top-dead, compressing a mixture of boosted air and fuel, and the ignition spark happening before the piston reaches the top of it's stroke. BAM! - ignition occurs around 16 degrees of crank rotation before TDC but it takes a few nanoseconds for the force of the explosion to peak...and during that time the piston continues on it's upward travel - passes TDC - and begins it's downward travel. It's all tied together with the speed the piston is traveling relative to it's position in the cylinder.