Sundial Moto Sports
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gt350 ramair project
http://www.suzuki2strokes.com/forum/viewtopic.php?f=21&t=13524
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Author:  dollydog [ Mon Sep 17, 2018 6:04 am ]
Post subject:  Re: gt350 ramair project

hi indy, 33???????????????????? :D jeez, i'm old enough to be your dad - easily :D
i too like jenning's book, i think it's a great read and not as difficult to take in as graham bell's book [i have both]. but like you say, they were good for the 70's. there again, the most modern 2 stroke i've worked on was an 86 rg250 gamma engine, but after stripping the engine completely, apart from being watercooled, it wasn't much different from the gt250's i usually work on :D
modern 2 strokes are so complicated now [to me] and they've wandered too far away from the simple piston port 2 stroke of yore. i'll stick with the gt250's, they don't tax my brain too much :D
right, the gt350 i'm building. i had the heads milled out to 65mm diameter [supposedly], fitted a 0.5mm thick copper head gasket, annealed it and finished building it. result? absolutely impossible to kick over. after taking the heads off again and measuring them - should have done that first in hindsight - the left one was only 63mm diameter, so that needs addressing. after measuring the volume of the head minus the volume of the piston dome and doing the maths, it seems i've got 16:1 compression ratio, which is somewhat high :D i'm sending them back to the bloke that milled them and he can first of all take them out to 65mm, slope the squish down to the original inside edge and take 2mm depth from the combustion dome. this should take me down to a more reasonable 8.8:1 c/r. that, of course, is when i can get hold of him :D until then, i'll try and figure some other way round it, but at the moment, this is all i can think of to lower the c/r considerably.
cheers, dd.

Author:  tz375 [ Mon Sep 17, 2018 8:54 am ]
Post subject:  Re: gt350 ramair project

16:1 geometric is a little high, but 8.8 is way too low IMHO. aim for say 7:1 corrected or 13:1 geometric as a place to start.

You can also just do a compression test and then decide how much to adjust it. I run 165psi on a water cooled motor and you should stay below that for an air cooled. Say 150 psi as a a target and see ho it runs at that.

Author:  dollydog [ Mon Sep 17, 2018 1:46 pm ]
Post subject:  Re: gt350 ramair project

hi tz, the 8.8:1 was the corrected c/r i was hoping for :)
i'd just like to be able to kick the damned thing over to start with, at the mo it's impossible - i only weigh 12 stone :D the piston at tdc is correct and going down the stroke all the ports are completely uncovered/covered correctly. i tried a thicker [1mm] head gasket and that was slightly [and i mean slight] better. so all in all i'm going for the removal of metal from the combustion dome. sorry, on the pc atm, i'll edit this when i get to the laptop with my figures. if anybody works it out differently, please let me know :D
cheers, dd.

Author:  Zunspec4 [ Mon Sep 17, 2018 1:50 pm ]
Post subject:  Re: gt350 ramair project

Hello dd,

The CR on my T500 race engines is set to something like 6.5:1 . A bit on the conservative side possibly, but not by much.

Cheers Geoff

Author:  dollydog [ Mon Sep 17, 2018 2:56 pm ]
Post subject:  Re: gt350 ramair project

hi geoff, thanks for the input. after a word with sep this morning, he reckons that the gt250 at 7.5:1 was very conservative and said i should have no problem going up to 10:1 :D but like i said in another post [somewhere] i don't want to go that mad and i'd keep it at a reasonable 8.8:1 c/r. at that c/r i'll give it a whirl and see what the engine runs like, if i have to reduce it somewhat, then there's no problem :) the squish i copied off was the ts185, which are the standard pistons i've got fitted. i didn't copy the ts dome - ts c/r is only 6.2:1 c/r. and the combustion dome is very deep.

tz: my figures are volume of head atm is 20cc and according to my figures - and i'm generally hopeless at maths - with a flat top piston it would give me a 9.675:1 c/r. unfortunately, i've got domed pistons in it and the dome measures 8.5cc, making my head volume at tdc around 11.5cc, giving me about 16:1 c/r. if you know any better, then please post before i do anything rash :D my volume figures are as spot on as i can possibly get them, but whether the rest of it is is in the lap of the gods :D thanks for the input.
cheers, dd.
p.s. don't forget this engine is 347cc. 64mm bore x 54mm stroke - same as the rd350.

Author:  tz375 [ Mon Sep 17, 2018 6:21 pm ]
Post subject:  Re: gt350 ramair project

Using bore 64.0, stroke 54 and cv of 11.5cc gives a corrected CR of 9.1:1 with a fairly high performance exhaust port opening at 87 degrees. That's a little high and if your EO is closer to stock, CR would be around 10.1:1 corrected (@95 degrees ATDC)

A little less compression would be beneficial. Surprised you can't kick it over though. I can kick over a TZ750 top end on an RD350 twin and it's probably around 8.7:1 or 16:1 uncorrected.

