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Discussion Starter #1
I'm trying to figure out what fuel I should first try to run in the new motor once it's ready to go. Don't wanna dump 100 in there if I need 110 etc.

This is with iron heads, flat tops with 2 valve reliefs, and a quench distance of .039". SCR will be around 11.2 depending on what the exact chamber CCs are (63-64 ccs). Just looking for something to get me in the ballpark of what fuel to try out.
 

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If it was mine I'd use Cam2 Purple or Torco Purple to start with. My motor has 13.4 CR, I run 40 degrees timing and I tend to run my motor lean and those fuels have worked well for me with no problems. Been apart two times after about 250 passes each time and it looked like new inside. No signs of any detonation and the pistons and chambers looked good. Those fuels are rated as 110 octane or standard racing gas. If you can ,get your fuel from a reliable source so you are confident they haven't doctored it up. Some suppliers we know have been known to doctor the fuel up to make more profit. I buy fuel by the barrel for consistancy plus I get a better deal on it too. I feel using higher octane than you need reduces the power in the motor by slowing the burn but that is JMHO, RM
 

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DriveWFO said:
I'm trying to figure out what fuel I should first try to run in the new motor once it's ready to go. Don't wanna dump 100 in there if I need 110 etc.

This is with iron heads, flat tops with 2 valve reliefs, and a quench distance of .039". SCR will be around 11.2 depending on what the exact chamber CCs are (63-64 ccs). Just looking for something to get me in the ballpark of what fuel to try out.

If you ever find a way to figure this out please let me know cause I have asked this and similar questions many times before and have yet to get an answer that was truely helpful in figuring out the octain requirements of a motor........Paul posted a answer once based on cranking compression you might be able to find it if you look...but it was for pump gas
 

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here is Pauls post;

Paul Wright said:
Here's an easy tip. Do a cranking compression test before the cam swap and one after. 150-160 psi is good for regular gas. Premium is usually required if the pressure is 170-180 psi.If the pressure is too high (over 200 psi) then you can retard the cam to straight up. Most cams are ground 4 degrees advanced so when the dots line up the cam is not truly straight up. I think there's a thread in Best of Tech about this technique.
Maybe this will help ya out...maybe not:p
 

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Discussion Starter #5
It would be of better help with my scenario if the "scale" continued going up beyond 200psi/premium ;)
 

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DriveWFO said:
It would be of better help with my scenario if the "scale" continued going up beyond 200psi/premium ;)
ya I know what you mean...like I said its hard to get an answer that really answers the question....I'm not sure there is a cut and dry answer that will say if your CR or cranking pressures are "X" then you need "Y" amount of octain....not sure such an answer exists....

Or if it does exist those that know it seem unwilling to share it :(
 

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Discussion Starter #7
Like RM said, I don't wanna run any more octane than I need to though either.
 

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DriveWFO said:
Like RM said, I don't wanna run any more octane than I need to though either.
And from a monitary standpoint you dont want to run more then you need either:beat:
 

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His origional question was, What to use to start with. Using standard racing gas which is 110 would be a good place to start. While it may not need 110 it is close enough to not hurt anything, will allow him to get good plug readings, exhaust gas temps if he has a probe in it, and let him get jetted up right. Once he has all the basics right he can try stuff and evaluate the results off a good base line. Years ago many of my buddies and I used Av Gas in our race cars. It's high octane but unless they have changed it alot I wouldn't use it in my car today. It wasn't made for a car. If you can't afford the right fuel for the machine then maybe find a different machine.......LOL. JMHO, RM
 

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Discussion Starter #11
Is there much of a difference (other than specific gravity) between 110 octane of various fuels (CAM2, Turbo Blue, Sunoco or VP)???
 

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for what it's worth the purple cam2 hasn't worked the best in my car. i've had my best luck with 100 octane avgas
 

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Discussion Starter #13
sproosemoose said:
for what it's worth the purple cam2 hasn't worked the best in my car. i've had my best luck with 100 octane avgas
What's up with the purple cam2 and your car?
 

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One anylitical method would be to use a knock sensor.

Fuel up with 110 and see if you get knock counts.

If none then reduce to 105 same senero etc.

Till you get knock. The moment you see knock let off the throttle.

A knock sensor begins to detect knock before you can hear it.

The one bug a boo would be if your engine produces lots of frequencies that mimic knock and "false knock" could be seen by the sensor.
 

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Cam2 is Sunoco. Torco purple and Cam2 are very close to being the same. I ran Torco til last week then switched to Cam2. Motor didn't sense any change at all. Turbo Blue has a specific gravity all it's own, nothing else is close to it. Local dirt tracks have mandated it's use so they can easily check the fuel for cheaters. I ran it for a couple years with no ill effects but I just don't like it. Very hard to read plugs with it due to the goofy dye they use plus it stains everything up in the engine. Had dye tracks all over the carb and inside the intake runners. It didn't slow the car down though. RM
 

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No easy answers to that question.
You'll have to experiment to find out but what they are saying is err on the high side to start and work your way down.There's too many variables to predict exactly what octane it will need. The higher the cranking pressure the higher the octane. If you are cranking higher than 200 than you'll need 100 octane all the way to true race gas. If you get to 300 psi you've gone too far. I would try to keep to closer to 200 than 250.
 

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Discussion Starter #17
Paul Wright said:
No easy answers to that question.
You'll have to experiment to find out but what they are saying is err on the high side to start and work your way down.There's too many variables to predict exactly what octane it will need. The higher the cranking pressure the higher the octane. If you are cranking higher than 200 than you'll need 100 octane all the way to true race gas. If you get to 300 psi you've gone too far. I would try to keep to closer to 200 than 250.
Can I predict cranking pressure now with my current cam specs?
 

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What's up with the purple cam2 and your car?
can't say as i know...i suspect it has something to do with the 300/310 duration cam bleeding a lot of air out. the guy i get it from in town is a little different...i don't know if maybe he's thinning it out, maybe it's a knockoff brand, maybe the nights i ran it i just had bad air or something else up with the car but avgas seems to have come out on top so far.
 

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With my iron headed 406 and 14.5-1, the quickest passes were with VP C-12 (108 octane) cut with some Arco 91 pump gas. I drive the car to the track with very little fuel in the tank(couple gals.) and dump in about 2-3 gals of race gas on top of whatever is left when I arrive.
With my aluminum headed 414(also 14.5-1) I started running the stuff from the pumps at Irwindale (now Sunoco)they have 100 unleaded, and 110 leaded. I still drive in on Arco 91 and dump in either the 110 or 100 depending on how much 91 is still onboard--If I have more than 4-5 gals already, I go to the 110.
When I ran the bottle, I added a little C-16 to the C-12 for insurance.
The rep at V.P. told me C-12 was fine with 14.5-1 compression if the rpm was under 8000.
With 11-1, I'd probably dump in a gallon of race gas to about 5-6 of 91. If it doesn't rattle, I don't waste the money.
 

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Bowtie-with that much compression can you get away with pump gas only (Arco 91) if you have to or do you always have a little race gas on board?
 
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