47mm Vs 50mm Upgrade Worth It?

[magoconnor]

Hello. 
So I have been running with a 47mm cylinder for awhile now, still using the stock head. 

Lately I've been wondering, if it would be worth the money, upgrading to a 50mm cylinder. 
Do you guys think, the change would be significant? 
Or is it better just pushing the 47mm to the max? 

Btw If I where to install the 50mm, I would be pairing it up with a 47mm Bvh.
To keep the compression up, but gain the benefits of the bigger valves. 

[AtariGuy]

The big valve upgrade on any size bore kit increases the combustion chamber to 13cc's vs stock valve size head of 8cc's - a pretty large margin of increase. The only change in the bvheads in "bore size" is the miniscule squish band they use. The reason for the head's combustion chamber size is to fit the larger 21/19mm valves - they need space one way or the other - and that can only be accomplished by making the dome 5cc's larger.

I did get a 50cc bbk with the stock size valves and i'm quite pleased with the setup though i know i can get more compression with a better piston. A glixal 50mm kit comes with a lower compression dished piston. I wish i would have held out and got the much higher compression ssp-g setup with their domed piston from partsforscooters.com - but like i said, i have noticed a considerable increase from a 47mm kit with stock head to a 50mm borekit. The proof is in the takeoffs and rolling starts.

I would consult with dexameth as he has a monster setup that's holding strong for him and if i recall, the building blocks are a 52mm bvh, bore, domed piston, 44mm stroker crank, and ssp-g performance cam.

On that quick cam note, i plan on ordering the ssp-g cam shortly to compare it beside the glixal a9 cam. I've been hearing good stuff about the ssp-g profile!

[dexameth]

Aug 19, 2018 5:59:52 GMT -5 AtariGuy said:

The big valve upgrade on any size bore kit increases the combustion chamber to 13cc's vs stock valve size head of 8cc's - a pretty large margin of increase. The only change in the bvheads in "bore size" is the miniscule squish band they use. The reason for the head's combustion chamber size is to fit the larger 21/19mm valves - they need space one way or the other - and that can only be accomplished by making the dome 5cc's larger.

I did get a 50cc bbk with the stock size valves and i'm quite pleased with the setup though i know i can get more compression with a better piston. A glixal 50mm kit comes with a lower compression dished piston. I wish i would have held out and got the much higher compression ssp-g setup with their domed piston from partsforscooters.com - but like i said, i have noticed a considerable increase from a 47mm kit with stock head to a 50mm borekit. The proof is in the takeoffs and rolling starts.

I would consult with dexameth as he has a monster setup that's holding strong for him and if i recall, the building blocks are a 52mm bvh, bore, domed piston, 44mm stroker crank, and ssp-g performance cam.

On that quick cam note, i plan on ordering the ssp-g cam shortly to compare it beside the glixal a9 cam. I've been hearing good stuff about the ssp-g profile!

Yeah, unfortunately our 50mm piston is not the nice Hoca one anymore ): it is coming from a different supplier. The 52mm is still Hoca for now... We did bring in the Airsal pistons and cylinder kits tho (: and Airsal claims they test with stock heads!

As to the cam: I had the old Growsun A9 cam witch was AMAZING compared to stock. I had installed a stock cam with my first 50mm kit and rode for a while. Was gifted the A9 and noticed instantly the pickup all around the rev range.

I installed the SSP-G right before the stroker build and it really opened up my low end but felt similar mid- and high range. I want to cross compare with the Growsun A9 but the bearings are shot ): I would love to test vs the Glixal one... Maybe I'll get one just for testing cuz aren't they only like 20 bucks?

[AtariGuy]

I think i paid 12 bucks before shipping from china. You could easily have one for 15 + 3-5 bucks shipped from amazon here in the us tho. Afaik the "A" profiles are guidelines to certain sets of profiles, i'd seen a big post on a gy6 website that talked about a5-a16 profiles and a few performance cams cut from outside that list (taida, ncy, etc) so i would imagine the growsun a9 cam would be nearly identical to the glixal - plus/minus chinese manufacturing tolerances of course lol

Otherwise check out aliexpress for the glixal a9 cam?

Edit : ebay glix cam for 15 and change, free shipping

[ratdog]

I think it would be helpful if we knew where you are now. What are you doing as far as top speed and RPM

The reason I ask it because the three bikes I have built lately are really limited by RPM. That is, they more then enough power to get up to the 45 mph range. But they are doing so at 8500-8700 rpm. Basicly they are limited by the final drive ratio. I have head the some bikes are faster, but I can’t help but think they would have to have a bigger rear tire, or a different gearing.

IMHO, a pisto upgrade is not going to be worth it, unless your combine it with other goodies. We have done the A9 cam on 3 bikes in the last couple of days. It has produced a slight inprovment in top speed and increases acceleration a bit. I would say it was worth the little bit they cost

One of the over bore kits we got came with a big valve head. We had a chance to compare the big valve head with stock porting to a big bore head with stock valves. Belive it or not, the big valve head with stock porting and 18mm carb reduced preformance by a little over 1 mph. We have since ported that head, got a metal intake runner, and a 22 mm carb. That combo made a measurable inprovment in acceleration but of course the top speed is still limited ny the gearing. That bike gets up to 30 real quick, any will pull 47 mph on flat ground but at 8700 RPM.

