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Joined: Fri Aug 19, 2016 6:48 am
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Vehicle: Suzuki Grand Vitara 04 manual

Post Posted: Fri Jul 12, 2019 8:56 pm 
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A head with all the bits has come up for sale near me. Two bent valves after a timing chain issue.
I'm concidering buying it to have rebuild ready for the future when I need to do the timing kit. Mine already burns oil, presumably through the valve guides.
I would assume replace lifters bent valves guides too. Apart from cleaning the whole head and reassembly are there any improvements that can be made at the same time?
Compression, flow increase. What would it gain? Economy? Performance?
I've discuss it with a local machine shop. About $600 to recondition and up to $400 more for performance grinding improvements.

What advice do you have?

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Post Posted: Sat Jul 13, 2019 8:38 am 
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Leave it alone.

Removing material from the head castings is all about increasing airflow through the ports. This may allow the engine to generate more power, but only at high rpm. 4WD's, if anything, require smaller ports an valves to maximise low RPM flow velocity, increasing torque and low RPM drivability.

Increased compression is great but you're already at 9.3:1. you could probably go to around 10:1 (the the J20B from the SX4 is 9.7:1, J24 is at 10:1) but I'd suggest you may find you'll need premium fuel to prevent detonation. I think the J24 Kizashi was a 95 or 98 octane car, and you'll need the ECU to be mapped for it, likewise if any significant flow improvements were made.

In any case, you're very quickly going to spend a lot more than $400 making that $400 head grind do anything.

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Post Posted: Thu Jul 18, 2019 9:48 am 
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Understood.

What about Abrasive Flow Machining of the ports? Claims of 25% higher flow velocity.

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Post Posted: Thu Jul 18, 2019 10:41 am 
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There’s some complex ideas to unpack in that statement. I’ll reply in detail tonight.

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Post Posted: Thu Jul 18, 2019 7:39 pm 
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OK, in front of a computer.

I'm not a fluid dynamics engineer but I might know just enough to get into trouble.... and I'm sorry if this is way overkill and blows your mind, but I'm enjoying the process of working through this, so humour me.

I think the vendors of extrude honing are using some fancy footwork to promote their services. Interestingly, some quick googling turned up an article from 2002 Mustang and Fast Fords magazine, which is about the last time I remember seeing much about this. This article talked about 20% increased "flow velocity" Flow velocity is a pretty strange way of describing an improvement and sounds like they're hiding something. The right term (if you're a Yank) is CFM and that's achieved at a certain "/hg vacuum. that's measure of air volume not speed.

So that "20% increased velocity" comment needs unpacking. Yes, I'm sure you can grind some material out of a head to make it bigger inside and it will move more air, but will that air actually move faster once it's in your car?

I don't doubt there's good reason to remove casting seams and imperfections from heads and manifolds. Of course they will inhibit flow and create turbulence, but in a modern EFI engine, that's not as critical as you might think, because the fuel is injected at high speed and pressure and basically sprayed at the back of the valve. In an old-school carby or TBI engine, the air/fuel mix has to travel through the whole length of the manifold and head port and remain atomised. Turbulence and surface condition can condense the fuel, so port finish is quite critical.

Now increasing flow velocity is a very different thing, because a port or manifold has two points of restriction - the throttle body, and the valve. the speed that the air moves between the two restrictions is a function of the volume of the port/manifold, and the frictional loss of the shape and surface condition of the walls of the port.

Think about what's happening when an engine is running. The air is being drawn into the cylinder by the downward stroke of the piston, past the valve. the highest velocity will be past the point of biggest restriction - generally the valve. Making a port bigger upstream of the valve slows the air in that chamber. It MUST move slower, it CAN'T can't move faster - physics doesn't allow that. If 500cc of air/fuel is being moved into the cylinder and the volume of the port and manifold has increased from, say, 1l to 1.25l, the air will move through the port 25% slower.

