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Maximum Compression ratio #3257380
09/12/24 11:20 AM
09/12/24 11:20 AM
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Kalispell Mt.
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HotRodDave Offline OP
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So a while back I asked about the biggest cam and now I am wanting to hear what is the most compression anyone has ran? What fuel and what is the highest static compression ratio you successfully used... out of curiosity in general not any particular build just wondering how hard people have pushed it... maybe as an added bonus throw in your cranking compression PSI...

Last edited by HotRodDave; 09/12/24 12:13 PM.

I am not causing global warming, I am just trying to hold off a impending Ice Age!



Re: Maximum Compression ratio [Re: HotRodDave] #3257418
09/12/24 02:58 PM
09/12/24 02:58 PM
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I've done a few engines in the 15:1 range. What I've learned is that high compression doesn't make more power unless the cylinder heads are designed to give you high compression. That is, stuffing a big dome in a big combustion chamber usually doesn't make more power. One of the very best high compression engines we did was a SBC with special Pontiac racing heads. Those heads had a tiny combustion chamber so that engine made 15:1 with flat top pistons. That engine was very, very efficient.
My Ford 521 has Kaase SR71 heads on it and it makes good compression with flat top pistons. Depending on the bore and stroke a person can get close to 15:1 with flat top pistons with these heads. That combination should make good power.
The one BB Mopar that I did with 15:1 didn't pick up power over a 13:1 version. That engine had Indy EZ heads on it and I think the bigger dome got in the way. If I was going to build a high compression BB Mopar I'd look for a better head than the EZ.
The very best heads I've worked with are the RS-X big block heads from GM. They have a fairly small chamber at 70 cc but are designed for large bore, large displacement engines such as the 632 crate motor. You can make a lot of compression with a flat top piston when you are 632 inches and a 70 cc chamber! The RS-X heads flow a ton of air so you can easily make more than 1000 hp even with a fairly mild camshaft.

Re: Maximum Compression ratio [Re: HotRodDave] #3257420
09/12/24 03:08 PM
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jwb123 Offline
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I think maybe you answered your own question by asking the cranking compression of different combinations. Because that which some call dynamic compression is much more important than static compression. I have built engines for limited truck pulling classes that required 93 octane pump gas as fuel, and I used 13 to 1 compression pistons. Lots of engine simulation programs out there but I use Performance Trends, every engine combination I build, I run it through the program, and those numbers generally are within a few percent on my real engine dyno. The program will predict idle vacuum, dynamic compression, cranking compression, plots an advance curve, predicts detonation, and a bunch more, plus it is relatively cheap. And with any computer program garbage in garbage out. You have to take the time to put accurate numbers into the program. And to top it all off it is just fun sometimes to play with.

Re: Maximum Compression ratio [Re: jwb123] #3257469
09/12/24 07:55 PM
09/12/24 07:55 PM
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My current race motor is 15.1-1. 580" with -1 heads that are 57cc using a flat top piston. Cam 285/296 .825"/.800"@111lca. Never dynoed but it will run 9.0x@147-149@3350 at 1.050-1.090 correction summer air.
Doug

Last edited by dvw; 09/12/24 07:56 PM.
Re: Maximum Compression ratio [Re: HotRodDave] #3257491
09/12/24 09:25 PM
09/12/24 09:25 PM
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montana
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The last motor I got for the dragster was at 16.2 to 1, according to David Rehr. It made right at 1400 hp @7300rpm, and had 1100 ft lbs of torque when they started pulling it at 6300 rpm. He recommended VP X-16 fuel for it. Has been running good so far. Jim.


64 Dodge Coronet 440. In progress
1998. Dodge Avenger. 8.35@165. 4400 DA
1980 Plymouth Arrow 572 Hemi.
242" Mullis Dragster. 6.90@ 200mph
Re: Maximum Compression ratio [Re: HotRodDave] #3257513
09/13/24 02:06 AM
09/13/24 02:06 AM
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Bend,OR USA
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My last E85 fuel bracket motor had between 14.98 to 1 up to 15.03 to 1, depending on the combustion chamber sizes. I didn't make the combustion chambers exactly all the same realcrazy That motor had also around .003 differences on the deck height differences due to rod length variances shruggy
That motor wouldn't vary .004 from the first run in the morning until the last race in the afternoon boogie, very easy to dial up devil

Last edited by Cab_Burge; 09/13/24 02:13 AM.

