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Sportster Motorcycle Engine Conversions Advice, questions, and tips for 1200, 1250, 1340, 1450 etc... for Sportster and Buell motorcycle engine conversions

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  #1  
Old 26th November 2004
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Question Compression Ratios

Is there much differance in horsepower and/or Torque with 10.5:1, 10:1, or 9.8:1 compression ratio? Looking at doing the 883 upgrade, soon!!!!
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Old 26th November 2004
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All my experience is with the LS1 Chev and Turbo Buicks (GN). Not Harleys ... But, I would think the same logic applies here ... If you are going to change cams to the longer duration/high overlap variety that everyone uses, I would opt for a higher compression ratio ... Why? ... You will loose cyl. compression/charge pressure when going to a greater duration/longer overlap cam (ie: one of those bigger cams). This is due to the greater amount of time the valves are in a partially open position in comparison to your stock cams. While this makes for greater cylinder filling, it also reduces compression .... So, bigger cam .... Opt for a little greater CR to make up for what's lost.

Ron,
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Old 26th November 2004
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for each full comp ratio point I believe that 10 h.p is the net. so the dif. should be around 7 h.p. This is a change with no cost so go higher because either way you have to use 92 octane gas anyway. This is a rough guess but the fastest way to h.p. is compression.
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Old 26th November 2004
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Not really sure about the compression question, but HQ says my actual compresion is 10.6-1, I have always run 91 octane, best they have around here. But on several occasions had to fuel when only 87 was available, wasn't completely out, just on reserve and only added 1 gal to get to next station but never had any issues.
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Old 27th November 2004
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Cool compression

There will be a little hp gain with a little compression jump. I'm not really into actual figures but Ralph's response sounds pretty close. This has been discussed to death, but the more you raise compression the more issues pop up at ya. A point or less up on comp with nothing else changed will make a bit more power across the whole rev band, and may not bring up other issues. Any more than a point will bring up detonation ( pinging) issues. There are lots of different ways to cure detonation. I'm running almost 12:1 with no issues, but I have done just about all the detonation avoidance and cure tricks that there are. A lot of thought and planning went into my engine before it was built. Most was to make nice power and the rest was to fix the issues associated with making power. Stevo is a lot farther along on research than I ever was, so I'm sure he will jump in and elaborate. More specific questions?? Please feel free to ask.
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Old 27th November 2004
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G'day Mike

Yup there is gains to be had from upping the compression and HD's like a bit more

Everything is academic unless it's actually measured...very few advertised 10:1 set ups are running much over 9:1.

As Ted said there's a lot of tricks to keep your engine together at high compression ratios.

I'm running 11.25:1 on pump fuel..and I can actually run on low octane fuel with that BUT I have to knock some timing out when I do it.

I wouldn't recomend anything over 10:1 ACTUAL measured compression on a street bike, there is too many trade offs.

Excessive wear on the bottom end....ask Crim
harder on everything in the engine
More difficult/fickle on correct fuels...makes goin out of town difficult

You need to understand your engine and be able to tune it if ya get a bad batch of fuel

Now these compression ratios can only be used with cams with very late closing inlets.

Your compression ratio is closely linked to your cam spec

I used ta know the numbers for how many degrees extra inlet close equated to how much extra compression ya could run...but I've forgotten it.
Theory is great in theory but I'm gettin enough runs on the board with different combos now to be able to get it right first go


again...where do you want the power??? what sort of ridin are ya doin?? etc etc etc
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Last edited by stevo; 27th November 2004 at 01:33..
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Thanks for the input..

Stevo
I normally don't run over 5000 rpm usually between 3500-5000 rpm, like to get into the twisties now and again!! Solo riding 98% of the time!!
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OK mild compression around 9.5-10:1 with a set of N4's and if ya like the 3500+ mark then a 2-1 exhaust isn't a bad idea...they come good around 3500 but have a dip around 2700
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I have n8's with 10.5 to 1 with thunderheader and s&s and mine goes like hell after 3500. (I had a buddy with an 03 sport and he could not stay close) Also vibes get lower and bike is more fun. I would guess I have low to mid 80's horsepower wise. Only dynoed once at 71 hp but that was before cams, carb, headsmilled 50 thou. plus thin head gaskets and 5 angle valve job + port job + titanium valves and ignition single fire crane hi4 ignition. My bike will bring the front end up in first and second without using the clutch. Just whack the throttle at 3500 and up she comes but only when the rear tire is not spinning. Compression is good.
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Very Seldom run under 3500!!! I guess, I'll go with the 10:1... Thinking about some head work, but money is tight for the Scoot right now!!!
Is there some pistons I can use now and not have to change when I do get the heads done??
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