question for blown 1500 and others
question for blown 1500 and others
05-02-2003 | 08:46 AM
#2
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Many things affect the rate of burn (pressure) in a combustion chamber from fuel particle size to turbulence in the chamber. Pressure is also a big factor. The higher the pressure, the faster the burn. In a big block Chevy, total timing commonly runs from about 34-42* in na and 34* or less in low boost to about 25* at higher boost. The deal is, you try to get the highest cylinder pressure just as the piston starts down in the power stroke.
In na, the fire burns more slowly than when supercharged and the rate of burn in na causes the fire to still be burning when it goes out the exhaust with 20* timing. When forced induction is used, the cylinder pressures are much higher and the burn rate much faster so the burn can be completed when started later. There are books written on this subject and I do not know everything about this subject but this is a general idea of how it works. If you want more, I will help all I can or refer you to some people and articles that may further cloud the subject
In na, the fire burns more slowly than when supercharged and the rate of burn in na causes the fire to still be burning when it goes out the exhaust with 20* timing. When forced induction is used, the cylinder pressures are much higher and the burn rate much faster so the burn can be completed when started later. There are books written on this subject and I do not know everything about this subject but this is a general idea of how it works. If you want more, I will help all I can or refer you to some people and articles that may further cloud the subject
Last edited by blown1500; 05-02-2003 at 08:55 AM.
05-02-2003 | 09:09 AM
#3
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Joined: Jul 2002
Posts: 2,867
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From: Waterford,MI
In the March edition of Powerboat, Bob Teague says that retarding timing too much on blower motors raises egt. He tries to run as much as 34 degrees to save exaust valves, seats and exaust manifolds. See the Teague on Tech section.
05-02-2003 | 10:47 AM
#4
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TEAGUE needs to have his stuff (nice verbage) proof read!
We run alot of timing 32 to 34 degrees also but it is determined by the boost,,plus most of the time at 25 degrees if the boost is to low its the intake valve that burns as the cylinder gets so hot and stays that way the intake can't handle the heat
but just to run 34 plus degrees at random is wrong
What really works well is a high speed retard or boost retard
if you ever use one its like changing into another gear just to knock off 4 degrees or so of timing at full boost and close to full rpm ,again you need the proper set up to get the best results
ie 8.5comp or more and 7.5 or more boost 34 degress timing knocked back to 29 on chevy heads or brodix !
I'm no expert on this either ,just from dyno and boat experance !
We run alot of timing 32 to 34 degrees also but it is determined by the boost,,plus most of the time at 25 degrees if the boost is to low its the intake valve that burns as the cylinder gets so hot and stays that way the intake can't handle the heat
but just to run 34 plus degrees at random is wrong
What really works well is a high speed retard or boost retard
if you ever use one its like changing into another gear just to knock off 4 degrees or so of timing at full boost and close to full rpm ,again you need the proper set up to get the best results
ie 8.5comp or more and 7.5 or more boost 34 degress timing knocked back to 29 on chevy heads or brodix !
I'm no expert on this either ,just from dyno and boat experance !
Last edited by bobby daniels; 05-02-2003 at 10:57 AM.
05-02-2003 | 11:15 AM
#5
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Joined: Oct 2000
Posts: 486
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From: claymont, DE, USA
Just to try and expound on blown 1500's post;
Pressure and temp are directly related. A supercharged engine increases cylinder pressure. So, in order to use pump gas, you must retard the timing in many applications. Now, that's a general statement, if you want to get deeper into the subject, we can get into charged air temps.
Anyway, back to the subject. If the blower increases pressure, (adds density) the temp in the cylinder gets hotter, faster...(in relation to the piston comming to TDC) because this happens the spark doesn't have to ignite the mix till later in the travel of the piston because of the amount of time it takes for the mix to burn.
Octane slows the burn of the gasoline. The more octane, the slower the burn. So, for example; if you have a blown motor that runs 6 lbs of boost on 91 octane and the timing is 28*, You could use 104 octane and bump the timing to 36*. Again, that's just an example, they are not real numbers.
For another example, if you had a n/a motor at 8.75:1 and you put domed pistions in and raised it to say, 11:1, it's the same concept, You're decreasing the space for compression, therefore raising the pressure, therefore raising the heat. That will result in the need to use higher octane and possibly decrease the timing.
I hope this helped
DAVE
Pressure and temp are directly related. A supercharged engine increases cylinder pressure. So, in order to use pump gas, you must retard the timing in many applications. Now, that's a general statement, if you want to get deeper into the subject, we can get into charged air temps.
Anyway, back to the subject. If the blower increases pressure, (adds density) the temp in the cylinder gets hotter, faster...(in relation to the piston comming to TDC) because this happens the spark doesn't have to ignite the mix till later in the travel of the piston because of the amount of time it takes for the mix to burn.
Octane slows the burn of the gasoline. The more octane, the slower the burn. So, for example; if you have a blown motor that runs 6 lbs of boost on 91 octane and the timing is 28*, You could use 104 octane and bump the timing to 36*. Again, that's just an example, they are not real numbers.
For another example, if you had a n/a motor at 8.75:1 and you put domed pistions in and raised it to say, 11:1, it's the same concept, You're decreasing the space for compression, therefore raising the pressure, therefore raising the heat. That will result in the need to use higher octane and possibly decrease the timing.
I hope this helped
DAVE
05-02-2003 | 03:57 PM
#7
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Joined: Oct 2002
Posts: 50
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From: Morgan Hill, CA
I recently had a 600" BBC built and when we put it on the dyno, we started seeing flames start shooting out the headers at higher RPM (6,000 to 6,600). We were running 30 degrees of timing. The BSF and A/F ratio numbers showed it going pretty fat at the same time. It got worse as we put more boost to it. We tried bumping the timing up to 34 and it didn't seem to make any difference. Has anyone else experienced this problem?
