decking a 200x cylinder head

[torque]

hey guys im starting an engine build one my first gen 200x. i have a weisco piston 10.25:1. i want to get a bit more bang, but dont want to go all the way to 12:1 and have to use full race gas. so ive been investigating decking the head. i cant seen to find any information on the subject. i would like to get close to the 11.1 ratio.

has any one ever done this?

if so what were you results?

[leevarnado]

cheaper way is to buy 12.1 piston and double up your base gasket

[torque]

ya i read about that in dirtcrashers 20 day build. if memory serves he was hoping to run 93 octane, but it did not work out for him. thats why i was hoping to go the other way, and take 15-20 thou of of the head. plus i already have the piston 10.25:1 piston.
thanks

[leevarnado]

a certain member has some 66mm pistons that are 11.1.i want say his name but maybe he will chim in.

[torque]

great thanks

i would not mind having a look at one of those 11.1 pistons

[Vealmonkey]

Just get some lapping compound and a lapping board. Lap the cylinder top and the head base in and assemble without a gasket! Or handmake a headgasket by tracing the headgasket onto a piece of shimstock and carefully cut it out. Voila!

[torque]

very good ideas that i had not thought about thank you.


i just happen to have some copper sheets 18 thou,i think. i will have to compare a already compressed head gasket.

[Vealmonkey]

I get a good idea on occasion! The big atc90s don't use a head gasket at all, but the cylinder base is somewhat recessed down in to the cylinder making it easier. There were high performance cars that didn't use a head gasket, the ford SOHC and DOHC engines come to mind. I'm sure they have a little more pressure and a non gasketed head than almost any trike will have! LOL

[greenhuman]

Buy a CRF230 genuine head gasket. They are 3 seperate layers with a little rivet holding them together. Take out the rivet and you can use one or two layers to up the compression.

[torque]

you guys are freaking awesome. so many possibilities now. thanks

[WilliamJ]

Torque,
You need to do some checks before you remove the gasket or put a thin one in place. Standard gasket is 1.34mm new and probably doesn't compress much - maybe to 1.32mm

Here's an example - standard piston is 9.6 to 1CR - take the gasket out and you would have an 11.8 to 1 CR!! If you do that with a 10.25 piston you'll get 12.8 to 1.
The other thing you should check is piston to valve contact using some putty or modeling clay on top of the piston.

There is a simple way to calculate the chamber size you need for the CR you want - the snag is that measuring is not easy, unless you have the kit and have done it before. By chamber size I mean the total volume of the gasket, the piston dish (or dome), the ring land and the chamber itself.

The calc is:- swept volume, divided by desired CR minus 1. So for a standard 200x swept volume is 191.8cc, your desired CR is 11, so the chamber volume is 19.2cc.
The standard chamber is 22.3cc - so the difference is only 3.1cc - less than the gasket volume which is 4.6cc.

Ideally for 11 to 1 you need a gasket which is 0.9mm thick - or as you were thinking, deck the head. I think you need to take a little more off if you deck the head because of the squish band. You are probably looking at about half a millimeter but check the piston to valve clearance.

[torque]

this is a fantastic post ^^^^. have a beer on me.

i found a cr calculator on line and have been playing around with it for some time now. i am making a few assumptions, deck height is zero and using a gasket thickness of 1.32 mm.

the total volume of the combustion chamber with head gasket 22.3 cc this is base off of the above post.

bore 66 mm
stroke 57.8mm
total chamber size 22.3 cc
head gasket thickness 1.32 mm
head gasket volume 4.6 cc
actual combustion chamber size 17.8 cc
piston dome .923 cc

by plugging these numbers into the calculator i came up with a theoretical cr of 10.243:1. that comes very close to the actual piston spec of 10.25:1.

now if i take these same numbers and replace the head gasket thickness with .9
cr of 10.9:1!!!

it would appear that Williamj knows exactly what he was talking about. have another beer you earned it

please check my figures here is the link http://www.csgnetwork.com/compcalc.html

[ama009]

I think this is the most I have learned from such a short thread in the time I have been on this forum!! awesome info guys!!

