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what are some good blowdown numbers to aim for?

Discussion in HPI Baja Engine and Exhaust forum, started by sclark.

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  #1  
Old 04-23-2010, 09:01 PM
sclark sclark is offline
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what are some good blowdown numbers to aim for?

hello

I was wondering what are some good blowdown number to aim for? I know blowdown is exhaust duration - transfer duration with that said if a exhaust is 176 what would be a good number for the transfer port? I know blowdown is times 2 cuase you have the same ports on the other side of the head as you all know blowdown is extremely critical factor in the modification of a 2 stroke engine I see people taking about exhaust duration and intake duration BUT no one really talks about what are some good blowdown numbers to GO with thoses duration? i hope i make senses..lol

thanks

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Old 04-23-2010, 09:05 PM
mickcraftycarper mickcraftycarper is offline
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what??? You've lost me....
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Old 04-23-2010, 09:14 PM
sclark sclark is offline
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Originally Posted by mickcraftycarper View Post
what??? You've lost me....
sorry iam not good at explain when if comes to writing i guess what iam trying to say is from all the reading i been doing i should ALSO be pay attention to the transfer duration and blowdown and not just exhaust duration and intake duration...I guess from the threads on here from what i have saw so far people say ohh make your exhaust duration 172 make the intake duration 150 then widen the transfer ports,etc BUT no one mentions what the blowdown is or what they are trying to aim for??
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Old 04-23-2010, 09:17 PM
Timmaay Timmaay is offline
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Maybe take the same amount of material off as you did with the other ports? Seems like from the factory they must have gotten all the timings to work with each other so taking off the same amount from all of them might keep it in that sweet zone?? Just a thought...
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Old 04-23-2010, 09:17 PM
sclark sclark is offline
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so for example if the exhaust is 170 and the transfer is 120 the blowdown is 50 so my question is is 50 a good blowdown number??
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Old 04-23-2010, 09:19 PM
sclark sclark is offline
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or why not keep the exhaust duration at 170 BUT modfty the transfer to 110 instead so then the blowdown is 60 so what is better 50 or 60? and why?

Last edited by sclark; 04-23-2010 at 09:24 PM.
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Old 04-24-2010, 01:54 AM
turboglenn turboglenn is offline
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IIRC lower numbers work better with can muflers and higher numbers need better tuned pipes to be effective or a power loss wil be shown. read in the 2t tuners's handbook, there's only a touching of the subject, but google and reading is your friend
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Old 04-24-2010, 06:12 AM
Martin_ Martin_ is offline
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On a pipped motor you want your pipe the most possible in the blowdown area to get the pipe effect in that area, other than that, depending on what you want from your engine, the blowdown is a result of what you have make with your EX and transfert duration.

There is some great Exhaust and transfert number wich work great on two stroke engine............. blowdown is set itself.

There is too much possible configuration to tell what is the best or not.
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Old 04-24-2010, 06:49 AM
sclark sclark is offline
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Quote:
Originally Posted by Martin_ View Post
On a pipped motor you want your pipe the most possible in the blowdown area to get the pipe effect in that area, other than that, depending on what you want from your engine, the blowdown is a result of what you have make with your EX and transfer duration.

There is some great Exhaust and transfer number which work great on two stroke engine............. blowdown is set itself.

There is too much possible configuration to tell what is the best or not.
I see what you are saying but there has to be some "general" numbers if ya know what i mean

turboglenn - I been reading some much i cant see straight any more..lol
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Old 04-24-2010, 07:57 AM
turboglenn turboglenn is offline
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sclark, if it helps you, WITH MY PIPES (the ones i make myself) i've been running blowdowns as high as 75* when measured your way,, ( i measure differently) I measure the degrees from when the exh port opens to when the transfers open (much easier to me, jsut a flashlight and no carb or pipe on the engine)

with a can or regular pipe i liike to keep it down about 50 - 55 on your measures
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Old 04-24-2010, 08:43 AM
sclark sclark is offline
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Quote:
Originally Posted by turboglenn View Post
sclark, if it helps you, WITH MY PIPES (the ones i make myself) i've been running blowdowns as high as 75* when measured your way,, ( i measure differently) I measure the degrees from when the exh port opens to when the transfers open (much easier to me, jsut a flashlight and no carb or pipe on the engine)

with a can or regular pipe i liike to keep it down about 50 - 55 on your measures
thanks for that info at least that gives me some "general" ideas one thing tho is why did you pick 75 for your pipe? i guess if you can explain the reason for that that may help me understand this more

