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Tech Information => Technical Discussion => Topic started by: Truckedup on December 14, 2012, 04:00:38 PM

Title: Gearing for Bonneville
Post by: Truckedup on December 14, 2012, 04:00:38 PM
 I did a search but??? When you increase the altitude above sea level the power drops.Is the power that is left still peak at the same RPM before? Or will the engine not rev as high with increase in altitude?
Title: Re: Gearing for Bonneville
Post by: SPARKY on December 14, 2012, 05:02:46 PM
It has to with TE  what your tires put to the ground---if you are geared too HI because your eng does not have enough power to overcome the total drag ---  it will not tach as hi
Title: Re: Gearing for Bonneville
Post by: Truckedup on December 15, 2012, 04:04:55 PM
 Ok,But for this example let's say the engine is on a dyno at sea level and HP peaks at 7000 rpm.At 4000 feet with the carburetors adjusted for alttitude,will the engine still make peak HP(somewhat less because of altitude) at 7000 rpm?
Title: Re: Gearing for Bonneville
Post by: SPARKY on December 15, 2012, 06:26:34 PM
that should not change
Title: Re: Gearing for Bonneville
Post by: Truckedup on December 16, 2012, 07:59:57 AM
 Thanks.This is a 650 cc Triumph pushrod gas cycle engine in non streamlined frame.Typically these engines peak at 6800-7000 rpm when tuned for 1.5-1.6 HP per cubic inch at sea level.Less power at elevation ,more drag on the surface,go from there...
Title: Re: Gearing for Bonneville
Post by: SPARKY on December 16, 2012, 10:32:04 AM
If you have any idea what a dyno sheet on that eng looks like how high you can keep it together and how fast the torque falls off will tell you how to gear it--- I gear for the rpm through the first light to be just about peak power and the last --past peak power for a torque curve that falls of fast.---If its pretty flat I and it is still climbing I would lower my gearing--- :-o YMMV
Title: Re: Gearing for Bonneville
Post by: Truckedup on December 16, 2012, 11:48:24 AM
 In general,these 2 valve hemi engines get peaky when modified.But it''ll have a somewhat flat power curve from 5000-7000 rpm.And 7000 is a good rpm limit but 7200 is ok for the last 10-15 seconds.The record is 124 mph,the bike is geared now for 130 at 7100 rpm,and it will run up against the rpm limit at low elevation on hard pavement.I am new to Bonneville and have to guess .I think running 124 on the first outing will need a little divine help  :-D
Title: Re: Gearing for Bonneville
Post by: SPARKY on December 16, 2012, 12:26:58 PM
I would gear for 124.5 at 7050 at the beginning of the mile and try to have 7075 or 7100 at the exit!!!  and see where you are going to find DA from best ever heard of 2800 to 8800 so it is a crap shoot!! :roll:  you will be at the mercy of the track, the DA, the wind gusts ect ect ect---work on your tuck and try to fair your rear body all you can---any minute think is going too be the difference  over bore to your absolute maximum!! :evil:
Title: Re: Gearing for Bonneville
Post by: Stainless1 on December 16, 2012, 12:57:18 PM
Running NA, leave it the way it is now, take gear changes up and down, and run it in October.... the air is a lot better in October for NA Motors.  It is always a crap shoot running the salt.
Title: Re: Gearing for Bonneville
Post by: 55chevr on December 16, 2012, 01:06:31 PM
Just about everyone over gears their first time out.   The informed sources believe that there is about a 15% power lose at the altitude of the salt flats. Bring enough sprockets to cover all possibilities. 
Title: Re: Gearing for Bonneville
Post by: SPARKY on December 16, 2012, 01:15:14 PM
SS #1 and 55 you guys said it best  :cheers: :cheers: :cheers:
Title: Re: Gearing for Bonneville
Post by: JustaRacer on December 16, 2012, 01:43:56 PM
I did a search but??? When you increase the altitude above sea level the power drops.Is the power that is left still peak at the same RPM before? Or will the engine not rev as high with increase in altitude?

