Closing up the squish band clearance on my modified head! My testing continues....

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

if you want to totally rechamber the head i would say 180psi and a 40 thou. squish would be safe on a FORGED piston with 30 thou. PtW clearance and a long 3 day break in procedure. with a 3 degree advance on timing. IF and ONLY IF you have the jetting down before you start and are not wildly guessing at jetting before the engine is good.

ALSO DO NOT do this on a stock ported cylinder, the air charge will not have enough velocity to and temps would be to high.

AND do NOT use dual stage reeds on large volume engines especially if you mostly ride around at WOT.

 

[sicivicdude]

Re: Closing up the squish band clearance on my modified head! My testing continues...

if you want to totally rechamber the head i would say 180psi and a 40 thou. squish would be safe on a FORGED piston with 30 thou. PtW clearance and a long 3 day break in procedure. with a 3 degree advance on timing. IF and ONLY IF you have the jetting down before you start and are not wildly guessing at jetting before the engine is good.

ALSO DO NOT do this on a stock ported cylinder, the air charge will not have enough velocity to and temps would be to high.

AND do NOT use dual stage reeds on large volume engines especially if you mostly ride around at WOT.

Be careful making brash recommendations there hoss,

You're not defying the laws of physics only slightly bending them in a way most riders don't....

You've corked that engine with a relatively low rev exhaust and stock silencer. Running that engine with that exhaust will work ok but if someone took a much stronger pressure return wave exhaust and put it on an engine built to those ^^^^^^^^ specs will be putting a piston in it in 6 months from predetonation.

I should add to my previous post earlier that all of engine testing experiences were with a Power Pro's right bend pipe with a strong pressure return wave.

 

[sicivicdude]

Re: Closing up the squish band clearance on my modified head! My testing continues...

and that's why Awk08 says "every engine and every situation is different" and leave it at that.

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

not exactly corking, just shaping, upping the contained amount and forcing more through allows more heat to dissipate into the charge. and even on a more free flowing exhaust things could be altered to keep the high compression and retain loads of torque, with out sacrificing much top end.

Be careful making brash recommendations there hoss,

You're not defying the laws of physics only slightly bending them in a way most riders don't....

You've corked that engine with a relatively low rev exhaust and stock silencer. Running that engine with that exhaust will work ok but if someone took a much stronger pressure return wave exhaust and put it on an engine built to those ^^^^^^^^ specs will be putting a piston in it in 6 months from predetonation.

I should add to my previous post earlier that all of engine testing experiences were with a Power Pro's right bend pipe with a strong pressure return wave.

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

i would not run a drag pipe on the motor i describe. but thats not to say 180psi would not work on a drag set up engine. different head design and non-stroker motor would be more the route to go. with ports designed to point a different direction to aid with forcing the heat charge out.

 

[sicivicdude]

Re: Closing up the squish band clearance on my modified head! My testing continues...

not exactly corking, just shaping, upping the contained amount and forcing more through allows more heat to dissipate into the charge. and even on a more free flowing exhaust things could be altered to keep the high compression and retain loads of torque, with out sacrificing much top end.

I don't want you to take this the wrong way so I'm gonna try to be delicate. I'd take that challenge in a NY minute.

I do have high compression, currently 165psi with .052" squish (on the safe side in my opinion/testing) but working around a relatively low pressure return charge pipe in order to increase the compression is sort of "working around your elbow to get to your butt" (as my mother has said it before).

That setup may work really well for your terrain and situation but be very careful about making engine recommendations without including all of the caveats. Someone's going to use your recommendations without your particular set of circumstances and run afoul of predetonation problems.

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

thats to be said about ANY advice in ANY situation. and the pipe doesnt effect static compression too much. theres simply not enough backwave at the low kicking speed for it to do anything yet.

I don't want you to take this the wrong way so I'm gonna try to be delicate. I'd take that challenge in a NY minute.

I do have high compression, currently 165psi with .052" squish (on the safe side in my opinion/testing) but working around a relatively low pressure return charge pipe in order to increase the compression is sort of "working around your elbow to get to your butt" (as my mother has said it before).

That setup may work really well for your terrain and situation but be very careful about making engine recommendations without including all of the caveats. Someone's going to use your recommendations without your particular set of circumstances and run afoul of predetonation problems.

 

[sicivicdude]

Re: Closing up the squish band clearance on my modified head! My testing continues...

thats to be said about ANY advice in ANY situation. and the pipe doesnt effect static compression too much. theres simply not enough backwave at the low kicking speed for it to do anything yet.

Well, that's actually part of the reason why you're able to run that squish and compression. At kicking speed static compression is ONLY the trapped volume.

However, a properly designed tuned expansion chamber acts like a super charger at it's efficient rpm range (powerband). THAT'S where you run afoul of predetonation. Notice I said earlier about running an engine hard without giving the top end time to cool down between "runs" or "pulls"? The pipe stays efficient at supercharging the cylinder for enough time to heat the piston up...

