Compression ratio for 92 octane?
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I dont mean to hijack the thread but I think Im in the same boat as op. I only have acces to 92 as well and may have a poor cam choice. Is all dcr measured @ .006? anyone know what the tsp 228R 112 lsa cam is @ .006?
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Simple physics 101 here in play. You are not going to gain 50-75 hp going from 20 degrees to 25-26 degrees of spark advance on a naturally aspirated LS engine build. You are not even gaining 20 hp from that change. If your power is not as expected, it is a lack of airflow issue at the RPM range you are running. That is greatly affected by the cam timing events. If the IVC is too early your engine is not going to breathe at high rpm, plain and simple. Dropping the static compression ratio is not going to suddenly add 50-75 hp with 5-6* more advance. Low compression setups require more timing and high compression setups require less timing. That is simply from physics alone. The higher the dynamic compression ratio, the tighter the air/fuel molecules are packed together, thus once the spark ignites the flame kernel the flame front is able to propagate more quickly. This is the same reason boosted or nitrous engines require the timing to be retarded. I tuned a cammed LS3 not long ago that made peak power at 22* of ignition advance. I rarely see a LS head want more than 22-24* of advance for peak power. As the VE starts to nose over past peak HP they may want 2-4* more timing but that is very combination dependent. In an ideal world an engine would require no spark advance. The fuel would be burned instantly with a nice smooth pressure rise and the cylinder pressure would act on the crank with it at an ideal angle to maximize power. The more efficiently your engine burns fuel the less timing advance it needs or wants. My daily driver, tow pig has a DCR of 8.9:1, has 220 psi of cranking compression, and runs all day long on 91 octane.
Last edited by Fast355; Feb 25, 2026 at 03:14 AM.
Exactly!! Well said. Someone in my distant past once explained overlap to me thusly; Overlap is the time both valves are open. What many lose sight of is that while more overlap increases the time both valves are open, it also increases the time both valves are closed. While this is a simplified explanation, it made it easier for me to understand.....
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Both valves closed is from IVC event until EVO event, both of which are completely independent of the events that determine overlap.
Last edited by grinder11; Feb 26, 2026 at 06:49 AM.
AFAIK, what I stated is fact. The longer the valves are both open, the opposite reaction is they are both closed longer. To exaggerate this, line up both lobes exactly the same. Yes, the engine wouldn't run. This is for demonstration purposes. If the intake and exhaust are both open 160° of crank rotation, at the same time, they will both be closed for 160° of crank rotation at the same time. Actually, they aren't independent of overlap. If you change the IVC/EVO, you will alter the time the valves are closed. Hey, Scott-Can you explain this better? Or tell me I've been wrong all these years. Maybe I have been. I'd just like to know for 100% sure, right or wrong, no worries. Grubinski has me wondering now, lol!! @Che70velle
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I get what he is saying and it is correct. If you have lobes that were both ground to open the valves for 180* and they are lined up you have 180* where they are both closed. If you offset them 90* there would only be 90* left where both valves are closed.
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From: Carnation, WA
Cam 1 -- IVO 5 BTDC EVC 5 ATDC ... 10 degrees overlap. IVC 45 ABDC, EVO 55 BBDC, so both valves closed 260 crank degrees. Cam is 230/240 112.5+2.5
Cam 2 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC can still be 45 ABDC, EVO can still be 55 BBDC, again both valves closed 260 crank degrees. Cam is 232/242 111.5+2.5 More overlap doesn't mean longer time with both valves closed.
Cam 3 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC 43 ABDC EVO 53 BBDC, so both valves close 264 crank degrees Cam is 230/240 111.5+2.5 ... same lobes as Cam 1 (230/240)
So **IF** (IFF ... If and only if) we are talking about the SAME LOBES, just moving them towards more or less overlap, then yes, more overlap ==> both valves will be closed for a longer duration.
If that's what you meant, you should have said what you meant.
Cam 2 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC can still be 45 ABDC, EVO can still be 55 BBDC, again both valves closed 260 crank degrees. Cam is 232/242 111.5+2.5 More overlap doesn't mean longer time with both valves closed.
Cam 3 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC 43 ABDC EVO 53 BBDC, so both valves close 264 crank degrees Cam is 230/240 111.5+2.5 ... same lobes as Cam 1 (230/240)
So **IF** (IFF ... If and only if) we are talking about the SAME LOBES, just moving them towards more or less overlap, then yes, more overlap ==> both valves will be closed for a longer duration.
If that's what you meant, you should have said what you meant.
I think he means that the intake lobe of XXX duration opens the valve during 180 degrees of rotation and closes it during 180 degrees of duration. The same goes for the exhaust lobe. So during one rotation the valve is open as long as it is closed. Or is that incorrect?
Sorry for the confusion. I guess my explanation sucked. I kinda thought it did. Anyway, perhaps it would be clearer to word it differently. The tighter the LSA is, the longer both valves are open. Which increases overlap. You could still have negative overlap, but the tighter the LSA, you either have more overlap, and/or less negative overlap. We all know that for every action, there is an equal and opposite reaction. With both valves open longer at the same time, both valves will be closed longer, at the same time. I have seen this in print, as well, years ago in Hot Rod and Car Craft magazine. I'm hoping a cam vendor will chime in to explain it better.....
Last edited by grinder11; Feb 26, 2026 at 11:50 AM.
Cam 1 -- IVO 5 BTDC EVC 5 ATDC ... 10 degrees overlap. IVC 45 ABDC, EVO 55 BBDC, so both valves closed 260 crank degrees. Cam is 230/240 112.5+2.5
Cam 2 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC can still be 45 ABDC, EVO can still be 55 BBDC, again both valves closed 260 crank degrees. Cam is 232/242 111.5+2.5 More overlap doesn't mean longer time with both valves closed.
Cam 3 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC 43 ABDC EVO 53 BBDC, so both valves close 264 crank degrees Cam is 230/240 111.5+2.5 ... same lobes as Cam 1 (230/240)
So **IF** (IFF ... If and only if) we are talking about the SAME LOBES, just moving them towards more or less overlap, then yes, more overlap ==> both valves will be closed for a longer duration.
If that's what you meant, you should have said what you meant.
Cam 2 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC can still be 45 ABDC, EVO can still be 55 BBDC, again both valves closed 260 crank degrees. Cam is 232/242 111.5+2.5 More overlap doesn't mean longer time with both valves closed.
Cam 3 -- IVO 7 BTDC EVC 7 ATDC ... 14 degrees overlap IVC 43 ABDC EVO 53 BBDC, so both valves close 264 crank degrees Cam is 230/240 111.5+2.5 ... same lobes as Cam 1 (230/240)
So **IF** (IFF ... If and only if) we are talking about the SAME LOBES, just moving them towards more or less overlap, then yes, more overlap ==> both valves will be closed for a longer duration.
If that's what you meant, you should have said what you meant.
Last edited by grinder11; Feb 26, 2026 at 06:36 AM.
Thank you for bringing up my error on the 160° post. I meant to type 180°, lol. I'm aware there's 360° in a circle. I don't know how 160° got in there. I'll edit that post .....
There is an excellent cam article online that was originally printed up in the September, 1989 issue of Hot Rod magazine, and was posted online by the hot rod network in January, 2023. It is an excellent article that is explained much better than I can do. Just Google hot rod network cam talk....