Octane
Thread Starter
Instructor
Joined: Apr 2010
Posts: 185
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From: Gilbert Arizona
Octane
I am so confused about this octane issue. I am having my L-46 350-350 rebuilt. I would like to keep it 11:01 pistons, but my engine rebuilder says it would need 98 to 100 octane gas. It's a 1969 power everything. He say's if I don't do that it will run like crap. States that if I go smaller pistons it will run better and I can use regular gas station with Octane of 91. I have the camel hump heads that came with the car.Everyone has different answers. Money is an issue for me and I sure can't afford that expensive fuel. I really need advice on this.
Race Director
Joined: May 2006
Posts: 18,597
Likes: 2,540
You dont have 93 octane gas available? If not you will have to retard the timing, if a stock distributor you will have to retard initial timing from 8 degrees to 4 degrees. If you already have a recurved distributor you will have to change total timing from 36 degrees to 32 degrees.
Thread Starter
Instructor
Joined: Apr 2010
Posts: 185
Likes: 8
From: Gilbert Arizona
You dont have 93 octane gas available? If not you will have to retard the timing, if a stock distributor you will have to retard initial timing from 8 degrees to 4 degrees. If you already have a recurved distributor you will have to change total timing from 36 degrees to 32 degrees.
Dr. Detroit
Joined: Mar 2012
Posts: 10,090
Likes: 4,015
From: New Braunfels Texas
Make it 10.0 to 1 and you will never miss the extra point.....
Don't shoot yourself in the foot......the 10-15 horsepower loss can be made up elsewhere in many ways......
At 11.0 to 1 it will most likely knock on pump gas.
If you retard it to run pump fuel...it will run like a turd.
10 to 1 and have a 5 angle valve job with mild port work and backcut valves.....it will run hard.
Jebby
Instructor
Joined: Apr 2016
Posts: 104
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From: Denver Colorado
I am so confused about this octane issue. I am having my L-46 350-350 rebuilt. I would like to keep it 11:01 pistons, but my engine rebuilder says it would need 98 to 100 octane gas. It's a 1969 power everything. He say's if I don't do that it will run like crap. States that if I go smaller pistons it will run better and I can use regular gas station with Octane of 91. I have the camel hump heads that came with the car.Everyone has different answers. Money is an issue for me and I sure can't afford that expensive fuel. I really need advice on this.
I suggest putting dished pistons in it to drop the compression ratio down to around 8.5 to so it can run on 87 octane like most of the other cars use now days.
Dr. Detroit
Joined: Mar 2012
Posts: 10,090
Likes: 4,015
From: New Braunfels Texas
There is not a single car built today that runs less than 10 to 1....but they have efficient chambers and other tricks to do so.
My LS GMC truck runs 10.5 to 1.....on 87......but it is a much more modern chamber design....with an engine management system....
An 8.5 to 1 motor will not be a happy one.....especially with any kind of camshaft with any kind of duration......
Hot Rod once built two LT-1 engines....a 70' and a 71'.....
The 71' low compression version (9.? to 1) made more power because the chamber was larger (unshrouded).....
Compression is great....but leave high compression for purpose built engines.
Jebby
My LS GMC truck runs 10.5 to 1.....on 87......but it is a much more modern chamber design....with an engine management system....
An 8.5 to 1 motor will not be a happy one.....especially with any kind of camshaft with any kind of duration......
Hot Rod once built two LT-1 engines....a 70' and a 71'.....
The 71' low compression version (9.? to 1) made more power because the chamber was larger (unshrouded).....
Compression is great....but leave high compression for purpose built engines.
Jebby
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Race Director
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93 Octane
I see Arizona doesnt have 93 octane so I would definitely go with the suggestion that you use a piston setup that will give you no more than 10:1 compression. I would guess your builder wants to go from a domed piston to a flat top piston which makes sense.
Melting Slicks
Joined: Jul 2006
Posts: 2,266
Likes: 477
From: Salmon Arm, BC
I had my car down in AZ last October and it ran like $hit....10.3 to 1 compression. I had to take a bunch of timing out of it to get it to run better.
Ran fine as soon as I got out of AZ. With 11 to 1 you'll be in a world of hurt.
As others have said, limit your compression to 9.5 and you and your engine will be happier.
Ran fine as soon as I got out of AZ. With 11 to 1 you'll be in a world of hurt.