Using a stock silencer on a FMF pipe may actually TRAP enough heat to alter the wave return characteristics. Remember that sound waves move differently as the heat increases.

You're not defying the laws of physics with your engine setup, just "bending" them in some way that works. You've managed to find a "coffin corner" of area where the engine will continue to operate outside of "normal" design parameters by a combination of jetting, plugs, porting, head design, pipe and silencer. Change one parameter and you could get out of that "magic box" where you're currently at.

 

[joeak47]

Re: Closing up the squish band clearance on my modified head! My testing continues...

When everybody is stating they're compression readings,are these corrected for altitude?

Compression readings vary at different altitudes with the same engine.

You should add 3.4 % for every 1000 feet in elevation to get "sea level" readings.

I'm running .035 to .041 squish clearance (that cheap cast piston isn't machined on top),and 130 psi compression witch is corrected to 150 psi for my elevation.

No detonation.

I wonder how little I can go on the squish clearance and how high I can go on the compression psi?

I wonder if castor fortified two stroke oil and 20 to 1 mix is keeping things running awesome?

 

[sicivicdude]

Re: Closing up the squish band clearance on my modified head! My testing continues...

When everybody is stating they're compression readings,are these corrected for altitude?

Compression readings vary at different altitudes with the same engine.

You should add 3.4 % for every 1000 feet in elevation to get "sea level" readings.

I'm running .035 to .041 squish clearance (that cheap cast piston isn't machined on top),and 130 psi compression witch is corrected to 150 psi for my elevation.

No detonation.

I wonder how little I can go on the squish clearance and how high I can go on the compression psi?

I wonder if castor fortified two stroke oil and 20 to 1 mix is keeping things running awesome?

I'm at ~300 feet above sea level so you can do the math While your compression readings may need to be corrected for "comparison" with other people, your actual compression is still only 130 psi. It's "relatively" higher compared to someone who has denser air around their engine but inside your engine, 130psi is still only 130 psi which contains only a certain amount of oxygen in it.

You're beyond what I consider the "safe" limit on squish depending on your tuned rpm simply because with a .040" squish clearance, if you try to raise your static compression a bunch, you'll run into predetonation issues quicker when it gets hot. You're pushing the charge too fast too early to then compress it more and have a hot cylinder. There's a "grey area" somewhere between .065" and about .045" where the trade off is slightly less torque but you can raise the cranking compression pressure higher (by having a smaller combustion chamber) and see gains in the top of the RPM range where the decreased squish will help at lower RPM's.

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

everything has a magic box, i use to custom car stereo installs and we built custom speaker boxes. the recomended box design for a speaker will net you a broad but poor sounding experience. a custom box design tuned to what the owner listens to 90% of the time will sound amazing, and in that 10% of the time where its not in its eficiencey range it won't explode, it will just sound like an average box.

the trick is finding the magic in things. its when when you get out almost as much power as what you put in. its the bleeding edge of efficiency. and its not actually that hard, instead of tuning an ATV for DRAG or WOODS or for MX, tune the atv for YOU!. your the one that rides the dang thing.

Well, that's actually part of the reason why you're able to run that squish and compression. At kicking speed static compression is ONLY the trapped volume.

However, a properly designed tuned expansion chamber acts like a super charger at it's efficient rpm range (powerband). THAT'S where you run afoul of predetonation. Notice I said earlier about running an engine hard without giving the top end time to cool down between "runs" or "pulls"? The pipe stays efficient at supercharging the cylinder for enough time to heat the piston up...

Using a stock silencer on a FMF pipe may actually TRAP enough heat to alter the wave return characteristics. Remember that sound waves move differently as the heat increases.

You're not defying the laws of physics with your engine setup, just "bending" them in some way that works. You've managed to find a "coffin corner" of area where the engine will continue to operate outside of "normal" design parameters by a combination of jetting, plugs, porting, head design, pipe and silencer. Change one parameter and you could get out of that "magic box" where you're currently at.

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

sounds relatively safe with the point about the cast piston being a good example of how they work. cast pistons hold heat diferently and are actually full of air more so then a forged piston. ind youve discovered how the top of the piston is not as nicely shaped as a forged piston. all these work against you. that said if your on your tuning you should be good on pump gas.

When everybody is stating they're compression readings,are these corrected for altitude?

Compression readings vary at different altitudes with the same engine.

You should add 3.4 % for every 1000 feet in elevation to get "sea level" readings.

I'm running .035 to .041 squish clearance (that cheap cast piston isn't machined on top),and 130 psi compression witch is corrected to 150 psi for my elevation.

No detonation.

I wonder how little I can go on the squish clearance and how high I can go on the compression psi?

I wonder if castor fortified two stroke oil and 20 to 1 mix is keeping things running awesome?

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

one more thing, pump gas is crazy crap, the stuff can contain large amounts of water and ethonol and they don't have to tell you about it. and if someone says 'buy here, our gas contains no ethonol' they are LIEING to you.

now, water and ethonol actually fend off preignition, and so does 2-stroke oil. some better then others.

