Vtec + turbo good or bad?
06-14-2010, 07:08 AM
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DITB-AHHH
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Vtec + turbo good or bad?
So ive been hearing alot of differnt things about this, some people say vtec and turbo dont go together well. Some say it does.
Can anyone set the story straight for me.
and explain why.
Can anyone set the story straight for me.
and explain why.
06-14-2010, 07:52 AM
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Re: Vtec + turbo good or bad?
from what ive read alot of it has to do with the valve overlap. here is what i found.
The old theory was that the valve overlap would produce excessive blowdown and, with it, a loss of power. However, one just can't overlook the fact that the more air you move into a motor, the more power you make, period. VTEC allows a stable idle while opening up to allow more airflow at higher rpm. True that you will have more blowdown as an occurance of overlap, but here's a few points to consider. First of all, blowdown means more traveling out the exhaust path. So what's good about that in a turbo car? Well, since a turbo runs off spent exhaust gasses, a little extra pressure going down the exhaust end can help spool it up a little earlier. (Especially if you move the VTEC crossover a down a bit.) Secondly, even though pressurized air is pushing against the intake valve, the scavenging effect is not completely wasted. The vacuum created by the exiting exhaust gasses can still add a little bit of velocity to the incoming charge. As everyone knows, air that's in motion tends to want to stay in motion. What does that mean? More air getting in the cylinder. Third, it is the nature of a turbocharger to create massive backpressure. Think about it, the turbo is directly in the path of exhaust flow. The bit of overlap creates a force against the backpressure and can prevent reversion from occuring. Finally, at high rpm, the speed of valve occurance is so swift that blowdown is really a minor issue. However, the additional lift and duration guarantees that you get a healthy charge into your cylinder. According to the book "The High Performance Honda Builder's Handbook" by Joe Pettitt, the 450 hp GSR buildup they did suffered some blowdown under 4000 rpm. That's under the point in which the VTEC high lobe kicks in. (4400 rpm) The top end was just fine. Anyways, the gist of it is that you really can't hurt from having dual cam profiles.
The old theory was that the valve overlap would produce excessive blowdown and, with it, a loss of power. However, one just can't overlook the fact that the more air you move into a motor, the more power you make, period. VTEC allows a stable idle while opening up to allow more airflow at higher rpm. True that you will have more blowdown as an occurance of overlap, but here's a few points to consider. First of all, blowdown means more traveling out the exhaust path. So what's good about that in a turbo car? Well, since a turbo runs off spent exhaust gasses, a little extra pressure going down the exhaust end can help spool it up a little earlier. (Especially if you move the VTEC crossover a down a bit.) Secondly, even though pressurized air is pushing against the intake valve, the scavenging effect is not completely wasted. The vacuum created by the exiting exhaust gasses can still add a little bit of velocity to the incoming charge. As everyone knows, air that's in motion tends to want to stay in motion. What does that mean? More air getting in the cylinder. Third, it is the nature of a turbocharger to create massive backpressure. Think about it, the turbo is directly in the path of exhaust flow. The bit of overlap creates a force against the backpressure and can prevent reversion from occuring. Finally, at high rpm, the speed of valve occurance is so swift that blowdown is really a minor issue. However, the additional lift and duration guarantees that you get a healthy charge into your cylinder. According to the book "The High Performance Honda Builder's Handbook" by Joe Pettitt, the 450 hp GSR buildup they did suffered some blowdown under 4000 rpm. That's under the point in which the VTEC high lobe kicks in. (4400 rpm) The top end was just fine. Anyways, the gist of it is that you really can't hurt from having dual cam profiles.
06-14-2010, 08:29 AM
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Re: Vtec + turbo good or bad?
It sounds like the only problems they're talking about are directly related to the stock cam profiles, not the VTEC system itself.
06-14-2010, 08:43 AM
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Re: Vtec + turbo good or bad?
so I don't think you should have a problem
06-14-2010, 08:54 AM
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Re: Vtec + turbo good or bad?
valve lap is critical on forced induction motors, Too much valve lap and the forced air would leave the combustion chamber at that split second both intake and exhaust valves are open, causing the motor to fall flat on its face. There is more valve lap when VTEC kicks in, obviously due to both in/ex valves opening up more. That was the purpose of explaining it above.
But on a side note, all of this can be tuned out. So really no worries.
But on a side note, all of this can be tuned out. So really no worries.
06-14-2010, 08:59 AM
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Re: Vtec + turbo good or bad?
from what ive read alot of it has to do with the valve overlap. here is what i found.
The old theory was that the valve overlap would produce excessive blowdown and, with it, a loss of power. However, one just can't overlook the fact that the more air you move into a motor, the more power you make, period. VTEC allows a stable idle while opening up to allow more airflow at higher rpm. True that you will have more blowdown as an occurance of overlap, but here's a few points to consider. First of all, blowdown means more traveling out the exhaust path. So what's good about that in a turbo car? Well, since a turbo runs off spent exhaust gasses, a little extra pressure going down the exhaust end can help spool it up a little earlier. (Especially if you move the VTEC crossover a down a bit.) Secondly, even though pressurized air is pushing against the intake valve, the scavenging effect is not completely wasted. The vacuum created by the exiting exhaust gasses can still add a little bit of velocity to the incoming charge. As everyone knows, air that's in motion tends to want to stay in motion. What does that mean? More air getting in the cylinder. Third, it is the nature of a turbocharger to create massive backpressure. Think about it, the turbo is directly in the path of exhaust flow. The bit of overlap creates a force against the backpressure and can prevent reversion from occuring. Finally, at high rpm, the speed of valve occurance is so swift that blowdown is really a minor issue. However, the additional lift and duration guarantees that you get a healthy charge into your cylinder. According to the book "The High Performance Honda Builder's Handbook" by Joe Pettitt, the 450 hp GSR buildup they did suffered some blowdown under 4000 rpm. That's under the point in which the VTEC high lobe kicks in. (4400 rpm) The top end was just fine. Anyways, the gist of it is that you really can't hurt from having dual cam profiles.
The old theory was that the valve overlap would produce excessive blowdown and, with it, a loss of power. However, one just can't overlook the fact that the more air you move into a motor, the more power you make, period. VTEC allows a stable idle while opening up to allow more airflow at higher rpm. True that you will have more blowdown as an occurance of overlap, but here's a few points to consider. First of all, blowdown means more traveling out the exhaust path. So what's good about that in a turbo car? Well, since a turbo runs off spent exhaust gasses, a little extra pressure going down the exhaust end can help spool it up a little earlier. (Especially if you move the VTEC crossover a down a bit.) Secondly, even though pressurized air is pushing against the intake valve, the scavenging effect is not completely wasted. The vacuum created by the exiting exhaust gasses can still add a little bit of velocity to the incoming charge. As everyone knows, air that's in motion tends to want to stay in motion. What does that mean? More air getting in the cylinder. Third, it is the nature of a turbocharger to create massive backpressure. Think about it, the turbo is directly in the path of exhaust flow. The bit of overlap creates a force against the backpressure and can prevent reversion from occuring. Finally, at high rpm, the speed of valve occurance is so swift that blowdown is really a minor issue. However, the additional lift and duration guarantees that you get a healthy charge into your cylinder. According to the book "The High Performance Honda Builder's Handbook" by Joe Pettitt, the 450 hp GSR buildup they did suffered some blowdown under 4000 rpm. That's under the point in which the VTEC high lobe kicks in. (4400 rpm) The top end was just fine. Anyways, the gist of it is that you really can't hurt from having dual cam profiles.
06-14-2010, 09:08 AM
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DITB-AHHH
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, i gave you some as well.