Octane Opinion
 

I have a SBC with 11.25:1 heads and 38 degrees of total advance. What do you think is the lowest octane level I can safely run?

100+

I don't know if I would even run that in a boat. If you do roll back that timing to around 32.

You're gonna have to be a lot more specific about your mods. What cam specs? head design, are the chambers polished? If properly tuned the degrees of advance will actually have less to do with octane than cam & head design.

Roby

I ran 110 with 10.8:1, Gonna be running 110 again with my new setup of 12.6:1.

Wette vette's dad runs a small block which i believe is 11.25:1 and iron heads he runs 110 octane.

This is the boat that belonged to Wette Vette's dad. It just seems like 110 is a little overkill for 11.25:1.

better safe then sorry in my book :)

Need a lot more information to answer that question. Try to calculate the DCR rather than look at the SCR, depending on your camshaft, you may find that you are lot safer then you think on pump gas.

We ran it using Travis's "better safe than sorry" attitude. It helped that we have a supply for 110 octane near our lake. I believe 38 degrees works so well with this motor for 2 reasons. One is the domed pistons require a little more advance than flat tops or dished pistons that are typically in marine engines. The other is 110 octane is slightly overkill for that motor combo. If the timing is backed off to 34 or 36 there is a noticable loss in power and running the 110 octane we never had any signs of detonation at 38*. I am sure some mixture of 93 and 110 will work just fine, but some timing adjustments may be required. The problem is there is no real set formula as to what mixture and what timing should be used. Careful experimentation is the only real way to find out. I haven't been a big fan of octane boosters, but does anyone know of any that work? That could be another option, but again be careful!! :)

Toluene & Marvel Mystery Oil.

PhD,
Got a recipe for the Toluene and Mystery Oil? I wanted to mix up a small batch to carry for my B&M 250 502 in case I cannot get 93.

I run 93 Octane in my 10.5:1 632". Aluminum heads and BIG solid roller to bleed down cylinder pressures though. 32 degrees timing. Never any Detonation signs

Sure,

Toluene is 114 octane.
Following mixtures are with 92 octane unleaded:
1 gal toluene to 10 gallons 92 gas: 94.0 octane
2 gal toluene to 10 gallons 92 gas: 95.6 octane
3 gal toluene to 10 gallons 92 gas: 97.0 octane
4 gal toluene to 10 gallons 92 gas: 98.2 octane
5 gal toluene to 10 gallons 92 gas: 99.3 octane

Just a tblspoon per gallon of MMO for a touch of topend lube.

Mix with 87octane?
1 gal toluene to 10 gallons 87 gas: 89.5 octane
2 gal toluene to 10 gallons 87 gas: 91.5 octane
3 gal toluene to 10 gallons 87 gas: 93.2 octane
4 gal toluene to 10 gallons 87 gas: 94.7 octane
5 gal toluene to 10 gallons 87 gas: 96.0 octane

** It is not linear scale, but figure on 2 points of additional octane for each gallon per ten you use, as long as you aren't going for more than 94 octane. This way, whatever you find: 87,89,91, 92, 93, you can "ballpark" your additives...

As you can see, when you pass 94 octane, the benefits get smaller and smaller.

Damn, mc; That is so on the #s I have, but I never had the guts to try to tank mix it, so I go with an engine that will run 92 oct. and add the booster if I get stuck having to fill up where I have to settle for less. But, 38 deg. and 11-1/4:1 C/R.? ----WOW!--- Guess I would run nothing under 100 oct. and still do some judicious plug reading. --- Jer

Mmm.

As I sit here, I still find myself saying in my head "38 degrees is past the optimum for a SBC". It's sposed to be 36 for optimum. While 2more degrees isnt night and day, in my mind it should keep making more power unless the fuel burn is too slow.

The stuff after this sentence is just me thinking out loud............
If we figure big cam, 34 degrees, NA, 92 octane, 10.5:1 would be about the limit. If we raise CR with octane, the guess would be 10.75 @ 93, 11.0 @ 94, 11.25 @ 95, 11.50 @ 96.