 

[joeak47]

Re: Closing up the squish band clearance on my modified head! My testing continues...

I forgot to metion that I was riding it in 106 degree weather. No detonation what so ever!
SICIVICDUDE... I think I'll try a smaller chamber (for more compression),and increase the squish band clearance to .045 and see what happens.

Will this give me more high end,high rpm torque? I have plenty of low end torque,and could lose some and not care.

Cutting these heads are easy and fun. I'm still learning. The head I'm on now is ABSOLUTELY IMPRESSIVE!!!

I'm thinking that more compression will make more torque...obviously,but how high can I go on 92 octane pump gas? What would be the maximum you would suggest?

 

[sicivicdude]

Re: Closing up the squish band clearance on my modified head! My testing continues...

There is no perfect number for every engine out there. The MSV calculating programs take all sorts of parameters into consideration in order to calculate the one perfect head design for a particular engine...

That being said, a squish thickness of .050", a squish area of approximately 45%, and a cranking compression of 160psi is generally regarded as being SAFE on high test pump gas (93 PON here). Anything more (or less in the case of squish thickness) and you either have a special case or you're asking for trouble/trying the line. You won't know FOR SURE how much is too much until you've melted a hole through the middle of the piston so those ^^^^^ numbers are generally safe numbers.

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

each engine has a perfect set up, it will not be identical for every engine. no matter how similar the engines seem to start with.

and i still say 180 psi is safe on 93 octane as long as you have not gone crazy on the squish and i would prefer to see a forged piston just for its treuness and ability to shed heat more efficiently.

 

[joeak47]

Re: Closing up the squish band clearance on my modified head! My testing continues...

ZEDICUS... I've been reading that 180 to 190 psi is the maximum for pump gas. I guess I'll just keep reading and gather more info.

I plan on upping the compression some more. SICIVICDUDE says that 160 psi is the safe zone and that makes sense. I'm sure I could push it further,but how far. I'll start machining another head soon with a smaller dome and a bit more squish clearance to see what happens.

As for a forged piston, I'm not impressed with them. I get the fact that they can handle detonation a bit more,but I cant stand they're drawbacks such as swelling up too much in the bore even with properly set cylinder to piston clearance. I've even seen them crack like cast pistons can. much noisier,etc.

I've been running cast pistons for many years without any issues what so ever,and I flog the hell out my blaster! I also inspect things too,and do routine maintenance. Most blasters that I've seen that are out of commision or "blown up" are do to the lazy slobs that own them. These are the same people that don't know how to open the hood of they're car.

I was thinking about machining the top of the piston smooth in the lathe and going from there. I wouldn't have to take off much at all...maybe .005 to .006 over the entire dome. Easy to do. That way I have a nice smooth top.

I've been comparing cast and forged pistons for a while>>>http://www.blasterforum.com/engine-13/joeak47s-thread-cast-piston-vs-wiseco-forged-piston-49255/

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

cast pistons arent bad, as ive mentioned ive got a et in my banshee. just hafta know the differences. my 250R has a forged and will only get forged. i would chuck your cast piston in a lathe and finish the top if you have that ability.

180 - 190 psi is what a lot of other forums say, its got a lot to do with the engine but with the right head design i feal its fine in a blaster. the banshee guys say don't go over 150, i think they are fools. BUT there is no possible way to get over about 140 psi on a stock banshee head, the water jackets get paper thin, just not enough cuttable depth with out going to a cool head. with a cool head on a banshee 180-190 should be totally fine.

the pic i posted is right at 180 static psi on my wifes blaster. it runs fine in scalding heat and high load, never had an issue with it whatsoever.

 

[sicivicdude]

Re: Closing up the squish band clearance on my modified head! My testing continues...

On a liquid cooled engine or the right setup (zedicus, your pipe/engine combo may be a rare combination) you may be able to get away with 180 psi. In my experience, that's too high (TO BE SAFE IN ALL CIRCUMSTANCES!) for a pump gas engine. As YZOtis says, your results can and will vary!

Joe and Zed, you already know I'm not a fan of cast pistons but only for the destruction they cause when they come apart. While they're running, they run fine.

If you feel comfortable "turning it up" until you start to hear it (and just love tearing your engine apart) I suggest welding a boss on the side of the intake window or transfer port and threading it for a knock sensor. A small battery and an LED light on the handlebars would be a VERY good indicator. That would be a very accurate indication of when you "hit the limit" versus speculation.

 

[zedicus]

Re: Closing up the squish band clearance on my modified head! My testing continues...

my forged piston 250R with a procross jug that has the watercooled exhaust area on it will run 210psi on 91 oct pump gas. it actually crosses the limit a bit and if i run it long and hard i get detonation so to be safe i generally mix about 1 part race gas 10 parts pump gas. that tiny amount is enough to keep it from detonating at all even when run HARD in the desert heat.


yes, i have a banshee and a 250R. the banshee is for sale, big bore kit and pwk carbs and the dang 250r is still faster.