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Discussion Starter #1
This is a pruely a hypothetical question. As I have not yet CC'd anything and I am hoping the actual CR will be somewhat less then both of these projected numbers.

But lets just say I have a motor with a true actually measured & calculated compression ratio of either 11.82:1 or 12.75:1 (depending on configuration) Would the same fuel run both or would the higher CR motor require a step up in fuel octain requirments?

Basicly what I am wondering if I have to buy a higher octain fuel to run the 11.82 motor and that same fuel would also run the 12.75 motor (without having to switch to a more expensive fuel) then it would make sense, well to me anyway, to go with the higher CR motor as it would be a little bit like getting free power as the cost to run the motor will not have went up significantly:)
 

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There's no quick answer to that question. The quench distance will certainly be important, along with piston shape, combustion chamber design, ignition timing, etc. An adjustable timing controller may be helpful as well.
 

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Discussion Starter #3
well when I crunched the basic numbers I used a quench of .040"...in realility it might end up a little larger then that...:)

the rest is secret double knot stuff;)

J/K....lol:D
 

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I have a lot of text books and no where could I find a chart that shows a one to one relationship of static CR and octane.

The compression ratio is not a number. It's the ratio of swept to fixed volume. Just as you can't determine your MPH just from axle ratio without knowing rpm, trans gear, tire OD, etc. It's only one of the factors required to determine octane requirements.

Back in the sixties 11:1 required high octane to run. In modern times premimum is recommended for an 11:1 engine but with the aid of knock sensors, the computer can dial back timing and richen fuel mixture to allow it to run on 87 octane. Performance is not as good in this mode so running an 11:1 on 87 doesn't make sense.

For the same reasons, as you push the CR limits you may find that to safely run a higher CR with the same octane it requires less spark timing/more fuel which might cancel out any CR gains.

I think that when people are talking about quench they don't fully understand what it is and that it's not the panacea fix they think it is.

Quench and Squish are two separate but related terms.

Quench surfaces are there to transmit heat into the cooling system. This reduces detonation. Getting them closer helps conduct excess heat from compression more efficiently.

Squish is a phenomenom of when the piston comes very close to the quench surface the air trapped between the quench surfaces is squished so hard it shoots out into the center of the combustion chamber which causes A/F mixture to burn more efficiently.

I need to make a drawing to better illustrate it. If you've ever worked on 2 strokes they often have a "squish band" on the outer circumference of the piston and head.

I have to emphasis that minimum squish is something you sneak up on. It's a near collision with the head. If you get it wrong or your static dimension closes up dynamically, then the piston will hit the head and at the very least the rings will be pinched in the bores. At the worst you have catastrophic engine failure. As Dave (driveWFO) found out deck clearances can vary significantly on a rebuilt engine from a regular machinist.

A 12.75:1 engine with .040" squish is an advanced project. You'll need a crack machinist and a virtual suitcase of money to get it right.
 

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Discussion Starter #5 (Edited)
Paul Wright said:
A 12.75:1 engine with .040" squish is an advanced project. You'll need a crack machinist and a virtual suitcase of money to get it right.
thanks for the reply Paul and I agree which is why I said this in a previous post;)

69NovaSS said:
when I crunched the basic numbers I used a quench of .040"...in realility it might end up a little larger then that...:)
 

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Paul Wright said:
I have to emphasis that minimum squish is something you sneak up on. It's a near collision with the head. If you get it wrong or your static dimension closes up dynamically, then the piston will hit the head and at the very least the rings will be pinched in the bores. At the worst you have catastrophic engine failure.
Just thinking out loud here, but wouldn't you also have to factor in the rod construction material too? I.E., certains rods of type X may stretch more than rods of type y? I suppose ulta light-weight pistons would help.
 

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Discussion Starter #7 (Edited)
DriveWFO said:
certains rods of type X may stretch more than rods of type y?
Thats the down side of using rods made of rubber bands...they stretch too much....of course the upside is you can buy a bag of 100 for a buck:rolleyes: :D

But seriously thats a very good question:)
 

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im no expert and dont claim to be or to have all the answers. with that said it makes me think back to the days when we didnt worry about the science of it all, and just matched parts and went racing. what i mean is over the years ive had several performance motors for my own toys. the one that comes to mind is a 396 i had in my 75 nova pro street. when building this motor the main concern was what fuel i wanted to run. since i dont mind mixing or running high octane the sky is the limit. so i went with 12.5-1 slugs with the old oval closed chamber style heads with the usual port polish and all that. i picked a cam that had the rpm band i wanted and all other parts matched around that.quench,squish cam bleed off, static vs. dynamic compression none of that was considerd. and the funny thing is this motor produced fantastic numbers on the dyno with out the roller cam, without all the tech and science thats talked about now days. now i understand that with technology and computers and all, things arent "old school" any more but thats what i find funny. from what i read and what some who seem to have all the answers are saying the old school type engines shouldnt perform at all and yet they do! now one could say it was luck but im not talking about just 1 or 3 motors i mean dozens over the past 20 yrs. now i understand that technology has made a huge improvent in engines making power but sometimes i think we put to much into the science of it and dont go with what works.
 

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DriveWFO said:
Just thinking out loud here, but wouldn't you also have to factor in the rod construction material too? I.E., certain rods of type X may stretch more than rods of type y? I suppose ulta light-weight pistons would help.
That's why I mentioned that static clearance may not be your dynamic clearance. It's trickier than most people think.
If I remember right, I had .038" squish on the Wright Stuff 331 but I was sweating bullets to get there safely with a 35 year old 2 bolt block and cast crank.

