What comp. ratio - Road Glide Forums
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post #1 of 25 (permalink) Old 05-20-2016, 05:15 PM Thread Starter
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What comp. ratio

I have the 103 engine, and always run premium, or av gas. Looking at buying some heads, question is what comp ratio should I try to get. I was thinking 10:1. But local indy says that will just cause me pinging and other issues. He thinks 9.3 would be better. Basically keep my stock piston, and add heads. To get 10:1 I'd have to add a new pop up piston as well as the heads.
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post #2 of 25 (permalink) Old 05-20-2016, 06:49 PM
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That's just not correct. With the right cam, heads, and compression ratio with a good tune, no pinging will be happening. Nowadays I like to keep them below 10:4-1 and really like 10:2-1 ratio. It's all about the tune and the cam. First you have to make your mind up on what you want it to do and the way you ride. Pretty simple.
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post #3 of 25 (permalink) Old 05-21-2016, 07:17 AM
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You cannot just abitrairily throw compression at an engine.
The cam's intake closing number has a lot to do with the compression needs of the powerplant.
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post #4 of 25 (permalink) Old 05-21-2016, 09:22 AM Thread Starter
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I have the 551 cams installed. S&S say that cam does not like anything over 10.1, and strongly reccomended, I don't exceed that number. Here's the details of these cams.
Exhaust duration 2888
Ex open 41
Ex close 17
Ex lift .170
Intake
Duration216
Open 17
Close 19
Lift .178
Hope this data helps.

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post #5 of 25 (permalink) Old 05-21-2016, 10:35 AM
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Stock 103 with a .030 head gasket is about 10.1-1.

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post #6 of 25 (permalink) Old 05-22-2016, 08:30 PM Thread Starter
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They are telling me, tomuse to use 89 CC heads with the bigger valves. Keep the stock piston, but that looks like it gives me lower compressions. i want to be sure I do my home work, before dropping that kinda money..
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post #7 of 25 (permalink) Old 05-22-2016, 08:54 PM
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I'm at 10.5 and run premium gas, 93 octane. No pinging that I've noticed.

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post #8 of 25 (permalink) Old 05-22-2016, 10:57 PM
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Quote:
Originally Posted by Desertwndrr View Post
Stock 103 with a .030 head gasket is about 10.1-1.
So what hp and tq gain would you be looking at to do this? Let's say you already have exhaust and intake done.
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post #9 of 25 (permalink) Old 05-22-2016, 11:13 PM Thread Starter
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Question

My question exactly. Both intake and exhaust is done, cam is in, looking to move to next upgrade. The head stuff is a bit much to understand.. Torn between heads or big bore? Or both. Want to make sure I buy what ever, so that it fits later upgrades.. Lots of questions and little answers so far. If I go with the heads, I am leaning to using the 79 cc chambers, regardless of what the manufacturer says. Compression should remain close the limit of 10.1 CCR
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post #10 of 25 (permalink) Old 05-23-2016, 08:16 AM
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Quote:
Originally Posted by Diverrick View Post
My question exactly. Both intake and exhaust is done, cam is in, looking to move to next upgrade. The head stuff is a bit much to understand.. Torn between heads or big bore? Or both. Want to make sure I buy what ever, so that it fits later upgrades.. Lots of questions and little answers so far. If I go with the heads, I am leaning to using the 79 cc chambers, regardless of what the manufacturer says. Compression should remain close the limit of 10.1 CCR
If you are planning on changing cams in the future, I would suggest you build your heads and cc the combustion chamber to fit the cams you are leaning to. If you just arbitrarily choose heads and compression and don't change the cams, you could actually loose performance. What ends up happening is intake stall. This is where the intake ports and valves are too big for the cam you are using. Fuel will actually puddle in the intake track and performance will go down. It is very important to understand what S&S was telling you about compression and how the heads are set up for the cam you have. Good luck.

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