thanks
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Old 04-24-2010, 10:13 AM
2strokefolk 2strokefolk is offline
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I think you guys are measuring your blowdown completely wrong ...its not based on duration its strictly the difference in degrees from when the exhaust first opens ATDC to when the transfers open ATDC so it my exhaust opens at 94 degrees ATDC and transfers 112 degrees ATDC then that is 18 degrees blow down im not sure what you guys are doing..I know on my pocket bike motor people aim for 25 degrees blow down but not really sure what is all around good for these...basically the shorted blow down gives you a longer power stroke for more torque and longer gives you more rpm so its all personal preference
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Old 04-24-2010, 10:49 AM
sclark sclark is offline
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Originally Posted by 2strokefolk View Post
I think you guys are measuring your blowdown completely wrong ...its not based on duration its strictly the difference in degrees from when the exhaust first opens ATDC to when the transfers open ATDC so it my exhaust opens at 94 degrees ATDC and transfers 112 degrees ATDC then that is 18 degrees blow down im not sure what you guys are doing..I know on my pocket bike motor people aim for 25 degrees blow down but not really sure what is all around good for these...basically the shorted blow down gives you a longer power stroke for more torque and longer gives you more rpm so its all personal preference
hummmmm the way i understand it is:

BlowDown - Measured in degrees of crankshaft rotation from Exhaust Port opening to the Transfer Ports opening.

so with that being said

exhaust duration - transfer duration unless iam not understanding this right? or the definition i have is wrong?
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Old 04-24-2010, 11:28 AM
turboglenn turboglenn is offline
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Quote:
Originally Posted by 2strokefolk View Post
I think you guys are measuring your blowdown completely wrong ...its not based on duration its strictly the difference in degrees from when the exhaust first opens ATDC to when the transfers open ATDC so it my exhaust opens at 94 degrees ATDC and transfers 112 degrees ATDC then that is 18 degrees blow down im not sure what you guys are doing..I know on my pocket bike motor people aim for 25 degrees blow down but not really sure what is all around good for these...basically the shorted blow down gives you a longer power stroke for more torque and longer gives you more rpm so its all personal preference


ea, i know how to properly measure, that's why i stated "by his measures" I'm running 25 - 27* figured the proper way, have gone as low as 15once too
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Old 04-24-2010, 11:34 AM
2strokefolk 2strokefolk is offline
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oops my bad all didn't read the whole thing I just saw sclarks numbers and scrolled down and just saw your numbers without reading and was like wtf how did they come up with that hahaha my bad...sclark you are measuring it wrong....just subtract the degree at which the exhaust opens atdc from the degree at which the transfers open atdc and that is you blow down...25 -27 how is the low end with that..

Last edited by 2strokefolk; 04-24-2010 at 11:39 AM.
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Old 04-24-2010, 11:46 AM
turboglenn turboglenn is offline
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Quote:
Originally Posted by 2strokefolk View Post
oops my bad all didn't read the whole thing I just saw sclarks numbers and scrolled down and just saw your numbers without reading and was like wtf how did they come up with that hahaha my bad...sclark you are measuring it wrong....just subtract the degree at which the exhaust opens atdc from the degree at which the transfers open atdc and that is you blow down...25 -27 how is the low end with that..
with a custom pipe and 19/55 gears i only bog for a few feet on a piston port engine (30.5) and with the ts reed 28.5 i have no bog at all...reversion must be what's killing me on most of it
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Old 04-24-2010, 12:14 PM
sclark sclark is offline
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Quote:
Originally Posted by 2strokefolk View Post
oops my bad all didn't read the whole thing I just saw sclarks numbers and scrolled down and just saw your numbers without reading and was like wtf how did they come up with that hahaha my bad...sclark you are measuring it wrong....just subtract the degree at which the exhaust opens atdc from the degree at which the transfers open atdc and that is you blow down...25 -27 how is the low end with that..
so that is wrong by taking the exhaust exhaust - transfer duration?

so the info i have been reading is worng?
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Old 04-24-2010, 12:18 PM
sclark sclark is offline
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ahhh maybe iam missing a step!!!

170 - 120 = 50 then 50/2 = 25?
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Old 04-24-2010, 12:38 PM
sclark sclark is offline
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50 would be both sides
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Old 04-24-2010, 01:08 PM
tizer tizer is offline
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yeah when using differnet reference points its gets confusing...
94 ATDC exhaust opens and 112 ATDC transfers opens equals 18 'blowdown'
This would be 172 exhaust duration and 136 transfer duration which gives 36 blowdown
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