What makes a good “turbo cam?”

[TheDrZero]

So what exactly makes a turbo cam a turbo cam? I always subscribed to the Richard Holdener theory that every cam is a turbo cam. Whatever power curve you had before a turbo you’ll have the curve, just more power. It makes sense because all the turbo does is compress the air to be more dense. It still flows the same volume of air as it did before, just denser air.

After @pmuller9 chastised me for buying a Melling MTF6 cam I’ve been thinking more about it. He said it’s a bad turbo cam because it has too much exhaust duration. In Richards video he shows multiple dyno graphs to illustrate his point, but on the dyno it’s always wide open throttle. So maybe it doesn’t matter at WOT but under regular cruising it would? Is the difference the spool rate? Cam A will spool the turbo quicker than cam B, and that’s what makes it a better turbo cam?

As always any info is greatly appreciated. The amount of knowledge on this forum is unbelievable and everyone is always so helpful. Any pointers to other resources to learn more would be appreciated! I included the link to the video mentioned in case anyone is interested.

 

[aussie7mains]

Ok, Ill submit a text for you to read, A Graham Bells "forced induction" is a very good one IMO. This subject isnt widely covered. I would add from my own experience that for street type of driving sticking to something around a stock cam works best. If you think about charge mass going into the engine, when blown this is mostly influenced by whatever compressor you use. So RPM as always is limited by the mass of mixture you can get through the engine, in an NA application this is limited by internal resistance to mass flow, when blown, its not so much, so the engine will achieve the rpm unlimited by the cam (until valve float plays out). If you can shove the mixture in, it will rev. So dont over cam, you only succeed in ruining a really drivable engine. Ill give a example, in my blown 200ci crossflow, I run a cam with only a 208 degree duration, this will run to 5000rpm without difficulty, it does give a really usable torque curve that spreads form idle to wherever you want to, I have not explored the top end of the curve, but there is no need to with so much torque. Positive displacement compressors, especially Lysholm type are really great at providing fantastic torque that you really notice, my 200 behaves like its a 325ci engine, totaly drivable with a smooth idle. Do read the book.

 

[TheDrZero]

Ok, Ill submit a text for you to read, A Graham Bells "forced induction" is a very good one IMO. This subject isnt widely covered. I would add from my own experience that for street type of driving sticking to something around a stock cam works best. If you think about charge mass going into the engine, when blown this is mostly influenced by whatever compressor you use. So RPM as always is limited by the mass of mixture you can get through the engine, in an NA application this is limited by internal resistance to mass flow, when blown, its not so much, so the engine will achieve the rpm unlimited by the cam (until valve float plays out). If you can shove the mixture in, it will rev. So dont over cam, you only succeed in ruining a really drivable engine. Ill give a example, in my blown 200ci crossflow, I run a cam with only a 208 degree duration, this will run to 5000rpm without difficulty, it does give a really usable torque curve that spreads form idle to wherever you want to, I have not explored the top end of the curve, but there is no need to with so much torque. Positive displacement compressors, especially Lysholm type are really great at providing fantastic torque that you really notice, my 200 behaves like its a 325ci engine, totaly drivable with a smooth idle. Do read the book.

Thanks for the reply. The Melling cam I installed is just an RV cam so it’s nothing radical. If I remember correctly the intake duration is 204 and exhaust is 214. I’ll definitely check out the book, I appreciate the recommendation.

 

[pmuller9]

After @pmuller9 chastised me for buying a Melling MTF6 cam I’ve been thinking more about it. He said it’s a bad turbo cam because it has too much exhaust duration. In Richards video he shows multiple dyno graphs to illustrate his point, but on the dyno it’s always wide open throttle. So maybe it doesn’t matter at WOT but under regular cruising it would? Is the difference the spool rate? Cam A will spool the turbo quicker than cam B, and that’s what makes it a better turbo cam?

