Highest comp ratio for pump gas
Thread Starter
Instructor
Joined: Nov 2003
Posts: 177
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From: Rockingham NC
Highest comp ratio for pump gas
I am getting ready to order new pistons for freshly bored block(350). I want it as close to the 350/350 as possible. The machine shop is telling me i have to run a dish piston to be able to run pump gas. Is this true????
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Re: Highest comp ratio for pump gas (brs6482)
What CR you can run on pump gas depends on many things. For instance, aluminum heads lets you run more compression. Longer duration camshaft also lets you run more.
With iron heads, and a 350/350 cam I'll take a stab and say you probably want to limit it to the upper 9's.
With iron heads, and a 350/350 cam I'll take a stab and say you probably want to limit it to the upper 9's.
Team Owner
Joined: Feb 1999
Posts: 34,454
Likes: 665
From: Republic of Texas
Re: Highest comp ratio for pump gas (zwede)
What CR you can run on pump gas depends on many things. For instance, aluminum heads lets you run more compression. Longer duration camshaft also lets you run more.
With iron heads, and a 350/350 cam I'll take a stab and say you probably want to limit it to the upper 9's.
With iron heads, and a 350/350 cam I'll take a stab and say you probably want to limit it to the upper 9's.
Melting Slicks
Joined: Apr 2000
Posts: 3,286
Likes: 8
From: San Jose California
Re: Highest comp ratio for pump gas (brs6482)
The L46 or the early LT1 will run on pump gas with factory 11:1 pistons (which are really only 10.5:1). If you use a 180 thermostat, keep the fuel mixture at 14:1 or lower (13.5:1 to be safe), and are not driving under extended high load situations like racing on a road coarse it will work fine. If you let carbon build up, you will acquire a detonation problem as the carbon will start to glow like little charcoal briquettes firing the a/f mixture long before the spark plug is allowed to do so. A few full throttle passes every now and then (called an Italian tune-up) will prevent this. I've driven tens of thousands of miles with both configurations. I miss the "bite" you get from a high compression engine. For this reason, the L82 never measured up for me. The L82 is a low compression version of the L46. Remember that the L82 was designed to run on 87-89 octane gas. Fortunately the gas is considerably better now than it was when the first un-leaded stuff came on the market during those dark years in the '70's. The unleaded gas also makes less combustion chamber buildup. Leaded engines were designed to anticipate this buildup which is why they called 10.5:1 pistons 11:1.
Chuck
Chuck
Race Director
Joined: Mar 1999
Posts: 10,258
Likes: 1
From: Calif.
Re: Highest comp ratio for pump gas (brs6482)
Another thing to consider is the engine's "quench." That is, the distance the piston is in the cylinder at TDC. You also have to figure in your compressed head gasket thickness. Accorgint to Lingenfelter and others, you want this distance to be between .040" and .045".
For example, if your block is decked so your pistons are .005" down in the cylinder at TDC and your head gasket's compressed thickness is the typical .039" your total quench distance would be .044". That's in the ballpark.
But, like others have said, you also have to consider your cam specs, etc.
It's possible to run 10:1 with iron heads and premium pump gas if you build correctly. I've done it. I suppose you could go to 11:1 with aluminum heads since they'll typically allow about 1 point more than iron.
For example, if your block is decked so your pistons are .005" down in the cylinder at TDC and your head gasket's compressed thickness is the typical .039" your total quench distance would be .044". That's in the ballpark.
But, like others have said, you also have to consider your cam specs, etc.
It's possible to run 10:1 with iron heads and premium pump gas if you build correctly. I've done it. I suppose you could go to 11:1 with aluminum heads since they'll typically allow about 1 point more than iron.
C6 the C5 of tomorrow
Joined: Nov 2000
Posts: 6,665
Likes: 2
From: Twin Cities Minnesota
Re: Highest comp ratio for pump gas (The Dude)
I'd run nor more than 10:1 on Alum heads 9:1 on ironheads. the performance advantade of the extra 1/2 point of compression is almost 0 and being able to run regular gas is priceless!
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