Take with grain of salt.  Not an expert.

This comes from drag racing naturally aspired engines in thin air (LACR, Bandimere, LVMS, etc).  My Bville experience is limited to turbocharged engines.

B'ville sees wild changes in DA.  IIRC, I saw from 3500' DA to 8800' DA.

Get a DA computer, but DON'T lend it out.  My first one grew legs and ran away.

You might have to advance the timing.  Thin air takes longer to burn.

Depending on carb design, you might have to drop main jet size.

Thin air acts like a smaller cam.  Your HP peak rpm will drop.

Or at least that's what worked for me.

Remember to tighten the axle bolt when switching sprockets.  I learned that one the hard way when I got in a rush at OCIR.

Title: Re: Gearing for Bonneville
Post by: Truckedup on December 16, 2012, 01:54:55 PM
October huh???? I'm the builder,a younger friend of mine,experienced dirt racer, is financing the bike and trip out there.I'll tell him...Old Triumphs do like cooler weather ...And it can be run a little leaner without the need for  safety cooling by using a slightly rich mixture...
 
Title: Re: Gearing for Bonneville
Post by: SPARKY on December 16, 2012, 02:27:10 PM
Remember,  Aircooled engs cooling capabilities are affected by DA!! as well as air temps
Title: Re: Gearing for Bonneville
Post by: Truckedup on December 16, 2012, 02:35:49 PM
 DA is air density?
Title: Re: Gearing for Bonneville
Post by: JustaRacer on December 16, 2012, 02:38:57 PM
DA is air density?

Density Altitude.  It is the effective "thickness" of the air.  Stolen from aerospace.
Title: Re: Gearing for Bonneville
Post by: panic on December 16, 2012, 05:13:57 PM
If your tune results in peaky power near 7K, consider using the close-ratio (24/22) high gear set to reduce the RPM drop on the 3-4.
Title: Re: Gearing for Bonneville
Post by: JimL on December 16, 2012, 10:26:49 PM
That peaky power is a pesky problem.  My pushrod 650 (or 680) is prone to wheelspin in the first two gears because the power hits very hard as it gets "on the cam".  I started with 18/42 sprockets and am now using 18/38 which has helped the traction issues and added about 15mph from the same power.  If you can get closer ratios on the top two gears, it'll have to help.  I plan to go 17/34 next year with a slightly lower 5th gear.  This video might show the issue; youll hear the problem.  I have never been able to use full throttle in the first two gears, so learning how to get the most will be key to better speed downtrack...we just dont have a lot of acceleration at speed with these little pushrod engines.

http://www.youtube.com/watch?v=6ErXLJwSRcE&feature=youtube_gdata_player

Regards, JimL
Title: Re: Gearing for Bonneville
Post by: stay`tee on December 17, 2012, 02:03:22 AM
if you havent done the laps at Bonneville its impossible to tune for it when ya some place else, :roll:,, my suggestion would be to get your bike on the dyno and get the best possible tune there, take RAD (Relative Air Density, (RAD guages are avliable from all good speed shops)  readings in conjunction with final dyno pulls,, keep notes,,, once at Bonnie, readjust your jetting to suit RAD on site, make small timing changes required for humidity, this will put you right in the "ball park", 8-), with consistant data for future events,  :-)