The question should be "What makes a good turbo cam for a 300 six"
What Richard said in the last 3 minutes of the video is what I was saying.
In a 300 six you want a camshaft that has excellent response off idle.
Then when you add a turbocharger, it will respond accordingly.

Yes, the turbocharger amplifies the engine's NA response.
If you install a cam with a lot of overlap, the NA engine may be choppy at low rpm and come in suddenly later on.
The turbo will copy that response and produce very little boost until the engine comes suddenly into its power band.
Not ideal for a 300 six in a truck.

In a 300 six it also depends on the exhaust system as to how a turbocharger responds.
If you notice in the video, the test engine had a turbocharger header with fairly long primary tubes which makes the engine and turbocharger a lot less sensitive to camshaft overlap.
We don't have that luxury with the 300 six unless you make your own custom manifold.

What works best for a turbocharged 300 six truck engine is a single pattern camshaft.
If you are using the log exhaust manifold, a reverse pattern with less exhaust duration works better.

 

[drag-200stang]

Definitely get " A. Graham Bell forced induction performance tuning'' book.

 

[TheDrZero]

The question should be "What makes a good turbo cam in a 300 six"
What Richard said in the last 3 minutes of the video is what I was saying.
In a 300 six you want a camshaft that has excellent response off idle.
Then when you add a turbocharger, it will respond faster.

Yes, the turbocharger amplifies the engine's NA response.
If you install a cam with a lot of overlap, the NA engine may be choppy at low rpm and come in suddenly later on.
The turbo will copy that response and produce very little boost until the engine comes suddenly into its power band.
Not ideal for a 300 six in a truck.

In a 300 six it also depends on the exhaust system as to how a turbocharger responds.
If you notice in the video, the engine had a very good single primary tube turbocharger header which makes the engine and turbocharger a lot less sensitive to camshaft overlap.
We don't have that luxury with the 300 six unless you make your own custom manifold.

What works best for a 300 six truck engine is a single pattern camshaft.
If you are using the log exhaust manifold, a reverse pattern with less exhaust duration works better.

So the Melling cam in the truck now runs fantastic and is definitely more responsive. I assumed the extra exhaust duration was to help with scavenging. Since the 300 doesn’t have a cross flow head and the air needs to go in and do a u turn to push the exhaust out, more duration on the exhaust might help with that?

I’m just trying to understand why reducing the exhaust duration would make it run better once you add the turbo? If I like the way the truck performs now, adding a turbo should just make it better right? A properly sized ball bearing turbo should spool quickly. Will having the cam reground to have exhaust match the intake make the truck more responsive? Turbo more responsive?

I’m definitely going to order the book. Unfortunately I’m on the road for work now so I probably won’t be home to read it until June or July.

 

[pmuller9]

The extra exhaust duration does help evacuate the exhaust at the upper end of the power band.
The earlier opening exhaust valve also helps drive the turbine by extending the “blow down” portion of the exhaust cycle.
It is the late closing exhaust valve that causes reversion in the intake port at low rpm that delays the intake cycle.
However, the 300 six having undersized ports is not that sensitive if you are using at least the EFI exhaust manifolds with the turbocharger.
Not worth the effort to change anything at this point.
It will run well as is with the addition of a turbocharger.
Your main goal will be to match the turbocharger’s compressor to give you maximum width power band.

 

[pmuller9]

What does the power bands rpm range look like?

 

[TheDrZero]

The extra exhaust duration does help evacuate the exhaust at the upper end of the power band.
The earlier opening exhaust valve also helps drive the turbine by extending the “blow down” portion of the exhaust cycle.
It is the late closing exhaust valve that causes reversion in the intake port at low rpm that delays the intake cycle.
However, the 300 six having undersized ports is not that sensitive if you are using at least the EFI exhaust manifolds with the turbocharger.
Not worth the effort to change anything at this point.
It will run well as is with the addition of a turbocharger.
Your main goal will be to match the turbocharger’s compressor to give you maximum width power band.

