Higher compression octane requierments

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Lefty71

Lefty71

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Anybody have a link to a meaningful chart showing octane requierment at a given compression at a given temperature. I'd like to find something that uses cranking pressure as a baseline, if such an animal exists. I know the calculated static and dynamic compressions for the build, but the cranking pressure is more of the reality, if you know what I mean. TIA, Lefty71
 
Lefty71 said:
Anybody have a link to a meaningful chart showing octane requierment at a given compression at a given temperature. I'd like to find something that uses cranking pressure as a baseline, if such an animal exists. I know the calculated static and dynamic compressions for the build, but the cranking pressure is more of the reality, if you know what I mean. TIA, Lefty71
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There is no such thing. Cranking compression means nothing, unless you have changed cams in the same engine, and even then it's nearly meaningless.

You can run 12:1 on pump gas and not have issues. You can have 9:1 compression on pump gas and it will rattle it's brains out.

It's all about getting the parts that work together before you start the build.
 
i agree.
Ballpark with a stock style cam would be comp ratio = octane
i.e. 8.7 = 87, 9.1 = 91 etc
 
Yep, how such squeeze and the cam will make all the difference. I'm in N.E. Ohio and run this Lunati 10230703 cam with Speed Pro H116CP pistons in a 360. Block is decked so they are level with the deck. Heads are 71cc. I can use 91 octane no problem 89 and it will ping. Runs great. I used to run a xe268 Comp Cam and no matter what I did it rattled. That was the only change and it is a completely different animal.
 
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No,because there are so many variables, there's no such animal. I can give a good example. @EL5DEMON340 get in here. Tell um about your combo. I'll let him give the details, but it's an iron headed small block, over 11:1 pump gas motor. That's but one of many examples.
 
I have a 9.6/1 273 that runs pretty comfortably on 91. I have ran 87 E10 with no ping but I didn't push it either. It's all in the chambers.

11_14_0.JPEG
 
ir3333 said:
i agree.
Ballpark with a stock style cam would be comp ratio = octane
i.e. 8.7 = 87, 9.1 = 91 etc
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Ok, but do you mean static comp, or dynamic. At 11.5 to one static, my minimum is now 110+ octane, or are we now in a different ballpark, lol? And what is a stock style cam?, .550+ not at factory...

RustyRatRod said:
No,because there are so many variables, there's no such animal. I can give a good example. @EL5DEMON340 get in here. Tell um about your combo. I'll let him give the details, but it's an iron headed small block, over 11:1 pump gas motor. That's but one of many examples.
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Thx, sounds similar, but this is a BB383. Motor is 383, 11.5 to one old school TRW pop-up slugs, iron 915's and a small Hughes mechanical cam. Thx everyone for responses, always good to learn, especially on plan for next build as well.
 
very rough guide using static comp and stock style muscle car cams
which were around .450 lift and .220 dur "give or take"
 
Don't remember which gaskets just that they are Felpro. Bore is 30 over. Heads are cast iron.
 
Lefty71 said:
Ok, but do you mean static comp, or dynamic. At 11.5 to one static, my minimum is now 110+ octane, or are we now in a different ballpark, lol? And what is a stock style cam?, .550+ not at factory...



Thx, sounds similar, but this is a BB383. Motor is 383, 11.5 to one old school TRW pop-up slugs, iron 915's and a small Hughes mechanical cam. Thx everyone for responses, always good to learn, especially on plan for next build as well.
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How are you measuring dynamic compression ratio? Are you sure, for a fact your CR is that high. I'm asking, did you actually measure dome volume, chamber volume, gasket volume and do the math?

If you didn't, you don't know what you have. Usually the CR is much lower than advertised. But you could easily be off a point or more either way.

11:1 on pump gas is relatively easy to do.
 
yellow rose said:
How are you measuring dynamic compression ratio? Are you sure, for a fact your CR is that high. I'm asking, did you actually measure dome volume, chamber volume, gasket volume and do the math?

If you didn't, you don't know what you have. Usually the CR is much lower than advertised. But you could easily be off a point or more either way.