The rods have to be close tolerance for length and the pistons for CH. You'll spend more for light weight, close tolerance pistons but it worth it. Never assume that a 5.7" rod is exactly 5.7".
You can grind the crank throws to account for slight variations, but I'd rather have accurate, equal sized parts. Even so, there's lot's of mocking up, measuring, dissassemble, machine some, repeat.
 

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Discussion Starter #10 (Edited)
Paul Wright said:
A 12.75:1 engine with .040" squish is an advanced project
I was thinking about this last night and something struck me. Are you saying that to build a 12.75:1 engine with a .040" quench is harder to do that to build a 10:1 eninge with the same quench? To me that doesnt make sense...I can see that a .040" quench would require a great deal of skill to hit but I dont see how the cr would factor into that making things more or less difficult...:confused:

Really this was as I stated in my original question just hypothetical...and the .040" quench was just a number I used with the calculator....I'm sure once parts are measured, mixed and matched, mocked up, machined, etc, etc I will have a much more realistic number for quench. Just like the cr numbers I stated...they are just based on the manufactures stated cc's of various parts...and as we know parts rarely cc out to these values...So even that number is very likely to change as the real values become known.

I guess the problem with my original question is that there are too many variables involved for someone to say ya the two motors would run on the same fuel or that no they wouldnt run on the same fuel... The difference in cost between the two motors really is negligible and it just made me wonder if they might be able to run on the same fuel...cause if they could then the higher cr motor really would be like getting free hp....:)
 

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.040" is more difficult than leaving it down .025" and using the typical thick gasket regardless of CR. For a 13:1 engine to run the same octane as 11:1 might be doable but not without more effort and attention to detail.
Getting almost 13:1 from a 350 almost certainly requires a dome if you are using a 23 degree 64cc head. The bigger the dome, the more likely it has to be fitted to the chamber. Flame travel is obstructed by high domes and for it to work right requires extra work.

The comment about people building reasonable power without messing with all this stuff is interesting and probably true to some extent for hobby builders, but for racing engine builders that's not an option these days if you have to win.
 

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Discussion Starter #12 (Edited)
Paul Wright said:
.040" is more difficult than leaving it down .025" and using the typical thick gasket regardless of CR. For a 13:1 engine to run the same octane as 11:1 might be doable but not without more effort and attention to detail.
Getting almost 13:1 from a 350 almost certainly requires a dome if you are using a 23 degree 64cc head. The bigger the dome, the more likely it has to be fitted to the chamber. Flame travel is obstructed by high domes and for it to work right requires extra work.
thanks Paul; well with a tiny little dome:rolleyes: and some extra stroke your in the ball park of 12.75:1;) Ya the fitting to the chamber is something I have to look forward too..BUT its a must do...dont want anything possibly touching that shouldnt touch...:eek:

Paul Wright said:
The comment about people building reasonable power without messing with all this stuff is interesting and probably true to some extent for hobby builders, but for racing engine builders that's not an option these days if you have to win.

You know I think about this stuff every so often..that it used to be so much simpler back in the day....of course most people back then dont go as fast as most people seem to do today. 400hp SB's and hitting the 11's in the quarter was a big deal back then...well today cars that do that seem to be a dime a dozen...heck you can buy 400HP SB's completely built for very little money today from several venders...its a great time to be into High Performance cars...IMO you can make more power today with less time, effort, and money invested then they could back in the day....
 

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A couple things hit me when I was reading the post. First one was the comment about the old days and not knowing or checking much yet making good power. Back in the 60s and 70s we did make good power but we used almost all factory parts. There were no great aftermarket heads really. Sort of the same thing with pistons, there were only a few pistons available, their were heavy and used big rings too. Few people if any played with rod lengths. Does anyone recall painting the tops of pistons to get more heat in the engine when we went with aluminium heads? Gas ported pistons to try and seal up a motor cause we didn't use deck plates and the bores weren't true and the rings available were junk? Technology, tooling, the wide variety of aftermarket parts, the ability to find and share information, and the computer allows people to create power today that we only dreamed of when I was a kid. My bracket car runs as fast as an old pro stocker did so while we did make decent power for the time, we are way past that today. The last one was nobody mentions aluminium blocks which are getting cheaper and more common. The blocks grow alot as they get hot. If you plan to use one you need to do your homework if you expect it to make good power. Cold clearances for deck height alone can change by as much as .020 or more when they get hot. RM
 

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Vizard has an article in the June 06 issue of PHR called "Compression Comprehension". He gets in to higher compression ratios along with quench, dynamic compression, expansion ratio, and thermal efficiency among other things. It was an interesting article.:)
 

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Discussion Starter #15
Repointer said:
Vizard has an article in the June 06 issue of PHR called "Compression Comprehension". He gets in to higher compression ratios along with quench, dynamic compression, expansion ratio, and thermal efficiency among other things. It was an interesting article.:)

Chris, thanks for the tip on this article....I gave it a quick look online and I'm going to run out and buy a copy...its a very interesting article:)

BTW here is the link to it

http://popularhotrodding.com/tech/0606phr_understanding_compression_ratio/index.html
 

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Wow, they have the article online. Guess I didn't need to buy a copy.;) Actually I think I'm gonna sign up for a subscription. I've read a few good article's in their mag. What's one more.:rolleyes:

I like the idea of doing a leak-down test on my motor. The short block was something I built in the late 80's and I was real green back then.:D I'm curious as to how well those rings are sealing. Some interesting stuff.:)
 
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