low end problems with head?

[themountain]

the piston as far like that in the bore and almost no dome...?? you are lucky if you have the same compressionrate like the stocker!!
1.8mm inside with a flat piston(which it isn't) would be 4.5cc more to the headvolume which is 15.3cc....that would be a ratio of around 7.7-8.0:1...that would be good too kickstart..cough,cough!!
Fortunately there is the dome, so my bet is you are at stocklevel or sligtly under!

 

[damann33]

akunar wanted 150 for the cylinder piston kit, and gaskets plus 50 dollar shipping. I have only bought a piston, head,gaskets off him for a total of $210.00, and I might hold onto the piston for later in the future, or ill buy the cylinder and do the set up. It sounded to easy to be true, everyone was saying to jus sbuy this do that, do this, and now that I buy it its a whole other story I will see if I can get my money back for the piston, and will probably run the bigger head, with thinner head gasket to regain the compression I will loose.

 

[damann33]

If the valves hit the piston, which they probably will with a thinner head gasket, i will have to notch the piston a bit, which will drop my compression a bit, but hopefully I will have a good amount of compression since the head will be about .5mm lower

 

[Mack]

don't be too hasty there damann33...just wait and see what Akunar has sent you, then mock it all up and measure everything

 

[damann33]

just emailed him back and told him to ignore the last email I sent him tellin him that I didint want the piston lol. I will see what I can do then when I get the piston. I can either use my stock cylinder/piston and run at a bit lower then stock compression since i would ahve to have the pistons notched, or I can buy the 57mm cylinder and try working something out, running that set up so that I wont loose compression. Lets say I lowered the cylinder so that it is flush with the piston, how would I know how many links to remove off the cam chain? I will probably lower the cylinder a bit to raise up the piston to somewhere around 1 and 1.2mm, the dome on the piston I am getting is about 4mm high, the piston I am getting isnt on his website, but he showed me an identical piston in terms of looks, dome height, and probably deck height as well. Is it possible to bore the stock cylinder to 57mm?

 

[Cactus Jack]

just emailed him back and told him to ignore the last email I sent him tellin him that I didint want the piston lol. I will see what I can do then when I get the piston. I can either use my stock cylinder/piston and run at a bit lower then stock compression since i would ahve to have the pistons notched, or I can buy the 57mm cylinder and try working something out, running that set up so that I wont loose compression. Lets say I lowered the cylinder so that it is flush with the piston, how would I know how many links to remove off the cam chain? I will probably lower the cylinder a bit to raise up the piston to somewhere around 1 and 1.2mm, the dome on the piston I am getting is about 4mm high, the piston I am getting isnt on his website, but he showed me an identical piston in terms of looks, dome height, and probably deck height as well. Is it possible to bore the stock cylinder to 57mm?

If you are going ahead with the Akunar piston you'd have been better off getting one with the biggest dome possible ...... You should be able to bore the iron cylinder to 57 mm as long as the casting is clean metal and free of defects ..... Mack just said it takes several passes of the boring bar and they'll charge you for each pass .......

As to the cam chain ...... say for instance one link is 10 mm long from centre of pin to centre of pin ...... if you removed one link you'd have to drop the head by 5 mm to get the stock chain tension back ..... but you'd have to remove TWO links to be able to rejoin the chain so the head would need to be 10 mm lower to regain stock tension ....... then there's the fact that the degreeing of the cam goes out even when you use different thickness gaskets or mill the head ...... To understand that try to move the wheel backwards and forwards on your bicycle with the chain tension maintained ..... WITHOUT any of the sprockets turning

As you can see it's best to keep everything as close as possible to the stock height ...... the absolute BEST thing to do would be to have a piston manufacturer custom make you a hi comp piston with the proper pin size , pin height , dome shape , compression ratio , weight etc ....... OR get a custom length conrod made to get the Akunar piston to the proper deck height ...... They used to do all that at the snap of a finger with XR75's etc back in the '70's ... wonder where all those brilliant , spontaneous modifiers are these days ? .... although Powroll concocted a worrying method of shortening conrods by heating and compressing them to save $$$$ ........ maybe they could stretch a Lifan 140 rod using the same method in reverse .......

 

[damann33]

perfect, I will get the bigger domed piston from akunar, I will email him now and let him know the piston I want. The one piston on his site has the 6mm dome and the 13.0mm pin top to deck height. With the bigger dome I guess it is better because even after notching it, I will still have gained compression. So if I get this I can see how low I can bring the cylinder, maybe to make the piston only 1mm down in the cylinder. I can see what you mean by the chain links, i will measure everything and see what I can do, and how low I can bring the cylinder. I wonder if I could lower the cylinder just a bit without touching the cam chain. I have found a company that custom makes pistons, but that would probably be a lot of money. I guess I will have to wait and see what happens once I get all the parts. I guess the only risk of me running the bigger domed piston in the stock height cylinder is detnation?

 

[Cactus Jack]

I guess the only risk of me running the bigger domed piston in the stock height cylinder is detnation?

You'll need to run higher octane fuel with anti knock additives ...... standard unleaded won't cut it .......

