Ok well im also ordering cams, valves, and springs... My question
I want to go for high end power, and im lookin at the stage 3 cams for 3500-8000 rpms from TTR... however seein as im going to be runnin a FI setup should i buy the turbo cams? Im willing to sacrafice some bottom end considering the only time im below 3500 is.. Well never
My question though is will i lose POTENTIAL power because of valve overlap?? Where ill be able to make more power using the turbo cams?
<img src="http://memimage.cardomain.net/member_images/3/web/831000-831999/831395_11_full.jpg">
BTW im ordering factory valves unless the ferrea valves are cheaper
<img src="http://memimage.cardomain.net/member_images/3/web/831000-831999/831395_11_full.jpg">
Overlap is what helps make pwer on NA cars and is bad for turbo cars. You could get the NA cams now, and once you are turbo throw a set of cam gears on them and cut down the overlap.
Well i decided im goin with the stage 3 cams after talkin with TTR, should be a fun ride
<img src="http://memimage.cardomain.net/member_images/3/web/831000-831999/831395_11_full.jpg">
LIVID wrote:Overlap is what helps make pwer on NA cars and is bad for turbo cars. You could get the NA cams now, and once you are turbo throw a set of cam gears on them and cut down the overlap.
The over lap will not have a negative effect on top end power on a turbo motor. The negative effect of long duration cans on a turbo motor is added turbo lag due to moving the power band up and reducing the exhaust energy needed to spool a turbo at lower rpms.
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Skilz10179 wrote:The over lap will not have a negative effect on top end power on a turbo motor. The negative effect of long duration cans on a turbo motor is added turbo lag due to moving the power band up and reducing the exhaust energy needed to spool a turbo at lower rpms.
I'm not talking about duration, I'm talking about the degrees of overlap between the intake and the exhasut valves. This overlap is typically a result of increase duration, but it is possible to have longer duration cams with less valve overlap. The increased overlap on NA cams which aids in the scavanging effect means both intake and exhaust valves are open at the same time. In a FI car this results in a portion of the intake charge being blown through the chamber as apposed the being trapped by it.
LIVID wrote:Skilz10179 wrote:The over lap will not have a negative effect on top end power on a turbo motor. The negative effect of long duration cans on a turbo motor is added turbo lag due to moving the power band up and reducing the exhaust energy needed to spool a turbo at lower rpms.
I'm not talking about duration, I'm talking about the degrees of overlap between the intake and the exhasut valves. This overlap is typically a result of increase duration, but it is possible to have longer duration cams with less valve overlap. The increased overlap on NA cams which aids in the scavanging effect means both intake and exhaust valves are open at the same time. In a FI car this results in a portion of the intake charge being blown through the chamber as apposed the being trapped by it.
That's in a supercharged or blown car, a turbo motor has quite the opposite effect (at least while spooling). Also, blowing air through the cylinder in a wastegate-controlled turbo motor would have no noticeable negative side effect.
fortune cookie say:
better a delay than a disaster.
I know the common recomendation for turbo cams are low duration/low overlap, but that is for a stock style turbo profile.
Corky Bell wrote:
Make no mistake in the fact that the turbo performance cams are very different from atmospheric performance cams. The characteristics of long duration and high overlap for atmo cams are unwelcome in the turbo system. The street turbo, which is generally small, operates on exhaust manifold pressure somewhat higher than intake boost pressure. This situation, when presented with long-duration, high-overlap cams, creates a huge amount of reversion. Thus the "turbo cam" tends to become a low duration, very limited overlap camshaft.
Some random article wrote:
Duration:
Duration is critical to a turbo setup since its probably the single most important event of a turbo motor (i.e. time valve sits open and closed). Since the air is being forced instead of drawn into and out of the combustion chamber, duration will be your largest variable on how that incoming/outgoing air is managed.
Duration when using a manifold or log design on most turbo cams is usually about 6 degrees more intake duration than exhaust duration. This is mainly because a manifold/log design will typically see higher then a 2:1 pressure ratio in the exhaust ( as high as 4:1 with some logs). By using a reverse split duration this will somewhat help prevent from getting exhaust gas reversion.
