turbo lag
05-02-2002, 09:30 AM
#1
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turbo lag
What is the best way to reduce turbo lag? My boost comes on hard just over 4000rpm and stays strong to my shift point. Below 4000 the power is not there. Would replacing the stock intercooler pipes or adding a MAF help reduce the lag?
2.5L, large Garrett, Tial WG, 1 bar boost,3" exhaust w/o cats. Strictly a track car.
TIA
2.5L, large Garrett, Tial WG, 1 bar boost,3" exhaust w/o cats. Strictly a track car.
TIA
05-02-2002, 10:10 AM
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Sloth -
A couple ways to reduce the lag are:
1) Open the intake as much as possible. That means MAF/hard pipes, etc.
2) Coat/Wrap the crossover pipe - retain the heat energy in the gasses that drive the turbo.
3) Open the exhaust - though yours seems to be pretty open now...
4) Smaller hot-side on the turbo. Will spool faster - but you'll loose some higher end oomph.
I'd try the #1 and #2 and see where that getts you.
HTH!
A couple ways to reduce the lag are:
1) Open the intake as much as possible. That means MAF/hard pipes, etc.
2) Coat/Wrap the crossover pipe - retain the heat energy in the gasses that drive the turbo.
3) Open the exhaust - though yours seems to be pretty open now...
4) Smaller hot-side on the turbo. Will spool faster - but you'll loose some higher end oomph.
I'd try the #1 and #2 and see where that getts you.
HTH!
05-02-2002, 02:11 PM
#3
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We're actually talking about boost-lag and it's always present on a turbo car. You just have to make design decisions about how much is tolerable. One of the primary problems is the low compression 8.0:1 which doesn't provide a whole lot of power to spin up the turbo at low-RPM. The more modern turbo cars (996TT, Audi/VW) all seem to have 9.5:1 compression.
However, people always brings up max-power potential here, so I'll address it. Sure 8.0:1 compression gives you higher max-HP potential because you can run up to 25-30psi of boost for 400-450rwhp. But very few people are doing that. Whereas the 9.5:1 compression makes much more sense. The boost limit on that compression might only be 16-17psi and most Audi/VW folks bring up the boost to 14.5psi without any problems. And you get oomph down below too since the higher-compression give you more torque just from non-boosted engine power alone, and there's more exhaust pressure to spin up the turbo.
However, people always brings up max-power potential here, so I'll address it. Sure 8.0:1 compression gives you higher max-HP potential because you can run up to 25-30psi of boost for 400-450rwhp. But very few people are doing that. Whereas the 9.5:1 compression makes much more sense. The boost limit on that compression might only be 16-17psi and most Audi/VW folks bring up the boost to 14.5psi without any problems. And you get oomph down below too since the higher-compression give you more torque just from non-boosted engine power alone, and there's more exhaust pressure to spin up the turbo.
05-02-2002, 03:45 PM
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So are you sayin gthat if you could build a 951 motor and weren't particularly concerned with Max power but rather all around driveability you would take the HP hit and build a motor with higher compression.
05-02-2002, 04:03 PM
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"So are you sayin gthat if you could build a 951 motor and weren't particularly concerned with Max power but rather all around driveability you would take the HP hit and build a motor with higher compression. "
Yes, sort of. You only have a HP hit if you don't run the boost as high as the compression allows. Which is the case of the 951 engine. The 996TT and Audi S4 engines run the same 0.7-0.8bar boost, with their higher-compression they get higher specific-power output as well as less turbo lag than we do.
I'm saying that I would design the engine so that I would run it at close to the maximum boost that a certain compression would allow. When I build my 968 Turbo engine, I would use 9.5:1 @ 15psi boost for about 400rwhp. Or I could go with 9.0:1 @ 18psi for about 450rwhp, or 8.5:1 @ 21psi and 500rwhp.
But it would be silly to use 8.5:1 @ 15psi because then I'd only be getting 350rwhp. I might as well use 10.0:1 @ 8psi for that same power output and have more low-end torque with less lag.
Yes, sort of. You only have a HP hit if you don't run the boost as high as the compression allows. Which is the case of the 951 engine. The 996TT and Audi S4 engines run the same 0.7-0.8bar boost, with their higher-compression they get higher specific-power output as well as less turbo lag than we do.
I'm saying that I would design the engine so that I would run it at close to the maximum boost that a certain compression would allow. When I build my 968 Turbo engine, I would use 9.5:1 @ 15psi boost for about 400rwhp. Or I could go with 9.0:1 @ 18psi for about 450rwhp, or 8.5:1 @ 21psi and 500rwhp.
But it would be silly to use 8.5:1 @ 15psi because then I'd only be getting 350rwhp. I might as well use 10.0:1 @ 8psi for that same power output and have more low-end torque with less lag.
05-02-2002, 04:07 PM
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That is very interesting Danno. I didn't realize that it worked like that but now that you illustrated it like you did it is perfectly logical.
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05-02-2002, 06:00 PM
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Danno,
Does anyone do any weld-filling of the "pocket" in the head to raise compression. Is there a better way? Can you plane the head enough to do it? I'd like to know because I've got my head off right now!
Does anyone do any weld-filling of the "pocket" in the head to raise compression. Is there a better way? Can you plane the head enough to do it? I'd like to know because I've got my head off right now!
05-03-2002, 04:51 AM
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sloth:
A few other things to consider is:
1) Get an electronic boost control with fuz logic (apexi greddy,hks,etc). This will help lower spool time.
2) Lightweight flywheel and or lightweight clutch. Help the engine rev up fast.
Good luck
ken
86 951
79 930
A few other things to consider is:
1) Get an electronic boost control with fuz logic (apexi greddy,hks,etc). This will help lower spool time.
2) Lightweight flywheel and or lightweight clutch. Help the engine rev up fast.
Good luck
ken
86 951
79 930
05-03-2002, 07:27 AM
#10
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"Does anyone do any weld-filling of the "pocket" in the head to raise compression. Is there a better way? Can you plane the head enough to do it?"
Russ, reducing combustion-chamber volume via welding is tough because you can't easily control the amount and shape. A lot of people taken off 0.040" the head. This won't remove as much volume as shaving the block because part of the head will already be flush with the deck partway through the milling. I'll see if I can't dig up some CC numbers before and after to give you a resultant compression ratio. I'm guestimating between 8.3:1 and 8.5:1 at the moment. I'll at least do a calculated number if I can't get a simple CC number from some folks...
Another way to do it is with a thinner copper headgasket. This also allows you higher boost-levels, but you're not blowing stock headgaskets anyway. You can get these from Garrity with the 'special' sealant they need.
Russ, reducing combustion-chamber volume via welding is tough because you can't easily control the amount and shape. A lot of people taken off 0.040" the head. This won't remove as much volume as shaving the block because part of the head will already be flush with the deck partway through the milling. I'll see if I can't dig up some CC numbers before and after to give you a resultant compression ratio. I'm guestimating between 8.3:1 and 8.5:1 at the moment. I'll at least do a calculated number if I can't get a simple CC number from some folks...
Another way to do it is with a thinner copper headgasket. This also allows you higher boost-levels, but you're not blowing stock headgaskets anyway. You can get these from Garrity with the 'special' sealant they need.
