6.7l with VGT

[Hamilton Cams]

We have been contacted by a lot of owners with 6.7l engines wondering what they can do to help with blown head gaskets on their 6.7l.

Doing a cam will allow a higher volume of air through the engine at most rpm. in other words a cam will push your turbo harder and since the VGT charger has a hard time keeping drive pressures in check with mild tuning a cam will increase this effect. All that said, the best cam for the 6.7l is the 188-208. With the smaller exhaust profile, it will add the least amount of drive pressure of any of our cams.

The most important thing that cam be done though to help the situation if you plan to keep the VGT charger is heavier valve springs. One major cause to higher cylinder pressures is reversion. With drive pressures exceeding 80psi in many instances, the exhaust valve is blown off the seat when the piston is on it's downward travel on the intake stroke. When only fresh air through the intake is supposed to be entering the cylinder, the exhaust valve, in this instance will allow a large volume of spent gasses to enter the cylinder. This creates a situation when there is very high cylinder pressures which aid greatly compromising the head gasket.

In my personal 6.7l we ran about 900hp with no fire-rings on the stock gasket with only studs. Valve springs, WILL help you keep that gasket in check if you plan on keeping the VGT charger.

Zach

 

[rjp]

Hey Zach,

Hope you dont mind that i reposted this in another forum? I could see where it may help some of the 6.7 guys that are having the dreaded head gasket issue. Take care and see ya in October.

 

[Hamilton Cams]

A little excerpt from CF about how the 188-208 produces less drive pressure etc.

First of all, "drop In" cams as it pertains to our cams means that if you took a truck off of the lot and did nothing to it, but verify the cam was on the correct intake centerline, there would be no issues with installation or piston to valve clearance. That being said, I have seen people use stock cams and old 181-210's and run their valves into the pistons. One of the things to look for when looking for a "drop in cam" is looking a the total open time for the intake and exhaust lobes. With a .010" intake lash on a 24v with an intake rocker ratio of 1.38:1 the 188@ .050" intake is open 228 degrees , the stock cam at about 159 degrees @.050is open 232 degrees and the old 181 is around 250 degrees if memory serves me correct. The longer a valve is open, the greater the chance that it will interfere with the piston. This is how we are able to fit a bigger intake with out issues.

Troubles arise when people start to have the head or block machined, new valve seats; or valves, Aftermarket head-gaskets Non- OE piston protrusion and cam gears other than the one the truck came with.

It is sad, but most people do not measure anything in their engine. When you start to machine things and swap parts Clearances DO CHANGE! This is why it is hard to be in the cam business. If someone uses a cam gear from a 1989 Cummins 12v in their 1997, the intake center-line is most likely off, and I will get cursed because I implied that it is a drop in cam. ARGGGHHH!

As far as performance and fuel economy, regardless of the cam you are looking at, you always want to look at the ratio of total open time to duration at .250". On the intake, the fewer degrees the valve is open, it will be open fewer degrees after bottom dead center on the intake stroke. If the valve is open too long after BDC on the intake stroke, it will push a lot of air that was in the cylinder, back into the intake runner. This is why big duration cams do not make power at lower rpm and why they have little to no vacuum(in gas engines)

If they have a short open time they will trap a lot of air at low rpm and spool up very nicely. The magic with the 188 intake is that it is open less time than the 181 or the stock intake, but at higher lift such as .250" it is open around 20degrees or more depending on the cam you are comparing it to. Since it is open longer at the higher lift points, it will also flow more air at the higher rpm which will allow it to have more high-rpm power in addition to more Low-rpm power.

When looking at different exhaust profiles total open time is very important as well. AS soon as the exhaust valve opens, the pressure is released from the cylinder and for the most part stops pushing on the piston. This is the end of the powerstroke. When a cam has less total open time the pressure in the cylinder is allowed to act longer on the piston, in effect increasing efficiency or total work output without using any more fuel. The problem arises if the exhaust is opened too late, then there is not enough time for all of the exhaust to exit, and the exhaust is restricted as it tries to leave the cylinder and the piston has to push very hard to get the exhaust out as opposed to allowing it to leave with little effort. This is called negative torque, which robs the engine of HP and in many cases cancels any gain from letting the pressure act on the piston longer with the smaller exhaust profile. Another negative effect of small exhaust profiles is that more heat is released in the cylinder which lowers the drive pressure since "cooler" air takes up less volume. The 220 exhaust is open a similar amount of time than the 210 exhaust, but it has MUCH more duration at higher lift. This means it kills the powerstroke at about the same time so efficiency is very similar, but it opens faster and higher which means the exhaust can exit the cylinder easier. Since there is less pumping losses or negative torque, you get FREE power.

