How much actual performance would be lost by dropping 1 point in CR

[gryphon]

I'm posting this in the Shovel and EVO sections rather than General because I kinda feel that some threads get overlooked in general. Okay, in taking a loss of one point in compression ratio from 9.5:1 down to 8.5:1 how much would this actually  effect perceived performance. Not necessarily what will it show on a dyno but, is this something you would really feel or would the effects be negligable. I assume that running a cam with a very early intake closing would help offset it somewhat also as would tuning my exhaust.

[Buddy WMC]

Gryphon,

Been doing these for over 40 years on and off. Compression is what brings an engine to life amongst other things and yes, you can feel the difference. I have never had an engine below 10.5 to 1 unless it was lower that that before I changed it. To be fair I usually waited until the winter downtime when living in Western N.Y.

The only exception to that was the only new Harley I ever owned, a 1977 black FXS for warranty reasons. The only real reason I can see to lower the CR would be to run a lower octane fuel. With today's ignition systems as you know, there will be no pinging on 93 octane with a higher ratio if set up correctly. You really would not believe the compression ratio's being run on a Pro-Stock race engine today.

BTW, I'm old enough to remember buying Sunoco 260 for my 88" souped up Shovel for .42 cents a gallon and those really were the days. I also remember having to change the valves and seats in everyone's heads when they removed the lead from gasoline.

What are you actually trying to accomplish?

[gryphon]

I found a killer deal on a new 107ci crank assembly with cylinders and pistons. Problem is they somehow ended up with 113ci cylinders. According to S&S if I shave the heads by .06 I'll end up with a loss of only 1 point. If I end up having to buy the correct cylinders or getting higher comp. pistons then it starts to make this less of a good deal. I can get the entire assembly for $650 balanced, assembled, trued and pistons fitted to bores.

[gryphon]

By the way. I'm old enough to remember filling my panhead with super for less than 40 cents a gallon.

[Buddy WMC]

IMHO,
It will kill the benefit of having the extra inches and stroke. Do you have to run S&S pistons, or can you go with CP's? Call Andrew at Revolution Performance 1-866-892-2109. Tell him Brian from Choppers Spirit referred you. They do or have alot of custom CP stuff in stock and on sale and you might get lucky.

It does not hurt to call and find out and let me know what he says. They were dead on with my combo and he'll tell you that!

[gryphon]

Nope. I just called the guys at REV. Perf. but they don't have any 4" bore pistons. If I end up spending money on items that I've already bought then the deal starts to lose it's appeal.

[rdkng]

Hey Gryphon, that sounds pretty interesting having the S&S parts &
I assume you'd be going to a 4 inch bore and have 107 inches?
With cubic inches being the same, I did notice a pretty good increase
in power when I took my 80 inch Evo from 8.5 to 9.5 CR as the EV27
woke up.   If youre increasing cubic inches, then the reduction may not
be an issue.  At least low compression motors last forever
Let us know what you do and all.
enjoy
rdkng      

[gryphon]

I'm presently running an S&S 4" bore 97 incher. This motor runs great but I was attracted by the relatively cheap price of the 107" setup. Sooner or later it's gonna be time to re-do this motor. I had also thought about swapping it out this winter and keeping the 97" setup as a backup. In any event it's not a necessary thing right now. Just sounded like a pretty good deal.

[stroker800]

  To tell you the truth,,,I don't think you would even notice the difference...Your bike might run better do to not getting as hot . You could esily make up some more power buy running a cam designed for a lower comp motor or even messing with timing...I have found over the years S&S seems to over rate the comp..I had a 74" low comp motor few years ago and it would mess hotrodded evos....I should have left that motor alone and built another.

Dave

[Hillside Motorcycle]

Gryphon,

Been doing these for over 40 years on and off. Compression is what brings an engine to life amongst other things and yes, you can feel the difference. I have never had an engine below 10.5 to 1 unless it was lower that that before I changed it. To be fair I usually waited until the winter downtime when living in Western N.Y.

The only exception to that was the only new Harley I ever owned, a 1977 black FXS for warranty reasons. The only real reason I can see to lower the CR would be to run a lower octane fuel. With today's ignition systems as you know, there will be no pinging on 93 octane with a higher ratio if set up correctly. You really would not believe the compression ratio's being run on a Pro-Stock race engine today.

BTW, I'm old enough to remember buying Sunoco 260 for my 88" souped up Shovel for .42 cents a gallon and those really were the days. I also remember having to change the valves and seats in everyone's heads when they removed the lead from gasoline.

What are you actually trying to accomplish?

Well said.
We too remember the good old Sunonco 260, and as far as compression, our AHDRA Modified bike loved 17.25:1 CR.
We strive here to maintain cylinder pressure, correctly. Certainly no good once you go over the top.
I would not arbitrarily, pull squeeze out of an engine, unless you want it to be sluggish.