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Author Topic: GT380 ground up build  (Read 4644 times)

Offline themotoworks

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Re: GT380 ground up build
« Reply #10 on: May 27, 2018, 12:48:21 »

trying to finish designing the swingarm and rear frame mount, which is a big p.i.t.a. because the rear engine mounts are kicked way off to the right of the engine centerline, not ideal but I can make it work

Offline themotoworks

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Re: GT380 ground up build
« Reply #11 on: May 31, 2018, 18:09:46 »


I'm planning on slightly modifying the engine, cutting the piston skirt and adding piston ports as well as reed valves and tuned pipes, anyone on here know much about 2 strokes?  Will this timing be alright?  It's a bit modified on the intake side to allow for the reed valve to stop reversion back into the carbs instead of the piston blocking the back flow. 

also made a lot of progress on designing the rear suspension, need to finalize the shock choice so I can finish designing all the linkages etc.

Offline teazer

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Re: GT380 ground up build
« Reply #12 on: May 31, 2018, 18:51:36 »
Where did those data points come from? Is that what you measured stock timing as or where you want it to be?  For a free revving motor, those numbers are not close to where you want them to be.  IMHO. try EO at 90 ATDC, TO at 120 ATDC and Intake at say 1110 BTDS as a starting point and work out the time area at target peak revs and see how wide you would have to go to reach the sort of BMEP levels you want at target RPMs. 

Of course you will also need to design a pipe to work with the target port shapes - assuming there's enough metal to port it to those numbers. Look at what Barton motors did with the water cooled Sparton 500 which was based on that bottom end. 

Suitable sized reed boxes are a good way to add additional intake time area but that may cause clearance issues with the rear cylinder base studs.

Just as a point of reference with a more modern motor, an RS250 Aprilia lightly cleaned up makes over 60hp from 250cc, so a triple based on the same ports could theoretically make around 90hp.  Your motor will not have exhaust power valves or the port area, or crankcase reed area of an RS250 so that gives you a base to consider.  By comparison, a full race GT250 or T20 makes closer to 40hp with a TZ crank, later style multi port barrels etc .

You may want to look at what anyone with a T20/GT250 or even a T315/350 does to make power and start there. I recall riding a beautiful T350 with 6mm spacers under the cylinders many years ago. It looked totally stock and went remarkably fast.

How much HP at what Revs is your target?

Last time I looked at one of those, I ended up with EO at 80 ATDC, and transfers at 110 ATDC peaking at almost 10,000 with restricted intakes to keep revs down and some semblance of bottom end (ie below 6k).   
« Last Edit: May 31, 2018, 19:06:54 by teazer »

Offline themotoworks

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Re: GT380 ground up build
« Reply #13 on: May 31, 2018, 21:37:38 »
those exhaust and transfer numbers were based on measuring the stock engine, the intake to crank timing is based on adding windows into the piston skirt and cutting the base of the skirt back, which gave longer duration, requiring a reed valve, but possibly making more power.  I downloaded some long book on 2 stroke performance, looked like it was from the 70's, but it has a lot of info on port size based on rpm etc., I want to make this thing peppy but not go so overboard that I lose months to engine tweaking.  The idea for this build was to really test the idea of building a bike completely in the computer, then machining and welding to the prints, then assembling, and seeing if it would work, or what would need to be changed to make it work.  I hesitate to go neck deep into the engine, but at the same time I don't want to leave it as sleepy as stock, especially when the rest of the bike is really build for performance.  also since my slow time at work is usually the summer months, if this goes past september, it may not see the light of day until next summer

Here's where I'm at with the rear suspension so far, plots look good for the geometry, clearances look pretty good in SW, just need to flesh out things like the linkage and swingarm, I'm also a little hesitant to finalize the design without having the shock in hand, and since ohlins doesn't give out cad info or drawings, that might force a bit of a wait...



« Last Edit: May 31, 2018, 21:39:41 by themotoworks »

Offline irk miller

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Re: GT380 ground up build
« Reply #14 on: May 31, 2018, 22:03:03 »
What's your wheelbase?

Offline themotoworks

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Re: GT380 ground up build
« Reply #15 on: Jun 01, 2018, 07:10:30 »
I'm aiming for 54.5" at static sag

Offline teazer

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Re: GT380 ground up build
« Reply #16 on: Jun 01, 2018, 08:09:24 »
I dug out a previous simulation run for a more or less stock 380 and changed inlet timing to yours and lost power everywhere.  The ports are way too large and lose intake velocity and therefore crankcase filling. Raise the exhaust 2.5-3.0mm, transfers by 2mm and drop the intakes 2mm to start.  Check those numbers with your engine builder.  There are infinite number of combinations and a few of them work better than others.  You will have to machine teh head to get compression back  too.

Offline themotoworks

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Re: GT380 ground up build
« Reply #17 on: Jun 01, 2018, 08:22:08 »
the stock intake is 69 degrees, what simulator are you using?

Offline teazer

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Re: GT380 ground up build
« Reply #18 on: Jun 02, 2018, 01:45:19 »
283 ATDC or 77 BTDC modeled in Mota 6.1 or -x beta version.  283* is 24.2mm BTDC and your 123* works out to 44.1mm BTDC (plus piston length of course). 

MOTA ignores actual intake port dimension when using reeds because it assumes that the reeds will effectively modulate flow but with excessively large ports, it loses flow velocity even with reeds. The sections of the reeds and port must be matched to work effectively.

BTW, you still get harmonics in the intakes with reeds and reversion - just not quite as bad. 

Offline themotoworks

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Re: GT380 ground up build
« Reply #19 on: Jun 02, 2018, 20:13:44 »



finished up the rear hub, it's modeled in the solidworks bike so it's nice to know already that things will fit, next going to try to finish the front hub sunday before I need to do actual work at the shop haha