[WilliamJ]

Torque,
Glad to be of help. If you need more info I have a calculator for the relationship between the CR, air/fuel ratio, intake temperature, head temperature, octane and ignition timing. The valve overlap also has an effect on the numbers.

Bill

[oscarmayer]

i run a 12.5:1 wiseco in my 200x and have no issues with 93 pump gas. not sure why you guys would have issues.

[torque]

Torque,
Glad to be of help. If you need more info I have a calculator for the relationship between the CR, air/fuel ratio, intake temperature, head temperature, octane and ignition timing. The valve overlap also has an effect on the numbers.

Bill

thanks bill

i would love to see this calculator. i want to build a killer engine and this info is priceless when planning and knowing how to peace together parts that compliment each other.

[torque]

i run a 12.5:1 wiseco in my 200x and have no issues with 93 pump gas. not sure why you guys would have issues.

i have been reading about cr's in these bike's for quite some time. conventional thinking would say that you can not run 12:1 on 93 octane. but i know there is a small contingency of guys on this forum that say it can be done and works great. how ever its one of those things that i would have to see to believe.

[Vealmonkey]

You can run a high compression piston on pump gas, but the thing is, why? The maintenance will tend to be very high and the margain for something bad to happen is very large. The higher octane fuel will give you a way larger cushion on the engine for possible calamities. My theory is, if you don't want to run high octane, then don't build your engine for it. It can only hurt your engine and your wallet in the long run. But what do I know, right?

[torque]

You can run a high compression piston on pump gas, but the thing is, why? The maintenance will tend to be very high and the margain for something bad to happen is very large. The higher octane fuel will give you a way larger cushion on the engine for possible calamities. My theory is, if you don't want to run high octane, then don't build your engine for it. It can only hurt your engine and your wallet in the long run. But what do I know, right?

ok ok

i will take your word for it. but i will take the other half of your advice. i want to keep the margin of error on the fat side and easier on the kickers. so i will stick to the 11.1 if i can swing it. and a steady diet of 93 octane and a little booster juice

[WilliamJ]

thanks bill

i would love to see this calculator. i want to build a killer engine and this info is priceless when planning and knowing how to peace together parts that compliment each other.

If you PM me your email address I'll send it over - its a little excel spreadsheet.
Bill

[torque]

e mail addy sent

thanks bud

jarret

[torque]

this thread has been very useful to me. thanks to all that have helped out and provided me with information, and the options to reach the 11.1 ratio i am after.

i am going to start a new thread about this engine build. i hope to get some participation from some of you, as i hope you can help me identify some aftermarket parts that i have come across

[250rRoostmaster]

Torque,
You need to do some checks before you remove the gasket or put a thin one in place. Standard gasket is 1.34mm new and probably doesn't compress much - maybe to 1.32mm

Here's an example - standard piston is 9.6 to 1CR - take the gasket out and you would have an 11.8 to 1 CR!! If you do that with a 10.25 piston you'll get 12.8 to 1.
The other thing you should check is piston to valve contact using some putty or modeling clay on top of the piston.

There is a simple way to calculate the chamber size you need for the CR you want - the snag is that measuring is not easy, unless you have the kit and have done it before. By chamber size I mean the total volume of the gasket, the piston dish (or dome), the ring land and the chamber itself.

The calc is:- swept volume, divided by desired CR minus 1. So for a standard 200x swept volume is 191.8cc, your desired CR is 11, so the chamber volume is 19.2cc.
The standard chamber is 22.3cc - so the difference is only 3.1cc - less than the gasket volume which is 4.6cc.

Ideally for 11 to 1 you need a gasket which is 0.9mm thick - or as you were thinking, deck the head. I think you need to take a little more off if you deck the head because of the squish band. You are probably looking at about half a millimeter but check the piston to valve clearance.

One of the best posts i've seen in awhile!! Props to your excellent math skillz!!! :beer

[Scootertrash]

Excellent post WilliamJ!!!

