What are peoples different takes on turbocharged cam theories...?


Things like...Overlap, duration, lift, opening and closing events, area under the curve, best DCR, fast vs slow ramps, asymmetrical vs symmetrical lobes, valve bounce and valve float and the problems they create under boost, proper spring rates/pressure in boosted applications and anything else I might have missed...

It would be nice to pick other peoples brains and get a good discussion going...

 

"duration, lift, opening and closing events, area under the curve"

All depend on the rest of the combination just like any other cam

"best DCR"

Depends on the VE% and the amount of CFM being pushed into the motor along with the chamber design and SCR, not to mention the turbo and cubes that are being used... very hard to determine without knowing everything

"fast vs slow ramps, asymmetrical vs symmetrical lobes"

Depend on the RPM mostly..... and how much money you have to control them.

"valve bounce and valve float"

equally as bad under boost as in NA applications....

"proper spring rates/pressure in boosted applications"

All depends on the amount of pressure on the back side of the valve and on the underside of the valve... if the difference is not major (meaning your trapping at lot of boost) then you don't have more of a issue here as you would on a NA motor.... 150lbs on the seat is more than enough to handle 10lbs pushing on the back of it..... the intake closing event is where you look at the pressure on both sides of the valve.

"Overlap"

Last but not least..... important and you need to know the backpressure in the system and how good the exhaust design is. A low backpressure intake track (meaning lower boost for the same CFM fed to the motor) will give you a higher backpressure to boost offset ratio so you need to watch the overlap more there, either way backpressure is a killer.

Bret

 

what about split duration cams vs. standard cams. What effect does the exhaust duration have on effectively utilizing boost or CFM flow?

Overall overlap seems to be a tricky question, however, in my case, my car will be driving more on the street and less at the track, therefore, im trying to understand cam theories so i can maximize my power productivity.

408ci, S88 t6 est. 1.01 a/r single, hyd. cam, lq9 heads, single plan 90mm+ tb, 9.0:1 C.R., 4l80e est. 3k stall, 3.23 final.

Not asking for a cam selection, just what should i/we be leaning towards in cam selection?

 

After plenty of dyno testing I ended up liking single pattern cams with relatively low duration and tight LSA's. It seems the trend is toward very wide LSA's, but from my experience overlap = power just as it would with a naturally aspirated engine.

I like to run alot of valvespring pressure. Around 175-185 on the seat and 420 open is getting us 7,500+rpm with 20lbs of boost. (hydraulic roller).

I don't believe boost pressure has any contribution to valve float or bounce, as the piston is on it's way up the bore at IVC and the compression cycle starts as the intake is closing, immediately eclipsing boost pressure. (which helps keep the valve closed)

I believe backpressure is the cause of valve bounce on the exhaust side. Backpressure can be 2x+ boost pressure and the intake cycle is starting as the exhaust valve is closing. (meaning a big pressure differential working against the exhaust valve closing).

 

You don't want lot's of overlap at high boost on almost any turbo but the higher the back pressure is compared to the boost the worse it hurts you. Top end power on some turbos improves greatly when you kill overlap even by running SHORTER exhaust lobes which just shows you how much reversion they are seeing as this would be the opposite on an NA engine. If you run a reverse split on these NA only engines they LOSE top end power.

 

So,you want small diffrence from exhaust lobe to intake lobe but low lift on the exaust and high on the intake?

Is the exhaust lift determined by the backpressure and IC?

What about overlap?If say your spinning a 350cid to 6000rpm on low 10psi you want no overlap but same psi on a 408cid at 6000,would you then want overlap due to better breathing ability?

 

Ari,

No I meant the overlap needs to be low not the lift. I wouldn't want much overlap on either engine in your example but again it depends on what the pressure differentials are on that particular system.

 

What info could I provide a cam designer so an optimum cut can be made?

You mentioned Intake and exhaust pressures, temps? Turbo flow capacity? Intake? heads? etc?

Also, with the intake being pressurized are the same principals with wave tuning applied? Or do they differ?

 

I'm not qualified to talk about wave tuning but yes the engine acts mostly the same from what I have really seen as long as the back pressure and manifold pressure do not diverge too much.

 

Give them the head and manifold data and the expected rpm range and anything like extremely high back pressure and they can steer you right. In reality you can get close with a cam but the dyno and track will help you wittle it down after that. If there was any perfect cam-picking program then we wouldn't all be dynoing all the time.

 

Thats a first.................someone on this board admitting they are not qualified to answer a question. Kudos. Wish more people would do that so others wouldnt spend 50+ post just to prove them wrong.

Brandon

 

Depends on the header/manifold design along with the Turbo charger turbine housing wheel and size. Overlap is a tricky pig in a turbo application. sometimes depending on the combo overlap may help or hurt. There is however no such thing as a perfect boost cam. My general experience has been that if you do a good job designing your turbo kit IE the IntakeManifold/ExhuastManifold or IMP EMP pressures are kept in check and you have a ratio of 1:1 IMP to EMP almost any good running NA cam will work well with turbocharging. now when your IMP EMP is 1:2 then its time to rethink everything. If you are 2:1 IMP to EMP then again its time to rethink everything again. Also the most confusing part of IMP EMP is going to be how much is the camshaft itself affecting these ratios. I tend to try to develope a good running NA cam that hits the power target that i can quantify in terms of total boost and compression for a fuel then build develop the turbo kit around it. Had good luck doing this.

SO if you wondering here is the process i use.

Develop NA Combo with compression target in mind IE what ever comrpession your fuel can tolerate with boost basically. If not find more means of intercooler detonation prevention.

Then design turbo kit around siad engine.

Theoretically if you use pulse charging type header designs with the properly sized turbo chargers you can acihive IMP EMP of 1:1 fiarly easily and when you do your boost increases vs HP NA should be relatively straight forward and clear. IE 10 PSI should work out to a very clear cut 80% increase in HP. these are just general rules of thumb. thats not to say however that you shouldn't rethink your cam choice if packaging etc denies good header/turbo/exhuast design.

Is a murky subject and no one here including myself is going to be able to give you a straight answer as to exactly what cam will generate the best power with any turbo without alot of testing.

 

that ain't no ****, kudos 7088

 

Indeed, refreshing to see somebody not spew usless rhetoic that they read somewhere in place of a correct answer just for the sake of answering. Although Erik is usally pretty well dialed in.