pc 2000 compression

[dove]

just rebuilt up an engine with a pc 2000 cylinder it is a 275cc ,i did a compression test and it read about 270psi,it has a 20cc dome in it,does that sound about right?i set the squish at 1.08mm,using a 5mm spacer plate,it is a 69.5mm wiseco piston,i am hoping to run 98 ron octane fuel,what would be a safe compression?to get to a safe compression what size dome will i need yo put in it?when i am trying to start it it seems to be kicking back a lot,i have never run this cylinder before so it is all new to me,any info greatly appreciated

[trx250rider]

wow thats high..i think you need jet fuel

[rustyATV]

Wow! Really?

For pump gas (93 Octane) the general limit is 185psi. I run 210psi for 110 race gas. You'll want to be somewhere in between.

270psi is astronomical. What size dome are you using?

Your squish is fine, but could actually be tightened up a bit if you could drop your compression.

[dove]

it is a20cc dome.what size dome would i need to drop compression to around 200 psi

[trx250rider]

duncan sells domes for 15cc to 22cc...id deffently try the 22cc first.

[rustyATV]

Something's not right here.

Is it just me, or should this compression not be possible with the setup as described?

[trx250rider]

go to auto zone and rent a compression tester and test it again..make sure it reads the same.your tester might be off..there like 35$ and you get the money back when you bring it back..

[dove]

my tester is a draper one,it reads ok as i used it on my 250 sparks cylinder and it read 180psi.but will try get a shot of another one to double check

[2muchquad]

yeah 270 is way up there,what is that a diesel..:D

[dove]

i see on duncan racing web site that you can get domes from 12-30 cc.wich size do you guys reckon i will need to drop 70 psi,20 cc dome in there just now,my tester is accurate,i have a lathe do you think i can just machine a bit out of it,without touching the squish angle.need to get this up and running pronto,will take ages to get one shipped over here

[rustyATV]

If you know how to cut nice curved surfaces, then sure. I'd have one on order, though, in case you need to go back.

[2-330s]

i know a few ftz race motors that run that squeeze and still rev to the moon. but normally that much compression is a mid range tt or mx motor. are you sure you didn't order a 86 piston for a 88-89 set up motor?

[dove]

i ordered it from duncan racing,i gave them the number on the cylinder pc20058 and they sent me the piston,i had to use a 5mm spacer plate under the cylinder,i also have 89 crankshaft.i think it is an 86 style piston as the gudgeon pin height is different from the 89 piston i took out.would this make that type difference?i can recall the cylinder having midrange engraved onto it

[Langbolt]

I'd be checkin' your compression gauge ???

Even at 100% volumetric efficiency @ your displacement (273.14cc) and a 20cc dome you'd only be hitting 180.81psi (12.3:1 compression)

Does the PCI Cylinder use a head gasket or is it an O-ring set up ?

To tell if you have an '86 style or '87-'89 style piston:

'86 has the half moon cut out at the base of the intake skirt.

'87-'89 has a rectangular window cut-out in the intake skirt.

:cool:

[dove]

it is an 86 style piston,the cylinder head has o-rings not a gasket,i put my air compressor on to the end of my tester and got it up to 6 bar and both read the same.can this have anything to do with using a spacer plate?everytime i try to start it it kicks back,it never used to do this with my sparks cylinder on it,i cant adjust timing,and have not changed anything else on the motor apart from the other things that came with it pyramid reed valve 39mm carb.is it comon for a pc2000 to kick back? i ve never run this setup before.

[dove]

what is the equation you use to work out your compression ratio?you will need to know the height from your highest port to do this i assume,the bore diameter,the dome cc

[Langbolt]

The equation I used is:

Total volume / head volume = uncorrected compression ratio

273cc+20cc / 20cc = 14.65:1

but if you have no head gasket this is what you'd get.

I assumed you had a 1mm thick head gaket which adds 3.793cc

To be really accurate....set your piston to top dead center with a dial indicator.....then using a graduated syringe ......drop oil into the spark plug hole till if fills to the base of the threads.

Then you will get your EXACT head volume.

Even at 14.65:1 you still wouldn't see 270psi ....(maybe 215)

:D

[rustyATV]

I calculated that as well, but I wasn't sure it was that easy.

What thickness base gaskets are you using with your base plate? (though with your squish at 0.042, it shouldn't matter)

[dove]

thats what i got too,but is the dome cc 20 when the piston is at top dead centre or when the piston is not protruding into it?

so if i do the maths backwards i get a compression ratio of 18.37-1 that is with 270psi.that would give the combustion chamber a cc of 15.95 when the piston is at top dead centre.does that sound right.so the dome of the piston would equal 4.05cc


so inorder for me to get a compression ratio of 13.3-1(196psi)i would need to use a domesize of 22cc and that calculation will then have to add in the extra 4cc that the piston takes up when at top dead centre,the actual dome size i will need is a 26cc dome


i will try and cc the cumbustion chamber when at top dead centre to see if what i wrote makes any sense
does anyone know what gap should be in the piston rings as i did not check that?

[dove]

sorry folks but my last calculation is not 100% accurate
what it should be is

273.145+26 cc dome=299.145ccwhich you divide by22cc(compressed combustion chamber volume)26ccdome-4cc piston dome =13.6-1 compression ratio x14.7(atmospheric pressure)=199.9psi cylinder compression@100%volumetric efficiency

i checked the cc of my compressed combustion chamber and it was 16cc so the piston dome is4cc

i believe this to be correct but please reply if you think otherwise

[trx250rider]

wow...all that math makes my head hurt.just buy another dome.