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Discussion Starter · #1 ·
Well I want to calculate the compression ratio on my 421 and I was hoping someone would post up the formula and plug some numbers into it for an example.

I am running alky so what ratio am I shooting for ?

I was also wondering if the dome size accounted for the squish clearance?

Not sure of the volume of a blaster dome?
 

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How the dome is cut in conjunction with the piston deck height determines your squish clearance as well as squish band width.

16:5 to 18:1 is what I'd shoot for on a 300 foot bike, a little less for duner/longer distance bike.

Search for a compression ratio calculator, there are several measurements you will need for an accurate reading.
 

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QUOTE (dajogejr @ Jun 24 2009, 05:06 AM) index.php?act=findpost&pid=47899How the dome is cut in conjunction with the piston deck height determines your squish clearance as well as squish band width.

16:5 to 18:1 is what I'd shoot for on a 300 foot bike, a little less for duner/longer distance bike.

Search for a compression ratio calculator, there are several measurements you will need for an accurate reading.

thats i bit high for my taste, I like around 15-15.5 :1

there is 4.7cc's on a 68 bore 573 piston if that helps

comp.ratio(uc) = cylinder volume + chamber volume / chamber volume. lets start with a 19 cc flat plate dome with a .040 step and a zero decked 421 cylinder

19ccs - 4.7 piston dome = 14.3 chamber volume at .040 clearance

210.5+14.3 / 14.3= (15.7:1 CR) (210.5 is the cylinder volume of one cylinder)
 

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Discussion Starter · #4 ·
What compression should I be shooting for with alky? I thought I read some where that I should be in the high side of dajogejr recomendation of like 18:1. I just had some alky domes cut and they are 19cc with zero deck height and .050 squish and i only have 145 psion each side. The gauge is accurate and the topend has a trip to bobs on it only, exhaust port timing @197 degrees. I thought I would be 180ish psi
 

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QUOTE (Rumbleshee @ Jun 24 2009, 07:42 AM) index.php?act=findpost&pid=47933What compression should I be shooting for with alky? I thought I read some where that I should be in the high side of dajogejr recomendation of like 18:1. I just had some alky domes cut and they are 19cc with zero deck height and .050 squish and i only have 145 psion each side. The gauge is accurate and the topend has a trip to bobs on it only, exhaust port timing @197 degrees. I thought I would be 180ish psi

a 19 cc dome on a 421 should be in the area of 190psi +or- 5,, if your guage has adapters on it to fit the plug hole they are adding volume to it in a big way, 1 cc is 10 psi.. I never trust a gauge unless it is to check for loss of compression and I just use the same gauge comparitively*... the math is the only way to build by.. 18:1 is to high for a banshee, in my opinion,, that would be around 220 + psi
 

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Discussion Starter · #6 ·
Yea I am starting to think something is up with these domes, my gauge uses no adaptors and it reads correctly according to the gause on my air compressor when I checked it. I had 185 psi with the old vito's pistons and some different domes but I changed to blaster pistons and some new domes cut for them and I got 145 psi. I may have to cc these domes and see what they come out at. Otherwise I need like 16cc domes to get the compression I want and no one else needs em this small so thats not right.
 

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This is off topic but seems fairly relevant.

The domes in my 421 cub are supposedly 18cc domes and when I first checked the compression I was only getting 160-165. It is unported, so I thought that was pretty low. I never investigated more b/c I was happy with that compression but thought it was odd with that size dome.
 

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Remember that domes are marked, and they could be marked before cutting for a certain application.

As sculpter said 18:1 is WAYYYYY to high for a Banshee. It would make for great bottom end, but top end power and engine life will be no good.
 

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Try the corrected compression ratio, bore x bore x exhaust port height(replaces full stroke)= cc, the higher the ex port the less cc head needed to acheive your target ratio. Most builders will shoot for 8.0 - 8.5:1 for race gas
cc + installed head volume divided by head volume =corrected ratio
 

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QUOTE (Moser Racing @ Jun 26 2009, 07:59 PM) index.php?act=findpost&pid=48274Try the corrected compression ratio, bore x bore x exhaust port height(replaces full stroke)= cc, the higher the ex port the less cc head needed to acheive your target ratio. Most builders will shoot for 8.0 - 8.5:1 for race gas
cc + installed head volume divided by head volume =corrected ratio

At 8.1:1 CCR the UCCR is still 15.7:1, at 198 ex*, and 15.2 at 194 ex*, a bit to high for gas on a reliable rider IMO.. I shoot for around 7:1 to 7.5:1 for gas some times 6.5:1 CCR(12.5:1 UCCR) at 196. most guys who want gas are riding the bike on a regular basis
 

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wow.. again blaster pistons are a lil shorter than banshee pistons is your deck height actually 0? do the edge's of the pistons stop at the very top of the bore.. if they are a lil less OR if your ASSEMBLED and measure'd squish is higher than 50 that may be your problems..

maybe and may not matter at all...

does it kick like it has 210lbs of compression??

or does it feel like 145lbs...

got any pics of the domes?
 

