Vacuum Adjustable TBI Fuel Pressure Regulator

[dave w]

I've finished the CAD for a Vacuum Adjustable TBI Fuel Pressure Regulator. The simple design is low cost to CNC Machine. The downside to this design is having to use spacer washers to adjust the maximum fuel pressure. At low KPA's the fuel pressure is reduced to allow for better idle and reduced emissions. At high KPA's the fuel pressure is at maximum pressure to provide optimum performance.

dave w

[EagleMark]

Does it really add cost to design to add the vacuum nipple in the side so it could be adjustable too?

There was a guy here awile back who dialed in fuel pressure on a TBI on bench with air pressure. Pretty sweet setup he made from parts laying around. Was same pressure in truck with fuel when he was done.

Do a group buy! I'll take a couple.

[dave w]

Does it really add cost to design to add the vacuum nipple in the side so it could be adjustable too?

There was a guy here awile back who dialed in fuel pressure on a TBI on bench with air pressure. Pretty sweet setup he made from parts laying around. Was same pressure in truck with fuel when he was done.

Do a group buy! I'll take a couple.

I don't have any exact quotes, so I'm thinking, yes adding a nipple on the side adds cost. On the topic of having a side nipple for a vacuum port, I've asked a few potential customers what they thought. Some commented that having a vacuum hose routed inside the air cleaner housing was something they did not want. Most liked the idea of a spacer washers to set the maximum fuel pressure. Most potential customers commented that they usually had the remove the regulator body to make adjustments. Most potential customers said it was very difficult to access the set screws for the aftermarket pressure regulators they had experience with.

As a guess, the machining cost of the housing is about $65 each depending on quantity. Add a spring, some spacer washers and pressure regulator, it's likely to be in the $100 ~ $120 range, depending on quantity. The only CNC machine shop I've talked to said 15 parts is about the minimum number they would be willing to make. I'm thinking that CNC machine shop won't do much of anything until they see about $1000 come their way. I asked about having 20 made for the $1000, ~ not happening.

I like the idea of using compressed air to preset the pressures!

dave w

[EagleMark]

That's fairly pricey...

It is a PIA to adjust on the truck, that's for sure. I usually leave injector pod off but fuel lines on, and adjust. Then bolt the injector pod on.

The same guy who did the air adjust on bench had made a cable attached to adjustable regulater with a knob on end. Of course he was a machinist.

[PJG1173]

what would the pressure operating range be?

[dave w]

It depends on the spring used. My testing had a 15 PSI spring range from 12 PSI at idle to 15 PSI at high RPM's. My testing had a 22 PSI spring at 18 PSI at idle and 22 PSI at high RPM. My testing had a 30 PSI spring at 25 PSI at idle and 30 PSI at high RPM's.

dave w

[gregs78cam]

Dave, I have a guy locally that does some stuff for me periodically and he gives me pretty good prices, if you want to send me the .dxf/.dwg I can forward it to him and see if he can do better.

If I had a rotary table I could probably make these pretty easily on the mill. Or depending on the overall dimensions, I could probably turn the round parts in the lathe.

[PJG1173]

a while back I read some threads about VFPR's on TGO and the consensus seemed to be that you needed a pcm (ebl or megasquirt) that supported it since the injectors are not exposed to manifold vacuum.

[dave w]

a while back I read some threads about VFPR's on TGO and the consensus seemed to be that you needed a pcm (ebl or megasquirt) that supported it since the injectors are not exposed to manifold vacuum.

The question, as I see it, is not about manifold vacuum, or computer. The question is, can the TBI VE table be adjusted with a fixed fuel pressure? I've seen dozens and dozens of TBI data logs where the High RPM's / High Kpa's are lean, and the Low RPM's / Low Kpa's are rich. In most stock or nearly stock TBI applications, adjusting the VE table will correct for Rich / Lean conditions described above. In TBI applications nearing or exceeding 1 HP / Cubic Inch or more of performance, the VE Table alone is not able to correct for Rich / Lean described above. Adding a few more PSI of fuel pressure at High RPM's / High Kpa's will correct for the Lean Condition described above, and provide enough fuel at Low RPM's / Low Kpa's without running rich.

The basic theory of operation of using the engine main vaccum to adjust fuel pressure is simple. At Idle, the engine main vacuum is about 30 Kpa ~ 40 Kpa, which pulls against the pressure regulator spring lowering fuel puressure. At near WOT (high RPM / High Kpa) the main vacuum pull against the pressure regulator spring is less allowing the fuel pressure to increase.