Duration when using an efficient header setup usually be slightly longer. The thinking is with the exhaust backpressure being only 2:1 you can leave the exhaust valve open a little longer then if the exhaust backpressure was 3:1 or higher. Also some of the new turbo designs produce a much lower backpressure with the advent of better flowing turbine wheels and housings which further decrease the total amount of backpressure created by the system.
Overlap:
Overlap definition, is the time period when both the exhaust valve and the intake valve are open at the same time. The exhaust valve needs to stay open after the piston passes TDC in order to use the vacuum created of the exiting exhaust gases to maximize the amount of exhaust gas drawn out of the cylinder. The intake valve opens before TDC in order to use the vacuum created by the exiting exhaust gases to start drawing the intake charge into the cylinder.
This sequence of events above are controlled by the duration and LS (Lobe separation) of the cam. On a typical N/A motor this is essential since you have no pressure being developed on the intake side to push the charge into the combustion chamber. The problem with this event is a turbocharged motor will create a larger amount of backpressure on the exhaust side. Due to this event the above definition will not apply. Reason being is, when the intake valve opens at BTDC, the burned gasses in the chamber will exit out the intake since the pressure is lower than the exhaust. Since this is true you would not want to open the intake valve until the piston has started going down, ATDC. This will lower the combustion chamber pressure till it's below the intake manifold pressure.
Typically on the set ups I build, I like to use a high lift/high duration cam with very litle (2-4 degrees) of overlap. For peak power output I retard the intake cam to cram the most air in while the valve is open, and advance the exhaust, so that the exhasut is still expanding as it leaves the chamber and hits the turbo.
OHV notec wrote: blowing air through the cylinder in a wastegate-controlled turbo motor would have no noticeable negative side effect.
I disagree. The valves are open for a certain period of time. The turbo is going to force air into the chamber as long as the valves are open, and the turbo is still capable of pushing against the preasure in the charge system. If some of that mixture is escaping out the exhaust side then it's wasting potential power. Granted that is not a negative side effect really, but it is something I consider important. Another unpleasant side effect of unburnt mixture is that is will make the car appear to be running richer then it is, because the o2 sensor reads everything in the exhasut pipe, not just what is getting trapped in the chamber. Increased overlap can make you spool up faster though, the particles of unburnt fuel have more mass and more potential to push the turbine wheel (this is how antilag systems work). Also gasoline has a relativly low auto ignition point, and when the turbo/header gets hot enough the unburnt fuel will ignite in the header and expand rapidly as they pass through the trubine, increaseing exhasut velocity. This is hard on the turbo though, which is also why antilag systems are not recommended for constant use. So where are having increased overlap can result in faster spool up, it comes at the cost of peak power, turbo life, accurate readings and could be accomplished by advancing the exhaust cam.
Guys thats not true. Modern turbochargers flow quite well, and do not require low overlap events.
112 - 114 LSA is fine.
LIVID wrote:OHV notec wrote: blowing air through the cylinder in a wastegate-controlled turbo motor would have no noticeable negative side effect.
I disagree. The valves are open for a certain period of time. The turbo is going to force air into the chamber as long as the valves are open, and the turbo is still capable of pushing against the preasure in the charge system. If some of that mixture is escaping out the exhaust side then it's wasting potential power. Granted that is not a negative side effect really, but it is something I consider important. Another unpleasant side effect of unburnt mixture is that is will make the car appear to be running richer then it is, because the o2 sensor reads everything in the exhasut pipe, not just what is getting trapped in the chamber.
My thinking was that having the valve open and blowing air through is not wasting any potential that would be had by not having the valve open at all. The turbo can push more air, cylinder pressure won't go down as it would on a blower motor. Also, by the time the cam with less overlap starts opening, the more aggressive cam is at a greater lift, so when the exhaust valve closes more air is allowed to enter than at the lower lift point. This is my thinking at least, it makes sense in my head.
Also, as long as you're tuning based on knock and running open loop, the 'appearence' of being rich wouldn't make a difference, but in part throttle closed-loop it might be a little confusing.
fortune cookie say:
better a delay than a disaster.
