I blew the head gasket on my 96 and was wanting to get the porting done while the head was taken apart I was thinking of using pure diesel power to do the job they seemed helpfull and Knowledgeable. but I was wondering what kind of workmanship they do Im not trying to stir any thing up but Im trying to do some reserce before rather than after I get the job done I dont what to do two heads. thanks for any advice.

 

Y'know, I've never seen anyone demonstrate measureable improvements in porting a head. Most times, people do a lot of mods at the same time, so they can't report back how much EGT they dropped (if any), or any HP improvements they've seen.

I'm not saying there aren't any to be had, but I just wish be could get some feedback on the performance per dollar.

I personally think a cam might be more worthwhile than a head port, but that's just gut feeling, not based on anything measureable or scientific.

Please let us know what results you see from the porting.

Justin

 

idacal, I agree, do it if you have the means while the head is off. That way you won't wish you had done it down the road.

Hohn, you're right, it is hard to gauge because when you do head work, you are generally doing a lot of different things. What I can attest is that the head work on my 24v seemed to pull all of my mods together. I had a cam prior and it seemed to help some but after the porting/head work the egt's were down but mostly boost was down due to the improved air flow. With twins and concerned with boost and drive pressures, I was happy to see boost lowered. I suspect that drive pressures were relaxed as well although I haven't measured them. I have everything I need to do it, just haven't gotten around to it yet...too many other projects with priority.

edit: I should add that I think the F1 valve springs had a lot to do with pulling the mods together. I think that really helped to take advantage of the F1 cam's benefits.

 

I was wanting to install a cam while I was at it so I will not know what improved it. I would think though if you opened the ports up a person could flow more air at the same psi so I could use the stock turbo longer. I know its not the most effetive way to make power for the dollars but it should help with lag problems or am I totally off base here.

 

Pure diesel power was trying to put a test together, running the truck before and after the porting. But time restaints and other projects put that on the back burner. But one person that bought the head put it on as the only mod, and the reports back were great, noticable inprovement. Loking forward to getting my head ported buy them.

 

I bought my truck pretty much completed, but had the head off last winter anyway. The ports were cleaned up in some key areas already, but the runners and the plenum were not touched at all. I knocked out that large obstruction just after the air elbow where the fuel filters used to screw in, and I chopped off what I could of the big bump by the #1 cylinder, and I cleaned up all the bowls a little more.

I've been wondering what kind of flow would result if you changed the 12 port into more of a traditional gasser port - get rid of that swirl hump in the intake port . . .

I was able to get 69lbs. of boost before (measured at the plenum) where the most I've seen since is 63. - I also have my drive pressure gauge hooked up now and I get anxious when I get above 1:1, so I may be able to get more boost than what I've seen.

I don't have any numbers beyond that. Can't say one way or another as it was about a month after taking it apart that I finally got it back together.


Take care,
Phill

 

I should hopefully have my head on my truck this coming Monday and I did the port work myself to what I would consider "Stage Two-ish" lol. The only mods I will be doing is the "Stage two-ish" Porting, gasket matching all the exhaust stuff, larger exhaust valves, 3-angle grind, HD valve springs, and Firerings. After the gasket is setup and I do a 2nd retorque I am going to bump the timing to 20*.

 

And also anything PureDieselPower has to offer is going to be top notch quality, If it was not for being poor right now I would have just bought a new head with all that stuff done to it from them...

 

These are low-rpm engines that breathe pressurized air a lot of the time. I wouldn't be sweating things like little port humps and such.

Port velocity matters a lot more than absolute flow-- even with a pressurized intake.

JMO

 

What size exhaust valve did you go with, where did you get them from? who did you have do the work for you? I might have to try that.

 

Im trying to figure out what larger valves do. does a person machine the seats out or how do they actually improve airflow Im not understanding what the point of them are because the hole in the head is the same size.

 

Hohn in your post you mentioned port velocity over absolute flow and trying to understand what you mean whats the difference? Im trying to learn as much as I can about these engines sorry for the dumb questions but I will never learn if I dont ask.

