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Discussion Starter · #1 ·
I am replacing my transmission and I am building an engine to replace the old one. I have a few non turbo 2.0 engine blocks a set of 85 turbo pistons, an 85 head, 2.1 head, and an 86' head with oil feed tubes. I think I just about have all the 16 valve cams possible from 85'-93' except the 7560808 Intake cam. I can't decide what direction to go with this car.

I am very close to using the 85 turbo pistons and 2.1 head (with a slight head skim to remove imperfections on the deck). With the low compression 85 turbo pistons, and lower compression 2.1 head will this make the car dismal to drive on a daily basis? Does anybody know what compression ratio this would likely have? I only have the stock T3 and Im not sure when my finances will allow for a turbo upgrade. I would probably just run really high boost through the T3. I'm a little worried about the transmission though as I have been through 3 in 2 years.

or

My other thought is to use one of the non turbo 2.0 blocks and the 2.1 head and have a high compression turbo engine running about 10 psi. Has anyone tried a setup like this? I think this might make the car more enjoyable on a daily basis, but I have a feeling I will be craving for more power within the first 3 days.

Any thoughts?
 

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wow - flash from the past! Sounds like my old plans I had for my 86 SPG :)

I like the pre-APC pistons - 8.5:1 compression is a turbo car's dream - but getting it tuned to run best both for daily use and track use would be key. Are you selling the 85' Turbo Exhaust cam? I'm all out and would love to have another laying around on hand...

A stock T3 can pull up to 18PSI steady and last a reasonable amount of time. The later TE05 turbos found on early 90's c900's will give you must faster tranny blowing spool - but the T3 is a more ballanced turbo IMHO. If you want more out of it - I'd suggest sourcing a T3/T4 hybrid or other.

The trans should be a well built 5 speed (unless you have a nice 4 speed to build) Get the thing gone over and ensure it's running the best gear lube possible in it. DeLorean, PaulH, 90SPG and many other's here can tell you more about c900's and their transmission faults/improvements.

Use the Turbo 2.0 block - a non turbo block makes no sense. The 2.1 head is a nice upgrade, but if you wanted to fool with compression you could have it milled a little to bump the dropped ratio up a little more. I assume the 2.1 intake will be used as well.

If you stay with the stock T3 for now - run the base boost @ 7 PSI and get the APC box set to handle 16-18PSI (more then a red box) and that car would fly.
 

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I am not sure what compression you would end up with an 85 block & a 2.1 head, however it would be lower, that's for sure. If you are used to the way it drives with the standard 85 engine, it might feel worse with the lower compression... however, you will have an engine that will be able to boost higher without knock, so you would have to do that ;) If you are doing other modifications, like some sort of proper ignition control, this would easily make up for the lower compression at low RPM's

I find with my standard engine in the 86 900-T notchback, it's a really nice "all-around" driving car. It is not the fastest, but it has reasonable low RPM power, and good around-town drivability. You might lose this if you drop the compression and change nothing else. I would say you need the extra boost & timing control to make things right again if you go the 2.1 route.

As for using a non turbo bottom end, I would not. The Pistons are just not as good.
 

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Discussion Starter · #4 ·
BurnsSide42 Im keeping the 85 exhaust cam as I will definitely be using it either on this car or another.

I've already been running 16 psi with a modded APC and I wasn't getting knock. I tuned an eprom for it and I am using a 3 bar FPR and the Red injectors. The power has not been that impressive with the current engine. It was wimpy off boost, and not much better until about 12psi and then it picked up a bit. I'm thinking the engine is a bit tired, which is why I am rebuilding a spare motor to drop in. The current motor is not the original motor so it does not have the low compression pistons or 85 head. When I was running on megasquirt I was getting boost spikes of over 20 psi without knock. Im trying to work out a crank trigger setup so I can try out megasquirt with the 4 bar map sensor. Now if I was running 15+ psi before with no knock, what should I push it to now?

I have a low mileage 89 transmission ready to drop in, with Redline Lightweght shockproof gear oil, and a saabrally steel diff cover.

The 2.1 head is getting milled slightly and it looks like I will be using that head and the 2.1 intake manifold. I hope to get the crank trigger set up soon, and have megasquirt running with wasted spark but Im not sure when that will be ready. If I can't make megasquirt reliable I will switch to T5.

Would lightening the flywheel help with the low end or will it make it worse?

How can I be certain that these are the 8.5:1 compression pistons? I got them off of craigslist and don't have a standard turbo piston to compare to.
 

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APC pistons with a 2.1 head and negligible amount taken off the deck and cyl. head surface should be around 8.25:1.

Non-APC and APC turbo pistons are both dished, and have about the same diameter dish (I think), but on the early non-APC ones the dish is deeper, the difference is visible if you have two side-by-side.

On the APC piston from my '85 SPG I have on my desk the dish is ~63.5mm diameter at crown height, and is ~5.0mm deep.

Best,
Drew
 

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APC pistons with a 2.1 head and negligible amount taken off the deck and cyl. head surface should be around 8.25:1.

Best,
Drew
No -
stock T16 APC cars were 9.5:1 Compression
Stock Pre-APC Turbo cars were 7.2:1 Comp

A set of Pre-APC Turbo pistons mated to a stock 2.1 head will make 8.5:1

If the head is milled - it is creating MORE compression as it reduces the chamber volume. If a milled head made less comp there would be other issues or variences. I would suspect more like 8.75:1 by your math - all depending on how much the 2.1 head was milled.

