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Discussion Starter #1
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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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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Discussion Starter #4 (Edited)
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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Discussion Starter #6
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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I'm at 10.5 and run premium gas, 93 octane. No pinging that I've noticed.
 

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Discussion Starter #9
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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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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Discussion Starter #11
I guess I'll have to trust what they are telling me. They say 9.5:1 is the sweet spot. If I get the heads I am looking at, it will put me at 9.3.
 

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Why not just get a kit from one of the vendors here and the quess work will be gone? It can get expensive experimenting and you might end up disappointed, good luck regardless!
 

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Discussion Starter #13
Exactly. Lots of questions but the difference in price between buying this used set VS new or sending mine in is significant.
 

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Exactly. Lots of questions but the difference in price between buying this used set VS new or sending mine in is significant.
I am far from an expert but I do know every time I've tried the cheaper route it's always cost me more down the road. I say do it right the first time and be done with it. If you can't afford it and I totally understand then wait until you can. Good luck with which ever route you go and keep us posted.
 

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Why on earth would anyone run Av gas in a HD. It would have to be one heck of a build build to use that.
91 to 93 is all they need or can use effectively.
High out put 103 2016 RGU is according to HD 10:1 That is for the wet head. The non wet heads are 9.7:1 They post it on the web sight under Specs. Having the wet head allowed them to up it a bit.
 

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Discussion Starter #16
Why on earth would anyone run Av gas in a HD. It would have to be one heck of a build build to use that.
91 to 93 is all they need or can use effectively.
High out put 103 2016 RGU is according to HD 10:1 That is for the wet head. The non wet heads are 9.7:1 They post it on the web sight under Specs. Having the wet head allowed them to up it a bit.
Because I work on Aircraft and have fuel from them at times. Why not run it, if you got it.
 

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Because the octane rating dictates how fast the fuel burns not how energetic it is while burning. So with 145 octane AV gas) you can actually lose performance by having the fuel burn off to fast (and to hot...) for the engine if it isn't designed for it. And yes this will do some damage in the process.

Years age while working as the senior test tech for an aircraft company, I thought it would be fun to dump 5 gallons of 145 into my GMC pickup. It ran really good for a short bit ... And then never ran right after that ever again.
 

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Because the octane rating dictates how fast the fuel burns not how energetic it is while burning. So with 145 octane AV gas) you can actually lose performance by having the fuel burn off to fast (and to hot...) for the engine if it isn't designed for it. And yes this will do some damage in the process.



Years age while working as the senior test tech for an aircraft company, I thought it would be fun to dump 5 gallons of 145 into my GMC pickup. It ran really good for a short bit ... And then never ran right after that ever again.

Higher octane gas doesn't burn any faster, hotter, or cooler than regular gas.


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