??? bout compression and fuel

[jrm03]

I'm on the hunt for a high compression piston to compliment my stage 1 cam I just purchased. In these threads I read that anything over 11:1 needs to have race gas ran. Is this compression ratio from the piston itself, or is it the figured dynamic compression of the engine? If its figured I'd like to go a bit higher to achieve 11:1 while still running 93 in the stock engine.

I'm going to be running a Stage 1 Hotcam and if anybody could figure the dynamic on the stock bore, with an 11:1 high comp piston it would be appreciated. Thanks.

[gtilley45]

I know people that ran pump gas in their stock YFZs that have 11.9:1 compression with no problems.

[GPracer2500]

YFZ to 400EX is apples to oranges--no conclusions can be drawn about what an EX requires from what a YFZ requires.

When people talk about anything over 11:1 needing race gas (for a 400EX) they are talking about static compression ratio. Since we usually can't choose our static compression to be whatever we want, we just pick from what's available (e.g. 10.5:1, 11:1, 12:1, 12.5:1, etc). Many poeple report building engines with 11:1 pistons and do fine with pump fuel so that's become a somewhat dubious "standard threashold" between race fuel and pump fuel when folks are making piston choices. Using static compression ratio is better than nothing when trying to figure out an engine's octane requirement, but it is NOT the final word. Not by a longshot.

As you know, dynamic compression takes into account valve timing. It's still a calculated figure, but it's a much better judge of the engine's compressing ability since its calculation eliminates the portion of the stroke that takes place while the intake valves are still open.

By my calculations with:

85mm bore
70mm stroke
5 inch rod length (estimated)
11:1 SC
intake valve closing 49 deg ABDC (HC stg 1)
at 1000ft elevation

That should produce about 9.5:1 DC or almost 200psi. Change that to a HC stg 2 and your looking at about 9.2:1 DC or 190psi. Stick with the Stg 1 but go to 3000ft of elevation at your at about 9.1:1 DC or 185 psi.

Those numbers are really just estimates, but they should be in the ballpark. The dynamic compression calculator I'm using is best used to make comparisons between different settups rather than pinpointing absolute figures.

What the above calculations aren't taking into account is things like gasket thickness, cam lobe ramp angles, imperfections in cylinder sealing, the fact that some compression does start to build before the intake valves are completely closed, and other details. Measuring cylinder pressure with a compression tester can give you better information (but of course the engine already needs to be built for that).

And then there's the issue of liquid cooling vs. air cooling, combustion chamber effeciency/design, ignition timing, and more.

The fuel itself is of course a major consideration too. There's lots of things we don't know about our pump fuels. One guy's 93 might not perform the same as another guy's. Pump fuel is measured with Motor Octane Number plus Research Octane Number divided by two. MON is the testing method that is most applicable to our engines--it's the better judge of detonation resistence. But we don't know the MON of pump fuel. We can estimate, but the MON in one pump fuel my not be quite the same as the MON in another even if the (R+M)/2 number is identical. Also, we don't know how the higher octane compenents of the fuel are distributed within it's make-up. One fuel might have most of its high octane compentents concentrated at a certain point within the fuel's distillation curve. That means two fuels with identical octane numbers might offer differing deto resistence under certain operating conditions. As much as I'd like the octane numbers printed on the pump to be directly comparible to the same number on a different pump--that's not something you can count on.

What I'm trying to get at is that, IMO, the best you can do is consider things like:

--Dynamic compression ratio (i.e. cam choice)
--Is 93 available or only 91?
--What's my elevation?
--What will I do if I build my engine and it knocks when I wasn't expecting it to?
--Am I willing to jet richer than best power to stave-off deto if deto occurs during testing?

Think about stuff like that, make your build choices, and then test. Try to get the engine to detonate to determine if your fuel meets the minimum octane requirement for the engine you've built. If it won't deto under the hottest, highest load conditions, then you're good to go. If it does, then be prepared to either learn to ride around it (if it's mild deto you can do this) or find an alternative fuel choice.

FWIW, I built a 416 w/ a JE 10.8:1 piston and a HC Stg 2 (old version). It knocks with the 91 pump fuel avaliable in my area (Phoenix). I wasn't expecting that. If you ride it medium to easy or at higher elevations it's not to bad and I can ride around it. But at the dunes (extemely high engine loads and near sea-level elevations) race gas is a must if you want to run hard.

$0.02

[jrm03]

I live in a small town where race gas is non existent which leaves me with little options aside from traveling bout 45 minutes to a city and searching for a place that sells it. Are the octane boosters on the market effective (if mixed in correct portions) with 93 which IS available, to raise the octane rating and prevent detonation if it does become a problem?

Thanks for the very informative response, I will resort back to this throughout my decision process.
From what I am gathering, you are saying that static compression really has nothing to do with the need for race gas, merely a benchmark to go from, ad the only real way to know is to build your desired engine and then see if it detonates??... (Which in a nutshell makes perfect sense) The theory of using higher octanes above 11 to 1 is simply the actual results of other builders success or lack there of with this choice of piston.??..

[GPracer2500]

Originally posted by jrm03
...Are the octane boosters on the market effective (if mixed in correct portions) with 93 which IS available, to raise the octane rating and prevent detonation if it does become a problem?

I wouldn't bother with the stuff you'll find at most autoparts stores or gas stations. But there are a few products that actually work. Toluene (which is just an aromatic hydrocabon often already present in our fuels, not a branded product) can be an effective octane booster. I believe you can get it in gallon pales at most hardware stores and paint stores. 1 gallon of toluene plus 4 gallons of 93 would get you around 97 R+M/2. Do a Google search for more info. There's some similar aromatic hydrocarbons (xylene) that could also be used but I hear about toluene the most.

Kemco's Octane Supreme 130 is the only lead (TEL) additive that I'm aware of. Lead is an excellent octane enhancer. It does its magic in a chemically different way than aromatics but the end result is the same--higher effective octane.

http://www.kemcooil.com/newcart/images/productInfo/Octane%20Supreme2.jpg (http://www.kemcooil.com/product_info.php?pId=61)

Note that when most companies (not Kemco though) talk about their octane boosting products raising octane by 5 "points", the points they are talking about are really only 0.5 points of octane. This language is decieving. Most commonly found octane boosters would have to be added in quantities similar to that of toluene (10%-30%) to get any meaningful octane boost.

If there's an airport in your town, 100LL aviation fuel (straight or mixed with pump) would be another option. It's MON rating is 99.5.


Originally posted by jrm03
...From what I am gathering, you are saying that static compression really has nothing to do with the need for race gas, merely a benchmark to go from, and the only real way to know is to build your desired engine and then see if it detonates??....The theory of using higher octanes above 11 to 1 is simply the actual results of other builders success or lack there of with this choice of piston.??..

EXACTLY! Well put.

[jrm03]

If it comes to having to run that type of fuel, I gots to do the homework on where and how to get it. I will check into the Supreme 130, it'd be my personal preference to mix myself. I also understand that the lead in higher octane fuels (race fuels) act as a cooling agent so I guess that is an added benefit. Thanks again for all of the help.

[wilkin250r]

Originally posted by GPracer2500
$0.02

No, that post was a good $1.50 at least.

[gtilley45]

Originally posted by wilkin250r
No, that post was a good $1.50 at least.

But, it was very good information. :)