Rodram2002

got ya. Rip it da man.

CL2000

Hey Mike! I would have your buddies injectors checked out. I knew some one who had the same prob, ended up beeing a bad injector.It ran fine but it was stuck or something not sure..... Never hurts to have them checked since the head gaskets are so fun!

paulb

There are a lot of variables when we start pushing the limit, when it comes to performance of the cummins. A standard rule of thumb is that it doesn't matter when we start the combustion, but we want it to go out between 7 and 13 degrees after TDC. Where in the range of 7 to 13 degrees depends on mainly the rod length to stroke ratio. There are actually quite a few things that will change the speed of combustion. Just to name a few...

The amount of "free" air in the combustion chamber. What I mean is, that the more air that is swirling in the combustion chamber, the easier it is for unburnt fuel to find air to burn.

The size of the injector. Even without changing when the injector starts spraying fuel, how fast it sprays will change the rate of combustion.

RPM. This one can be tricky. At first glance it would appear that if it take a fixed amount of time to burn the fuel, then you would have to start it earlier as rpm increases. This usually only is correct up to a certain rpm. At higher rpms, you will actually gain some hp by retarding the timing a little. There are actually many reasons for this, but one is that the swirling air is moving much faster as the piston speed increases. This speeds up the rate of combustion.

As far as timing itself, the VP44 and the ECM controls the dynamic advance. One of the little gotcha's is that when the ECM detects that more fuel is being injected then what it things should be, it cuts back on how much fuel it wants injected. Part of the stock ECM programing is that anytime it cuts back on fuel, it also advances the timing. The only reason I can think for it to do this is emissions related, but there maybe other reasons. Now we add big injectors, and a fueling box and the ECM is almost always cutting back fuel, or at least trying to. What happens is that the more fuel we add, the more timing the ECM supplies. So on a 400-500hp truck the additional timing that comes from both a Edge Comp and a HR pump, might not be a problem. On a 500-700hp truck it sure can be.

I didn't take it as bashing the HRVP44, and I hope nobody else does either. I don't know if all the HRVP44 are created equal, but I sure like the one I have! I also hope that people realize that to much advance can come from other things then just a HRVP44.

Sorry for the long post...
Paul

BigBlue

Do you think that maybe the 85lbs of drive pressure at 60psi of boost has something to do with it? That seems like an awful lot of drive pressure to me.

Mike D

The dp number was not a concern, Blue. From what I understand some are pushing 100 lbs of boost with close to 200 lbs of drive pressure.

Mike D

Paul, I remember reading some of what you said about timing advance when researching the TST. I think I read some where around 9 degrees advance just from the addition of the fueling box.

BigBlue

Maybe but the guys at 100psi also probably don't care to much if their headgasket goes because they are only doing it for a short period of time and not driving the truck daily.

sleds

Not to change the topic, but it's strange to me that I've ran both the drag comp and the tst/smarty stack and no matter what settings I tried on the latter, it would not dyno within 70hp of the drag comp! I've talked to other people that say the same and yet other say the complete opposite. On my truck the tst/smarty was unreal for bottom to midrange fueling, but, the drag comp kicked it's butt for all out hp. Oh, the comp would do everything the stack would do, it just took more throttle down low. JMO

BigBlue

What injection pump are you running? Hot rod or SO?

paulb

I don't know what the amount is. I would sure like to be able to adjust it. It would be very cool to be able to set a timing curve, that also takes into account what the fueling box is doing.

sleds

SO. The hot rod pump only seems to help bottom end or mildly bombed trucks. Guys that are pushing alot of fuel with a wire tap, don't gain much over S.O pump.

Tiger Rag

Mike's buddy and also my buddy (same guy) has been suspect of the drive pressures....so was I. Some of the folks I've talked to about the VP44 have said you can only add so much timing with the VP. I'm interested as to where this topic goes as I've got twins and a HRVP. I personally think the drive pressures are playing a part but the guy our buddy learned this from knows his stuff. Maybe it is a little bit of both factoring in.

signature600

85psi drive at 65psi boost is high...but I wouldn't think high enough to blow headgaskets by itself. Something else is helping it.

Does he have a cam/headwork? It shouldn't matter, I'm just curious

What would cause the HR VP44 to advance timing more than either of the other pumps?? I would assume it's some mechanical aspect of how they take the head and rotors of two different pumps and mate them together, because the VP electronics limit the max total advance from what I know...but I've never messed with a HR VP44 at all!

Chris

Tiger Rag

He has a studded and o-ringed head. It has blown once (slight water leak on front of block....pushed a little silicone out) since he oringed. He checked the plumbness of both head and block this last time. The head has been on less than a couple thousand miles I'd guess and is a high performance head. Head gasket is a 20 over.

HRVP to my understanding is a combination of the HO and SO pumps. Larger plungers of the SO pump that can push more fuel and the high pressure and electronics of the HO pump. I may have a generic understanding and hopefully someone can give more detail, but this should be a close description.

Mike D

He has had some head work done Chris. Not sure about the timing aspect of the HR pump, but their was some mention of the pump key...