The 208 exhaust differs considerably. It has a good bit less open time than either the 210 or the 220. This allows the pressure to act on the piston longer(free Power) at the expense of less drive pressure. Since more energy is released in the cylinder, there is less left over to spin the turbine wheel. The good thing is that when it does open, it opens much faster than the stock or the 181-210 cam, so that there is much less pumping losses and negative torque. Again, small amounts of free power.

The downside is that there is less energy to spin huge singles, so there is a greater emphasis on having a correctly sized turbo for your application. I would run the 188-208 with a 64mm or smaller turbo on a 5.9l or a 66mm or smaller with a 6.7l. People have driven 80mm singles with 6.7ls using the 188-220. This is a never ending battle. I see people run huge exhaust wheels and turbine housings to lower drive pressure, but lose most of their low rpm spool-up. In effect, the same higher rpm drive pressure could have been replicated, using the same compressor side, with a smaller turbine wheel and housing, but also a more conservative exhaust camshaft profile.

I will be more than happy to answer any questions about any cam you might have. We also offer a free service to have your cam analyzed on our cam measuring computer. That way you know exactly what you have.

 

[seeker1056]

whats the maximum lift before coil bind for either of your new 12v springs at 1.88 1.9 and 1.94

 

[crackerman]

Are these high cylinder pressure situations higher than the cylinder pressure created during the power stroke/ignition?

 

[Hamilton Cams]

whats the maximum lift before coil bind for either of your new 12v springs at 1.88 1.9 and 1.94

I apologize, but I am on the road and don't have those figures in front of me. I believe that the highest lift cam on a 1.880" is around .615-.625" which would put bind around 1.260" or so. It will require machining of the head. On my rail we ran .630" lift at the valve with a 1.900" installed height with 1.950"/1.850" valves and 6,000+rpm. I have gotten away with this many times but with larger valves and the additional weight, we have to go higher pressure than 170 on the seat and 440 on the nose. We are on the edge of what a single spring will handle when you go 6,000 with large valves.

If you are running over 5,000rpm and you are running more than .600"-.750", you need more spring pressure than our single can handle. The down side is that there is not enough room to fit a dual spring in the OE valve pocket. If you are pushing that much power, I guess machining spring pockets is small potatoes.

Crackerman,

I'm not sure what pressure you are speaking about. Please qualify your question.

 

[crackerman]

One major cause to higher cylinder pressures is reversion. With drive pressures exceeding 80psi in many instances, the exhaust valve is blown off the seat when the piston is on it's downward travel on the intake stroke. When only fresh air through the intake is supposed to be entering the cylinder, the exhaust valve, in this instance will allow a large volume of spent gasses to enter the cylinder. This creates a situation when there is very high cylinder pressures which aid greatly compromising the head gasket.

Zach

Maybe I misunderstood the above statement. I was thinking, how could just the exhaust pressure sneaking in through the valve, plus boost pressure, cause high cylinder pressures compared to the spike in pressure during combustion.

But basically you are stating above, that due to the leaking exhaust valve, the cylinder is effectively being "supercharged" from two directions, resulting in a higher volume of gases to compress then ignite, resulting in higher than normal, or even spiking pressures during combustion.

 

[Hamilton Cams]

I apologize for the misunderstanding.

Cylinder pressures are are higher than normal when aftermarket tuning is used. That is compounded by the fact that the 6.7l uses a larger bore. Pressure X surface area = force.
Adding a larger efficient cam with a large exhaust profile exaggerates this with higher total volume through the engine at the rpm the cam works.

These first two items allow higher total volume through the engine while the VGT is the bottleneck creating higher drive pressures. At a certain point the higher drive pressures open the exhaust valve and shove even more gases into the cylinder while the engine is trying to suck in fresh air. This can allow cylinder pressures that exceed the mechanical limits of the gasket. Higher seat pressure springs will lessen this effect, as will external wastegates that bypass the turbine housing. More power means more volume of air, at some point the VGT cannot handle the flow of a performance engine.