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Re: Maximum Compression ratio [Re: jwb123] #3257531
09/13/24 07:25 AM
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Frostbitefalls MN (Rocky&Bullw...
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Originally Posted by jwb123
I think maybe you answered your own question by asking the cranking compression of different combinations. Because that which some call dynamic compression is much more important than static compression. I have built engines for limited truck pulling classes that required 93 octane pump gas as fuel, and I used 13 to 1 compression pistons. Lots of engine simulation programs out there but I use Performance Trends, every engine combination I build, I run it through the program, and those numbers generally are within a few percent on my real engine dyno. The program will predict idle vacuum, dynamic compression, cranking compression, plots an advance curve, predicts detonation, and a bunch more, plus it is relatively cheap. And with any computer program garbage in garbage out. You have to take the time to put accurate numbers into the program. And to top it all off it is just fun sometimes to play with.

Sounds like great advice right here . Only you know how far down the rabit hole you will go to make power. After reading a bit abou ProStock and F1 engine development, it has shown me it is a very complicated thing once you go past generally known or accepted limits.
I run 15.25/1 using methanol, aluminum heads and block, 549 cubes, 4.50 bore, good sized dome designed by experts. Made good power with 572-13 heads, 982 hp.
With methanol, i was told years ago that beyond about 15.5/1 it takes a fatter mixture to be ok, not much power gain if any, and it is harder on parts. So there is no generic answer, especially when you hear that F1 engines are at 18/1, 93 octane fuel, but rpms as high as 21,000 rpm! So plan carefully, parts ain't cheap😊


8.582, 160.18 mph best, 2905 lbs 549, indy 572-13, alky
Re: Maximum Compression ratio [Re: gregsdart] #3257537
09/13/24 08:15 AM
09/13/24 08:15 AM
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Tulsa OK
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Not actual proof here but I have read there is diminishing returns as it gets higher. Going from 8:1 to 10:1 will yield more gain than going from 13:1 to 15:1. On the same token if you are building an engine with no fuel restriction you might as well go as high as you can.

There is some OEMs and modern engines with 13:1 compression on pump fuel but they have the sensors and monitoring to make it work. They can also control how the throttle rolls in separate from what the pedal is commanded to do etc.

I recently watched a video of a Porsche Caymen with a naturally aspirated 4.x liter stroked 6 cylinder in it. It was 13:1 and made like 560HP to the tire on california 91. The engine builder built it and tuned it for 100 octane, the guy had another tuner dyno tune it for 91 octane and didn't lose any power.


68 Barracuda Formula S 340
Re: Maximum Compression ratio [Re: AndyF] #3257550
09/13/24 09:39 AM
09/13/24 09:39 AM
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Southington Ct.
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turbobitt Online content
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Originally Posted by AndyF
I've done a few engines in the 15:1 range. What I've learned is that high compression doesn't make more power unless the cylinder heads are designed to give you high compression. That is, stuffing a big dome in a big combustion chamber usually doesn't make more power. One of the very best high compression engines we did was a SBC with special Pontiac racing heads. Those heads had a tiny combustion chamber so that engine made 15:1 with flat top pistons. That engine was very, very efficient.
My Ford 521 has Kaase SR71 heads on it and it makes good compression with flat top pistons. Depending on the bore and stroke a person can get close to 15:1 with flat top pistons with these heads. That combination should make good power.
The one BB Mopar that I did with 15:1 didn't pick up power over a 13:1 version. That engine had Indy EZ heads on it and I think the bigger dome got in the way. If I was going to build a high compression BB Mopar I'd look for a better head than the EZ.
The very best heads I've worked with are the RS-X big block heads from GM. They have a fairly small chamber at 70 cc but are designed for large bore, large displacement engines such as the 632 crate motor. You can make a lot of compression with a flat top piston when you are 632 inches and a 70 cc chamber! The RS-X heads flow a ton of air so you can easily make more than 1000 hp even with a fairly mild camshaft.