 

[damann33]

ya forsure, ill be runnin 94 octain pump gas and will use a little NOS octain booster with all the addictives in there. So if I go ahead and use proper addictives and gas, and the biggest domed piston from akunar, I wont have any problems with the piston down in the cylinder? I will still measure everything out and see what I can do to bring up the piston. I also ahve just emailed him and told him I want a piston with the biggest dome, which is this piston, AKPRP010-A. It has a 6mm dome, 13mm deck height and the 15mm pin. The dome starts at about 5mm from the edge as Mack was saying. Also the cam chain is wierd how it works, i measured two links to be about 24mm so would that mean I would have to lower the cylinder and head about 12mm total? I can move the cylinder 1.8mm lower to become flush with the piston, and then I could go out and buy that head gasket which lowers the head .5mm, which gives me a total of 2.3mm lower. What would happen if I just dropped the cylinder .5mm to bring the piston up a bit, and not even touch the cam chain?

 

[Cactus Jack]

That's about all you can do without copping a host of other problems and you'll just have to be careful with timing advance ..

The piston being down the bore means your squish and quench won't be as efficient as it could be for max power .....

Anyway we'll see what Mack comes up with .....

Here's a pic of bone stock CB/SL175 pistons from back in the days when fuel was top quality .....

 

[damann33]

yup true enough, those piston have massive domes, is that when fuel contained lead?

 

[damann33]

well akunar got back to me and stated that the bigger domes pistons are only 55mm in diameter and the 57mm piston only has the 3.5mm dome

 

[damann33]

well i cam to a conclusion to buy a 55mm akunar piston with the big 8.5mm dome. I did this so that I wont have to pay 150 bucks for a bore, or have to go out and buy a cylinder. Also this piston will give me a lot more compression then stock and I have a little more to play with when it comes to notching the piston to make room for the valves. I could even bring up the piston a bit maybe by .3 or .4 if I use a sealent instead of gasket. I have a special lue that withstands pressure, heat and chemicals and is used to seal crankcases on 2 stroke engines. The piston will be about 1.4mm down in the cylinder and about 7.4mm of the dome will stick out from on top.

 

[Cactus Jack]

Did Akunar tell you what most people are doing with the 140's ?????

 

[Cactus Jack]

yup true enough, those piston have massive domes, is that when fuel contained lead?

Those pistons are ready to run ..... notice how every edge is nicely rounded off to prevent hot spots and detonation ..... (including the edges of the valve reliefs) . When you deepen the pockets you want everything to be radiused ....

Those pistons would require high octane leaded fuel or race gas ...... just like how formula 1 cars use Extra Leaded Fuel (ELF)

 

[Mack]

as I said no suitable pistons + cam chain problems. But wait theres more....now that your grasping these issues try this. Now if you find the workable deck height and gasket thickness you will be forced without further modifications to live with the static compression ratio this gives. BUT you now add cam of choice. Your choice has already been made and that is a z40 profile cam. This cam I have doctored and can tell you that it gets its top end performance by not only increased duration but also its centrelines and the amount of retard it runs. This cam has one of the latest intake valve closing times of all commonly available cams. This cam has the inlet valve still open 1mm at 53.6 degrees ABDC (after bottom dead centre) and actually doesn't fully close until a little past 80 degrees ABDC. As therefore compression does not begin until the piston is fairly well on its way up the stroke your actual running compression ratio (dynamic) is dramaticly lowered. So even if you acheive stock compression ratio with one of these pistons your running ratio with THAT cam may end up 6-1 or something crap like that. You may be better running a stock cam!

...infact I just ran some numbers and estimate you will be around 6.3 to 1 running that cam if you only acheive 10 to 1 static compression. Thats only 148psi cranking pressure. In order to run about 8 to 1 or 210psi crank pressure with that cam, you would need to build the engine with a 13 to 1 static ratio. To acheive 8.7 to 1 running and 230psi you will need 14-1 static...

 

[Mack]

i forgot to add that, when lowering the head you are retarding the cam even further, so if you don't get the cam timing back to where it should be, you will be running a great amount of retard on a z40 cam. Not only reducing compression further but having a later exhaust valve closing time also will cause problems...some of them quite big and catastrophic...

 

[damann33]

thanks Mack for that information, I am working on getting this z40 cam sold, and I may just buy a milder cam from AHP. It states that I can use the stock springs with that cam, but would it hurt if I add in stiffer springs which I bought for the z40? Also Akunar, after putting up with me for a while, is going to have my ported/polished 28/24 valve head and big domed 55mm piston shipped out to me on monday. If he doesnt screw up the order, and I hope he doesnt, ($60.00 shipping from him to me) I will get that cam from AHP, lower the cylinder by .3mm or .4mm and get the piston notched to the proper specs. I will measure the stock setup piston to wall clearences and use that as a minimum measurement.

 

[damann33]

Did Akunar tell you what most people are doing with the 140's ?????

he just told me that he had 3 people over the last weekend that went with the 55mm piston, he also told me alot of people buy the kit he provides.

 

[damann33]

well today I was going to measure the stock piston to wall clearences, but the problem is that my stock cam is toast, there has been galling on the actual cam lobes, since it was starved of oil, so I dont think it will be able to open the valves to the full potential. Someone told me it has to be around
.070, so I will measure the piston to wall clearence with the toased cam and see what I come up with that way.