Ranking Increased Head Flow Against More Expensive Upgrades

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Need For Speed?

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The recipe for gaining horsepower seems conventional. Start with more air---get either a large single or compounds---then add fuel---larger injection pump and injectors. "Air first, then fuel," the saying goes, accompanied by the appropriate supporting mods of course.

Spending $3K to $5K on compounds, $2K to $3K on an injection pump (or two), and another $3K to $4K on injectors, in that order, seems the common practice, at minimum.

But why is this the conventional order of upgrades as opposed to starting with increased head flow first?

Stated differently, why add more fuel and air to a stock head rather than tailoring the fuel and air setup to a higher flowing head?

Wouldn't it make more sense to purchase a machined head that flows 220 cfm for $2250, for instance, and then "build" your air and fuel combination around that particular "foundation" to begin with?
 
Most people build in stages and for most people a stock head flows plenty on any year Cummins. If you know your goal and it exceeds the flow of a stock head then get a ported head on it now, but theres no reason for anyone wanting 600hp or less to spend so much money on that. Also another reason i think people do it later is because most people dont know their goals, what they will be happy with, or their goals change altogether. Someone may plan to go 1000hp then drive a 500hp truck and think twice about doubling or tripling their money into the truck for that extra 500hp.
 
Turbo's are rated by flow (lbs/min) and obviously supply the flow. They're also rated based on pressure.

If a stock turbo can make it's greatest rated flow rate and stay on it's island pressure wise, you're not going to get much out of the head.

The turbo will flow the same lbs/min because it's already 'maxed out' and your drive-ability will fall because the velocity through the head will decrease. Think of the head as an orifice.

Now if you've got a turbo that can flow a ton of lbs/min, but the head is too restrictive. You're stressing the turbo, and making the head hotter because of it. You make the head less restrictive, bring the pressure down so the turbo is running on it's islands, and increase flow, which increases HP.

So... If you got a head done up to double it's flow with a stock turbo. You're probably going to stay at the same HP rating, but reduce drive-ability.


That's my thoughts on the whole process.
 
What makes even more sense is buying a trans to support all this extra power before doing anything...

Granted this hasn't already been addressed and this is strictly an air/fuel discussion.

But like DDually said, head work usually isn't needed until way later down the line. Unless your shooting for 1200hp right off the line.
 
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Glad I asked. Thank you for the replies.

I am confused about "drive-ability," though. In your scenario, Cornelius, a stock turbo paired with a ported head results in reduced drive-ability because of the reduction in velocity? Velocity vs. volume?
 
When you start making enough power that you blow your head gasket and need O-rings, it time to think about porting the head while it is off.
 
Need For Speed? said:
Glad I asked. Thank you for the replies.

I am confused about "drive-ability," though. In your scenario, Cornelius, a stock turbo paired with a ported head results in reduced drive-ability because of the reduction in velocity? Velocity vs. volume?
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Heads are designed for velocity and swirl OEM. That helps spool a turbo at lower RPM, and get a better distribution of air in the cylinder. This is a driveability move, not a power move. If you took a stock truck and tossed a Hamilton Head on it and nothing else, you'd lose driveability.

Your velocity vs volume point doesn't make a ton of sense. If you're keeping volume constant, a port job will decrease pressure/resistance and lower velocity. This means less compression of the air to flow the same volume, less heat, and an easier life for the turbo.
 
I still don't understand the loss in driveability part. I obviously need to study up on the relationship between volume and velocity. Thanks.
 
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In simple terms big ports won't spool as quick on a small charger because the charger is not forcing enough air through the head to create the same pressure ratio to spin the turbine. Therefore the ability to put the truck on the street and make it drive is diminished. If you've ever driven a non-turbo diesel, or a truck with a big ass boost leak you will understand. Yes, read more on the subject these guys can educate you world's beyond that on the subject if you read
 
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Need For Speed? said:
I still don't understand the loss in driveability part. I obviously need to study up on the relationship between volume and velocity. Thanks.
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Blow through a drinking straw and then blow through a garden hose. Same volume of air but velocity through the straw is way greater then the garden hose. Put a spinning thing on the end of each and see which is quicker to react to sudden changes and that is where driveability is gained and lost. Big ports have their place i f the engine will stay in the higher rpms all the time
 
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