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camshaft durrations for turbo

 
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Chadwards
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Joined: 08 Jun 2013
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Location: utah
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PostPosted: Sun Jun 30, 2013 7:18 pm    Post subject: camshaft durrations for turbo Reply with quote

so got some questions for a guru.

i know 4cylinder engines are not exactly popular but thats whats in my car.my car is 2.5 liter, dohc and was dyno'd at 318wtq at 3200 and 276whp @ 6500. my turbo is fairly small. and my cams are in the 230 range i believe. which is why my torque falls on its face.
we are at 4600ft elivation.
and i plan on building a new motor with a precision 58 series turbo.
my transmission is fairly weak so i dont want to go over 350ft lbs at the wheels. but i want to hit 450whp so to do it i need to move my tq later in the power band.

first question: is 280 cams big enough to move the ve up in the 4000 -7400 rpm range?

second question: ive seen some BIG turbo builds running, say 4 degrees more durration on the exhaust than the intake. if im thinking right it is to open the ex valve earlier to sacrifice a little cylinder pressure for more back pressure to spool the big turbo. am i right? would i bennifit from this
?
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af2
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Location: grassvalley, ca
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1933 Willys Coupe

PostPosted: Sun Jun 30, 2013 8:48 pm    Post subject: Reply with quote

Interesting as I have talked to 2 cam grinders that say with the proper turbo or turbos the duration is not a factor but with a turbo not correct the duration difference is a band aid. I am going to say they were talking to small but I could be totally wrong..
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af2
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Location: grassvalley, ca
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1933 Willys Coupe

PostPosted: Sun Jun 30, 2013 10:23 pm    Post subject: Re: camshaft durrations for turbo Reply with quote

Chadwards wrote:


second question: ive seen some BIG turbo builds running, say 4 degrees more durration on the exhaust than the intake. if im thinking right it is to open the ex valve earlier to sacrifice a little cylinder pressure for more back pressure to spool the big turbo. am i right? would i bennifit from this
?


I could give you a phone # of a good grinder close to me that deals with a lot of Porsche builds.
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Chadwards
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PostPosted: Mon Jul 01, 2013 1:37 am    Post subject: Reply with quote

i was gonna call delta and ask them. the weird thing is i found a few that is like that from the factory...
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Big Dave
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PostPosted: Mon Jul 01, 2013 3:08 pm    Post subject: Reply with quote

There is no set in stone answer to this question as each engine is designed and built differently. You can not compare your four valve per cylinder dual overhead cam head to the small block Chevy head that hasn't changed it's basic design all that much since it was on the drawing board back in 1953. Though the SBC has improved it's combustion chamber the size and layout of the exhaust port has remained the same.

You increase exhaust duration on a car with a power adder normally to get the increased volume of exhaust gas out of the cylinder as fast as you can. You can not draw a fresh charge until the cylinder is at a atmospheric pressure at BDC. I mention the increased volume of exhaust gas as a blower or turbo is stuffing more air into the cylinder by way of a mechanical pump, but the exhaust has nothing other than atmospheric pressure to draw down the spent gasses before the next cycle can start.

In the case of a big block Chevy you run a split duration cam because even when normally aspirated the exhaust ports are so bad on a the BBC that it needs all the help it can get. Add a blower or a turbo and you only aggravate a bad situation.

So what makes a good port or a bad port? It is determined by the ratio of intake to exhaust flow. You will see that term advertised on some head manufactures web sites stating how they have improved one or the other or both.

The flow rate on the exhaust is determined by the port's volume (size) and the diameter of the valve, but equally important is it's shape. Since the intake is more important generally than the exhaust (up to a point) most designers optimize the intake at the expense of the exhaust. The only head design that doesn't suffer this robbing Peter to pay Paul is the true Hemi head where both intake and exhaust valves can be the same size.

Getting back to port shape the SBC currently uses a "D" port (round on top and flat on the bottom). This was chosen after modeling head flow in a computer to remove material where flow was fastest and adding material where the flow rate had slowed down to increase the port speed. Chevy has had square ports and round ports and D ports on both the SBC and the BBC (with the BBC even experimenting with a "W" shaped port that had a vane in the port to direct the gasses).

Once again adding a turbo increases the volume of gas exiting the port so the harder it has to work to do this. Increasing the time the exhaust vale is off the seat helps but you don't want to open the valve before you have extracted all the work you can from the hot gasses. The Otto cycle isn't as easily affected as the Diesel cycle that burns the fuel from top to bottom of each power stroke. Once the piston is about a third of the way down the cylinder you can vent the gasses to the outside with the Otto cycle because the fuel burns so much faster.

It is the heat you want as that is what does the work (not back pressure, which is a measure of fluid friction). The hotter the gasses are when they arrive at the turbo, the more work they can perform expanding against the blades of the scroll. That is why turbos are mounted as close to the exhaust port as it is physically possible to do so (and why a rear turbo is going to suffer heat loss, and as such, loose power compared to one in the engine bay).

You design a cam to match your heads flow rate and to work with the induction (power adder) as well as the exhaust. A restrictive exhaust can kill a turbo car or a diesel engine, as both have to vent the spent gasses as quickly and as efficiently as is possible (a six inch exhaust can keep a 350 horse diesel happy, but you go to dual six inch stacks above that power rating). Same applies to a turbo powered car. You should have twin six inch pipes exiting through the hood to do it right with a twin turbo car (if you have just one big turbo grab the exhaust stack off of a Pratt & Whitney P6 jet engine for your exhaust pipe).

Big Dave
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