Around here, 100 octane race gas is more than $7/gallon.

10 gallons of 92 is $13.99
5 gallons of toluene (in drums) is $15.00
15 gallons of homebrew 96 is $29 ($2.00/gallon)

Working backwards from 100octane,

1 gal of 100= $7
1 gal of 92= $1.39
2 gals of 50/50 mix(96 octane)= $8.39 ($4.20/gallon)

If you have a 60 gallon tank, you need 20 gal of toluene to get 60 gal of HB96.

Now, you gotta get a handle on exactly what your motor needs. I've got an old MSD knock sensor that you can "borrow" to get some electronic help.

throw in some 100 octane. Set the knock sensor on high sensitivity, and make some passes to get the sensitivity set properly (we know you aren't detonating at your current settings). back the sensitivity down a little at a time until it starts giving false positives. Go back up till it doesn't.

Drain her dry. Back the timing down to 30 and put in some HB96. Make a pull, note the WOT rpm at the same trim setting, look for the MSD light and listen for detonation. Keep advancing the timing 2degrees at a time. Each advancing should give you a little more revs. When you reach a setting that lights up the MSD OR doesn't give you any more revs, then you are at the limit for that fuel (actually, you are 2 degrees past it).

If it don't run as well (as fast) as it did with 100, then your engine combo definitely wants more octane. Keep in mind, though, that when you "over - octane" a combo, you slow down and spend more money.

Here is my disclaimer: if you blow up your motor by trying to follow my advice, I am not responsible because you must not have followed my instructions properly.

(plug heat range is also an issue - keep an eye on the heat band on the electrode).

Hey guys here is some more motor info for you. I am sure the new owner of my dad's old boat is very appreciative. Oh by the way 110 octane leaded racing fuel goes for about $3.00 / gallon where I am from. The cam in this motor is an Erson solid roller 246 /254 @ .050" lift with .568" int. and .529" net lift on a 112+4 installation. How well will a knock detector work on a solid roller motor with the extra valvetrain noise? Heads are Pro Topline 200 CC non ported (2.02/1.60) with a Holley Strip Dominator intake. The carb is an 800 Holley DP. Plugs are NGK 5673A-8, which are colder than MR43T's and slightly hotter than MR41T's and are non projected tip. Ignition is full MSD with the timing locked in. Exhaust is CMI etop and JE pistons make for 11.25:1 compression. The motor water runs at 150* to 160* and 15 PSI. Colder water temps will also help to stay out of detonation with less octane, but too cold has negative effects as well. As the motor sits with 110 octane leaded racing fuel and 38* timing there are no signs of detonation after WOT operation. I have become very well versed with a spark plug viewer and this is the only way to really check on how a motor is doing. The heat range of the plugs are correct, however colder plugs could be used at the cost of some performance. I like the way mcollinstn suggested testing the different timing / octane ratings, just be careful to take accurate info.

Craig

I've seen knock sensors on mechanical motors.
Supposedly the "frequency" that the acoustical pickup is tuned to is different than the lifter tap.

Obviously, the degree of acoustical accuracy on the mechanical motor would not be as wide since all sounds form cascading-level harmonics on even splits of frequency and bleedover into adjacent frequency bands as well. Knock sensors are usually mounted on the side of the block, though, and that mounting gives some degree of insulation from tappet noise...

At $3/gallon, there not a strong economical reason to home brew, aside from the need to occasionally boat where 100+ is not available. Downmixing 100 and 92 evenly will provide 80c/gallon savings if 96 turns out to be an acceptable number for the little monster.

Build specs sound great on that motor. Bet its mean.

