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10.5 : 1 Compression Ratio. Too much?

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Old 11-25-2007, 05:50 PM
Clifton63 Clifton63 is offline
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Default 10.5 : 1 Compression Ratio. Too much?

I have an EVO 80ci with 10.5 KB pistons bored 020 over standard. Has standard heads with Andrews EV27 cam. Dyna S ignition. Is this ratio too much stress for the engine. How do I prevent "Detonation"?
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Old 11-25-2007, 07:17 PM
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Default RE: 10.5 : 1 Compression Ratio. Too much?

10.5 will take some engine life away but it isn't that bad....however it's way too much compression for that cam, your going to get detonation with it.
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Old 11-25-2007, 08:56 PM
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Default RE: 10.5 : 1 Compression Ratio. Too much?

Would 9.5 ratio pistons be better suited for this cam? How much power will I lose going from 10.5 to 9.5?
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Old 11-26-2007, 08:43 AM
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Default RE: 10.5 : 1 Compression Ratio. Too much?

The ev 27 is a great cam at 9.5 up to 9.8 compression 10.5 is a bit steep. If you want to keep the engine the way it is you can swap cams. There are several that will work with the compression. I have used several of the Comp cams extreme cams and have had very good luck with them.
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Old 11-28-2007, 08:16 PM
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Default RE: 10.5 : 1 Compression Ratio. Too much?

I found some forged ones for cheap and they are 10:1. Do you think that is pushing it or should I be OK with the Andrews EV27?

I was considering the Keith Black 9.5:1 but I think they are cast. I heard forged are better. Anyway I would like to know what you think. Thanks.
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Old 11-29-2007, 10:11 AM
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Default RE: 10.5 : 1 Compression Ratio. Too much?

Well if you had our ported heads no , head porting is going to effect how the engine controls temps , etc. I still think you may run into problems. As I listed the ratio that I would run. You may get by on 10.1 with timing alterations and not lugging the engine. As well fuel quality is going to play a part in your issue. Why not swap the cam out?? Leave it at 10.5 and just use a later closing cam??? There are plenty of cams that iwll make great power with that compression. I have used several of the V-thunder extreme cams and have found them to really pull hard and not be a loud cam. Gentle ramps , rather middle of the road lift and nice duration. Just a thought
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Old 11-29-2007, 02:47 PM
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Default RE: 10.5 : 1 Compression Ratio. Too much?

I would agree with Steve, I would check and set the quench around .030"- .035" and go with the compression and like a .560 Viper cam.
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Old 11-29-2007, 06:46 PM
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Default RE: 10.5 : 1 Compression Ratio. Too much?

Thank you for the great advice. The reason I am considering different pistons is because one of mine is cracked and I have to replace them. I was going to go with10.5 replacements since that was whatwas in there before,until you guys told me that was too much for that cam. I really don't want a race engine, I would like a little more performance than stock, but at the same time I want problem free and smooth. I think I will go with the 9.5 KB pistons bored .030 over (I need a good hone). With your advice it sound like the EV27 would work. I really didnt want to have to buy another cam. I am looking at getting the Dyna 2000 ignition module. Currently the previous owner removed the ignition module and direct wired a DYNA S pickup to the coil. It is Dual fire. I also want to convert to Single Fire. This bike really was mis-treated. Mismatched bolts, bent rim, never washed. Tonight I discovered it has a "Indian" rear rim. This is almost like a "junkyard" bike. I am working feverishly to restore it. Ebay has been a saviour making that affordable. I also am replacing the engine case that cracked around the cylinder bolts. (Probably due to the detonation and high compression). Thank you again for the great advice you have given me. I am a new Harley owner (just 3 weeks).
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