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Evil Tweety
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« on: June 11, 2008, 05:27:26 PM » |
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Any opinion on optimal CR for an air cooled twin, 2 valve/cylinder running in Bonneville? Running gas, naturally aspirated.
Thanks!
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panic
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« Reply #1 on: June 11, 2008, 06:15:52 PM » |
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Not enough info.
80 VE, good quench, small R/S ratio: 15:1.
105 VE, large bore with single plug, large R/S ratio, bad quench: 12:1.
Tell us more?
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Evil Tweety
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« Reply #2 on: June 11, 2008, 06:55:44 PM » |
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Not enough info.
80 VE, good quench, small R/S ratio: 15:1.
105 VE, large bore with single plug, large R/S ratio, bad quench: 12:1.
Tell us more?
I'm running in the middle of that range right now. It's a single plug, bore around 80mm (3.14") - had a bad quench with the last head - but I've got one where I can take a lot of the dome off the piston and improve it. Sorry - don't know what you mean by R/S ratio . . . but please let me know if this info helps. |
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John Noonan
Hero Member
 Offline
Age: 45
Location: 6 X 200 club member: El Mirage 2 Club, Bonneville 2 Club, Australia 2 Club, Maxton, 2 Club, Mojave 2 Club, Bubs 201 Club
Posts: 3306
245 200+ mph time slips. 252 mph on a dirtbike
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« Reply #3 on: June 11, 2008, 07:01:50 PM » |
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R/S is Rod to Stroke ratio..
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On 9-27-08 we lost a great friend and fellow racer, he was the BEST, he helped anyone with anything at anytime. His name is Dave Owen and he will be missed by all that knew him and I am glad to have met him.
Seeing him at Bonneville during 2008 Speedweek was the tops, Dave was in awe of the salt and as usual was there helping out anyone who asked..simply put we lost a great man who will be missed by all.
J
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Evil Tweety
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« Reply #4 on: June 12, 2008, 11:41:36 AM » |
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R/S is Rod to Stroke ratio..
Thanks John! So measuring the rod (center of wrist pin to center of big end) it worked out to 2.3:1 or 2.44:1, depending on which crank I am able to use. |
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Evil Tweety
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« Reply #5 on: June 16, 2008, 12:33:13 PM » |
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Thought I'd give this a bump and see if anyone had any comments . . . any more input?
Thanks!!!!
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panic
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« Reply #6 on: June 16, 2008, 05:14:04 PM » |
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Rod ratios that high are more sensitive to knock than for example 1.7, etc. but as to actual numbers IKBA. There isn't much diff (if any) between your 2 ratio choices.
Remember than the weak density/atmo of Wendover is your friend in designing in a static CR, since some people plan on closer to sea level and can't use such big numbers.
Not sure how much effect, but the narrower the RPM range the engine must pull near top speed (viz., close-ratio with high progression is better than street ratios) the higher the static CR, closer to max power the jetting, more aggressive the spark setting. In any event, does your drive-line allow keeping the engine above 5,000 RPM under power?
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Evil Tweety
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« Reply #7 on: June 17, 2008, 04:48:23 PM » |
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Rod ratios that high are more sensitive to knock than for example 1.7, etc. but as to actual numbers IKBA. There isn't much diff (if any) between your 2 ratio choices.
Remember than the weak density/atmo of Wendover is your friend in designing in a static CR, since some people plan on closer to sea level and can't use such big numbers.
Not sure how much effect, but the narrower the RPM range the engine must pull near top speed (viz., close-ratio with high progression is better than street ratios) the higher the static CR, closer to max power the jetting, more aggressive the spark setting. In any event, does your drive-line allow keeping the engine above 5,000 RPM under power?
With the electromotive EFI we can do whatever we want with spark and gas. RPM is higher than 5,000, more like 8,000 plus. |
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