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Old 08-11-2007, 07:56 AM   #1 (permalink)
r turner (Snj5)
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Default Here are what the JE 10.5:1 an

Here are what the JE 10.5:1 and stock US 9.2:1 static compression ratio pistons look like. There were off an existing Norwood pattern and designed after the combustion chamber was cc'd. This car has a requirement to run on street premium, so it was felt about 10.5:1 was about as high to go with some safety margin with the ported heads. I've never done this before, but I understand that raising the compression ratio really makes a noticible difference in seat of the pants driving. We'll also probably start tuning at a reduced advance of 30 degrees btdc and slowly advance for safety regarding detonation. Any other recommendtions or experience with increasing CR?
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Old 08-12-2007, 05:09 PM   #2 (permalink)
Chris Fricke (Tomdial)
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Default Going from 9.2 to 10.5 seems t

Going from 9.2 to 10.5 seems to be uncritical, even on pump gas. Just be sure only to fuel up on recognized brands stations.

For performance: from 9:1 to 10:1 gives much more than stepping from 10 to 11... your engine should feel much more lively. Expect 4-5% more HP from pistons alone.
Even as this is not of greatest interest, you MPG should increase noticeable. On most cars the exhaust note gets more aggressive too!
Some simple work on the head, a good valve job and a matched manifold can find the same power increase.

I don’t think you will have to reduce the pre ignition, just be careful the combustion chambers are well matched and have no sharp edges. Same on the pistons.

Don’t go hunting for exotic spark plugs, if we talk NGK a D8EVX should be just right, it is already very hot!

Maybe replace the ignition wires if they are old, once disturbed they tend to fail with the higher compression.

If you retard ignition, you will lose some of the power gain the higher compression gives you.

On high rpm (6000 and up) you don’t have to reduce it anyway. Normally you need 36° at the upper end. Knock (in your case) is usually a low range problem. It sure is a good idea to have an eye on it.
But be aware! Retarding 5° can cost as much as 3% in power!
Modern ignition systems are very constant and don’t change between service. So we need less build in safety margin than on a contact breaker systems or a mechanical advance mechanism.

Maybe, if you can adjust it, you have to go a little richer, but in most cases the injection will “see” the increased air flow and adapt to it.
I prefer testing the mixture while driving. INNOVATE has some very interesting instruments that work great and are simple to handle. They are cheap compared to repairing Ferrari engines!
The XD-16 + LC 1 in combination are a great product and take the guesses out of tuning your engine. Less than $ 400.

The higher CR gives more power for two main reasons:

First, the negative pressure is higher when the inlet valve opens, this brings more air in the cylinder. More mixture means more power.

Second, the pressure the same amount of mixture puts on the piston is increased, this is the reason for the reduced fuel consumption on part throttle. Imagine a fire cracker exploding in a small room compared to launching it outside. The effect on your ears equals the piston in your engine.
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Old 08-12-2007, 07:14 PM   #3 (permalink)
r turner (Snj5)
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Default Thanks! Yes - I have an Innov

Yes - I have an Innovate LM-1 I will be using to set the mixtures - it has been TERRIFIC in the past when I first went to the 40 DCNFs. This time I am going with larger inlet valve, higher lift cam (410 from stock 348), porting and a 44DCNF Weber. The porting guy checked everything on the flow bench: while peak flow increased about 17% (92CFM to about 108+ CFM), he also increased port velocity over stock similarly. I am hoping for a very sharply responding engine.

Right now the engine is happy with 34 degrees by 3200 rpm with a Unilite fired MSD. We'll see!
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Old 08-13-2007, 07:28 PM   #4 (permalink)
r turner (Snj5)
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Default For future reference, here are

For future reference, here are some measured specs off a 1988 3.2 liter US spec engine from the Mondial 3.2:

Stock pistons:
Dish Vol.- 25.6cc
Piston Deck- .020" plus (above block deck)
Gasket- 3.312" x .061" (compressed)
Bore- 3.268"
Stroke- 2.90"

My new head data is not as applicable with the larger non-stock pistons, but knowing the numbers above you can figure out what the others should be.
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