My custom cam for example has both more duration and higher lift on the intake rather than on the exhaust. This is counterintuitive to me because turbos both need the exhaust to get them spooled and because there is more resistance in the exhaust which means that the cylinder should take longer to evacuate. Does this mean a turbocharged combination would prefer a smaller exhaust port than usual?

The only thing that I can think of is that without the scavenging effect inherent with n/a and blown cars, it takes longer to fill the cylinder, and at the same time the exhaust is restricted by the necessity to minimize overlap. Is that right or is there more to it?

Thanks,

Chris

 

From what I have been seeing the trend is starting to go the other way again. Cam motion are the ones that seem to like the reverse pattern cams.

 

it all kinda depends on many factors

 

I did not get my cam from cam motion. And turbo cat..... um thanks?

 

Lots of factors....but here are a few.

1. Turbo motors have a longer powerstroke due to the pressure in the exhaust manifold turning the turbine. You'll lose power by opening the exhaust too early.
2. The typical Factory EFI intake manifolds are very restrictive.....lots of bends & turns. These setups benefit from a larger intake profile.
3. Carburator style manifolds don't need as much intake lobe due to the less restrictive design.
4. Overlap.... There's more pressure in the exhaust manifold than the intake, therefore overlap *can* be an issue, espeically at low RPM.
5. Powerband, just like any other cam design, one needs to optimize the camshaft for the intended powerband. IE, a low RPM turbo motor would not benift as much from a large camshaft like an N/A motor. On a high RPM turbo motor, larger camshafts most definately make more power. Less physical time for the exhaust to contaminate the aircharge....just like on an N/A motor, but acentuated due to the pressure in the exhaust.
6. Exhaust reversion is more prevelant with a turbo application as there is more pressure in the exhaust, so shutting the exhaust valve early is a benefit at lower RPM's.

So, in general, a turbo motor will benefit from a retarded exhaust lobe profile that closes early to prevent exhaust reverison... What you end up with is an exhaust lobe that's a bit smaller than normal. The flow of exhaust can be offset by using a more aggressive exhaust lobe profile. Same lobe area as a 'larger' exhaust lobe, but with less advertised duration. This then presents another problem with controlling the valves... Valvefloat on a turbo motor happens on the exhaust side due to the added pressure on the valve from the turbine.

 

So it opens late and closes early... Wow. My cam card says that the exhaust valve opens 56* BBDC @ .050". In what range do most non-turbo cams usually open? Are we talking just a few degrees?

Hmm... My setup is a TFS-R with Boxed upper, and AFR205cc heads on a 331, so that doesn't apply to me or does it?

That makes sense. Mine actually has -2* overlap at .050"

How important is the boost level and specific turbo used? How radically will the cam profile change if these are changed?

Now that's interesting and might explain why my cam card calls for lighter valvetrain.

Wouldn't have expected that. Thanks that was good reading.

Chris

 

Exhaust opening at 56 BBDC @.050" is pretty early actually. What are the full specs of your cam?

TFS Box intakes are not a restrictive design & therefore shouldn't need as much intake lobe to fill the cylinders. You'll also get more RPM out of that intake than a 'traditional' EFI manifold.

Boost level & specific kits can be taken into consideration when designing a camshaft. This comes into play with how much back-pressure there is in a given setup vs another, at a given boost vs another. However, it's VERY difficult to know which setup does what without actually digging into that specific combo and finding out. Every setup is different. Yes, you can further maximize a camshaft given this information.....but it takes more than just calling up Cam Motion and telling them what kit you're running.

 

Having more information on your set-up would be a big help, Chris.

Tom Vaught

 

Depends on backpressure to boost pressure ratio.

You cant really "spec" a turbo cam without those numbers.... then of course, with the right cam, the numbers can change, so it becomes an iterative process.

 

Thanks guys. Honestly, I was just interested in the concepts behind the general theory more so than in my particular combination. I was just using it as an example, but I appreciate the interest and welcome any comments on my setup from the think-tank you've got going here, Tom.

The excuses: To be honest, I know I've got big heads and a very high flowing intake for the rpm the motor will run now, but purposefully decided to do that so I had room to grow to a high rpm bigger turbo combination in the future without having the redo the top end. Eventually, I believe I will go track only with the car and move over to a solid roller hi-revver. I'll probably up the compression, too. Here's what I've got:

Dart block w/ neutral balanced billet crank and rods 4.030" & 3.250" stroke
8.5:1 CP custom pistons
Hydraulic roller cam (I'll list the specs later)
AFR205 heads with the 80cc "nitrous" exhaust port
TFS-R w/ boxed upper intake & 75mm TB
HP Stage III kit w/ a Precision GT42-76R turbo 1.15 A/R exhaust housing
Shaft mount 1.6 rockers
120 lbs injectors
BS3 to tune with - planning for speed density

The cam is a custom HR with the following specs:
max lift = .604"/.573"
228/224 @ .050"
114 LSA

@.050" Open Close
Intake 4* BTDC 44* ABDC
Exhaust 56* BBDC -6* ATDC

Cam card says the power band for a 331 is 2700 RPM to 6800 rpm. I told the cam guy, who shall remain unnamed (here), that I wanted to shift at ~7000 RPM, and wanted the cam to remain streetable since this is still my street toy. On pump gas, I'm going to keep the boost at or under 15 psi. On race gas, I'm going to see just how far that turbo will go. On the street, the car will run a manual, and when it goes to track only duty, a 2 or 3 speed auto will go in. I'm really excited about all of it. I've been planning and/or building this car since the stock block and internals twin-turbo combo gave up the ghost in November of '05. The motor is going to be cranked and broken in sans turbo, tomorrow.

Is there anything pertinent that I've left out?

Chris

 

Exhaust should be opening at 50 BBDC with a 224 exhaust lobe on a 114LSA with the 4 degrees advance listed.

 

Well, I'm just reading off of the cam card. So, it's wrong then? I have tried using the calculations and have been told in the past that I couldn't apply them. Have you considered that because it's a custom cam the lobes are asymmetric? The cam card says, "Degree this camshaft using the specs provided on this sheet. The asymmetrical design of these lobes may not allow for accurate reading using the centerline method." The cam is also 4* advanced as you already mentioned. It says the recommended intake centerline is 110*.

Chris

 

Sounds like an FTI cam perhaps?
If so, probably just a type-o.

All you've got to do is degree it in....you'll know for sure what the cam is once you do that.

 

It is... I believe that the cam was degreed in when I was still overseas. I'll check with my shop in the morning.

Still, LSA should be the angle from peak to peak, not centerline to centerline, right? And since the lobes are asymmetrical, the centerline is not going to be the same as the peak lift, which to me means that whatever calculations you used are inaccurate without knowing the shape of each lobe.

Am I wrong?

 

subscribing

 

Nah, it's just a type-o.

BTW, I had an FTI custom turbo profile as well. 230/218 113 +2
That card reads:
ICO 4 BTDC
IVC 46
EVO 44
EVC 6 BTDC
I found it worked okay, but no better in the 1/4 mile than another turbo cam I had (219/219 113LSA -1)

I now run an F303, which is ~20hp behind these cams and a bit harder to dial in for drivability, but my valvetrain is now rock-solid stable....no matter how much boost I run.

 

Thanks man.. I appreciate the information.