Thanks for the info. I will be using EFI exhaust manifolds and stock EFI intake manifold. I might try to get a better single throttle body to fit the stock manifold but that’s it. In the future I’ll probably fabricate a custom exhaust and intake, but that’ll be a long term project that includes an engine rebuild with forged pistons and rods. And crank of I can get my hands on one. Melling advertises the cam as having an operating range of 1000-4800 rpm. But it feels to me to be better off idle than stock with a very strong mid range, but seems to fall on its face around 4000rpm. I’m assuming that’s a result of the poor flow of the stock head. I used to cruise on the interstate with the throttle well over halfway at 80mph and get 12mpg with my full travel setup in the bed (about 2000lbs). Now with the Holley Terminator X, Melling cam, LS coils, and fuel injector clinic injectors I cruise at 80mph at 25-30% throttle opening and I’m seeing around 15mpg. This is without a professional tune, just what I was able to put together myself based on advice from this forum and YouTube videos I watched. I planned on a professional tune but ran out of time.

 

[pmuller9]

The head and the intake manifold is killing the power after 4000 rpm.
The stock EFI intake was designed for peak torque at 2000 rpms.
It has a relatively small cross sectional area and combined with the long runner length, it restricts the airflow needed to make higher horsepower.

 

[pmuller9]

If you size the turbocharger for this engine for power up to 4000 rpm’s, the next engine with better components and breathing will make power to 5500 rpm and require at different turbocharger.

 

[TheDrZero]

If you size the turbocharger for this engine for power up to 4000 rpm’s, the next engine with better components and breathing will make power to 5500 rpm and require at different turbocharger.

I thought you sized turbo chargers by desired power level, not RPM. So I wasn’t planning to buy a second turbo, this is good to know and keep in mind. I might need to plan to do the rebuild and turbo at the same time, depending on the cost of a decent turbo. Or maybe just a cheap junkyard turbo for the midterm and a good ball bearing turbo later.

 

[pmuller9]

The turbocharger compressor has a certain range of airflow at each level of compression.
You match the engine’s range of airflow with the turbochargers compressor range of airflow.
If you want to make more horsepower then you design the engine for more power which means the power band will be at a higher rpm range and the matching turbocharger will have a larger compressor with a higher flow rate and is rated at a higher horsepower.

I will post a more detailed explanation tomorrow.

 

[Bronctopia]

I thought you sized turbo chargers by desired power level, not RPM

Power=RPM x Torque.
Turbochargers work by increasing torque at any given RPM, by increasing air density.
But detonation puts a cap on that density/torque with the Otto cycle.
So more power means higher RPMs, where you can move more air mass per second at the density limit.

 

[drag-200stang]

The best turbo book by A Graham Bell ''forced induction performance tuning'' is a great book to have. Trust me it is worth getting. It is worth repeating.
Also go to ''Borg Warner Match Bot'' and play with it some, it will help understanding sizing.

 

[pmuller9]

I thought you sized turbo chargers by desired power level, not RPM. So I wasn’t planning to buy a second turbo, this is good to know and keep in mind. I might need to plan to do the rebuild and turbo at the same time, depending on the cost of a decent turbo. Or maybe just a cheap junkyard turbo for the midterm and a good ball bearing turbo later.

I started a thread on "Selecting a turbocharger for the 300 six".
It will take me a few days to finish.

P

Thread 'Selecting a turbocharger for the 300 six'

Mar 26, 2025

One of the first things you often hear is about picking a turbocharger that will spool.
That is not the objective.
The main goal is to pick a turbocharger that is the correct size for the engine and its application.

An engine is an air pump that has a range of airflow from the beginning of its power band to the end of its power band.
The turbocharger's compressor is likewise an air pump that has a flow range from the surge line out to the choke line.

Most of the 300 sixes discussed on this forum are for street use in a truck that works best with a wide power band.
In order to achieve the...

• pmuller9

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