It's all in your exhaust timing. The lower (tamer) the EO time is, the higher the effective cr.

Author:  dollydog [ Tue Sep 18, 2018 2:21 am ]
Post subject:  Re: gt350 ramair project

thanks for the reply tz :)
should have added, when i was first kicking [or not] the engine over, there was no exhaust fitted, at all, just open exhaust port. sep, who fitted the new liners and did the port work for me did a mild tune on the ports, lifting the exhaust port by 1mm and widening it by 1mm. all taking into account that the barrels would be lifted by 4mm to compensate for the extra length of the pistons :) i can stand, full weight on the kickstart at tdc and as it finally eases over the kickstart lever drops just far enough to stop dead again when the other piston reaches tdc. it really is a nightmare to try and kick over.
cheers, dd.

Author:  dollydog [ Mon Oct 01, 2018 6:09 am ]
Post subject:  Re: gt350 ramair project

ok, plan b :D
heads sent off for machining this morning to a bloke up north, who has been recommended by a couple of lads on the 2 stroke forum. he's going to machine the diameter to 65mm [where it should have been in the first place] mill the squish to copy the ts185 head and reduce the dome by 2mm. this should give me the c/r i want and at least enable me to kick the damned thing up :D atm, i can get it to fire every kick, but only with one spark plug out. i tried it on both pots, so when the heads are done properly we should be set to go. 3 sets of different rated plugs, several sets of main and pilot jets at the ready :D oh yes, copper washers to lift the spark plugs up 2mm.
cheers, dd.

Author:  dollydog [ Tue Oct 02, 2018 2:49 am ]
Post subject:  Re: gt350 ramair project

i was going to scrap these, but machining them for the gt350 will give them a new lease of life :o courier picked them up yesterday morning, so should be up north by wednesday - in theory. first job he's doing is drawing me a before and after cross section, just so there's no mix up with my instructions :o i think they're pretty clear tbh :lol:
cheers, dd.

Image

Author:  Zunspec4 [ Tue Oct 02, 2018 3:19 am ]
Post subject:  Re: gt350 ramair project

Hello dd,

I found this C/R calculator on a scooter page and input the specs for one of my T500 race engines. The result is pretty much what I expected: https://lambretta-images.com/tuningh/port-timing-calculators/c-and-uc-cr-calculator/

Uncorrected C/R = 11.27

Corrected C/R = 6.61 (the important one)

I assume that when SEP said 10:1 was OK they meant "uncorrected" because IMHO that is way too high for the "corrected" C/R.


Cheers Geoff

Author:  dollydog [ Tue Oct 02, 2018 5:11 am ]
Post subject:  Re: gt350 ramair project

hi geoff, thanks for that, but i can't get it to work. when i hit 'calc' nothing happens :(
i entered stroke = 54mm, height from top of exhaust port = 32mm, volume of head = 11.5cc [taking into account the piston dome is 8.5cc]. it just says NaN in each box. no doubt i've done something stupid. if you'd like to try it for me i'd be most grateful. ignore all that. i went too far down the page and missed bore out :roll: what a plank :D anyway, it works out at 16.11/1 uncorrected and 9.96/1 corrected. i think that's way too high.
cheers, dd.

Author:  dollydog [ Tue Oct 02, 2018 5:21 am ]
Post subject:  Re: gt350 ramair project

thanks again geoff. i posted the link on the 2 stroke forum, it will no doubt come in useful.
cheers, dd.

Author:  Zunspec4 [ Tue Oct 02, 2018 6:59 am ]
Post subject:  Re: gt350 ramair project

Hi dd,

Funnily enough I've found several useful calculators on scooter pages. There is one which will convert Degrees BTDC to mm BTDC (and vice versa).

I would agree that a corrected C/R of 9.96 is way up there and will probably result in a very sensitive engine (before it goes pop anyway :D )

Cheers Geoff

Author:  johnu [ Tue Oct 02, 2018 1:32 pm ]
Post subject:  Re: gt350 ramair project

Zunspec4 wrote:
I say go for the Candy Lavender, but I might be a bit biased on that :D
Image

Those candy colors Suzuki did are awesome!

Author:  dollydog [ Mon Oct 08, 2018 6:53 am ]
Post subject:  Re: gt350 ramair project

cylinder heads are done and on their way back home :D seems to be a bit of porosity in them, but after reading many, many posts, including some on this forum, they seem nothing to worry about. from the volume he sent me they should be now 8.22:1 corrected compression ratio. i've got 3 sets of copper head gaskets to try, 0.3mm, 0.5mm and 0.7mm thick. 3 sets of spark plugs b7, b8 and b9. plus several pairs of mains and pilot jets. to start with i've fitted a standard gt250 exhaust, once it's running something like right, i'll switch over to the spannys and have another go :D
cheers, dd.
photo: machined heads, 65mm diameter and 2mm taken out of the dome.
Image

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