[oscarmayer]

guys, i still don't get why you are all cringing on 12:1 on 93 pump. it's been Deon for many many years in car engines, and even other bikes and ATVs. There are several stock biles that run that or higher compression form the factory now days. (ATV/motocross engines included in that). The biggest issue is to ensure proper jetting. Take the time to properly jet your carb and your good to go. Many import street cars run a 12:1-13:1 on high pump gas w/o issues. I think the deal is most people don't properly adjust their carb or properly jet it and this will cause you problems. my current 200x the one i did up for you guys remember, with the race can and head work and such, it run s a 12.5:1 as i run a thin gasket. but other than that, it's fine. we beat the hell out of it nearly weekly here (weekend warriors). Now mind you, we are NOT racing we do mostly steep hill climbs and other stuff, but still it stays running for a couple of hrs at a time. YES constantly not just a few min and shut down, so that should tell you something. Anyway, I hope this helps and I hope you guys think about the best way to handle your jetting you'll be fine. I run a 26mm mikuni carb instead of the 30mm that was having mid throttle hickups.

[WilliamJ]

Excellent post WilliamJ!!!


One of the best posts i've seen in awhile!! Props to your excellent math skillz!!! :beer

Thanks for your appreciation. I am not too smart at the math - I have to work it all out slowly and double check it!

Bill

[torque]

guys, i still don't get why you are all cringing on 12:1 on 93 pump. it's been Deon for many many years in car engines, and even other bikes and ATVs. There are several stock biles that run that or higher compression form the factory now days. (ATV/motocross engines included in that). The biggest issue is to ensure proper jetting. Take the time to properly jet your carb and your good to go. Many import street cars run a 12:1-13:1 on high pump gas w/o issues. I think the deal is most people don't properly adjust their carb or properly jet it and this will cause you problems. my current 200x the one i did up for you guys remember, with the race can and head work and such, it run s a 12.5:1 as i run a thin gasket. but other than that, it's fine. we beat the hell out of it nearly weekly here (weekend warriors). Now mind you, we are NOT racing we do mostly steep hill climbs and other stuff, but still it stays running for a couple of hrs at a time. YES constantly not just a few min and shut down, so that should tell you something. Anyway, I hope this helps and I hope you guys think about the best way to handle your jetting you'll be fine. I run a 26mm mikuni carb instead of the 30mm that was having mid throttle hickups.


i have never built a high compression engine before, so i did some research. it turns out that the static compression ratio(scr) is not as important as the dynamic compression ratio (dcr). The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume and a chamber with a volume of 1 has a 10:1 compression ratio. or Static Compression Ratio (SCR) . It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder. another huge affect on cr's and octane is cylinder pressure(cp). cp is a big function of dcr, but there are many variables Valve size, engine RPM, cylinder head, manifold , carburetor size, altitude, fuel engine/air temperature and compression ratio all combine to determine compression pressure. basically anything that affects volumetric efficiency. the biggest factor is cam design. the longer the intake valve is held open atdc the less the dcr and dramatically reduces cylinder pressure.

so in the end what really matters to the engine is cylinder pressure. thinking that a 10.:1 piston will build xxx number of cp. wrong you have to take into account your camshaft and a few other variables. all these variables are why some guys have successfully run a 12.1 piston on 93 octane while others fail.

oscermayer i like this kind of discussion , it make me think and learn stuff.

[Team ATC]

so what head gasket did u use?

[torque]

i am still looking in to a few different options. i have a pretty good parts collection, but i am still missing a few key components before i start this build. i'm going to start a build thread when i get every thing
lined up

[atc007]

guys, i still don't get why you are all cringing on 12:1 on 93 pump. it's been Deon for many many years in car engines, and even other bikes and ATVs. There are several stock biles that run that or higher compression form the factory now days. (ATV/motocross engines included in that). The biggest issue is to ensure proper jetting. Take the time to properly jet your carb and your good to go. Many import street cars run a 12:1-13:1 on high pump gas w/o issues. I think the deal is most people don't properly adjust their carb or properly jet it and this will cause you problems. my current 200x the one i did up for you guys remember, with the race can and head work and such, it run s a 12.5:1 as i run a thin gasket. but other than that, it's fine. we beat the hell out of it nearly weekly here (weekend warriors). Now mind you, we are NOT racing we do mostly steep hill climbs and other stuff, but still it stays running for a couple of hrs at a time. YES constantly not just a few min and shut down, so that should tell you something. Anyway, I hope this helps and I hope you guys think about the best way to handle your jetting you'll be fine. I run a 26mm mikuni carb instead of the 30mm that was having mid throttle hickups.

