Tried posting this in the forced induction section but not getting much help. Thought I would try here.

I'm thinking of buying a Comp cam that specs out at 581/588 224/228 @50 on a 114+4 LSA. This would be a great N/A or Nitrous cam but I'm concerned about it with a turbo. I'm going to be running a T6 flange T76 turbo with a 1.08 AR and shorty headers. I know that each and every cam on a turbo setup is different and should be speced according to the entire package. My thinking is that with shorty headers and being a T6 flange with a 1.08 AR the standard split cam should help spool the turbo and not hurt backpressure at all. I should have more than enough flow to accomidate this cam. However, I'm concerned about the advance built into the cam. The 114 LSA is fine but being a 114+4 that would put it on a 110 ICL. My understanding is that a 110 ICL would cause cylinder pressure to rise and increase my DCR. I'll be running stock heads for a while. Eventually going to go with #317's.

What are your thoughts on this cam or running a standard split with my setup?

 

You need to run a single pattern cam. he longer exhaust duration will hurt your power, and will not help "spool the turbo".
As for the 114, a 112 would be better.
as for the +4 advance, you can always just retard the cam back to straight up.

 

if you got +4 on the cam, can't you just go -4 on the timing chain to undo the effects of raised DCR? i'm no expert, i'm just curious...

paging Doctor Patrick

 

I agree that a single pattern cam would be better in my case but please explain why you think a longer exhaust duration will hurt my power. As for the 112 LCA I cannot agree on that. The lower LCA would actually bleed off boost as opposed to the 114. Also I figured that you could just retard the cam back to straight up, just didn't know if it would have any ill effect.

 

the way I understand it, longer duration drops your DCR, so you have to make up for it elsewhere, like milling heads, or pistons that increase your static compression. grab a DCR calculator, and create few different scenarios: stock, cam only (you'll see a DCR drop) and then bring the DCR back up with different methods: pistons, head mill, advancing the cam.

Why do you think all the cams for 'cam only' setups have +4 ground in?

 

I was unaware that longer duration will drop your DCR. Maybe, that being the case I can use this cam with stock heads and a turbo. I was under the impression that with this cam it would either have to be set up 4* retarded or go to LQ9 #317 heads.

AS far as your last statement. From my experience the main reason that most off the shelf cams have a +4 built into them is because 99% of the time people over cam their set up and the +4 helps to make the cam more liveable on the street. Of course this really doesn't apply to a race only set up.

 

Punch it up in a dcr calculator and you'll see that it drops. i saw it when i punched in some slightly different numbers as a result of going to 1.8 rockers. even that small of a difference dropped my DCR by a significant amount. Now I either have to mill heads, or advance the cam as much as possible to get it back.

yes, there's a lot of truth to your last statment. most people do overcam--they get a huge cam, and try to make it act smaller since they cannot take advantage of the good aspects of a big cam (namely: top end power) due to weak shortblocks, not well picked valvetrain, and not enough gearing. so this way they get the worse part of both worlds (no power up top and crappy power delivery down low). that's why i have gone the other way, and i have a built shortblock with a naturally retarded z06 cam. perfect daily driver that I can flog at the tracetrack to 7000rpm where I can take advantage of the top end power.

 

If you go large on the cam you can raise compression to increase dcr, which is why larger cams usually go hand in hand with higher static compression.

DCR is dependant on IVC, so you can advance or retard cam to tweak it a little.

 

Advance is for more top end but retard for low end power

EVO is critical

 

I could be wrong, but I think you guys are getting to caught up on DCR.

There is A LOT more to cam design than that. For starters your negating the pressure differences in the exhaust and on the intake side of the valve from the turbo system...

 

You've got that backward.
Advance = more low end
Retard = more top end

The main reason there is advance ground into most off the shelf cams is because most people over cam the motors and loose low end and drivability. Thus the ground in advance helps to retore that.

After thinking about it more I think I could just set the cam at 4* retarded which will set it at a straight up 114lca and 114 icl and I'll be alright.

 

Where is the lower LCA will increase the torque to help and help with turbo lag.
The longer LCA and the longer exhaust duration hurts the power on a turbo engine because they both make the exhaust valve open sooner while the piston is still moving down on the firing stroke. You also want the pulse length to be the same on both sides of the turbo.

As for why most cams are advanced 4 degrees. It's to help increase the torque at low RPM's where the air velocity in the port is too slow. If you had a port that got bigger as the RPM increased, you wouldn't need to advance the cam. In 90% of the cases with NA racing engines, a 4-8 degree larger duration cam advanced 4 degrees will out perform the smaller cam that is straight up.

 

So putting a turbo cam in on a 110 would help with spool, but kill some up top power?

 

Yes, but if the turbo is big enough you can up the boost to get the power back.

 

not quite, think about it, if you have a baloon (system with positive pressure) with a hole (overlap) then the pressurized air from the baloon with escape with rate proportional to the pressure.

