Re: Compression
[Re: cesar perez]
#3038677
05/01/22 05:12 AM
05/01/22 05:12 AM
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Joined: Jan 2003
Posts: 10,020 Frostbitefalls MN (Rocky&Bullw...
gregsdart
I Live Here
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I Live Here
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Posts: 10,020
Frostbitefalls MN (Rocky&Bullw...
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The difference between pump 91 to pump 93 and being able to run e85 will be a huge change. About 10.5 to one with aluminum heads is considered about as high as compression should be for e10, or pump gas. E85 on the other hand can tolerate much higher ratios. For e85 out of the pump, a race motor will tolerate 13/1, and e85 race fuel or e98 ethanol can be run at up to 15/1 from what i have read. So if you can dedicate an engine combo to e85 it will be able to handle some serious compression . I have a 2019 5.0 gt mustang with factory 12/1 compression, and after retuning it for e85 the difference was huge.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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Re: Compression
[Re: jwb123]
#3038727
05/01/22 10:46 AM
05/01/22 10:46 AM
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Joined: Jan 2003
Posts: 20,918 A collage of whims
topside
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 20,918
A collage of whims
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Cylinder pressure is indeed the big deal. The camshaft specs - and obviously ignition timing - will have a fair amount of "input" on octane requirement. Also, the load the engine's under in whatever the operating scenario is. Details like no sharp edges in the chamber matter as well. For something like a 3500-lb car. 3.55-4.10 gear, street 440, alum heads, 220-240 @ .050, on 93, I've run 10.0/10.3 static (measured). None rattled/pinged, all had great response, but that was pretty much the max CR on 93 for me. I do have a policy of never lugging a performance engine.
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Re: Compression
[Re: Cab_Burge]
#3038854
05/01/22 06:54 PM
05/01/22 06:54 PM
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Joined: Jul 2005
Posts: 712 California
BigDaddy440
super stock
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super stock
Joined: Jul 2005
Posts: 712
California
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I only have experience with gasoline. As others have mentioned static compression is basically a rule of thumb type figure as it doesn't take into consideration the valve events occurring while your engine is actually running. With a small block ford stroker engine, I have ran 200psi of cylinder pressure with aluminum heads on 91 octane gas without detonation. My dynamic compression ratio was 8.4:1 and my quench was dialed in at .035". On an iron headed motor with 91-93 octane, I'd keep my dynamic compression under 8.0 (on Pat's calculator) and with Aluminum heads, under 8.5. Quench is super important too, .030-.045" and keep an eye on piston to valve clearance as well. If you're building the engine and you have access to all the figures and measurements, I'd use a dynamic compression calculator. Personally, I use Pat Kelley's which can be downloaded for free. You have to know how to use the calculator, as the information must be entered in a particular order, but it's a great tool. You should read his webpage on the topic and download the calculator: http://members.uia.net/pkelley2/DynamicCR.htmlDM me if you need help with the calculator. -Dan
1969 A12 Roadrunner 1970 Plymouth Cuda 1968 Dodge Dart
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Re: Compression
[Re: cesar perez]
#3039142
05/02/22 04:47 PM
05/02/22 04:47 PM
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Joined: Jan 2005
Posts: 12,424 Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
Joined: Jan 2005
Posts: 12,424
Kalispell Mt.
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depends on a lot more than the info given.
I have about 11 to 1 and 230+ PSI in my daily driver 6.4 hemi ram and no knocking on 87 octane (well it does just a little when driven stone cold but that is a tuning problem), it does have a plastic intake and much better chambers that help. My dads 97 geo metro would crank 210 PSI and ran fine on 87 octane. A BB mopar could easily be built with the right components to run safely up to 11 to 1 in a street motor. If your just going to slap a bunch of random parts laying around and try to make it work you would probably not want to go over about 10 to 1.
For the other side of the equation e-85 acts like a higher octane than it actually measures because of the cooling effect of the evaporating alcohol, I have seen as high as 16 to 1 but 13 or 14 to one is pretty normal.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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