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Compression Ratio & Creating Piston Squish/Quench area…

Compression Ratio & Creating Piston Squish/Quench area…

Compression Ratio

I’m pondering the compression ratio for a 3.4 rebuild and would like some opinions. I plan to D.E. and time trial the car as well as street use and want it to be happy with 93 octane. From what I’ve read it looks like I’ll end up in the 10.5 to 11.3 range… But I want as much as I can get away with!

Here’s the pertinent info –
930/03 (79 SC)
74.4 mm X 98mm (3367cc)
Web “S” cam
Webbers
Headers
Twin Plug Heads
Ports cleaned up
Lightweight Flywheel
2400 Lb car
Standard gear ratios/R&P
530-700 Ft. average elevation
Not reaching WOT @ RPM’s < Peak Torque RPM can be accommodated by the right Foot…
JE Pistons
Compression ratio =


Creating Squish/Quench Area

Assuming that a close tolerance squish area will allow more compression without harmful detonation. How much more will it allow?

If the contour of the piston dome matched the cylinder head and the clearance between the two is set at let’s say .030” (deck height) it seems that would create a significant amount of squish area compared to the minimal squish area found only at the perimeter of the piston in the production configuration. This approach may require leaving some material on the dome and fly cutting it off after test assemblies to cc and zero in on the C/R. Is the gain worth the effort?

Compression ratio w/additional squish area =

Thanks
Scott
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84 Carrera Coupe M491
Old 01-11-2005, 08:41 AM
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Are you saying that with a turbo??.....I know after a pt of boost you start to get away from detn.....(quote Corky Bell)max boost.You might ask the dudes dwn at www.turbo911.com
Old 01-11-2005, 09:17 AM
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Scott,

Good set of questions.

The biggest issue with deck clearance and squish is to not leave too much space where detonation can occur. The clearance should be at its minimum in the area above the ring land and slightly increase progressively towards the center. It should never have more clearance at the periphery and then less toward the center. If you leave a “pocket” above the ring land and detonation occurs, the piston will be deformed and grab the compression ring – awful things then happen.

For several reasons you want to have the deck as close as possible; best turbulence, least unburned mixture from the periphery, highest compression ratio, among others. Of course the deck clearance is limited by mechanical and thermal issues. With care, 0.030” is a reasonable starting point, 0.035” is safer. The critical issue is there can never, ever, under any conditions be contact between the piston and head. Yes, you can creep up on very tight clearances by removing the heads every few hundred miles until you scare yourself.

With a twin plug 3367 cc the compression ratio is limited by your access to high octane fuel and temperature. High CR is great fun and makes power. You can keep the engine (heads and cylinders) cooler by using the 1.82:1 fan ratio. At DE events you can additionally use the “Rubbermaid Solution.” Of course you will need an efficient front oil cooler and large oil squirt case nozzles.
If you are willing, you can use race fuel – say 112 octane. It is expensive ($3.75-$4.50/gal) but is readily available. I have it delivered to my house and they take the empty drums. There are good, safe systems for dispensing the fuel.
If you want to be happy with 93 octane, you will need to keep the CR in the 10:1 and lower range. At 11.5:1 CR, 93 octane will damage the engine IMHO.

At the limit, you will also need to pay attention to tailoring the ignition advance to the cylinder pressure (torque) of the engine. What ignition system are you intending to use?


Tell us more about the engine, car and use. What suspension mods?

Best,
Grady
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Old 01-11-2005, 01:44 PM
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You should also note that out here on the left coast the best we can hope for is 91 octane. Having thoroughly broken all the ring lands on an M635 with 10.5:1 compression, a water cooled engine with better head and piston design, I would suggest adding a knock detector and sticking to say 9.8:1

That said, I just love the smell of race gas and the sound of 12.5:1 Porsche cylinders firing.

Last edited by snowman; 01-11-2005 at 03:37 PM..
Old 01-11-2005, 03:34 PM
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Quote:
Originally posted by snowman
That said, I just love the smell of race gas and the sound of 12.5:1 Porsche cylinders firing.
Finally, snowman is coming around!

Seriously - the issue of detonation is important with respect to your question. However, with twin-plugging, it becomes less of an issue because you have dual ignition on both sides of the piston. Single plug would be a big problem, but in this case, you're firing both sides of the combustion chamber.

Your compression ratio numbers seem reasonable for what you are looking to do. Important benchmark - the 964 engines are rated at about 10.5 - 11:1 ratio (there's some debate over what they really are). They have twin-plugs, but they also do benefit from a knock sensor, as snowman eluded to.

-Wayne
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Old 01-11-2005, 09:31 PM
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One more thing - The combustion chamber and Domes will be polished...

While searching for a picture I had previously seen of the Squish area built into (I think) Dodge's new Hemi I ran accross this.

http://www.hemidesign.com/tech.html

These people are already doing what I'm talking about by closely controling the head to dome gap...

Thanks
Scott
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Last edited by Scott Wolthuis; 01-13-2005 at 08:32 AM..
Old 01-12-2005, 09:00 AM
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No turbo here…

Ignition …
For Ignition Distribution I like the Jag 12 Plug cap used with an adapter to fit a Bosch Distributor. If the Burn Bros. are able to manufacture that rotor I’ll likely go that way. I’m also interested in what the Jaguar guys have to say about the longevity/reliability of that cap.

Adapting the 964 Distributor is probably a better option because the twin distributors eliminate the potential crossfire’s that could occur inside the 12 plug cap.

I haven’t studied the coils and such yet… any recommendations?

I was initially leaning toward the Crankfire HPV or HPX systems. Their adjustability is a real plus. I was surprised to read that the plug gap used with these systems is somewhat limited and that they didn’t produce a hot enough spark to keep the plugs clean/from fouling if you have a rich condition, which is a requirement with Webbers in the lower RPM range.

Suspension mod’s…
Not really unless you consider poly bushings all around and gas Koni’s set on firm a mod… It’s also lowered with a fair amount of negative camber. Any recommendations?

Edit: Almost forgot 22mm sway's...
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75 911S 3.0 to 3.2 now to 3.4 Twin Plug (Inwork...)
84 Carrera Coupe M491

Last edited by Scott Wolthuis; 01-14-2005 at 12:32 PM..
Old 01-13-2005, 08:35 AM
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I would limit the compression ratio to 9.5 /9.8 with a mechanical dist. set at 30 degrees MAX If you want to run 10.0 to 10.3 you can but you will have to give up some dist advance. 25-28 total depending on the advance curve.
Remember that you must also keep the head temp down to help prevent detonation at anything over 10.0 to 1.
Good luck.
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Old 01-18-2005, 09:27 AM
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