Andy,
Would like to here more info on your Ford Kaase SR71 build.. I also have a set on a mild build and so far love the power curve over the Edelbrock heads. I don't have any actual dyno power numbers though....
AG.


1970 Challenger w/572 Hemi street car and my pride and joy. 1986 T-Type with 272 Stage 2 Buick V6 engine - True 8 second street car. Just updated the engine and put down 928 HP @ 35# boost to the ground on chasis dyno. 1976 Cee Bee Avenger Jet Boat - 460 Ford powered.
Re: Maximum Compression ratio [Re: turbobitt] #3257559
09/13/24 10:12 AM
09/13/24 10:12 AM
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Las Vegas
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My last few engines have been 16.5-17.2-1 these are also aluminum blocks. We keep making more power. My 655 made 1200/1000 at 17.2-1 with a single 4 barrell on a cast intake with only 3" of vacuum due to a broken pulley. On the heads up car we have been squeaking it up and are now at 17.4-1 with no diminishing returns yet. It just came off the dyno last week and is up again on power it is making just under 2.8 per cube. It is also an aluminum block. Just did a 582"B1 that was 16-1 with an iron block. Made 1061 and is nothing fancy just a bracket piece.

Cranking on the 655 was 230, the Vette is 220, most range in and around that area. When the vette had 15.5-1 its cranking compression was 180..The cranking on the last B1 was 240. iron block. There are a lot of factors that go into cranking numbers for sure. But one thing with an aluminum block is you loose cylinder pressure at a very rapid rate compared to cast iron and a way faster than compacted graphite block.


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Re: Maximum Compression ratio [Re: Al_Alguire] #3257575
09/13/24 10:49 AM
09/13/24 10:49 AM
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WI
mopar873 Offline
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Originally Posted by Al_Alguire
My last few engines have been 16.5-17.2-1 these are also aluminum blocks. We keep making more power. My 655 made 1200/1000 at 17.2-1 with a single 4 barrell on a cast intake with only 3" of vacuum due to a broken pulley. On the heads up car we have been squeaking it up and are now at 17.4-1 with no diminishing returns yet. It just came off the dyno last week and is up again on power it is making just under 2.8 per cube. It is also an aluminum block. Just did a 582"B1 that was 16-1 with an iron block. Made 1061 and is nothing fancy just a bracket piece.

Cranking on the 655 was 230, the Vette is 220, most range in and around that area. When the vette had 15.5-1 its cranking compression was 180..The cranking on the last B1 was 240. iron block. There are a lot of factors that go into cranking numbers for sure. But one thing with an aluminum block is you loose cylinder pressure at a very rapid rate compared to cast iron and a way faster than compacted graphite block.


Al, Awesome numbers. I have to ask, is your SBC running an MBE or CFE head or something like a Dart 9 degree? I'm fortunate enough to get to spend some time around a local 655" Ford build that makes 2.45/cube and the time/money/tuning investment is huge in these applications. It's a whole different level....

Last edited by mopar873; 09/13/24 10:50 AM.
Re: Maximum Compression ratio [Re: mopar873] #3257619
09/13/24 02:29 PM
09/13/24 02:29 PM
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Las Vegas
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Originally Posted by mopar873
Al, Awesome numbers. I have to ask, is your SBC running an MBE or CFE head or something like a Dart 9 degree? I'm fortunate enough to get to spend some time around a local 655" Ford build that makes 2.45/cube and the time/money/tuning investment is huge in these applications. It's a whole different level....


It's a Dart Little Chief head, pretty much unicorns as no one really runs or ever did run them. ALL of our heads are done by Slawko. This set up is basically an Aussie pro stock type deal from 15 years ago with a "cast" intake single 4 and a wet sump oiling system.

