injector pop pressure vs timing advance

12vriviera

12vriviera

That Guy with that car
Joined
Mar 31, 2009
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Some of you may know, I am sticking with a vp44, but going to a 12v head this year.

I am wondering if anyone had done this research or know where I can find it?

VP pop pressure vs ppump pop pressure, what is timing advance going to lower pressure, or somewhere in between?
Also aside from the smoke, and longer burn time, what are other advantages, disadvantages to this that you can see?
If I had timing light and injector stand and dyno I would do this myself as best I could..

Thoughts?
 
I held off assuming everybody knew more then me on the subject, but one guy who has done that conversion is Zach Hamilton. His Fuel Miser used a vp44 with the 12 valve head on at least one of the two engines. It's a little different because he was messing with high compression on both engines, but he can offer a little insight.
 
I have the lines and head off that engine (or one that looks alot like it!).

What he was playing with is spray angle and depth of injector spray in the bowl.. not so much timing i don't think..
still have to ask him. But i am sure he kept pressures up to have a decent burn for mileage...
Also i am going bigger on injector than he did...

What i am looking for its how much more advance does a lower pop pressure equate to.
 
To compare apples to pears... When I was researching pop pressures for VP injectors on a P7100, I was told by one very knowledgeable individual, the higher pop pressure COULD equate to as much as 8-9 degrees of timing retard. Another told me 3 at most with non being most likely....This again being higher pop pressures. on a lower pressure pump if you will.

A well respected injector builder told me he sets all his mechanical injectors to about 285 bar (IIRC), 12 and 24 valves alike.

This may actually be oranges to lemons.
 
Here's my take on it:
Diesel is a non-compressible liquid, so if you stay under the speed of sound, you should be able to neglect the length of the lines for timing. You would have to worry about the wall roughness creating resistance, and the injector lines expanding more than the lengths (which those two really shouldn't account for much either).

Another thing is, I am 99% sure that you can have your 12V head injectors changed to VP pop pressures.

Since you aren't compressing the fluid (or better put, it won't compress), you are going to spike pressure almost instantly. It will probably delay your injection timing very very slightly the more you increase pop pressure.
 
I could have 12v injectors popped to vp pressures very easily.

No issues there, But with timing limitations of the VP, a lower pressure for pop, would in theory, Start injection sooner in the cycle, and allow for full draining of the Rotor(happens already), and allow a longer burn time to help keep fuel in for power stroke, vs lots of smoke and no power.

(in theory)
 
Just set to 300bar and be done. I played around with this and didn't see anything worth reporting. On a p pump I use 285bar as the minimum.
 
You do have a dyno a your disposal ;) You provide the timing light. On the injectors I was running on that engine, we had them set at 290 bar. IIRC VE injectors get set at around 245bar, p-pump at 260, VP-290-305. Since you are after max performance, check timing with light, run on the dyno, pull the injectors shim and repop then repeat. Should take less than 30 min. For posterities sake if nothing else.

About half way through the fuel miser build, my truck got broken into and wiring got trashed which is why brandon got the hook up. I have been too busy to track wiring issues. Thousands in damage and all they got was a $500 edge juice that they probably got $50 worth of meth for. Idiots missed the $2,000 EFR on the manifold and the $1,200 injectors on the floor board, probably would have gotten them another $20 or $30 worth of meth.
 
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CorneliusRox said:
Here's my take on it:
Diesel is a non-compressible liquid, so if you stay under the speed of sound, you should be able to neglect the length of the lines for timing. You would have to worry about the wall roughness creating resistance, and the injector lines expanding more than the lengths (which those two really shouldn't account for much either).

Another thing is, I am 99% sure that you can have your 12V head injectors changed to VP pop pressures.

Since you aren't compressing the fluid (or better put, it won't compress), you are going to spike pressure almost instantly. It will probably delay your injection timing very very slightly the more you increase pop pressure.
Click to expand...

Diesel fuel will compress roughly 10% at 200 bar. While thats on the high side of mechanical injection, its still a sizable figure.
 
CorneliusRox said:
Here's my take on it:
Diesel is a non-compressible liquid, so if you stay under the speed of sound, you should be able to neglect the length of the lines for timing. You would have to worry about the wall roughness creating resistance, and the injector lines expanding more than the lengths (which those two really shouldn't account for much either).

Another thing is, I am 99% sure that you can have your 12V head injectors changed to VP pop pressures.

Since you aren't compressing the fluid (or better put, it won't compress), you are going to spike pressure almost instantly. It will probably delay your injection timing very very slightly the more you increase pop pressure.
Click to expand...
Diesel is compressible because it always contains air. I did calculations of how much delay 50 cm injection pipes make but I don't remember anymore, it was something like 8 degrees at 2000 rpm. There's a formula for it in some technical book. There has been also some info about pop-pressure effect on timing, don't remember where though. Maybe this board ?
 
Thanks Leiffi.
Its subjective to pumps across the board it looks like..
And will have to be verified with a light, and then see if power gains are worth it.
 
I don't want to share too much because my testing was limited to (1) p-pumped 12valve truck, but on a fuel limited truck, meaning it cleaned up the smoke 95% of the way at full throttle full boost, I picked up power lowering the pop pressures. My conclusion was that the lower pop pressure was allowing more total fuel into the motor with the side affect of reduced throttle response in the mid-range.

It also seemed to me that pop pressure had more effect on the end of the injection event rather than the start of the injection event. Late "lazy" lower pressure fuel is great for added exhaust back pressure and higher EGT readings, it also helps to spool turbos, but it seems to hurt overall power production on engine/turbo setups that are sized on the small side as far as turbine size and exhaust housing A/R.
 
Tate said:
Diesel fuel will compress roughly 10% at 200 bar. While thats on the high side of mechanical injection, its still a sizable figure.
Click to expand...

That's pretty interesting. How did you get those measurements. 2900psi seems like a pretty high number to get accurate readings at. Not being a dick, genuinely interested.

Leiffi said:
Diesel is compressible because it always contains air. I did calculations of how much delay 50 cm injection pipes make but I don't remember anymore, it was something like 8 degrees at 2000 rpm. There's a formula for it in some technical book. There has been also some info about pop-pressure effect on timing, don't remember where though. Maybe this board ?
Click to expand...

I never knew that. You're saying that it gets air in it while going through pumps and what not, or it inherently has air as some kind of chemical property? This totally makes sense with a lot of things that have confused me over time with diesels though.
I've read through a couple ICE book and taken a class on it back in college days, but never came across that. If you've got the name of the book, you should PM me and let me know.

Thanks again or the info guys!
 
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