So imho, don’t go to the bigger bore unless you are also doing other mods. You can do a bunch of great stuff to get more power, but to make use of that power, you need to do some work on the trans. If you running in the 45 mph range right now, your just spending money to spend money

I’m helping with a full house build right now. He has done a cool one off caustic with a reverse cone megaphone and a 50 mm bore. Unfourtanly the “22 mm high flow” cab isn’t ( more like 18 mm). While he gets that stringhted out, this morning he convinced me you “try” the big valve,ported head with the 22 mm carb on his bike. We had to screw with the jetting as his monster likes lots of fuel. With a 95 main jet, that bike just went screaming by house at 12,400 rpm and over 50 mph. Can’t give you an exact speed, because the speedo was jumping around and we hadn’t bothered mounting a GPS. I should note that he had a half mile run from the main road to do that.

Ofcourse this is not an engine you want to run all day at 12,000 rpm, so he went home to order some more goodies for the trans. He is also trying to find a bigger rear tire to put on it.

Next weekend he is going to the strip with me. He has been riding bikes for 40 years but never went to the strip. I’m going to teach him how the ride the 600. He keeps dying Who’s chicken too, but he will need a lot more seat time before he can put a leg over that!

[magoconnor]

The combustion chamber on the bvh meassures 47mm, and the valves are 22/19.
I have tried this slighty larger head with my current 72cc cylinder, with bad results.
The compression was too low...

I dont wanna repeat that, do you think I'll be better off using the stock head, with the airsal 50mm?
Or will the compression be too crazy.

[magoconnor]

Well I'm just looking for slightly more top speed, I would like to do 50 mph or so.
But more important, I would like the cruise speed to increase.

I'm also doing top speeds in the mid 40's.

I will definitely do a gear upgrade later on, if necessary.

I plan to purchase the a9 cam, while I already have cleaned up the head and port matched it.
And lapped the valves.
The head's quality is the best I have ever seen, straight from taiwan.
Nice and heavy with good quality aluminium, non of that flaky stuff.

My exhaust is a leo vince titan, so the bvh should be able to breath.

[AtariGuy]

I did originally take my scoot from stock to a 47mm cylinder, stock head, upjet to 84 on the carb. All else was stock (including stock cvt configuration) - i went from 32mph sitting up on the flats stock to 36mph, downhill tucked 40mph to 44mph. With the 50mm bore and head (that head is essentially a 39mm head with a squishband ground out to 50mm) and a9 cam, i'm currently running 41mph sitting up flat ground and 52mph (speedo only reads to 50, used digihud gps speedometer app to verify) downhill slight tuck. I have 3.50-10 wheels, stock gearing and cvt, 18mm stock carb jetted at 96.

[magoconnor]

Very cool, Thanks for report.
It was exactly the information I was looking for.

Thou you haven't tried it, do you think the bigger valves would hurt the performance?
Being that I'm still using a 18,5 carb.

[magoconnor]

I think the main concern is to keep the compression up.
Do you think the compression ratio, will be okay using the 47mm head?

[ratdog]

Ok, just thinking out loud here, but if you are going to do it, why not do the 52 mm kit from scrappydog. After putting in 3 BBK from other suppliers, this one looks very atrictive to me. The other kits I see all have very rough piston tops. In fact on the latest build ( earsplittinglouenboomer), we took the time to machine the top of the piston as well as true the cylinder and head. If I had seen this this before, I would have bought it instead. The top of the piston looks like it’s machined in the pic. From what I can see, this kit looks like much higher quality

[AtariGuy]

A 47mm bvh will still have a 13cc combustion chamber to fit the 21.5/19mm valve openings - thats the only way they can fit "big valves" in them tiny heads. So 39, 47, 50, 52mm heads with big valves will all have that size combustion chamber. Again, they just chamfer a squishband around the edge to make it 47/50mm bore. So i'd just avoid big valves unless you have a high compression piston, a stroker crank, or the big 52.4mm bore kit

[magoconnor]

I think if I were to import that kit, I would pay maybe triple or four times the purchase prize

The piston that comes with the kit I'm going to use, comes with a nice high compression piston.
Here take a look :https://www.google.dk/search?q=airsal+81.3cc+gy6&rlz=1C1AVFB_enDK752DK752&tbm=isch&source=iu&ictx=1&fir=uKrFzpf-BQjukM%253A%252C4v_RFaiUZ_b9iM%252C_&usg=AFrqEze6iR2T1CQk0uU6uwVd0vqry7YKWg&sa=X&ved=2ahUKEwiS66WH2_ncAhWr-ioKHR2ADI4Q9QEwA3oECAYQBg&biw=1280&bih=697&dpr=1.5#imgrc=uKrFzpf-BQjukM:

[AtariGuy]

There is this kit for less than the hoca 52mm bvh head and sspg cylinder combo at pfs

Or go with a 50mm bore and fetch this lightened airsal domed piston

A lot of people report premature bearing failure at the big end of the conrod with standard 50mm pistons on stock (non-taiwanese) cranks - after all, you are pushing a lot more force from a 50mm kit and them pistons weigh a whole lot more for a crank designed to handle 39mm cylinders

[ratdog]

The thing I would watch in trying to get higher and higher compression is where are you going to get gas to run it? When we stat talking about this stuff I always assume you want to run on pump gas bthe squash bands on “big bore” heads is there to help prevent pre ignition. Even then you might be best to run high octane gas

Just wondering, but have you ever put a compression gauge on your bike. Assuming good valves and rings you should be up in the 160 range of better. ( in fact with a stock cam you may be up in the 180 range. To drive day to day on the street, with pump gas imho you should not go over 200. A hotter cam will bring that down a bit, but still I really don’t think you should be pushing for more compression, it is really hard to make educated guesses if you don’t know where you are starting!