Compression or Flow?

Interesting, I am planning the same build with a T7 head on a 2.1 block NA... To answer your original question, the improvement in flow is worth way more than trying to get more flow from the 85 head.

To get the compression up, I was planning on skimming the head but I don't think you will get your money's worth from a block skimming- the pistons already come up ~flush. Might save you the difficulty of a custom head gasket though. Which is my question for you: What are you planning for the head gasket?

I would love to run trionic for my build, but I am thinking pretty hard about ITB's- I have no idea how trionic would handle the vacuum fluctuations that are typical of independent throttle bodies. Never mind how trionic would do NA in the first place. Curious that you and I are planning basically the same build!


C

HI Guys

Sorry for the delay in replying.

I've been trying to get to grips with where the performance/cost ratio is in our NA favor and which order headwork and other NA tuning should be done.

I have decided to follow the T7 head route with a big skim (despite the fitting problems) because, as discussed, I think the flow benefits are worth it compared to the significant porting requirements and valve size limits in the already high compression 85 head.

From discussions on another SAAB forums it seems compression is mandatory on any tuned NA engine - not because it gives massive power gains on an otherwise stock engine and head but it maximizes the potential of all further refinements, especially the cam.

To benefit from a sport cam, a compression ration of 10.5-1 to 11.0-1 is needed (for a fast road car, higher on a track car) with a cranking pressure of 200psi.

So, a big skim then. The block skim referred to earlier was not to raise compression but to ensure a perfect interface with the skimmed head for the ultra thin metallic head gasket. Now that a big skim (above the 0.4mm limitations set by SAAB) seems possible I won't be going down that expensive route.

Next up, the valves. These need to be as big as possible. The T5 and T7 heads have bigger valves from the B2*2 heads with room for more. The difference between the T5 and T7 heads is 1) the valves and 2) the valves seats. If you are going to fit bigger valves from the start it doesn't matter, therefore, which head you use. Bigger, flat based valves will also bump up compression a tad.

The next stage - to make the most effective use of the higher compression and larger valves is the cam. I can't afford new sport cams at this stage so I will be compromising with adjustable cam sprockets to accurately dial in the stock cams. Because of the skim, the head will be be lower than before which will affect the cam timing, so this will be needed regardless.

The economical choice with cams is (rather than dropping £600 on a pair of custom profiles) is to go for the longest duration and overlap combination available from the SAAB parts bin:

Intake:
Part # 7560808
IVO 16 degrees BTDC
IVC 56 degrees ABDC
Duration 252 degrees

Exhaust:
Part # 7509218
EVO 56 degrees BBDC
EVC 16 degrees ATDC
Duration 252 degrees

Overlap = 32 degrees.

The Intake cam (7560808 ) is on SAAB 900 T16s '86-'93. The Exhaust cam ( 7509219) is from an 84-85 T16.

This is as far as I can afford to go at this stage. I will be taking measurements and documenting the process as I go so hopefully it will provide some substance to a lot of the theoretical debate about what can be achieved on a NA engine with some dyno data.

I will be using the standard LH2.4.2 ecu from the 2.1 motor. After the dyno I will take the data to a new fuel chip and re-dyno to see any further improvements. I think the stock system will be able to cope fine with these mods.

To go any further would require specific aftermarket cams, aftermarket ECU, throttle bodies, custom plenum, bottom and top end lightening and balancing. That will take more time and much more money so we shall see how we go with the current plan.

DrewP said:

What is your timeline?

My brother and I are in the testing phase for +/- 10 degree adjustable cam sprockets with the C900 offset, they will be cheaper than the ones available from Europe too, but not finalized.

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Timing! :roll::grin:

I would be interested to know the price once things are moving. As you now know there is another source for c900 adjustable sprockets in the pipeline as well. I'm in no rush - limited by time and funds. I'd like to know how yours turn out compared to the others. I think the problem with existing adjustable sprockets is their failure rates because not enough radius is out in the designs.

Re: T5 conversion - I briefly thought about it but cannot see any real benefit at this stage. The 2.4.2 system will be perfectly adequate for more than my aims. The benefit of the T5 is a lack of AMM which would make T5 a better choice for ITBs if you didn't want a plenum fed induction.

I have no need for the bells and whistles of T5. Once all the hardware was installed on the engine I was going to get the car dyno'd and data logged and then get a custom fuel chip.

If I progress to ITBs I would move to Megaquirt or the like which runs Alpha-n so there would be no intrusive air temp sensor disturbing the inlet tracts. And I could map fuel and ignition much more easily.

Disclaimer: I am not suggesting you run T5, only discussing some points.

adamgent said:

I think the problem with existing adjustable sprockets is their failure rates because not enough radius is out in the designs.

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I haven't seen photos of any which have failed, what are some issues you have seen? The hubs tearing off, or the wheels slipping timing from just having clamping holding the halves together (this is what makes me most nervous).

adamgent said:

Re: T5 conversion - I briefly thought about it but cannot see any real benefit at this stage. The 2.4.2 system will be perfectly adequate for more than my aims. The benefit of the T5 is a lack of AMM which would make T5 a better choice for ITBs if you didn't want a plenum fed induction.

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I only slightly agree with this - I think the big benefit is having native knock and boost control reactive to the other engine parameters, and the ability to use the 4x,000 volt coil on plug packs rather than the dizzy, along with the open source way you can tweak it.

adamgent said:

I have no need for the bells and whistles of T5. Once all the hardware was installed on the engine I was going to get the car dyno'd and data logged and then get a custom fuel chip.

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Can you write the chips? Because in my experience by the time you go back and forth once or twice and for what you pay for the chip you're most of the way (monetarily) to converting it anyway, especially if you can get junkyarded parts or buy a running junk car to pull everything from.

adamgent said:

If I progress to ITBs I would move to Megaquirt or the like which runs Alpha-n so there would be no intrusive air temp sensor disturbing the inlet tracts. And I could map fuel and ignition much more easily.

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This I don't really agree with:

1) With ITB's and natural aspiration there won't be an appreciable enough difference in inlet air temp from ambient air temp to even need the temp sensor in the inlet tracts

2) Having worked with MS I would not do it again - it's too much of a headache, and has similar DIY capability of the much more sophisticated Trionic system - it's not even all that much cheaper by the time you modify the wiring to the ignition amplifier and build yourself a harness pigtail

3) Unless this car will see significant track time, alfa-N control doesn't seem like a good idea for a street car to me. Yes, fuel and timing are direct acting rather than re-acting with the speed density systems, but your altitude/humidity/temperature/warmup/ etc. compensations are all re-active, rather than active like with speed density or temp/MAP. I wouldn't be happy having to fiddle with it every season if I was going to as much trouble as you sound like you might be going to.

Have you considered a set of sidedraft carbs?

Are you going to plenum your ITB's through an airbox with sealed runners, or just leave them out in ambient?

Best,
Drew

Sprockets:

I have never seen this personally but have heard of this happening more than once with the Speedparts sprockets - it seems to be the central boss shears, probably because the corner andle between the boss and the flat has not got a wide enough radius.



ECU:

I agree with alot of what you have to say.

Thankfully I am not at this stage yet - it is a bit of a hornets nest. If I was I would box in the inlet trumpets into a cold-air fed plenum. I suppose an Air Temp sensor could sit in there and limit turbulance.

I have considered side-drafts and have a nice pair of Dellorto DHLA 45D carbs and a twin-carb 16v manifold, for when I planned on building the engine for my 99 but I had to sell that and though I would continue with it in the 900. What with spending all the money on the head and cams I can't help thinking carbs would be a step backwards in the 900.