EVOs and modded compression ratio
Club Member
Joined: Aug 2008
Posts: 22,857
Likes: 17,908
From: Lafayette, Louisiana
If I said it once, I said it a thousand times. Cranking pressure is the important number, and is influenced by a number of factors including cam shaft selection.
As a point of reference to answer the OP, I am right at what I consider safe limit with my build
Andrews EV13 cam
Cosmetic gasket kit with .030" head gasket (original gasket is .045")
Andrews EV13 cam
Runs perfect with 91 octane
I ran out of gas once in the middle of nowhere and long story short, I had to put 87 octane in it to get me to civilization. I pay a lot of attention to various sounds regardless, and sure enough I heard the pinging / detonation when I first took off with the 87 in the tank. In order to avoid damage, I decided to stay in one gear lower than normal to reduce load on the engine, and that made the pinging stop. As soon as I got some 91 octane in it, everything was fine.
If I would go to with an EV27 cam, I would have lower cranking pressure and thus probably would not need the 91 octane. I like the EV13 because it fits my riding style better. It is made to use with stock compression, but all Evo's can benefit from a bump in compression, so that is why I went up with it. It is also known that there were sloppy tolerances with combustion chamber volume, so my intent was also to get the head volumes to be as close to the same as possible.
The result is I have an extremely smooth running Evo that makes great power at the rpm I need it. It also runs substantially cooler than it did when it was stock.
I never put it on a dyno.
Most people are probably making bigger numbers than I am.
That doesn't matter.
What matters is I like my results so much that this is the last bike I am ever going to buy, unless it gets totaled or something catastrophic happens to where it is not reasonably rebuildable. At that point, there is going to be a lot of tough decision making.
As a point of reference to answer the OP, I am right at what I consider safe limit with my build
Andrews EV13 cam
Cosmetic gasket kit with .030" head gasket (original gasket is .045")
Andrews EV13 cam
Runs perfect with 91 octane
I ran out of gas once in the middle of nowhere and long story short, I had to put 87 octane in it to get me to civilization. I pay a lot of attention to various sounds regardless, and sure enough I heard the pinging / detonation when I first took off with the 87 in the tank. In order to avoid damage, I decided to stay in one gear lower than normal to reduce load on the engine, and that made the pinging stop. As soon as I got some 91 octane in it, everything was fine.
If I would go to with an EV27 cam, I would have lower cranking pressure and thus probably would not need the 91 octane. I like the EV13 because it fits my riding style better. It is made to use with stock compression, but all Evo's can benefit from a bump in compression, so that is why I went up with it. It is also known that there were sloppy tolerances with combustion chamber volume, so my intent was also to get the head volumes to be as close to the same as possible.
The result is I have an extremely smooth running Evo that makes great power at the rpm I need it. It also runs substantially cooler than it did when it was stock.
I never put it on a dyno.
Most people are probably making bigger numbers than I am.
That doesn't matter.
What matters is I like my results so much that this is the last bike I am ever going to buy, unless it gets totaled or something catastrophic happens to where it is not reasonably rebuildable. At that point, there is going to be a lot of tough decision making.
Club Member
Joined: Aug 2008
Posts: 22,857
Likes: 17,908
From: Lafayette, Louisiana
In reality, the cranking pressure is what is important. Put a compression gauge on it. Don't forget to hold the throttle wide open.
Road Warrior
Joined: Aug 2007
Posts: 1,564
Likes: 70
From: Oakdale Ca.
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Seasoned HDF Member
Joined: Aug 2008
Posts: 5,080
Likes: 2,154
From: Ohio
Like THC mentioned 10-1 with a 40-42 intake close cam, 9.8-1 with a 38 intake close cam like Prot said for the 1001 time, camshafts matter lol! Can jump up to 10.5-1 or 10.8-1 if entering the world of large numbered cams which includes 256-260 duration and intake closing in the 50's, starting to tamper with temper mental engines critical on timing and heat can play a role plus compression releases come into play, still leads to starter fatigue even with releases.
9.5-1 is a good number for everyday riding and camshaft flexible according to your bike, riding style plus tuning simplicity. If you ride like your hair is on fire then 9.5-1 will get your hopes up but going to have a newer bigger cube bike pull away from you. Compression comes down to wallet size, experience in your corner and what your riding style is, wouldn't pump a engine up past 9.8-1 or 10-1 and plan to ride to to Sturgis. 1200 miles riding when you are there and ride it back home.
9.5-1 is a good number for everyday riding and camshaft flexible according to your bike, riding style plus tuning simplicity. If you ride like your hair is on fire then 9.5-1 will get your hopes up but going to have a newer bigger cube bike pull away from you. Compression comes down to wallet size, experience in your corner and what your riding style is, wouldn't pump a engine up past 9.8-1 or 10-1 and plan to ride to to Sturgis. 1200 miles riding when you are there and ride it back home.
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Tourer
Joined: Dec 2015
Posts: 366
Likes: 25
From: CONUS
It is also known that there were sloppy tolerances with combustion chamber volume, so my intent was also to get the head volumes to be as close to the same as possible.
Club Member
Joined: Aug 2008
Posts: 22,857
Likes: 17,908
From: Lafayette, Louisiana
I had a 91 Sportster I bought brand new while I was in Japan and brought back to the U.S. with me after a couple of years. I changed the cams in it (interesting getting all four of them in at the same time the way the gears are made), put some slip on mufflers, and a dynojet carb kit with the thunderslide. While I had it apart (I was rebuilding it after it got totaled from being hit from behind ), I converted it to belt drive. In 91 Sportsters were still solid mounted and used chain drive on all but the deluxe model. The end result of those few changes was a bike that had a lot of get up and go for relatively low effort and expense. The belt drive smoothed it out tremendously plus made keeping the rear wheel clean a lot easier.