It's basic physics, a TBI injector with 15 PSI of fuel pressure will flow more lbs / hr of fuel than the same TBI injector with 12 PSI of fuel pressure. Mechanically adjustable TBI fuel pressure regulators are adjusted for a desired fuel pressure, and that's it.

I'll be the first to admit, tuning a TBI engine with a Vacuum Adjustable Fuel Pressure Regulator is not going to be easy. Most TBI engines do not produce 1 HP / Cubic Inch, so most TBI engine owners won't need a Vacuum Adjustable Fuel Pressure Regulator. Most TBI engine owners what simple, easy, and low cost solutions for their tuning challenges. There is nothing simple, easy or low cost when using TBI for a performance engine nearing or exceeding 1 HP / Cubic inch.

dave w

[dave w]

Dave, I have a guy locally that does some stuff for me periodically and he gives me pretty good prices, if you want to send me the .dxf/.dwg I can forward it to him and see if he can do better.

If I had a rotary table I could probably make these pretty easily on the mill. Or depending on the overall dimensions, I could probably turn the round parts in the lathe.

Thanks for the offer. The pic below is not the final design or exact measurements, but should be enough to get a ballpark quote? I'm reluctant to post information that could be a potential product.

dave w

[RobertISaar]

i wouldn't think tuning around it would be difficult... i mean, you'll just scale the VE table based on MAP columns.

assuming you had it roughed in with a non-vacuum referenced at 15PSI at WOT(100kPa) and now with the vacuum unit you see 12PSI at 40kPa and still 15 at WOT, injector flowrate drops 11% due to the pressure drop. so take the 40kPa column, multiply it by .89, and recalculate for every other MAP column. should be PRETTY close to what it was with a 15PSI static regulator.

http://witchhunter.com/injectorcalc1.php

the bottom calc works great for this.

[dave w]

i wouldn't think tuning around it would be difficult... i mean, you'll just scale the VE table based on MAP columns.

assuming you had it roughed in with a non-vacuum referenced at 15PSI at WOT(100kPa) and now with the vacuum unit you see 12PSI at 40kPa and still 15 at WOT, injector flowrate drops 11% due to the pressure drop. so take the 40kPa column, multiply it by .89, and recalculate for every other MAP column. should be PRETTY close to what it was with a 15PSI static regulator.

http://witchhunter.com/injectorcalc1.php

the bottom calc works great for this.

The challenge I see with some performance applications, is keeping the VE values between the low 40's and mid 90's. When VE values get into the 30's the actual AFR / BLM is not very responisve.

dave w

[EagleMark]

The issue is getting enough fuel for a built engine... then not being able to get low enough fuel for idle... hence the vacuum regulator.

Tuning vacuum regulator is a pain exagerated by the low resolution VE table, I've found with high pressure and the need for vacuum regulator it is best to use both VE1 and VE2 and find where the vacuum regulator is opening.

The real solution would be fuel pressure to lower at idle, or raise at WOT. Not be varible. TPS switch somewhere in the mix...

[dave w]

The issue is getting enough fuel for a built engine... then not being able to get low enough fuel for idle... hence the vacuum regulator.

Tuning vacuum regulator is a pain exagerated by the low resolution VE table, I've found with high pressure and the need for vacuum regulator it is best to use both VE1 and VE2 and find where the vacuum regulator is opening.

The real solution would be fuel pressure to lower at idle, or raise at WOT. Not be varible. TPS switch somewhere in the mix...

I think, based on my testing the vacuum changes the fuel pressure in a linear fashion. I'm thinking linear increase would provide consistent data to tune with?

How about two external fuel pressure regulators, one for low pressure and one for high pressure? Maybe a vacuum sensor switch to select high or low pressure?

dave w

[EagleMark]

If it was linear it would be easier to deal with. What I have seen is POOF, open! With data at every 1.2 seconds I could never find linear, but then I never used one with less then 22 psi on a motor that idles at 50MAP.

Maybe some bench testing on spring to know what your dealing with?

Two regulators, vacuum switches, TPS switches, two fuel pumps, relays... it all get's costly and complicated. As a aftermarket item I'm not sure it would be a good investment. As a one off if you needed one.... cost is not a factor. I've always thought the TPS switch was the answer, they are readily available for nitrous, could be used as idle on or WOT on. I like WOT on, this is where you need the extra fuel and then tuning the rest of VE table would be simple.

Get Robert to find an pin on ECM and write some code and there's the on/off too!