11:1 on pump gas is relatively easy to do.
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Everything was measured, and everything is known. The worksheets are not right here in front of me (i'll grab them), but I personally made all calculations, checked all clearances and verified all cam timing with a wheel, CC'ed chambers with a burette, and .500 down fill for dome volume, gaskets are very thin Cometec units, etc, etc. I mocked it up several different ways to see which is the proper direction to go, and what is too much for iron heads, in all practicality. The static is over 11.5 to one with the block decked and the 915 closed chambered heads shaved, pretty close to what the ancient catalog said they would be, and the dynamic is a true 10.0 to one, as according to the cam timing.
But you make my point in asking the question in the first place. So many motors are built with relative known parts, and it works out just fine. But, and this is a big one, there are large numbers of folks that are running by the seat of their pants, and don't know for a fact what the dome is, the chamber volume is, the critical valve clearances, the actual minimum gas requirements, yada, yada, yada, then want to comment but are kinda scratching their heads. I'm trying to learn where the edge is, NO GUESSING. I had hoped cranking pressure would tell us something, as an added layer of dimension to the puzzle. Thx again to all who have jumped in on this thread, I appreciate the voice of experience!! Lefty71
 
Lefty71 said:
Everything was measured, and everything is known. The worksheets are not right here in front of me (i'll grab them), but I personally made all calculations, checked all clearances and verified all cam timing with a wheel, CC'ed chambers with a burette, and .500 down fill for dome volume, gaskets are very thin Cometec units, etc, etc. I mocked it up several different ways to see which is the proper direction to go, and what is too much for iron heads, in all practicality. The static is over 11.5 to one with the block decked and the 915 closed chambered heads shaved, pretty close to what the ancient catalog said they would be, and the dynamic is a true 10.0 to one, as according to the cam timing.
But you make my point in asking the question in the first place. So many motors are built with relative known parts, and it works out just fine. But, and this is a big one, there are large numbers of folks that are running by the seat of their pants, and don't know for a fact what the dome is, the chamber volume is, the critical valve clearances, the actual minimum gas requirements, yada, yada, yada, then want to comment but are kinda scratching their heads. I'm trying to learn where the edge is, NO GUESSING. I had hoped cranking pressure would tell us something, as an added layer of dimension to the puzzle. Thx again to all who have jumped in on this thread, I appreciate the voice of experience!! Lefty71
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Cool. You are ahead of 99.875% of the people doing this. That's the ONLY way to know your CR.

I'm a firm believer that the engine doesn't know or even care what materiel the cylinder heads are made of as far as it relates to detonation resistance. Neither do many people I know. Some guys continue to spew that, but I've never seen that as the case. When I pick a CR, i don't even ask what the heads are made of.

Running that CR requires good tuning, a damn good ignition and camshaft selection is by far the the biggest factor in detonation resistance verses CR.

And that doesn't mean buy a cam that's the biggest in the book.

It means calling a cam grinder on the phone and telling them what you are doing. If they don't make you confident in what you are doing, call another until you find one who agrees with what you are doing and makes you comfortable.

It sounds like you have the vast majority of the hard work done. A cam change my be in order.

I also forgot to mention that spark plug selection become much more critical as you try and run higher CR's on pump gas. You may need to drop another heat range to get detonation under control. Which went with my saying a damn good ignition is essential. That eliminates the HEI, Chrysler, Pertronix and probably a couple of others I'm missing.


Edit: I forgot to mention that the edge is an ever moving target. There are just too many variables to nail that edge down with any sort of accuracy.

I don't think you have too much compression. You may have the wrong cam, and maybe another small thing or two wrong but you are very close to get this combination to make power and not rattle it's guts out.
 
yellow rose said:
Which went with my saying a damn good ignition is essential. That eliminates the HEI, Chrysler, Pertronix and probably a couple of others I'm missing.
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True. I don't mind being called a Mopar fanboi by any means, but once I discovered MSD long ago, I tossed my chrome box about as far as I could.
 
My 367 has run as much as 11.3 with a 292/292/108 cam. Just under zero deck and OOTB closed chamber aluminum Eddies.
I then installed a 270/276/110 cam and reduced the Scr to 10.9
Then I installed a 276/286/110 cam and again adjusted the Scr to 10.95
My engine could easily run more , and I would, except I almost had a bad experience with .028 gaskets, with the 292 cam;almost. With the FellPro .039s I have had zero problems.
All three were advertised at .008 tappet lift.
I have run all three combos on 87E10 with 34* timing, all in no sooner than 3200.
The car was heavy at 3650, me in it. Now a lil lighter.lotsa miles
All three of these combos ran right around 177 to 185 psi cranking cylinder pressure.
 
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I disagree with AJ. But the more I travel, around the country, the more I realize the speed of quality gasoline from state to state. AJ makes claims that are laughed at here. We all say he has “Magic Gas” like “Jack and his beanstalk!” There is no way that happens. Here!
 
Did Hot Rod Daves' Super Duper Mileage Master 318 make 30 mpg plus without overdrive? I can't remember?
...and our Canadian gallon is 20% bigger!
 
LMAO, I think I read 32, but that was sooooo long ago. He out a lot of effort into it. Totally NOT cost effective it certainly fun!
 
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