How Much does a tight LSA REALLY affect a boosted motor
10-16-2009 | 11:54 AM
#16
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Joined: Oct 2001
Posts: 151
From: Austin Tx
Overlap with a blower and overlap with a turbo have completely different effects. LSA is irrelevant.
On a blower, you're trying to prevent losing air/fuel out the exhaust during overlap. On a well-tuned NA setup, you could actually lose air/fuel out the exhaust, so I believe this to be a credible scenario since you now have high pressure aggravating the situation. How much overlap is too much is up for debate, though, as no one who has done extensive testing shares the results.
On a turbo, the exhaust pressure is higher than intake, so you don't lose air/fuel out the exhaust. The danger is pushing exhaust into the intake port. However, the positive benefits of overlap seem to outweigh the negatives. As far as I know, no one has gained power from decreasing overlap with a turbo. In fact, the opposite has been the case, up to about +5 deg @ .050" at 2/1 backpressure/boost. The actual optimum overlap will depend on the backpressure/boost ratio, head geometry, etc., but again, if anyone knows the correlation between backpressure and optimum overlap, they aren't sharing.
About 30 years ago, David Vizard published a book that stated that supercharged engines needed higher LSA's than NA. Everyone assumed he meant 114 and it applied to turbo's too and ran with it. He actually stated that 110-114 should be considered (basing that on 106 being good for NA). Keep in mind that the audience 30 years ago wasn't as knowledgable as the today's FI enthusiast, and didn't know to include factors such as backpressure, actual overlap, etc...
On a blower, you're trying to prevent losing air/fuel out the exhaust during overlap. On a well-tuned NA setup, you could actually lose air/fuel out the exhaust, so I believe this to be a credible scenario since you now have high pressure aggravating the situation. How much overlap is too much is up for debate, though, as no one who has done extensive testing shares the results.
On a turbo, the exhaust pressure is higher than intake, so you don't lose air/fuel out the exhaust. The danger is pushing exhaust into the intake port. However, the positive benefits of overlap seem to outweigh the negatives. As far as I know, no one has gained power from decreasing overlap with a turbo. In fact, the opposite has been the case, up to about +5 deg @ .050" at 2/1 backpressure/boost. The actual optimum overlap will depend on the backpressure/boost ratio, head geometry, etc., but again, if anyone knows the correlation between backpressure and optimum overlap, they aren't sharing.
About 30 years ago, David Vizard published a book that stated that supercharged engines needed higher LSA's than NA. Everyone assumed he meant 114 and it applied to turbo's too and ran with it. He actually stated that 110-114 should be considered (basing that on 106 being good for NA). Keep in mind that the audience 30 years ago wasn't as knowledgable as the today's FI enthusiast, and didn't know to include factors such as backpressure, actual overlap, etc...
10-20-2009 | 08:03 AM
#17
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Joined: Jun 2003
Posts: 57
From: Texas
[QUOTE=engineermike;6141422]Overlap with a blower and overlap with a turbo have completely different effects.
Thank You...I cant believe no one said this. You are absolutely correct, Its not as critical on a supercharged application but does have a serious affect in a turbo application. I tuned a Turbo LT1 awhile back, Had a big nasty N/A cam and just would not make the boost / power it should of..Why? he was blowing most of the boost out the exhaust....
Thank You...I cant believe no one said this. You are absolutely correct, Its not as critical on a supercharged application but does have a serious affect in a turbo application. I tuned a Turbo LT1 awhile back, Had a big nasty N/A cam and just would not make the boost / power it should of..Why? he was blowing most of the boost out the exhaust....
10-20-2009 | 09:20 AM
#18
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Joined: Feb 2004
Posts: 2,182
From: Moore OK
[QUOTE=moehorsepower;6144879]
Umm,, it was said back in post #5...
Overlap with a blower and overlap with a turbo have completely different effects.
Thank You...I cant believe no one said this. You are absolutely correct, Its not as critical on a supercharged application but does have a serious affect in a turbo application. I tuned a Turbo LT1 awhile back, Had a big nasty N/A cam and just would not make the boost / power it should of..Why? he was blowing most of the boost out the exhaust....
Thank You...I cant believe no one said this. You are absolutely correct, Its not as critical on a supercharged application but does have a serious affect in a turbo application. I tuned a Turbo LT1 awhile back, Had a big nasty N/A cam and just would not make the boost / power it should of..Why? he was blowing most of the boost out the exhaust....
10-20-2009 | 09:49 AM
#19
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Registered User
Joined: Oct 2001
Posts: 151
From: Austin Tx
[QUOTE=moehorsepower;6144879]
Yeah and Moe...
Me and you have never met....but I am planning on you tuning this combo...says it right here in Line 13 of my mod list "Moe bailey tune"
Overlap with a blower and overlap with a turbo have completely different effects.
Thank You...I cant believe no one said this. You are absolutely correct, Its not as critical on a supercharged application but does have a serious affect in a turbo application. I tuned a Turbo LT1 awhile back, Had a big nasty N/A cam and just would not make the boost / power it should of..Why? he was blowing most of the boost out the exhaust....
Thank You...I cant believe no one said this. You are absolutely correct, Its not as critical on a supercharged application but does have a serious affect in a turbo application. I tuned a Turbo LT1 awhile back, Had a big nasty N/A cam and just would not make the boost / power it should of..Why? he was blowing most of the boost out the exhaust....
Yeah and Moe...
Me and you have never met....but I am planning on you tuning this combo...says it right here in Line 13 of my mod list "Moe bailey tune"
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