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Mookster's Avatar
Join Date: Jun 2003
Location: Tampa, FL
Posts: 343
Compression ratio questions

I have Wayne's rebuild book and looking at page 113 on Wayne's engine picks. I am no MIT graduate but just trying to understand some of the fundamentals of gaining hp without breaking the bank when the time comes to rebuild/modify the motor.

I have a 1983SC with a 3.0L USA engine with 9.3:1 CR based on the spec book. The euro version of the same motor has a CR of 9.8:1

First, what do the CR ratio numbers mean?

Hypothetical, if I were to rebuild my motor with euro P&Cs what additional modifications to the motor, ignition, and fuel system would be needed? In this hypothetical can I use my current CIS fuel system, cams and heads effectively.

For kicks and giggles (hypothetical), if the compression ratio were increased to 10.1-10.3:1 either by displacement of 3.2L P&Cs or custom JEs. Would this work using the normal CIS system, cams, heads, 92 octane gas and valves? Mods/parts needed.

The reason I ask these questions is I see some of the parts cost differ for small dollar amounts. If you are going to drop this kind of coin on some of these bolt on parts the logic seems, at least on the surface, seems to upgrade to more displacement or higher C/R. My goal is to make a reliable inspired motor without breaking the bank.

All answeres and explainations are greatly appreciated in advance.
If it has a skirt or tires, it WILL cost you money!
Old 01-17-2005, 05:06 PM
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dweymer's Avatar
Join Date: Nov 2004
Location: Tucson, AZ
Posts: 2,356

I am not a pro by any sense of the imagination but IMHO
CIS does not care about CR, it supplies fuel based on air flow. I would think the ignition timing curve would be the same as current also. I don't know if 92 octane would be enough for 10.3:1, or if there would be any valve clearance issues.

Currently Porsche-less.....
Old 01-17-2005, 06:44 PM
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D troop 3/5 CAV ,162 AHC
afterburn 549's Avatar
Join Date: Jul 2003
Location: Army, Retrograded
Posts: 12,772
Generally with todays you bump c/r pts. up you move your powerband up as this somewhat helps the ping issue.....and there are a hundred other things too...... a bumpier cam ( more overlap) helps as well as other stuff.
Old 01-17-2005, 07:53 PM
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