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Discussion Starter · #12 ·
QUOTE (camatv @ Jun 30 2009, 05:50 PM) index.php?act=findpost&pid=48768wow.. again blaster pistons are a lil shorter than banshee pistons is your deck height actually 0? do the edge's of the pistons stop at the very top of the bore.. if they are a lil less OR if your ASSEMBLED and measure'd squish is higher than 50 that may be your problems..

maybe and may not matter at all...

does it kick like it has 210lbs of compression??

or does it feel like 145lbs...

got any pics of the domes?


The bike kicked like it had 145 psi....very easy to kick but it didn't like to start. I do not have pics of the domes as they are now in the bike, I put them in the lathe and did a little work on them to get the compression where I wanted it. I now have 185 psi and .050 squish. I plan to take them out after this weekend and cc them to figure out what I truly have and get a new set made, I had to make shims to set on top of the domes to lower them down after I made my cut off the bottom, made o ring grooves deeper and cut squish band back some. I took .030 off the bottom and .035 off the squish band. The bike starts right up now......can't believe how much easier it lights up with more compression. I have had it out on the dunes the last couple days and it feels STRONG but with no one out there to race lately.....who knows. We will see this coming weekend as I expect there to be some big dogs out there saturday.

Thanks for the help guys, hopefully I can get to the track and lay down some faster times now.
 

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QUOTE (camatv @ Jun 30 2009, 04:50 PM) index.php?act=findpost&pid=48768wow.. again blaster pistons are a lil shorter than banshee pistons is your deck height actually 0? do the edge's of the pistons stop at the very top of the bore.. if they are a lil less OR if your ASSEMBLED and measure'd squish is higher than 50 that may be your problems..

maybe and may not matter at all...

does it kick like it has 210lbs of compression??

or does it feel like 145lbs...

got any pics of the domes?

thats right, a blaster piston has a dome rise of .105 where a 513 or 795 has a .160 rise , but the only pistons at 68 bore with a banshee dome profile is the wsm big bore and longrod pistons or trinitys big bore banshee (8963 series) thier 9768 series has a .105 rise but it uses the short rod
so you dont see to many 13 degree squish big bore domes out there on longrod 4 mills.. you would think that 19 cc's flat plate domes and the same piston, either way you look at it , 13 degree squish or 11 degree squish , both would have the same CR just different MSV(Max squish velocity)

I would like to see a pic of the domes also
 

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Discussion Starter · #14 ·
QUOTE (Sculpter @ Jul 2 2009, 06:51 AM) index.php?act=findpost&pid=48955thats right, a blaster piston has a dome rise of .105 where a 513 or 795 has a .160 rise , but the only pistons at 68 bore with a banshee dome profile is the wsm big bore and longrod pistons or trinitys big bore banshee (8963 series) thier 9768 series has a .105 rise but it uses the short rod
so you dont see to many 13 degree squish big bore domes out there on longrod 4 mills.. you would think that 19 cc's flat plate domes and the same piston, either way you look at it , 13 degree squish or 11 degree squish , both would have the same CR just different MSV(Max squish velocity)

I would like to see a pic of the domes also

The original 13 degree squish domes were a vito dome for a short rod with there goofy [censored] super stock piston. When I switched to the wiseco I bought these new domes for them that are now 11 degree squish. That vito piston had a real tall dome on them compared to the wiseco's
 

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QUOTE (Rumbleshee @ Jul 2 2009, 08:14 AM) index.php?act=findpost&pid=48965The original 13 degree squish domes were a vito dome for a short rod with there goofy [censored] super stock piston. When I switched to the wiseco I bought these new domes for them that are now 11 degree squish. That vito piston had a real tall dome on them compared to the wiseco's

Ah vitos lol forgot about them ,,, I dont have those piston specs in my database lol
 

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Wow this post was very informative. Especially since i just got my 7MM stroker with blaster pistons 210lb of comp running E85 Ethanol. It has phenomenal bottom but the top is a little lacking but as she breaks in and loosens up I think it will be just what I like.
 
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