98ci compression?
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Stage III
Joined: Aug 2021
Posts: 3
Likes: 1
From: Iowa
98ci compression?
I have a 2006 Heritage Softail that is bored to a 98ci from an 88ci. Im running S&S 509 cams with stock heads and I'm getting cold compression test readings at 210 on both cylinders. What can I do to get my compression down to 180 or 190. Would head work be the way to go? What's your alls opinion on this? Any suggestions?
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Stage III
Joined: Aug 2021
Posts: 3
Likes: 1
From: Iowa
[QUOTE=Silentgreyfellow;22088020]Why do you want less compression?[/QUOTE
I was told my compression is way to high, especially for pump gas. The bike runs great, ive had it dyno tuned and no issues with it. When i was told that 210psi was too much it had my mind racing and just wanted some different input from people.
I was told my compression is way to high, especially for pump gas. The bike runs great, ive had it dyno tuned and no issues with it. When i was told that 210psi was too much it had my mind racing and just wanted some different input from people.
Seasoned HDF Member
Joined: Sep 2008
Posts: 14,146
Likes: 11,219
From: San Diego, CA
At 210psi, you are most certainly in an area where detonation can happen without a great tune and possibly also running expensive, high octane fuel (race gas).
If it was tuned at that compression on pump gas, and it runs well, I see no reason to change the compression..... unless....
How easy is it to start? That compression can also wear down a starter. You'll need to have a high CCA battery, keep it in good condition, along with good, clean battery cables.
But even if there were starting issues, to save wear & tear on the starter, I would add compression releases before reducing the engine compression on a well made, well tuned, and great running engine...
Good luck and keep us posted on what you decide to do...
If it was tuned at that compression on pump gas, and it runs well, I see no reason to change the compression..... unless....
How easy is it to start? That compression can also wear down a starter. You'll need to have a high CCA battery, keep it in good condition, along with good, clean battery cables.
But even if there were starting issues, to save wear & tear on the starter, I would add compression releases before reducing the engine compression on a well made, well tuned, and great running engine...
Good luck and keep us posted on what you decide to do...
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,900
Likes: 8,006
From: poway
[QUOTE=OnlyHarley;22088065]
If it's running great, you found someone that knows how to tune the bike.. Ignore the guy that fed you the BS..
Why do you want less compression?[/QUOTE
I was told my compression is way to high, especially for pump gas. The bike runs great, ive had it dyno tuned and no issues with it. When i was told that 210psi was too much it had my mind racing and just wanted some different input from people.
I was told my compression is way to high, especially for pump gas. The bike runs great, ive had it dyno tuned and no issues with it. When i was told that 210psi was too much it had my mind racing and just wanted some different input from people.
Thread Starter
|
Stage III
Joined: Aug 2021
Posts: 3
Likes: 1
From: Iowa
At 210psi, you are most certainly in an area where detonation can happen without a great tune and possibly also running expensive, high octane fuel (race gas).
If it was tuned at that compression on pump gas, and it runs well, I see no reason to change the compression..... unless....
How easy is it to start? That compression can also wear down a starter. You'll need to have a high CCA battery, keep it in good condition, along with good, clean battery cables.
But even if there were starting issues, to save wear & tear on the starter, I would add compression releases before reducing the engine compression on a well made, well tuned, and great running engine...
Good luck and keep us posted on what you decide to do...
If it was tuned at that compression on pump gas, and it runs well, I see no reason to change the compression..... unless....
How easy is it to start? That compression can also wear down a starter. You'll need to have a high CCA battery, keep it in good condition, along with good, clean battery cables.
But even if there were starting issues, to save wear & tear on the starter, I would add compression releases before reducing the engine compression on a well made, well tuned, and great running engine...
Good luck and keep us posted on what you decide to do...
Thanks for your input guys. And compression releases are in the winter plan book, with some slight head work and a thicker head gasket. I think its a rock solid bike, that guy just got in my head a little.
Will post later..ride safe brothers
HDF Community Team
Joined: Jun 2006
Posts: 12,774
Likes: 2,606
From: san antonio
Agree with the previous posters. I run a 98" motor with CCP at 212psi; static is 11.75 but it was tuned by someone that knew what they were doing and I do run manual compression releases and a high CCA lithium battery which help on heat soaked hot starts. I do run premium fuel, it was 102* in south Texas yesterday and won't start cooling off until September; no detonation. If the motor does not ping, no need for octane booster. a waste of money.
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Grand HDF Member
Joined: Sep 2022
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From: Florida
230 on the street in Florida heat with a shovelhead using pump 93 octang gas
. Youre fine. 93 1970 Andrews M grind, DynaS, single plug heads 28 degrees timing G carb, Drag pipes.
Last edited by Rains2much; Aug 9, 2025 at 06:14 PM.
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