Not enough octane? what you guys think?

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mshred

mshred

The Green Manalishi
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hey guys,

went cruising with a few buddys of mine (none of which built their cars but have shops do it for them and so just blab what they hear) and they were giving me a hardtime about the gas im running my scamp.

the car is a 340 stock forged bottom end with .30 with old trq pistons that i beleive are the stock replacement style just .30 advertised at 10.5:1cr...the heads are stock x heads, and the rest of the setup is very mild as well...im currently running medium, which around here is 89 octane...i take the car to 6200rpm regularly and have no issues with pinging, although sometimes i get a lean pop or very slight hesitation when i tip in the throttle

anyways, that being said, these guys are telling me that i should be atleast running 91 octane gas in the car (one went as far as saying it will feel like it has nitrous....total goofball that one is) because the 89 is not enough...my reasoning is that if it isnt pinging then there is no point of going to the higher gas as i will just be wasting the unburnt fuel (higher octance burns slower i believe) but they say it doesnt matter if its pinging or not, needs more...91 isnt that much more expensive, but money is money...i for one dont even want to try it just to prove them wrong, but hell maybe they are right?

makes me wonder, should my daily driver run better with c12 in it? because thats pretty much what they are proposing

whast do you guys think?
 
I think you should be running at least 91 also.
Have you tried it????
 
POS Dakota said:
I think you should be running at least 91 also.
Have you tried it????
Click to expand...

no, not yet....but what is the reasoning for it..

i feel like people just put in the higher octane for the hell of saying "i run 91 in my car"...if it runs fine on the 89, why does it need higher?

im just looking for a reason as to why its better or facts to prove it
 
If your not pinging, and your sure, then you don't need higher octane fuel. Higher octane fuel will actually produce LESS horsepower than lower octane fuel because it burns slower.
 
if it don't ping /run on/ deisel on 89 use it. higher octain burns slower, so you would have to add more timing. the old trw pistons are rated at bluprint specs and are usually much lower in a real production block, add the much thicker felpro gasket on to that and i bet your CR is more like 9 to 9.5.
 
mshred said:
no, not yet....but what is the reasoning for it..

i feel like people just put in the higher octane for the hell of saying "i run 91 in my car"...if it runs fine on the 89, why does it need higher?

im just looking for a reason as to why its better or facts to prove it
Click to expand...

Hey Dude, read this http://www.wanderings.net/notebook/Main/IsHighOctaneGasWorthTheMoney
and then go buy some 91 or 93 and see if it makes a difference......... I use 93 just because.........and every once in a while I use octain booster...
 
I agree. No 10.5:1 iron headed engine can run on 89 unless you have severely retarded timing. Do a comp test and get some PSI #'s. That will give you a better idea of your static compression. I bet if you installed an MSD dist, with the 18* bushing, 2 light blue springs, and set your initial at about 16*, and add 91, your gonna run alot harder than you do now.

No it will not feel like nitrous....lol.
 
gts340 said:
If your not pinging, and your sure, then you don't need higher octane fuel. Higher octane fuel will actually produce LESS horsepower than lower octane fuel because it burns slower.
Click to expand...

this is exactly what my reasoning was

EL5DEMON340 said:
if it don't ping /run on/ deisel on 89 use it. higher octain burns slower, so you would have to add more timing. the old trw pistons are rated at bluprint specs and are usually much lower in a real production block, add the much thicker felpro gasket on to that and i bet your CR is more like 9 to 9.5.
Click to expand...

i figured that im not getting a true 10.5:1 as thats just what the piston was advertised at...thanks for the info!

HemiMark said:
Hey Dude, read this http://www.wanderings.net/notebook/Main/IsHighOctaneGasWorthTheMoney
and then go buy some 91 or 93 and see if it makes a difference......... I use 93 just because.........and every once in a while I use octain booster...
Click to expand...