Well said!! This IS a VERY interesting thread,,NOT putting anyone down! A good read for sure. Life is short,,keep it simple,choose your battles to waste time on. Sell your piston in the classifieds or Ebay. Buy a 12.5 to 1, live happy,,we've been running them for 3 decades with no problems. My .02. There ,I said it! good luck!

[WilliamJ]

If someone has the stock cam details I can work out what the dynamic CR is. Strangely there isn't much info on here about it.

The further up the bore the piston is from BDC the higher the CR you can run. With a full race cam using seat-to-seat timing of say 310 degrees you can go for 13:1 and more.

CR, octane, air temp. head temp, mixture ratio, throttle opening/load and ignition point are all interrelated. So with an extreme CR, any one parameter getting out of line can give you detonation.
Using ignition timing degrees as the baseline the following comparisons can be made:-

10 degrees C head temp rise = retard ignition 1 degree
7 degrees C intake temp rise = retard ignition 1 degree
1 octane number decrease = retard ignition 1 degree
one half air fuel ratio leaner = retard ignition 1 degree
one fifth compression increase = retard ignition 1 degree

The ignition is the easiest thing to alter on most engines, which is why the comparison is drawn up that way.

Air pressure and humidity have an effect but I don't have definitive info on that.

I don't have any information on throttle position effect either, but at low and mid revs a wide open throttle promotes detonation. That's why cars used to have vacuum advance retard adjusters - these days it's all taken care of by the ECU.
Bill

[torque]

If someone has the stock cam details I can work out what the dynamic CR is. Strangely there isn't much info on here about it.

The further up the bore the piston is from BDC the higher the CR you can run. With a full race cam using seat-to-seat timing of say 310 degrees you can go for 13:1 and more.

CR, octane, air temp. head temp, mixture ratio, throttle opening/load and ignition point are all interrelated. So with an extreme CR, any one parameter getting out of line can give you detonation.
Using ignition timing degrees as the baseline the following comparisons can be made:-

10 degrees C head temp rise = retard ignition 1 degree
7 degrees C intake temp rise = retard ignition 1 degree
1 octane number decrease = retard ignition 1 degree
one half air fuel ratio leaner = retard ignition 1 degree
one fifth compression increase = retard ignition 1 degree

The ignition is the easiest thing to alter on most engines, which is why the comparison is drawn up that way.

Air pressure and humidity have an effect but I don't have definitive info on that.

I don't have any information on throttle position effect either, but at low and mid revs a wide open throttle promotes detonation. That's why cars used to have vacuum advance retard adjusters - these days it's all taken care of by the ECU.
Bill

thanks again bill. i have done lots of research on dcr. dcr is a better indicator of engine performance and detonation characteristics than a static cr. i am posting a link. there is a fantastic read on dcr and volumetric pressure.

http://victorylibrary.com/mopar/cam-tech-c.htm

[torque]

Well said!! This IS a VERY interesting thread,,NOT putting anyone down! A good read for sure. Life is short,,keep it simple,choose your battles to waste time on. Sell your piston in the classifieds or Ebay. Buy a 12.5 to 1, live happy,,we've been running them for 3 decades with no problems. My .02. There ,I said it! good luck!

not fighting any battles here just want to build a strong engine that has the performance where i want it, with out blowing it up. i am still looking at a couple options, at the end of the day i might go with your suggestion.

i just chalk it up to learning.

[WilliamJ]

Well I read that half-way through but I have had to give up because I was getting really confused. It starts off okay but then, well, it's like the writer had a few beers or something. Anyway, I just couldn't keep track of the point he was making because he seems to change subject partway through each paragraph. I am not refuting anything he has written, but I cannot get my head round a lot what he says because of the way it is written. A couple of the statements seem to contradict what I have read elsewhere though. I will try again when I am wide awake in the morning.
Bill