What actual cu-in are we talking about here incase I missed it? A lot of people make the mistake of thinking that a knock sensor is tuned to a certain frequency so as to respond to a knock. This is supposed to be true but isn't. No 2 knock sensors will work the same. I have seen knock sensors too sensitive that it would go haywire in a gear drive engine and I have seen one react to screw driver tapping on a valve cover. A knock sensor should be tuned to the engine. You have to install it so tight or so loose. Sometimes you need to use a little Teflon tape for the really sensitive ones. I have pulled all of my hair out many times getting these stinking things to work reliably. (lucky my hair grew back) To do it properly it takes reading exhaust temperature, experienced plug reading and dyno time. Now in my opinion 38 degrees does seem a little early for this engine and compression. Is this what the compression actually is? Have the combustion chambers been cc'd? Also how about the condition of the balancer? Has it slipped its ring? maybe? As I’m sure you know the plugs should be read right immediately proceeding a prolonged WOT run with no cool down cruise or idle allowed. How do they look? Based on what I have read if you are getting more performance at WOT with a hotter plug, more timing and your cam is +4 than there is a sign pointing me to believe that you may be a little overcammed.

What:
RPM are you running?
Cubic inch is the engine
Are the combustion chamber size of the heads in cc’s?
Rocker ratio are you using?
Reason are you using leaded fuel?

Roby

I can answer the questions since Plum is the new owner and may not know. The engine is 357", and the heads are 64cc. Not only have the heads been cc'd, but the compression was actually checked by filling the combustion volume on the assembled motor with fluid, which is the best way to check compression. The motor runs real nice at 6000 RPM, which is where it is propped. As far as leaded fuel the lead helps keep the valves cool kinda like the mystery oil. In my opinion unleaded racing fuel should only be used if a catalytic converter is used, like in modern street cars. Really, 38* isn't that awfully much. On stock flat top SBC motors 36* will still be safe and will provide some power benefits over Merc settings. With the dome of the high compression piston essentially getting in the way of the flame it shouldn't be surprising that a couple of degrees more timing are required. These aren't "fast burn heads" which do run better with a few less degrees of timing due to the combustion chamber design and the swirl effect that is created. The Fluid Dampener is like new and is in good condition. 11.25:1 compression with iron heads is pretty high and good fuel must be run!! Less than 110 may be OK, but careful experimentation for the right formula will be required. The other option is to run the motor at 100* water temp, fatten up the carb a couple of jet sizes, run the timing at 34*, and run colder -9 or -10 plugs and maybe get by on 93 octane. At this point the motor will probably loose 75 + HP!!:D It is a game of give and take!! I like mcollinstn's idea and test.

Just one other note. My 13.2:1 compression 496 with polished combustion chambers also liked spark timing in the 36* to 40* range. I did a test like mcollinstn suggested and checked the speed on radar / RPM at timing settings from 30* to 42* in 2 degree increments. The boat got faster and quicker each time until I hit 40*. At 40* and 42* it was still quicker, but the top speed was unchanged. I almost put an RPM activated switch on and ran 42* for the acceleration and at 5500 had a few degrees of timing pulled out to keep it safe on top. I decided to just compromise and ran 38*. I ran this motor HARD and never saw a scent of detonation during rutine plug readings after long 6300 RPM WOT runs. Anyway higher compression motors are a little different in what they like than stockers. ;)

Sounds good, as long as you do keep the octane numbers over 100 and you continue to run this much compression & timing. I have another question. Every head manufacture whether they are OEM or performance have hardened valve seats on the exhaust side and as well on the intake side. I have to wonder why you would want to run lead in your engine. In my experience there is no performance gain to speak of and it will shorten the life of your plugs (especially if you like to run cooler than normal) The only reason why lead was refined in gasoline was to cushion the valve seats on the exhaust side of older non-modified heads. Some said that there was an octane increase with the use of lead but it was argued and in this day it is redundant. The use of marvel mystery oil was used to cool the valves too but using hi quality stainless valve has also made this practice redundant as well. You might want to thoroughly research this practice too because it has been documented that the use of marvel mystery oil caused a heavy build up of carbon deposits on the back side of intake valves. These chunks I have seen can be as large as a walnut and I don't have to tell you what this will do to your flow rate. Marvel Mystery Oil unlike lead (or diesel fuel sometimes was used) will breakdown due to the very high temperatures. It was this breaking down that would cause a large build-up of coked carbon deposits on the intake valve and port.