As others have said, limit your compression to 9.5 and you and your engine will be happier.
Melting Slicks
Joined: Dec 2009
Posts: 3,195
Likes: 632
From: Thunder Bay
Another vote for 9.5-10:1 compression. 11:1 is too much for a Gen I small block on today's gas. Calculate your compression carefully to ensure you have a true 10:1. Select a cam with no less than 224 degrees of duration at .050. Depending on the rest of the setup you should be able to run regular fuel. Heads have a lot to do with it. you can run higher compression with aluminum heads. Worst case scenario you'll have to run premium fuel.
Drifting
Joined: Oct 2005
Posts: 1,615
Likes: 150
From: Forth Worth TX
I have a 1970 350/350 and rebuilt my engine a couple of years ago. One of my goals was as much compression as I could get away with and still run on 93 oct.. After taking all of the appropriate engine tolerance measurements to see what c/r I actually had, a head gasket thickness was chosen to bring my c/r to 10.4:1. It runs on 93 oct. just fine and I still use the "11:1" pistons.
If your block has NOT been decked and is original, your pistons most likely sit .025 or so down in the hole. So you don't have 11:1 c/r to begin with. You really need the engine builder to actually measure your engine tolerances to find what kind of c/r you have. Just because GM advertised 11:1 doesn't make it so.
If you're limited to 91 oct. you should be able to get away with high 9.x:1 or 10:1 c/r.
If your block has NOT been decked and is original, your pistons most likely sit .025 or so down in the hole. So you don't have 11:1 c/r to begin with. You really need the engine builder to actually measure your engine tolerances to find what kind of c/r you have. Just because GM advertised 11:1 doesn't make it so.
If you're limited to 91 oct. you should be able to get away with high 9.x:1 or 10:1 c/r.
Thread Starter
Instructor
Joined: Apr 2010
Posts: 185
Likes: 8
From: Gilbert Arizona
There are some stations by me that have 93 octane
Last edited by bugzbunny; Apr 23, 2016 at 10:29 AM.
Team Owner
Joined: Oct 2004
Posts: 81,241
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From: altered state
St. Jude Donor '05
OP whats your elevation out there? That has plenty to do with it and what you can get by with.
76 used to have stations out here that sold 100 from the pump anything like that in AZ?
W/jebby on porting theres power to be had there dont be afraid of using long tube headers too. Let the purists cry just about any hot rod these days has a pair on
Last edited by cv67; Apr 23, 2016 at 11:01 AM.
Melting Slicks
Joined: Mar 2006
Posts: 3,456
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From: Piedmont Va
St. Jude Donor '11-'12-'13,'19-'20
I suggest to listen to these guy's. (Well, with one or two exceptions). 10:1 is as high as you should go for static compression with your stock L-46 Heads if you are looking good consistent performance. As Jebby said there are plent of ways to recover the HP and no real ideal ways to address the detonation issues once your 11:1 motor is assembled and in the car. Good Luck! 
Safety Car
Joined: May 2004
Posts: 4,340
Likes: 1,565
From: los altos hills california
Just how far along is your motor? Nobody here has mentioned aluminum heads. Aluminum dissipates heat faster and you can run with higher compression. I assume you would be paying for a head rebuild so that is a $$ trade against new heads.
Don't remember my SBC spec's but the switch to aluminum let me run with 91 here in CA for years and years. Also there is static and dynamic compression which would depend on your cam timing. I am not a motor builder of any kind, just mentioning more topics that may be of value for you to research. And yes, stay away from dished pistons whatever you do, bad experience there, not cheap to unwind that decision.
My LS7 CR is 11:1 and 93 is recommended but I can only get 91. Runs fine, but of course it has knock sensors, so who knows what's going on. But my experience sort of suggests 10.5:1 would be OK.
Don't remember my SBC spec's but the switch to aluminum let me run with 91 here in CA for years and years. Also there is static and dynamic compression which would depend on your cam timing. I am not a motor builder of any kind, just mentioning more topics that may be of value for you to research. And yes, stay away from dished pistons whatever you do, bad experience there, not cheap to unwind that decision.
My LS7 CR is 11:1 and 93 is recommended but I can only get 91. Runs fine, but of course it has knock sensors, so who knows what's going on. But my experience sort of suggests 10.5:1 would be OK.
