SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: eliminator18 on May 16, 2015, 06:34:52 PM
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Running around 10.5-1 to 11-1 compression, how do you figure out what octane works best?
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There has been a lot of studies about this one! The best article I have read stated that depending on how close you matched all of the parts you won't have to run expensive race gas. If it were mine I'd start with super then try the mid stuff . I don't think the low grade will work to well but it may.
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If my $.02 matters I use 91 with Lucas octane booster. She's. 468 10.5-1 575 int 578 exh extreme marine comp cam. Victor jr. Single plane. Holley 800 dp she is more responsive getting on plane and motor seems to like it better than straight premium!!!
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Thanks guys, what about av gas?
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Yes to av gas ! If you can get it.
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X2 to av gas but might have to adjust timing back a few degrees on the big end!!
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X2 to av gas but might have to adjust timing back a few degrees on the big end!!
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I don't get it. When I run AV 100 I bump the timing about 4° more advance. Is that bad? I do run a touch of Marvel Mystery oil and Lucas fuel additive, but never had a detonation issue.
GT
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There are actually three different compression ratios that play a role in octane requirements of any given engine.
Static Compression Ratio - this is the actual measured volume above the piston at BDC divided by the volume above the piston at TDC.
Dynamic Compression Ratio - the air/fuel mixture can not be compressed until the intake valve is seated. Dynamic Compression Ratio is calculated the same as Static Compression Ratio except that instead of using the full stroke value, only the stroke from the point at which the intake valve is seated is used. A cam with longer duration (delayed intake valve closing) will reduce Dynamic Compression Ratio. Advancing the cam or a smaller cam with earlier intake valve closing points will increase Dynamic Compression Ratio.
Effective Compression Ratio - lastly, the actual compression ratio that the intake charge actually sees is affected by the volumetric efficiency of the engine. Multiply the VE and the Dynamic Compression Ratio. Effective Compression Ratio is the greatest contributing factor to octane requirement.
External factors that influence octane requirements include: atmospheric pressure; humidity; temperature (ambient, coolant, etc.); air/fuel ratio; and spark timing.
Hope this fuels a little more thought into the topic.
Cheers,
Joe
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Hope this fuels a little more thought into the topic.
Cheers,
Joe
:o lol. Um, ya, that was a lot to soak in.
This is an awesome subject.
:thumbup:
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As you can see there is allot to this subject, Flusher hit the nail on the head as he usually does, I personally would stay away from off the shelf octane boosters, if you have an engine that has high cylinder pressures then you have to dig a little deeper in your pockets to now give it the fuel it needs to avoid damage from detonation. A big mis conception that many people have is that octane makes power, it does not, this number is only to show the fuels resinstance to detonation. Tests have shown and I too in dyno rooms testing engines and different racing fuels that if you feed an engine a very high octane fuel that does not need it (mild cylinder pressures) it will loose power. I would set ign. timing to at a safe area and try 91 oct. pump gas and see how it feels, chances in a boat you will not hear the ping of detonation, so if you know what your looking at reading spark plugs this could give you an indication of what's going on in the combustion chamber. If you see little black peppered specs on the porcelain this is the first sign of detonation. These specs are carbon particles that have been ratteled loose from coliding flame fronts. You may have to splash a bit of good fuel along with pump gas just to be on the safe side, but every combo is different with allot of variables. Using AV gas is fine, that's what I do, I mix a 50/50 mix for my boat BBC engine and I'm dead nuts 10.0:1 compression with ign. timing set at 16 inital and 36 total but I set my dist. up so full advance is all in at 1,800 rpm so she pulls hard out of the hole, it works for me and I don't have anything really radicle, engine dynoed at 542 hp with that fuel blend.
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This is next on my to-do list. I am excited to try it. Thought I would post a link here since it is fitting here.
Yours in good tune,
Joe
http://www.autospeed.com/cms/article.html?&title=DIY-Detonation-Detection-Part-1&A=0348
http://www.autospeed.com/cms/article.html?&A=0353
Also
http://www.pro-system.com/sparktool.html
Does anyone have experience with otoscopes? Could probably get a really good deal on one of those.