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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 12-29-2004, 02:14 PM   #1
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max compression on pump gas?

the highest octane pump gas we have around here is 92, how much compression can i run while using 92 pump gas? i was hoping for 10.x:1 but i really have no idea, any input is appreciated
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Old 12-29-2004, 03:31 PM   #2
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10.5:1 or so is generally the safe limit, but it really depends on how big your engine is, what heads/pistons you use, and most importantly, what cam you use.
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Old 12-29-2004, 07:17 PM   #3
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Have a 1969 Chevelle with a 10.25 to 1 c.r. that won't run without major pinging on anything other than 100 octane racing gas. Probably has some serious carbon build-up which may drive the c.r. up even further.

Also, I've been told you can run a higher c.r. with aluminum heads, compared to iron, because of the heat rejection characteristics of aluminum.

As Air noted, there are some variables.

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Old 12-31-2004, 09:54 PM   #4
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Without going into a lot of what if's and so, if you want reliable street power performance on 92 octane, build your motor with just under 10:1 compression.
cast iron heads- carbed motor.
Build the motor with proper piston quench clearance (.040")

9.8:1 is perfered. about 10.2 with aluminum heads

You'll make more usable, reliable power if you don't get crazy with compression and optimize the total engine airflow, ignition timing and AFR for max output as opposed to too much compression ratio requireing retarded timing ,over camming and over rich afr's to keep it out of detonation.
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Old 01-01-2005, 01:43 AM   #5
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There's alot more too it than a simple #.

I have a iron headed (1965 327 462 heads) 355 10.18:1 compression, 217-219 psi cyl. pressure, and runs pretty as she pleases on 93 octane.

I have to limit total timing to 34*, and all in at 3000 rpm, adjustable vac. advance can


When you look at compression and fuel needs, you need to think more about dynamic compression, not static compression.

static compression (SCR)... Is based on vol. like head chamber cc, head gasket vol, bore, stroke, etc

dynamic compression (DCR).. Is above plus factors in cam size.

A bigger cam will bleed off more cyl. pressure, therefor makes dynamic compression lower, and lets you run more static compression.

To understand how cams and compression works together please read Pat Kelly's write up. Read it all, and you may have to re read it 4 or 5 times to fully understand

http://members.uia.net/pkelley2/DynamicCR.html


Also other factors comes into play... Iron vs. alum heads. Alum heads you can and need to go up close to or 1 full point in SCR.

head chamber shape.. Old style bowl chambers like mine cant take as much compression as the heart or kindy shaped chabers of alot of aftermarket and vortec heads. The better chamber will let you run more compression w/o detonation

Quench height (the height from piston at TDC **deck heiht**, to bottom of heads, counts the thickness of the gaskets plays a BIG role.. You need and want quench height in the .035"-.045" range. Going bigger (thibker head gaskets) will drop compression, but could cause even more detonation, going smaller could cause a piston to kiss a head/valve

Temps.. Hotter intake air, water, and oil temps will cause more detonation. Building a true sealed cold air set up, keeping water temps around 180*, and oil temps around 190* will let you run more compression

Total timing.. Too much will cause any engine to ping, too little with drop power off like a brick

Timing curve.. A curve that comes in too fast will cause detonation

Fueling... Running lean with cause detonation
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Old 01-01-2005, 01:43 AM
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