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Compression #3038669
05/01/22 01:13 AM
05/01/22 01:13 AM
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florida dade
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cesar perez Offline OP
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What would be a good max compression ratio for a bb wedge motor with aluminum heads on 93 pump gas or even e 85 540 ci

Re: Compression [Re: cesar perez] #3038677
05/01/22 05:12 AM
05/01/22 05:12 AM
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Frostbitefalls MN (Rocky&Bullw...
gregsdart Offline
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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.


8.77 153 mph best, 3055 lbs 528 indy 440-1 alky
Re: Compression [Re: cesar perez] #3038706
05/01/22 09:24 AM
05/01/22 09:24 AM
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Missouri
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jwb123 Offline
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Static compression has little to do with the octane needs of an engine. Cylinder pressure is the main indicator. A simple cranking compression which can be estimated on a lot of engine design software will give you the best indicator. Just from my experience for 93 octane you want to keep the cranking compression in the neighborhood of 140 to150 psi. I use performance trends software but a lot of them out there. Performance Trends actually calculates the cranking pressure and generates an advance curve to use whatever octane fuel you select. I find it pretty accurate, when running an engine on a real dyno that I set up with the software usually the max HP timing is within a couple degrees of the prediction. And HP predictions are always within 10% And also it is not so much how much compression you can run but how much timing you let the engine have as well. 91 octane to E85 is not an apples to apples comparison for a fuel type in regards to detonation. Just to see I pulled up an old engine build and between 91 octane and E85 the computer took out 10 degrees of timing for the gasoline. And it made 50 HP more with the E85

Re: Compression [Re: jwb123] #3038727
05/01/22 10:46 AM
05/01/22 10:46 AM
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So Near, Yet So Far
topside Offline
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So Near, Yet So Far
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.

Re: Compression [Re: cesar perez] #3038779
05/01/22 01:57 PM
05/01/22 01:57 PM
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Bend,OR USA
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Cab_Burge Offline
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my last pump gas stroker motor had 10.78 to 1 compression ratio, ran fine on Oregon 91 octane pump swill at 180 Ft. above sea level at Woodburn drag strip and at 3500 to 4500 Ft. above sea level here in Bend up


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Compression [Re: Cab_Burge] #3038854
05/01/22 06:54 PM
05/01/22 06:54 PM
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California
BigDaddy440 Offline
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California

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.html

DM me if you need help with the calculator.

-Dan


1969 A12 Roadrunner
1970 Plymouth Cuda
1968 Dodge Dart
Re: Compression [Re: cesar perez] #3039142
05/02/22 04:47 PM
05/02/22 04:47 PM
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Kalispell Mt.
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HotRodDave Offline
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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!



Re: Compression [Re: cesar perez] #3039166
05/02/22 06:25 PM
05/02/22 06:25 PM
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Bend,OR USA
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Cab_Burge Offline
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Bend,OR USA
E 85 carb. jetted for max power made more torque than 91 octane gas jetted for max power on the same day on the same engine dyno, but it did make less HP work
My engine dyno test done years ago show me that 15 HP less, 767 HP on E85 and 783 HP on pump swill, the E85 torque was around 30 Ft. lbs. more boogie
Torque moves the mass, not HP work scope


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)






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