L31 crate engine on regular gas? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**bhap** · 10-01-2022, 10:22 PM

I put an L31 crate engine into my 62 C10 about 800 miles ago. It has an HEI distributor, aluminum intake and a Quadrajet. I’ve been running premium since it was a brand new engine, but the other morning I filled it with regular, kind of by accident. When I turned the truck off when I got home, it dieseled for a few seconds after I shut it off. Everything I’ve read says an L31 should be fine on regular, but originally the L31’s were fuel injected. I think the engine has a compression ratio of 9.1 to 1. Should I be able to run 87 octane in this engine? If so, what can I do to stop the engine from running on? My idle is about 700-750. Fortunately it is a manual trans so I can stop it by letting out the clutch.
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memo43 · 10-01-2022, 10:56 PM

I'm running an L31 in my 85 K20 although mine has the Summit Racing Max EFI system. I'm running 87 in mine as well with no issues.
Engine dieseling is usually a result of a few things.
The fuel/air mixture could be heated to the point that it self-ignites, causing the diesel run-on effect. 4) Timing is out. The timing itself won't cause the run on as the ignition is switched off when the run in occurs. However timing too far advanced can cause an overheating engine which can attribute to over-run.

I would verify that everything is in proper order, even though its a new engine, something might have gotten loose (dist hold down).

Warrens69GMC · 10-01-2022, 11:20 PM

You could try to back off a few degrees and see if it continues.

geezer#99 · 10-02-2022, 12:26 AM

Or you could advance the timing or hook the vac pot up to manifold vacuum.
Either one will increase the idle which will let you back off on the idle screw and close off the primary throttle more.
No air sneaking past the throttle plates will stop the air and fuel being drawn into the intake.

As for the question of octane rating, I ran regular gas in both my 70 chevelles. The 11 to 1 LS6 and the 10.25 to 1 L34. Never had a problem but that coud be due to the elevation I lived at. 3500 feet.

**truckster** · 10-02-2022, 03:11 PM

I run a rebuilt L31 with approximately 9.7:1 compression, a little hotter cam, Quadrajet, and HEI. I've had no problem running 87 octane.

**tdangle** · 10-04-2022, 12:36 PM

I have a L31 variant from GM. The SP350/357 which is basically a L31 with a hotter cam. The instructions said to run premium. Wonder why, I might try regular and see if it works OK.

geezer#99 · 10-04-2022, 01:00 PM

Quote:

Originally Posted by tdangle

I have a L31 variant from GM. The SP350/357 which is basically a L31 with a hotter cam. The instructions said to run premium. Wonder why, I might try regular and see if it works OK.

The instructions likely also said to use 10 degrees initial timing and no vacuum advance and be installed by a certified technician.
Just a few of the ways the bean counters can void your warranty if you have a claim.

Tom · 10-04-2022, 05:42 PM

Its 9.4:1 compression to be exact, and it runs fine on 87. You need to adjust the carb and/or timing.

Bigdav160 · 10-05-2022, 07:27 AM

Too much throttle opening will be pulling fuel through the main metering circuit of the carb and contribute to run on.

Make sure your idle is set correctly

10-01-2022, 10:22 PM	#1
bhap Senior Member Join Date: Mar 2006 Location: Pittsburgh, PA Posts: 240	L31 crate engine on regular gas? I put an L31 crate engine into my 62 C10 about 800 miles ago. It has an HEI distributor, aluminum intake and a Quadrajet. I’ve been running premium since it was a brand new engine, but the other morning I filled it with regular, kind of by accident. When I turned the truck off when I got home, it dieseled for a few seconds after I shut it off. Everything I’ve read says an L31 should be fine on regular, but originally the L31’s were fuel injected. I think the engine has a compression ratio of 9.1 to 1. Should I be able to run 87 octane in this engine? If so, what can I do to stop the engine from running on? My idle is about 700-750. Fortunately it is a manual trans so I can stop it by letting out the clutch. Posted via Mobile Device

10-01-2022, 10:56 PM	#2
memo43 Registered User Join Date: Nov 2007 Location: Gardnerville NV Posts: 220	Re: L31 crate engine on regular gas? I'm running an L31 in my 85 K20 although mine has the Summit Racing Max EFI system. I'm running 87 in mine as well with no issues. Engine dieseling is usually a result of a few things. The fuel/air mixture could be heated to the point that it self-ignites, causing the diesel run-on effect. 4) Timing is out. The timing itself won't cause the run on as the ignition is switched off when the run in occurs. However timing too far advanced can cause an overheating engine which can attribute to over-run. I would verify that everything is in proper order, even though its a new engine, something might have gotten loose (dist hold down). __________________ Memo S. Retired US Marine Family Man

10-01-2022, 11:20 PM	#3
Warrens69GMC Registered User Join Date: Jan 2003 Location: Safford,AZ Posts: 3,613	Re: L31 crate engine on regular gas? You could try to back off a few degrees and see if it continues.

10-02-2022, 12:26 AM	#4
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,510	Re: L31 crate engine on regular gas? Or you could advance the timing or hook the vac pot up to manifold vacuum. Either one will increase the idle which will let you back off on the idle screw and close off the primary throttle more. No air sneaking past the throttle plates will stop the air and fuel being drawn into the intake. As for the question of octane rating, I ran regular gas in both my 70 chevelles. The 11 to 1 LS6 and the 10.25 to 1 L34. Never had a problem but that coud be due to the elevation I lived at. 3500 feet. Last edited by geezer#99; 10-02-2022 at 12:40 AM.

10-02-2022, 03:11 PM	#5
truckster Senior Member Join Date: Jan 2001 Location: Orem, Utah Posts: 7,791	Re: L31 crate engine on regular gas? I run a rebuilt L31 with approximately 9.7:1 compression, a little hotter cam, Quadrajet, and HEI. I've had no problem running 87 octane. __________________ I have sworn upon the altar of God, eternal hostility against every form of tyranny over the mind of man. - Thomas Jefferson

10-04-2022, 12:36 PM	#6
tdangle Senior Member Join Date: Mar 2005 Location: Jurupa Valley, Ca Posts: 1,209	Re: L31 crate engine on regular gas? I have a L31 variant from GM. The SP350/357 which is basically a L31 with a hotter cam. The instructions said to run premium. Wonder why, I might try regular and see if it works OK. __________________ Terry 1970 Custom Camper/C20 , GM Crate 350/7004R, Dana 60, factory AC

10-04-2022, 05:42 PM	#8
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,690	Re: L31 crate engine on regular gas? Its 9.4:1 compression to be exact, and it runs fine on 87. You need to adjust the carb and/or timing. __________________ -78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me.

10-05-2022, 07:27 AM	#9
Bigdav160 Registered User Join Date: Apr 2011 Location: Klein Texas Posts: 3,852	Re: L31 crate engine on regular gas? Too much throttle opening will be pulling fuel through the main metering circuit of the carb and contribute to run on. Make sure your idle is set correctly __________________ My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe



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