Air doesn't move at all by itself, it requires a pressure differential to make it move. A big port full of air at atmospheric pressure can move more readily into a chamber at low pressure due to reduced frictional loss, sure, but that's assuming a static state. An engine isn't static - it's a tuned system, and when you adjust one thing, you're affecting the tune.

Lets assume a stock throttle body, stock compression ratio and ported head and manifold., and a a stock rev limit.

A big, smooth port reduces frictional loss and slows the air entering the engine so the effect of the curves and compromises of the port are reduced. At 6500rpm, the air will flow more freely - there's less frictional loss, so the air has a more lamellar flow up to the back of the valve, so some horsepower is gained- a little more air is entering the cylinder. (and we're talking around 3-5% based on the figures I've seen on comparisons of inlet manifold surface) but at lower RPM, what is that big port doing? Nothing. There's plenty of time for air to be pulled into the cylinder and, in fact, the engine would benefit from SMALLER valves and SMALLER ports to maximise gas velocity to try and slingshot some air into the cylinder, because everything is moving so slowly.

it's why 454 big blocks used in chevy pickups had tiny valves and made only 210hp - they were built for torque. torque is about gas velocity for cylinder filling at low rpm. Trying to have a wide powerband is very difficult because an engine designed for high RPM power can't work effectively at low RPM to generate torque. It's also why forced induction is common - it's less about horsepower than filling the cylinder well at low RPM.

Here's an example of that - A LSA V8 makes 551 LB-FT of torque with a supercharger, That same engine architecture can make tslightly more torque from 7.4 litres of capacity, naturally aspirated (586 lb-FT) but needs another 1300rpm to do it (3800rpm vs 5100 rpm) the torque curve is much steeper and the torque peak is much higher in the RPM range- that makes the engine less drivable in a heavy sedan, less fuel efficient because it has to rev harder, and more irritating to drive (more gear changes to maintain road speed) - see two different approaches to HP and torque.

So what does all that mean for an otherwise stock J20? You're going to see tiny, likely undetectable increases in power high in the RPM range by porting your head and manifold, regardless of how you do it. If you could increase the RPM limit, you might see bigger increases, but that will require additional work and potentially highlight other engine weaknesses (rods, valve springs etc) and likely require reprofiled camshafts etc to move the powerband higher in the RPM range.

Those aren't things that make a 4WD a better 4WD, or a J20 a nicer engine in a (relatively) heavy vehicle.

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Post Posted: Thu Jul 18, 2019 10:08 pm 
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The other item I've seen mentioned is that inlet runners, or any runner /put can be extruded to a measured amount in order to get equal flow in each runner. I imagine these would mostly provide efficiency improvements as each cylinder is working closer to the same conditions as the next.

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Post Posted: Fri Jul 19, 2019 7:34 am 
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Anyone can do this with a pipette or similar and a die grinder. Measure volume of port in CC and grind until all ports match. The problem with extrude honing as a means of removing material is it is removed indiscriminately. Whilst 20 years ago it was revolutionary and may still have some application for stock manifolds, CNC porting has rendered it pretty obsolete.

As far as I can see it’s also expensive, not as expensive as CNC but far more expensive than conventional hand porting.

My recollection is that people got very excited about extrude honing in order to increase flow in the L98 SBC manifolds which have tiny runners for torque but won’t let the engine rev over 5.5k. The shape of the runners also prevented hand porting and the overall shape of the L98 manifold was critical to fit under the bonnet of a C4 corvette. That’s a relevant example for the worth of extrude honing. Trying to turn a stock manifold intended for a low revving stock engine into something that can support horsepower at high rpm or with increased capacity (like a 383)

Is that what you’re trying to do? It seems like you’re just trying to optimise what’s there with stock capacity and power band. If so, just have the head rebuilder clean up the ports with a die grinder like the old days. My guess is they don’t need much. Most Japanese alloy castings are pretty good.

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