 

Port velocity is simply how fast the air is flowing through the port. The larger the port is, the slower a given quantity of air will move through it in a given amount of time.

I can't tell you how many times I've seen a backyard port job on a cylinder where someone just carves out the ports as big as they think they can get them, smooth the protrustions down and slap the head back on the car only to find that they lost a TON of low end power and just barely broke even up top.

Port flow is measure in cubic feet per minute at some standardized pressure drop-- usually 28" water (iirc). This means that the air pressure on one side of the port is 28" of water column (about 1.01 psi) higher than the other. Obviously, the intake side is the higher pressure.

So, you set the flowbench to generate this pressure differential and the flow is measured.

Now, it SEEMS reasonable that you'd want the biggest ports possible to make it easier to draw the air into the engine (high flow, low velocity). This would be valid if air behaved perfectly and we could close the valve right at Bottom Dead Center and capture 100% of the cylinder's volume worth of air.

But air doesn't work that way. It has mass. Which means it has to accelerate up to speed, and also decelerate as it stops. Thus, we can actually "catch" more air in the cylinder if we close the valve AFTER the piston is already heading back up. How far after depends on the intake velocity (which depends a lot on RPM as well). The faster the air is flowing in, the later we can close the valve and the more air we "catch" because as the air is slowing down it is compressing. The trick is so catch as much as you can without pushing the air back out of the intake port-- which is what happens when you leave the valve open too long.

Smaller, higher velocity ports help things in two ways. First, because they don't flow as well forward, they don't flow as well backward, either. This means you can leave the valve open a little longer than with bigger ports. Secondly, the air coming into the engine travels faster through a smaller port and the potential to cram more air into the cylinder is greater with the higher velocity.

This is why big camshaft make power at high RPM and lose power at low rpm. At high rpm, the velocity is high and there's actually a mild supercharging effect going on. But at low rpm, the velocity is too low and the intake charge is pushed back out the port it came in-- a condition aptly called "reversion."

The difference between them is a term called "volumetric efficiency", which is just a fancy way of saying how much air we ACTUALLY got into the cylinder compared to the displacement of it. Let's say we have a 1L cylinder, but our configuration only gets .8L of air into that cylinder before the valve closes. In this case, our efficiency is 80% because we got 80% of that that cylinder's volume worth of air actually into it.

But at higher RPM, the velocity creates a slight supercharging effect, and with proper configuration, we can actually capture more than the cylinder's volume. A NASCAR engine, for example, will have a VE of up to 115% at certain point in the RPM range, capturing an additional 15% of the cylinder's volume of air just by optimizing cam timing and port sizes and so forth.


My personal rule of thumb is that if the port area is 80% of the valve area for 4V engine and 65% of the valve area for 2V engines, then the ports are big enough. This will get you right near the range where the incoming flow is almost supersonic on its way into the cylinder. Ideally, you'd hit Mach 1 briefly, and even though you can't acclerate the air faster than Mach 1 using suction, you CAN see power increases even with that "saturated flow." This, because the flow through the ports is not steady, but rather it acclerates up to speed and slows back down as the valves open and close. So if there's a point where you "max out" the flow when the valve is wide open, then that's OK-- you're more than compensating for that limitation when the valve is less than fully open and the flow isn't sonically limited.

All this is much more relevant to a higher-rpm gasser that relies heavily on the valve overlap event and the exiting exhaust "sucking" the intake air into the engine. But it's still relevant even to our turbodiesels that have little to no overlap in the cam timing, as you still want high port velocity to charge that cylinder as full as you can get it.

Sorry for all the hundred dollar words, but using the proper terminology saves confusion down the road.

Justin

 

Thanks Hohn it took me reading it three times but I think I understand what you are saying. I was thinking about putting the head on the mill and getting start hogging away but after reading what you wrote I dont think thats such a good idea thanks for explaining it

 

Could more velocity be created by smoothing out the ports and runners rather than enlarging them? There would be a small amount material being removed but it would be minimal, as apposed to hogging them out.