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Discussion Starter · #1 ·
Ive been trying to figure out why i have such low compression (145 lbs 400ft above sea level). I don't use a base or head casket, i use silicone so the port timing is correct. every thing is sealed up nicely even my rings. so i check the squish clearance and basically i need to shave off about 0.2-0.3 mm to reach the desired 0.8-1.0 mm squish clearance that i read in the the two stroke performance tuning bible i have. This is with my 370 long rod motor 66 bore 54 stroke 115 rod. I am running a PD cool head with changeable domes currently got 19cc domes in it. sorry its long just want to make sure you know what i am working with. First is 0.8-1.0 a good clearance to have or is that a little to tight? Second would it be better to mill the head and how? or would milling the top of my cylinders be better so i don't mess up the seating area of the o-rings? I have tried contacting Rudy Kurtz (my builder) with no luck if you seen my other thread. So im just trying to take matters into my own hands now.
 

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Assuming a 1.2mm squish clearance using a Blaster piston at 66.00mm. You will have a 13.71:1 UCCR with a dome having a FPV of 19cc.

Reducing the squish clearance by .2mm to 1.0mm clearance will increase the compression ratio to 14.11:1 UCCR.

Deck the top of your cylinders to reduce the clearance will be the easiest method. Otherwise you will need new domes.

What octane fuel do you plan to run with this motor??? IMO you are getting close to needing race gas. This is a prime example why building motors by a compression gauge isnt such a good idea. You're showing 140 PSI, some consider this a safe compression gauge reading to use pump gas and a moderate amount of ignition timing. The math of the geometry of your head (mostly empirical deriving the squish width, bowl volume, but still calculating the trapped volume at TDC) reveals that the compression ratio is nearly high enough to use race gas. Not knowing the health of your top-end (is the compression low due to excessive piston-to-bore clearance) or knowing how accurate that gauge is leaves too much to be desired to try to make changes based on those values alone.

If you are going to use race gas, want a moderate compression ratio that is good for duning- I would suggest a dome with a FPV of 18cc. This would set the compression ratio at 15.09:1 with a 1.0mm clearance. I would also suggest a SAR of around 45 to 50% of bore. Play with the timing, but around 6 degrees should work well with that octane and that head geometry.

Just remember- if we all did it the same- youd just go to Wal-Mart and buy your stuff. So this is my take on what to do.
 

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I come up with a slightly lower comp ratio. 13.3:1 UCCR. This is assuming dome volume for the 573 pistons, not the 513/795. Depending on your Exh duration, that is the primary reason for showing low pumping gauge numbers. This happens a lot with drag port timings. Do NOT build your engine around a compression gauge. I would not recommend cutting off the barrel as that could become a hassle for future setup changes. I think you need to consider different domes if you want to remain on pump fuels because the CR is just getting into the iffy zone. If you run race fuel, you could theoretically cut off the domes to optimize everything for race fuel but that will not work on a CH dome. You cannot just cut them off, you will need new domes. This is probably why Backcountry recommended cutting the cylinders; don't have to buy domes.


You need to check to see if your pistons are truly coming up to zero deck at the timing edge of the pistons, then determine if you need custom domes.
 

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Discussion Starter · #4 ·
Cool thanks both of you. that is alot of help. i am trying to get it running on alky. now i have 16cc domes that i was told never run with alky unless i want to melt my pistons. so if im border line to be on race gas where should i be for alky.
 

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Discussion Starter · #5 ·
this bike is suppose to be set up for alky but all my local buddies like to indicate that my compression is way to low be running alky and that i am most likly losing 10% of the power because of it.
 

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QUOTE (blowit @ Feb 23 2009, 08:10 AM) index.php?act=findpost&pid=31756I come up with a slightly lower comp ratio. 13.3:1 UCCR. This is assuming dome volume for the 573 pistons, not the 513/795. Depending on your Exh duration, that is the primary reason for showing low pumping gauge numbers. This happens a lot with drag port timings. Do NOT build your engine around a compression gauge. I would not recommend cutting off the barrel as that could become a hassle for future setup changes. I think you need to consider different domes if you want to remain on pump fuels because the CR is just getting into the iffy zone. If you run race fuel, you could theoretically cut off the domes to optimize everything for race fuel but that will not work on a CH dome. You cannot just cut them off, you will need new domes. This is probably why Backcountry recommended cutting the cylinders; don't have to buy domes.


You need to check to see if your pistons are truly coming up to zero deck at the timing edge of the pistons, then determine if you need custom domes.What is the most the cylinder can be decked for a stock cylinder, stock stroke? I have one cylinder that I broke a head stud off in and dinged the surface with a hammer.
 

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The 16cc domes will work with alky if you keep your squish clearance in 0.8-1.0 range AND go easy with the timing AND dont get it lean. Alky typically likes to run rich, but people try to tune them like a gas motor and poof!

With custom domes you could deck a cylinder top up to 2.5mm or so...
 

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Discussion Starter · #8 ·
QUOTE (Backcountry @ Feb 24 2009, 08:05 AM) index.php?act=findpost&pid=31893The 16cc domes will work with alky if you keep your squish clearance in 0.8-1.0 range AND go easy with the timing AND dont get it lean. Alky typically likes to run rich, but people try to tune them like a gas motor and poof!

With custom domes you could deck a cylinder top up to 2.5mm or so...

Thanks Backcountry you have been a really big help. thanks for taking the time and coming up with the numbers for a newbie to the sport
 
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