I dont get it, that article proves what i already think, that i dont need the higher octane gas...my engine is not pinging, so how would the 91 help? i dont know if the dealer called for 91 from the factory or not, so it seems like the lower the octane that works the more cents it makes (pun intended lol)

redfastback said:
I agree. No 10.5:1 iron headed engine can run on 89 unless you have severely retarded timing. Do a comp test and get some PSI #'s. That will give you a better idea of your static compression. I bet if you installed an MSD dist, with the 18* bushing, 2 light blue springs, and set your initial at about 16*, and add 91, your gonna run alot harder than you do now.

No it will not feel like nitrous....lol.
Click to expand...

Well like i said, its got stock x heads with trw replacement 10.5:1 pistons, so im not 100% sure what my true compression ratio is. I will have to do a compression test. I have a mopar performance electronic distributor and right now initial is set at 16 and total at 36...id rather not change distributors, but i am going to try and weld up the slots in the distributor to get something closer to 19 or 20 initial (i have heard this would be beneficial)...i think when i do this i may have to up to 91 octane, but all in due time i guess
 
Do you have a newer MP dist? My old one had like a 25* curve....just plain pitiful. If yours is the newer style with the socket cap screws, you can just tighten up the mech advance curve. More timing = more octane.....and vice versa.
 
redfastback said:
Do you have a newer MP dist? My old one had like a 25* curve....just plain pitiful. If yours is the newer style with the socket cap screws, you can just tighten up the mech advance curve. More timing = more octane.....and vice versa.
Click to expand...

I believe mine is the older one, but ill have to double check that
 
redfastback said:
I agree. No 10.5:1 iron headed engine can run on 89 unless you have severely retarded timing. Do a comp test and get some PSI #'s. That will give you a better idea of your static compression. I bet if you installed an MSD dist, with the 18* bushing, 2 light blue springs, and set your initial at about 16*, and add 91, your gonna run alot harder than you do now.

No it will not feel like nitrous....lol.
Click to expand...

Your statement about compression is far from the truth. I am running a measured 10.6:1 compression with 190-200 psi of cranking pressure on 89 octane and there are others on this forum with similar set ups doing the same.

It's not the static compression that determines the octane need anyway. It's how much cylinder pressure you develop. I get away with 200 psi by virtue of the excellent quench of the magnum head.

The OP very likely has well under 10:1 compression, they were under 10 new from the factory. Also, depending on the over lap in his cam his cranking pressure could very likely be down in the 150 psi range making the use 89 or even 87 octain within reason.

As for timing too much will reduce power just as to little will. The timing needs to be dialed in for the specific combination.
 
measured 11.25cr with 205 to 210 cyl psi on 93 with j heads, but i run very cold plugs, elec fuel pump, no heat crossover, spacer under carb, ram air, hi stall, lighter car, total timing about 31.
 
dgc333 said:
Your statement about compression is far from the truth. I am running a measured 10.6:1 compression with 190-200 psi of cranking pressure on 89 octane and there are others on this forum with similar set ups doing the same.

It's not the static compression that determines the octane need anyway. It's how much cylinder pressure you develop. I get away with 200 psi by virtue of the excellent quench of the magnum head.

The OP very likely has well under 10:1 compression, they were under 10 new from the factory. Also, depending on the over lap in his cam his cranking pressure could very likely be down in the 150 psi range making the use 89 or even 87 octain within reason.

As for timing too much will reduce power just as to little will. The timing needs to be dialed in for the specific combination.
Click to expand...

I don't believe it's far from the truth. I do agree with you about the magnum heads though. We are dealing with LA heads in this post. That is what my comments are based on. I know magnum heads take less timing and have a good quench area as well. From my experience, the LA heads that have above 10:1 comp can't max out the timing in the head, which can be up to 37-38* without 93 plus octane, because they start to ping. I also understand about the overlap but we still don't have all the facts.
 
If you are sure it isn't detonating then there is no need to go with a higher octane fuel.

Chuck
 
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