Octane question
- Thread starter White92
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there is not a straight forward answer to that question. It depends on a number of factors with the engine design: fuel injection setup, rod length, cylinder head design, piston design and so on. every engine will have different compression limits depending on the engine design and setup...
Sorry, I should have been more specific. I'm seriously looking into building a complete Toda 3.5 stroked motor. I'm just waiting on a few things. Mainly the release of their cams. They don't know the specs yet, or at least they won't release them now. They can't even tell me when the cams will be released for sale. They tell me that it might be around June or July of this year. I don't want to spent a load of money on these parts and then find out that I'm goind to have to wait another 2 years for the cams just to finish the project.
Anyway, the pistons have 12.5:1 compression ratio. I may or may not go with the ITB's. I'm not sure on that yet. I'm leaning towards it since I'm going all out with the engine, I might as well do the ITB's as well. But I keep thinking to myself, am I even goinng to be able to run this on pump gas? I'm not going to drive it everyday so it wouldn't be that big of a deal to get fuel from the track every once in a while. Or maybe even by a 55 gal drum of Sunoco fuel and keep that in the garage.
Anyway, the pistons have 12.5:1 compression ratio. I may or may not go with the ITB's. I'm not sure on that yet. I'm leaning towards it since I'm going all out with the engine, I might as well do the ITB's as well. But I keep thinking to myself, am I even goinng to be able to run this on pump gas? I'm not going to drive it everyday so it wouldn't be that big of a deal to get fuel from the track every once in a while. Or maybe even by a 55 gal drum of Sunoco fuel and keep that in the garage.
Actually, I don't think that 12.5 is pushing it at all. As already mentioned, the key is to tune for the compression ratio. If you use a fully programmable ECU then you can easily go 12.5:1 or more. The practical limit on compression is more often defined by things like valve to piston clearance and the ability of the remaining combustion chamber size/shape to support a clean flame front. A larger piston dome and higher lift cams eventually become a problem. Of course increasing stroke and/or bore effectively adds compression with all else being equal so you may not run into clearance problems.
In any case, higher compression is not all that different than forced induction, which roughly equates to a super high compression ratio under boost. So just as with turbos and SCs, you need to find the best balance of AFR and ignition timing (and valve timing, and other factors) to achieve the maximum safe power level throughout the RPM range. The difference of course is that the higher compression ratio is always in play, but tuning for it is much the same.
In any case, higher compression is not all that different than forced induction, which roughly equates to a super high compression ratio under boost. So just as with turbos and SCs, you need to find the best balance of AFR and ignition timing (and valve timing, and other factors) to achieve the maximum safe power level throughout the RPM range. The difference of course is that the higher compression ratio is always in play, but tuning for it is much the same.
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