 

I think that there is some misunderstanding here.. if there is overlap, I gaurantee that unless you are running a tremendous amount of boost and getting it for free meaning the pressure ratio between the preturbo exhaust and the intake is equal to or less then the pressure in the intake manifold, you will get what is called reversion. this will hurt power by keeping the combustion chamber pressurized with exhaust gases rather then the fresh incoming intake air charge. this is the main reason for keeping the lobe seperation angel wider and reducing overlap. you are not gaining any scavenging effect as you might from a very unrestrictive free flowing exhaust on a naturally aspirated engine. turbo cars have too much backpressure for scavenge effect to make a beneficial difference. as for camshafts.. it is the age old saying.. I don't think that any one cam is going to work for all applications even if they are similar.. there are too many variables involved.. including but not limited too the flow capacity of the exhaust preturbo and the restriction the turbocharger creates, the restriction post turbo, the flow dynamics of the cylinder heads and intake manifold, the boost level being run, the displacement of the engine, the desired power range for the motor in question.. just too many to be generalized that XXXlsa or XXX/XXX duration will fit properly in. larger motors running the most efficient turbo systems where the exhaust back pressure runs extremely close to or less then the intake pressure will benefit from a more traditional camshaft for a naturally aspirated engine as it will have closer characteristics to this type of setup. a engine with a very small turbo designed for low rpm torque but peters out up top from running out of breath is going to want as little overlap as possible to reduce reversion. again.. about the only thing common about camshafts is that the higher the lift the more flow and even this is dependent of the cylinder heads, but forced induction changes the flow capacity and cannot be measured in vacuum at 28* water as a naturally aspirated head would, the best way would be to flow them at the pressure that would be run in its intended application to determine what l;ift the port is going to be its most efficient. at a certain point you hit the wall of diminishing return and you just start adding undo stres to your valvetrain... this would better help decide the best matched lift for the camshaft. hope this sheds some light on the matter!

in general, the best way to select the most appropriate camshaft would be from taking multiple measurements of the entire system and determining what characteristics need to be addressed in duration, LSA, and lift. as far as advancing and retarding the camshaft... I won't speculate here, but trial and error will determine the best phasing for it.. one way will reduce the high end of the power band while the other will lengthen it at the cost of low end grunt.. you will have to find a happy median that creates the performance characteristics you have built the car for. its too bad no one has perfected a variable cam timing system for us conventional pushrod engine cars.. this would help greatly in achieving both ends of the spectrum. I know in the 60's, I read in some old hotrod mags of my fathers they had what was called the vari-cam.. and the cam sprocket would actually advance and retard with engine RPM.. not sure what happened to it, or why no one ever chased after it to perfect it and make it more reliable, but it sure would be interestign to see it work in conjunction with a staged intake manifold that increased port volume as rpm increased..hmmm sounds like a winter project..

Chris

 

Just my two cents...

I had a nice 90 Rx-7 turbo rotary setup.

I don't know alot about cams on piston engines, but, I can tell you a half bridge port rotary is WAY too much overlap on intake and exhaust timings and would not spool my T-76 P-trim .81.

IF the intake is open too much while the exhaust is open you will have boost problems.

I think I was getting "reversion"??? on my setup.

Basically, it made more power NA, and it wasn't able to keep exhaust out of the intake charge with the turbo's backpressure.

which i think caused spool up problems/ lag and low boost for such a large turbo on such a small engine.

Anyhoo...

Good luck!

 

In the first quoted paragraph, are you stating that boost motors should not
have any overlap to be effective?

What about reversion? It seems by reading that quote reversion will happen
anytime at any RPM if the cam has overlap?

In the second paragraph, you state that turbo cars have too much backpressure
in general. What sort of backpressure have you measured?

THe third paragraph implies that the exhaust backpressure should be close
to the intake pressure to be efficient? Do you mean that if the manifold
sees 10 PSI, the exhaust backpressure should be 10 PSI, or as close to 10 PSI
as possible?

Lastly, do you say that a turbo car should use a naturally aspirated camshaft
design rather than a turbo cam design?

 

First off, the 110 lobe center would be to help with the power before you've got to full boost.
It's OK for street turbos, but for racing, you want a wider lobe center to get the most out of the turbo.
As for the air that passes through on overlap, unlike a N/A engine, this air is not wasted because it helps spin the turbo.
As for pressures in the exhaust, this is where most people get screwed-up.
As the air is exiting through the exhaust port into the turbo, the pressure gets pretty high. As the piston slows down toward TDC, the pressure in the cyl drops. The turbo is still spinning and trying to pull the air out. This drops the pressure in the port which dropps the pressure in the cyl. At the time the intake valve starts opening at overlap, the boost pressure in the intake port is higher then the pressure in the cyl. This causes the intake air to flow into the cyl.
Reversion happens when the exhaust duration/lift is too small. The exhaust doesn't evacuate the cyl quick enough, and when the intake valve opens at overlap, the pressure in the cyl is still higher then the pressure in the intake port.

When looking at pressures, you need to look at the pressure differentials.
The pressures in the exhaust on a turbo engine with 30lbs of boost may be high, but not compared to the pressures in the intake port and the cyl.

Believe it or not, back pressure is a much bigger issue with N/A mufflered engines

 

"The turbo is still spinning and trying to pull the air out"

So, the turbo has enough stored energy to cross over between being driven by the exhaust and actually pulling it out of the cylinder, all while continually compressing the inlet charge? (and in most cases with measured exhaust pressure much higher than that of intake manifold pressure)

It would seem to me that the resistance of compressing the inlet charge would easily overcome any inertia of the rotating turbocharger assembly.