8.5:1 is turbo weather waiting to happend so - more boost the better. You would want low end spool up and a turbo which could keep spooling til the end of the power band. This would take the real research. I would guess a T3/T4 would be a basic starting point.
 

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My service manual says '85+ 16V turbo cars were 9.0 : 1, ???

You're right about cutting, I wasn't clear - if you don't cut, I was under the impression it'd drop it to about 8 1/4 from the extra volume in the chamber. The head and deck on my engine were both surfaced, but I never actually measured it out.

I think the turbo chambers are about 42cc for '85 only, 46cc for '86+ and the 2.1/2.3 chambers are 48cc's from this SaabNet post from Jon1

****************

And now, for some maths:

I measure the dish on my '85 16V piston at 63.5mm upper diameter, 47mm lower diameter, 4.9mm deep.

Approximate the volume as a truncated cone and compute as:

V=1/3*pi*(RB^2+RB*RT+RT^2)*h

with:

RB = radius of base = 3.17 cm
RT = rad54s of top = 2.35 cm
h = height = 0.49 cm

Gives dish volume approx. = 11.8 cc.

Volume of head gasket area - I measured a used head gasket at ~1.25mm thick crushed at the metal sealing ring gives volume:

V = 1/3*pi*R2*h

V_gasket = 2.65 cc

Assume 42 cc volume of combustion chamber (for my '85)

Swept displacement for one cylinder for 78mm stroke and 90 mm bore by:

V = 1/3*pi*R^2*h

with:

R = 4.5 cm
h = 7.8 cm

= 496 cc (which we already knew from 1985 cc / 4-cylinders, which checks)

Compression ratio = V_swept / V_combustion

with V_combustion = V_dish + V_gasket + V_chamber

V_swept = 496cc
V_combustion = 11.8 + 2.65 + 42 = 56.45

so:

Ratio = 496 / 56.45 = 8.8 : 1

(I think the error is coming from two places - the slight extra distance from the deck to the crown of the piston, which I can't measure right now, and the head gasket thickness since I measured an un-clamped gasket, if the crushed height of the gasket installed is actually 1.0 mm[/b] then the ratio goes to :

496 / 11.8 + 2.10 + 42 = 8.9:1, closer so I think the extra little bit is from the deck to crown distance.

With the 48cc 2.1L chamber the numbers go to (with 1.0mm crushed gasket height) =

496 / 11.8 + 2.10 + 48 ---> 496 / 61.9

= 8.0:1

Sooooo....... very interesting.

I am gonna have to measure what the deck - crown distance is, but I don't have a B202 handy to measure. I can go measure what the different dish heights on the '86+ pistons are, I have an '87 taken apart outside, but gotta run on an errand first.

Best,
Drew
 

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Discussion Starter · #10 ·
Got the 2.1 head milled today. The deck had some pitting and they took 8 thousands off.
 

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Discussion Starter · #11 · (Edited)
I was starting to assemble the engine but I realized I am missing a connecting rod nut. I thought I could use one from another one of my engines but the threads are different. The one I am missing is a 6 point nut with coarse threads (1.5 thread pitch) . Does anyone have a spare?
 

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just out of curiosity, has anybody ever thought about using a T7 rod with an upgraded piston?? i feel like the T7 parts are a little more rev happy.
I would go T5 honestly - the T7 bits like to fail under any real power... like holes in pistons... and warped or melted pistons. You don't see this too often in a nice T5 engine.
 

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I'm gonna do a mish-mash of T7 rods and pistons (or 2.3 N/A pistons, or something) in an N/A engine for the race car.

After seeing the T7 stuff we have stockpiled here at the shop I got all kinds of ideas.

The T7 rods really are like 2/3rds the weight of the C900 rods and take the same bearing shells, so it makes a lot of sense if you can get them free or cheap.

I will write up what combos fit when I get to it - I am thinking I may have to have the deck cut down some to get the C.R. where I want it, but it's a cheap drop-in.
 

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Discussion Starter · #16 ·
Just using the stock rods that came with the 85 pistons.

 

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The rods out of a '95 900 SE 2.0L are considerably longer than stock c900 2.0L rods, so I'd think that any t5 or t7 rods will be too long. IIRC, the '85 rods are some of the lightest c900 rods. I've been working on fitting the pistons from a '95 900 SE 2.0L onto the rods from my '85 c900 with good success so far. I haven't calculated the compression ratio yet though, so I'll try to remember to update with that later. Just by looking in person I had to wonder if the compression might actually be about the same as the '85 pistons provide. There's quite a lot of metal removed from the crown of the '95 pistons. Here's some photos comparing an '85 c900T piston to a '95 900 SE Turbo piston:





 

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Discussion Starter · #19 ·
Any idea when Saab switched the threads on the connecting rod bolts?
 

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Any idea when Saab switched the threads on the connecting rod bolts?
Possibly in '88 or '89? I've run into a different set on an SPG that was built in '88, but I can't say if it was technically a late '88 or early '89. The nut itself was quite different and I hadn't brought the right socket. I didn't notice if the tread was different specifically, but the rest was different enough that I'm sure it probably was as well.
 
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