Old pic back when it still had aluminum rockers..A few years ago

[Linked Image]


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Re: Maximum Compression ratio [Re: Al_Alguire] #3257624
09/13/24 03:03 PM
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Mine is budget piece made up of used and refreshed parts. By far not an all out piece. But it's goes 300+ passes before being looked at. And then springs and maybe rod bearings. My thought on compression is that it's cheap HP. It only uses about .5 gallon a pass. Diesel fuel to the track is a far larger expense. Haven't checked cranking compression in years. As I remember it was around 190-200.
Doug

Last edited by dvw; 09/13/24 08:23 PM.
Re: Maximum Compression ratio [Re: Al_Alguire] #3257628
09/13/24 03:31 PM
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Awesome!

Re: Maximum Compression ratio [Re: mopar873] #3257730
09/14/24 10:36 AM
09/14/24 10:36 AM
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Las Vegas
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I expect my bracket stuff to go at least 300-350 passes. Although of late I have not kept any of them long enough to get there smile Last time the Cuda was freshened it had 404 laps on it. 16-1 with just a tick under .900" lift. I try to cycle the rods out at 350 max but was chasin wallys and a championship so it got stretched out a bit


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Re: Maximum Compression ratio [Re: Al_Alguire] #3257740
09/14/24 11:28 AM
09/14/24 11:28 AM
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DCR is vastly over-rated as a comparator.


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Re: Maximum Compression ratio [Re: AndyF] #3257753
09/14/24 12:44 PM
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Originally Posted by AndyF
I've done a few engines in the 15:1 range. What I've learned is that high compression doesn't make more power unless the cylinder heads are designed to give you high compression. That is, stuffing a big dome in a big combustion chamber usually doesn't make more power. One of the very best high compression engines we did was a SBC with special Pontiac racing heads. Those heads had a tiny combustion chamber so that engine made 15:1 with flat top pistons. That engine was very, very efficient.
My Ford 521 has Kaase SR71 heads on it and it makes good compression with flat top pistons. Depending on the bore and stroke a person can get close to 15:1 with flat top pistons with these heads. That combination should make good power.
The one BB Mopar that I did with 15:1 didn't pick up power over a 13:1 version. That engine had Indy EZ heads on it and I think the bigger dome got in the way. If I was going to build a high compression BB Mopar I'd look for a better head than the EZ.
The very best heads I've worked with are the RS-X big block heads from GM. They have a fairly small chamber at 70 cc but are designed for large bore, large displacement engines such as the 632 crate motor. You can make a lot of compression with a flat top piston when you are 632 inches and a 70 cc chamber! The RS-X heads flow a ton of air so you can easily make more than 1000 hp even with a fairly mild camshaft.

With the current crop of aftermarket Alum heads, can you just mill the heads enough without downsides, to get to the higher CR's (13+?) and avoid needing the dreaded domes?


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Re: Maximum Compression ratio [Re: jcc] #3257875
09/15/24 09:20 AM
09/15/24 09:20 AM
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PA
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I bracket race and make a lot of passes each year so I never pushed the compression envelope. On alky I stay around 14.5 or so max. I have never set engine master records but my engines always run good, and need to stay together for hundreds of passes yearly. I think the tuning window gets tighter with the really high compression stuff too.

I will echo the flat top being the best way to make compression. Domes want more timing and you end up with a heavier piston.


67 Barracuda street/bracket car 11.27-119
68 Dart 502 BB 5.44-126
414 cid SB Dragster 7.65-174
Re: Maximum Compression ratio [Re: moparacer] #3257895
09/15/24 10:38 AM
09/15/24 10:38 AM
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Las Vegas
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Here is 17 plus with a small chamber head 2.8 per cube. While clearly not a bracket deal it had been on the car for two plus years when we took it apart this time.

[Linked Image]


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Re: Maximum Compression ratio [Re: Al_Alguire] #3257896
09/15/24 10:41 AM
09/15/24 10:41 AM
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Here is 17.2-1 on the 655..Sorry dont have just a piston picture,

[Linked Image]


"I am not ashamed to confess I am ignorant of what I do not know."

"It's never wrong to do the right thing"
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