i was wondering if this cam would work good with turbos. i was told by a comp cam specialist that its best for my lt1 intake ported 601s 355 engine. but now i have aquired a few turbo parts and have been researching it for a while. now i am thinking about making a setup but i want to use the cam that they recomended because it works good with my heads. would the cam work with a turbo?

cam

thanks
jeff

 

I think it's got way too much exhaust duration w.r.t to intake duration (226i and 236e) if you're running a normal (not cutting edge exotic) turbo. Go closer, keep intake the same as exhaust, or go more exhaust than intake, since the turbos present quite a bit of exhaust backpressure. Also, much overlap is not a good thing, with stockish restrictive turbos.

 

the cam I picked was 224/224 @ .050, 114 LSA, has a total of 4* overlap @ .050 timing.

the more restrictive manifolds / A/R you have on the turbo, I beleive the less overlap / exhaust duration you will want. I beleive it was Crossfire TA said, you may wind up with 2-3X the exhaust pressure in the manifolds during boost than you have actual boost in the intake. So its BAD to have exhaust flow back into the cylinder. he also said something about blow-down....

 

how about the xe268 ?

 

DOnt think any XE grind is a good idea, they are all Dual Pattern and typically with a turbo'd motor you want a single pattern cam. for instance, the XE262 is 218/224 @ .050. thats dual pattern. if you could find a cam with 218/218 it would be single pattern, a much better choice for a turbo'd motor, normally. something with 114 LSA ideally, all the XE grinds are 110LSA which is BAD for a turbo'd motor, once again. you can get an XE grind for MARINE applications, on a 114LSA, but again, its still dual pattern.

 

is this typical cam style general for all forced induction or just for turbos. say with a supercharger would it be the same?

 

It's going to be interesting to see how mine runs.

For now, I'm keeping the cam I have. I'll swap it later if I find I have trouble with it.

230/236 @ 0.050, 110LSA This is NOT a cam that was designed for a turbo.

 

When you add forced induction to a motor, your cam choice really depends on the same thing it always depends on, how do you want the motor to behave, where will you drive it, fuel economy, etc... Except for one minor detail... Overlap. TYPICALLY on a street motor with forced induction or N20(turbo, supercharger, nitrous) you want a bit less overlap. Why? It helps keep cylinder pressure up, during boost. why? because during boost, you have air being forced in, and during overlap, that means its also being forced right back out into the exhaust without doing crap, except wasting fuel and power. on a NON forced induction motor, the air isnt being PUSHED into the cylinder, its being PULLED in, so a little overlap can help bring in fresh charge by utilizing the exhaust as a scavenging system (which is why people lose torque when they run bigger exhaust, during overlap there is less exhaust gas scavenging so you dont get as much of a cylinder fill of air)... Forced inducted motors dont really like the overlap period because its pretty wastefull.

TYPICALLY, a turbocharged motor takes this rule 1 step further. see, since the turbo is being DRIVEN from the exhaust, it stands to reason the turbo itself presents a pretty hefty exhaust restriction. this FURTHER incites the need for less overlap, since now instead of the air being pushed from the intake to the exhaust, you actually have MORE pressure INSIDE THE EXHAUST, being pushed RIGHT into the CYLINDER! so the problem becomes TWICE as bad on turbo motors, since instead of wasting air/fuel out the exhaust, your pushing exhaust right back into the motor.

So overlap is generally BAD for supercharged motors, but its REALLY bad for turbocharged motors.

 

With a turbo, is exhaust manifold pressure always higher than intake manifold pressure? I thought I read somewhere that a well designed turbo system with the right sized turbo can actually have higher intake manifold pressure than exhaust (at certain parts of the powerband). I've actually heard of people sizing the turbine housing by comparing boost pressure to exhaust manifold pressure, trying to find the housing that has the least backpressure with acceptable lag times.

 

Well, from what I have read, a turbocharger shouldnt be thought of as a miniature motor... that is, it doesnt/shouldnt need to FLOW through like a motor to make your motor make power.

Lets use an example to make this easier. there is a typical turbo upgrade for the SR20 DET motor, (4 cylinder). the turbo is the GT25R. you can get it with either a .64 A/R housing, or a .84 A/R housing. obviouselly, the bigger A/R housing will flow better, because its bigger. this would obviouselly reduce backpressure in the exhaust manifold as well. is this what we want though? In theory, like you just stated, yeah, sure lets go with it. but in practice? A smaller A/R is more sensitive to pressure changes, and more responsive as far as the turbine goes. typically, people with the smaller A/R turbo wind up with better throttle response, better fuel economy, lots more low-end torque, and better drivability. the people who opt for the bigger A/R housing, wind up with a bit more top-end horsepower... because of the reduced exhaust restriction.

This entire example is based on a 4-cylinder motor, on a bigger motor such as a 350 (as opposed to that 2-liter) is obviouselly going to need more than a single .64 A/R turbo to make any power at all. so in the end, it comes down to sizing the turbo to your intended application. whether you wind up with exhaust backpressure or not, doesnt really matter, because as long as the turbo is sized properly, and the motor is built properly, your going to achieve the desired result in either case.