I would suggest that if your heads are built of high quality and using stainless valves the practice of using MM oil should be suspended.

Take care
Roby

Factory knock sensors are designed to listen to only a narrow band of frequencies. This frequency differs for some motors, and the sensor needs to be matched to the motor.

Then, some aftermarket flat band knock sensor transducers are not tuned to a specific frequency. The transducer itself does nothing to dampen or distinguish a particular level of sensitivity or frequency distinction - this is the job of the electronics in the head unit/display.

If you refer to needing to jump thru hoops to make a knock sensor behave properly in a closed loop system (like the current Merc offerings - or a stock automotive setup) then it is because those units have no built in provision for sensitivity adjustments. Aftermarket units (like the MSD and several others on the market) allow you to vary the sensitivity to tune it to trigger at a particular signal amplitude. In addition, the head units have internal frequency filtering circuits to make the unit "listen" only to specific frequencies. Some units are adjustable for that as well (not on the outside, but via internal pots).

My experience has been personally limited to BBC motors and I have used the factory GM transducer as found on the Corvette. It has worked well with the MSD sinal box.

A sensor may or may not be of assistance in this situation, but I would try it as if it proved to work, it makes short work of the rest of it...

I get goop buildup on the backs of intakes when I DONT run any oil in the fuel. It's the rare motor I see that doesn;t have some buildup there. I run a heavy shot of carb cleaner thru my motors on a regular basis (bikes, cars, boats). My 2000 5.3 Chev in the Yukon gets a sticky throttle plate every 5000 miles. Good ole carb cleaner takes care of that too.

I agree that leaded fuel has no advantage. I would ony run it if it were substantially cheaper for some odd reason. Makes plug readings all weird, too.

Thanks everyone for all the responses. I can get leaded 110 locally for $3.50/gal. I won't save much trying to cut it down to 102, so I'll just bite the bullet and run the 110 my first season with the boat. I don't want to cut any performance and definitely want the nitrous at the ready. During the following off-season, however, I intend to install a custom water-injection system. That should allow me to run 93 octane.

Sounds sweet Plum!! Don't forget the tunnel ram and dual quads!!:p

Oh, I haven't forgotten. I just need to give the wife a little time before I tell her an 82 MPH boat isn't fast enough.

It is common that some carbon burns and sticks to the back of the valves. However it is the oil that makes this carbon so hard that you can't clean with carb cleaner. I have seen instances to where they have to be bead blasted. Of course this measure is too extreme and valve damage will result. If it is built up this bad the valve needs to be replaced. Hi detergent gasoline can remove this stuff over time if its not too bad but for real thick build up will likely result in a burned valve because the carbon will not allow enough heat to dissipate.

And yes I can't imagine how you could get a clear plug reading when running fuel with high lead content.

Roby

Actually I think plug readings are quite easy with 110 octane leaded racing fuel. I have been reading plugs with this fuel for the past 10 years so that helps. There is still a fuel ring around the porcelins if the a/f is fat enough. If there is no ring developing within the 1st hour or so of running the motor is likely lean. At the drag strip we run a/f mixtures so lean there isn't a ring, but not in boats. There is a well defined heat line on the ground electrode which can be used to indicate if spark timing is too advanced or retarded. The center electrodes are easily read as well. If light detonation is encountered the aluminum speckles are very evident on the porcelins around the center electrode. White shiney areas on the center electrode are evident if pre-ignition occurs. Actually this type of fuel has additives to prevent excess depostis on the valves, piston tops, and spark plugs. Union 76 110 leaded racing fuel is what I have sued. Man this is the official fuel of NASCAR!!:D Now the 89 octane leaded fuel available in the 60's and 70's at the pump is certainly another story. In all honesty this racing fuel is the BEST fuel one could run in a high performance engine, too bad it isn't a little cheaper and available at the pumps on our lakes.:(

Been a long time since I've read a plug out of a leaded fuel motor. I guess the leaded fuels being used now are of a purity level of the ones I remember. Glad to hear it works out well.