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jmp
02-07-03, 08:33 PM
I've just recently found out that the compression on my 72 LT-1 had been bumped from 9.0:1 to 11.0:1. Now while I know this is generally considered "a good thing", I don't really know the specific advantages/disadvantages to changing the compression. Here's what I (think I) know:

- to run Turbo/Super chargers, low compression is a must
- higher compression means a need for higher octane

I assume that higher compression leads to a bigger "bang", hence more power.

Well... that's about it! So, why does a higher compression, for NA cars, give better performance? Should I advance my timing more or less? Is 91 octane good enough (I haven't had a problem thus far) or should I go up to 92 or 94 (the highest pump gas that I know of, but hard to find sometimes)? Does higher compression hurt fuel economy? What about emissions?

Thx.

JohnZ
02-07-03, 08:57 PM
All other things being constant, higher compression = increased efficiency = more power = better fuel economy. However, higher compression also = higher octane requirement and higher combustion temperature, which increases NoX emissions.

Calculated compression ratios are "static"; what matters most is "dynamic" compression ratio, which brings cam timing into the picture and gets a little complicated. This is why cars with longer-duration camshafts and more overlap (like yours) are less prone to detonation than an engine with the same "calculated" compression ratio with a mild stock cam with less duration and less overlap.

If you encounter detonation ("pinging"), the treatment is higher octane fuel and/or less advance; better fuel is more expensive, and retarding timing to use pump fuel costs performance.
:beer

jmp
02-07-03, 09:01 PM
Hmm...

So it's the NoX emissions that led Chevy to reduce the compression in the early 70's?

How noticeable in pinging? How dangerous? I was planning to advance my timing as much as possible, but I don't want to go too far. Is the process as simple as advancing until detonation, then backing off a little?

JohnZ
02-08-03, 07:50 PM
Until catalytic converter technology arrived in 1975, the only way to reduce NoX was to reduce compression and introduce EGR, which recirculated some exhaust gas into the intake charge, diluting it, to reduce combustion temperature.

Detonation ("pinging") can be extremely destructive, especially to pistons and rod bearings; if you hear it, back off timing two degrees at a time until it stops or back off on your centrifugal advance curve, or both (or start blending in race gas for situations where you don't want to compromise performance).
:beer

Stallion
02-08-03, 10:00 PM
>> higher combustion temperature <<

John, just a quick question. Say you have a cold air intake, which I can't imagine would be a high temp for the combustion, what would that do to your compression to get it to work?

TR

JohnZ
02-09-03, 03:31 PM
Compression ratio is a purely mechanical issue (basic physics) and isn't affected by incoming charge temperature; cold air intake systems can produce more power because (in general) automotive engines can gain about a 1% power increase for every 10-degree reduction in intake air temperature.
:beer

Stallion
02-09-03, 11:03 PM
What kind of hardware is involved in cold air intake? Expensive? Challenging modification?

Thanks, John. :)

TR

69autoXr
02-09-03, 11:27 PM
Cold air induction systems generally are for fuel injected engines, I've never seen such a system for a carbed car. The cold air induction system usually has a long snorkel coming off the throttle body, picking up the air through a filter mounted in front of the radiator, or next to it, but in general, away from the hot engine compartment.

Pobably the best induction setup for a carbureted car is a cowl induction system like an L-88 has, though I think some of the later Sharks had cowl induction hoods with the L-82. I'm not sure though (maybe someone else can confirm that). Cowl induction systems feature a hood with an opening at the rear, picking up air at the base of the windshield, where the air pressure is higher. The hood seals around the carburetor, so that the carb draws air only from the cowl area.

Ken
02-09-03, 11:55 PM
Originally posted by 69autoXr
... I've never seen such a system for a carbed car.

Well, there was the Ramcharger cold-air induction system, which was standard on Hemi cars; the 1965-launched Oldsmobile 400ci rocket powered V8 with a unique cold air induction system; the Buick-released the Stage 1 performance package, which included cold-air-induction; the cold air induction format for the Challenger was the Shaker hood scoop; the Firebird called theirs Ram Air, the factor being cold air induction; Chevrolet used a cowl induction system to draw air in cold air in from windshield area; and the list goes on... ;)

_ken :w

Tom73
02-10-03, 08:25 AM
Originally posted by 69autoXr
I've never seen such a system for a carbed car.

Pobably the best induction setup for a carbureted car is a cowl induction system like an L-88 has, though I think some of the later Sharks had cowl induction hoods with the L-82.
The 73 through 75 Vettes (all of them) had the cowl induction hoods. Starting in 76 I believe that the L82's had the snorkel type air cleaner assemblies.

tom...