horsepower readings will give an indication of what the machine is capable of, set your gearing to take the best advantage of this,  :-),,,
Title: Re: Gearing for Bonneville
Post by: Truckedup on December 17, 2012, 06:51:13 AM
 Ok,were got some 650 pushrod guy here...The rule book has the 2011 record runs...what is the 650cc MPG record? Still 123.9?
 My rear sprockets are 21-47 right now on a unit Triumph.With the current tire that's 125.9 at 7000 rpm.We have several rear wheels available so the sprocket sizes might be totally different in the end.And maybe rims with different diameter tires.Using the standard 4 speed.
 My engine should be a bit peaky but it has slightly less cam than some use and using a tight as possible quench in combustion chamber to get a faster cleaner burn to make power.
 My drag racing Triumph's was 40 years ago in a modified production class.Jetting changes were done if you ran did time trials in daytime and then serious racing at night when cooler.Sometimes I didn't mess with the carbs or timing and just launched harder and generally abused the equipment to go faster.I'm thinking that the harder launch won't work on the salt  :-D
 I believe that 750 Triumph twin flat track bikes tuned by Axtell made 71 reliable HP at 7000 rpm back in the early 1970's.A 650 about 63 HP.At low elevations of course.
Title: Re: Gearing for Bonneville
Post by: JimL on December 17, 2012, 09:44:42 AM
Those HP numbers sound about right for that era.  My T120 flattrack engine was set up for 1/2 mile, running JOMO 15B cams, Chevy valves in shortened Sportster valve guides (larger brass seats), 13:1 pistons, GP carbs, 26" exhaust, and a short 4th gear.  Just for comparison, that was high 13s at 5000 foot altitude on a warm day.  That was identical to my built CB450 on the drag strip, which was the same bike that ran a best unstreamlined pass of 118.733 at Bonneville 1969 (but only had 116.654 average).  With the fairing, best mile was 124.65, but not until I was coached by the veterans to be a lot more aggressive off the line!

Here we are 40 years later and these bikes are still about the same speeds, EXCEPT with AirTechs much better streamlining giving much more speed and being able to pull 10,000 rpm with a super short stroke.    My current bike is difficult to launch because there is no way to run a flywheel with the chain drive conversion; which is a deja vu flashback to that CB450 (which couldnt run a flywheel, either!) and was very peaky.  With a flywheel, that CB450 only ran 107 unstreamlined at Bonneville, in 2007, but I was off on jetting also.

You are correct; the M-PG record is 123.9, but I'm guessing Roosevelt was riding the 8-valve Weslake-Triumph.  The current MPS-PG-650 record is about 133 1/2, and that bike had the AirTech Charlies Toy fairing on a nearly stock CX500 chassis with stock shaft drive and rear gear ratio.  Streamlining is a lot of fun, and cheaper than horsepower!  About the video...not my posting on utube.  The bike only gets past 160 at the end of the 3rd mile.  It has never run better than 159 mile, with either engine. :?

Sorry for the rambling post; trying to connect the dots between the years, and see what we've learned.
Hope this is useful to your planning and building, which is some of the most fun. :-)

Regards, JimL
Title: Re: Gearing for Bonneville
Post by: Truckedup on December 17, 2012, 10:47:12 AM
 Yes,the 650 MPG record holder is an 8 valve.That doesn't mean a 4 valve can't go as fast,it means if he was off a bit and the 4 valve has the perfect day with perfect rider......We are looking at partial streamlining classes also. Elevation does make a difference,at closer to sea level a Triumph like yours will run high 12's.And on a cool night with an all out hole shot, closer to 12.5
Title: Re: Gearing for Bonneville
Post by: wobblywalrus on December 18, 2012, 12:50:27 AM
This way of doing the gearing helps me when I am running a new or unknown setup.  The gearing is set tall for the first runs so the engine will not reach peak power rpm.  Then I check everything, especially the mixture, and the chassis setup.  I drop the gearing after everything is OK to a ratio where the engine is running like 55chevr sets his up to do.

This is something I learned from racing those old British bikes.  Mine were not especially reliable when wound out hard, and by gearing the bike tall, I gave myself a chance to gather some good data.

You will see me do this with the new big motor this year.  My first runs will be with gearing set for 160 mph which I know it will not pull.  Then I will look at the plugs and exhaust header color, listen to the running engine with the motor stethoscpope, and a few other things.  Then a bigger sprocket will go on for the high speed runs if everything is OK.   

   
Title: Re: Gearing for Bonneville
Post by: SPARKY on December 18, 2012, 11:04:29 AM
Guys,  Thanks for these insights---for those of us who try to DATA MINE this site has no peer!!!! 


SSS  My yearly check will so be on the way