Injector & fuel pump sizing formulas based on HP

[My91TalonTSi]

Today, while doing research on AFPR installs, I found a great thread about fuel injector and fuel pump sizing based on horsepower goals. Steve (a DSM Wiseman) posted some formulas to figure this out. His post is close to the bottom of the thread. Here is the link:

http://dsmtuners.com/forums/showthread.php?t=211590&highlight=installing+afpr

Based on his forumlas, using an 85% max IDC, a .65 BSFC, and my goal of 375hp I figured that I need a 752.76cc injector. So let's say 750cc's to make it easier.

As for the fuel pump, I calculated that I need one that flows 183.6LPH. This figure is based on me using 750cc injectors and an added 20% LPH as a saftey margin. Basically, it sounds like a Walbro 190 will fit my needs.

My questions are: are the forumlas posted by Steve accurate (no offense Steve)? And if so, are my calculations correct?

My starting goal of 375hp is just that, a starting goal. In the future, once my goal is met, I may try to push higher. What I would like to do is is build a set up that will max out a 190lph fuel pump because it will eliminate me having to purchase and tune an AFPR. I am a novice tuner and feel that adding an AFPR would be one more thing that could make the tuning process harder. Do this sound rational?

Thanks in advance for the help.

 

[diablos991]

You might as well get the 255 and AFPR. That way you won't have to worry about it not being enough, and you will have room to grow in the future.

 

[steve]

My questions are: are the forumlas posted by Steve accurate (no offense Steve)?

You'll have to ask RC Engineering where they got them from. I just applied them.

Steve

 

[My91TalonTSi]

You might as well get the 255 and AFPR. That way you won't have to worry about it not being enough, and you will have room to grow in the future.

I know that I will need them 'IF' I decide to grow. But I may not.

Again, I want to make sure my calculations and formulas posted are correct and that I am going about things right given my horsepower goal.

 

[My91TalonTSi]

You'll have to ask RC Engineering where they got them from. I just applied them.

Steve

Thanks for the responce. I will assume the fomulas are accurate.

 

[steel_3d]

I would be curious where you got the .65 BSFC from. How much fuel you need for a specific amount of power depends on a lot of things. One of the biggest factors I can think of is knock resitance. Are you running pump gas, or race gas? Water injection? Small or large intercooler? The more you have to rely on fuel for knock suppression, the more of it you'll need.

If I'm not mistaken, empirical data seems to suggest that you'll be fine with 750's for 375whp on pump gas. But if you were running c16 and/or improved the knock resistance of your engine in other ways, you'd be able to get away with less.

 

[Sbeing1]

your fine with 550cc on 375whp....but yea, if your planning bigger hp in the future, go with some 750ccs I suppose.

 

[My91TalonTSi]

I would be curious where you got the .65 BSFC from. How much fuel you need for a specific amount of power depends on a lot of things. One of the biggest factors I can think of is knock resitance. Are you running pump gas, or race gas? Water injection? Small or large intercooler? The more you have to rely on fuel for knock suppression, the more of it you'll need.

If I'm not mistaken, empirical data seems to suggest that you'll be fine with 750's for 375whp on pump gas. But if you were running c16 and/or improved the knock resistance of your engine in other ways, you'd be able to get away with less.

I got .65 BSFC from Steve's figures in the link I left. I think he chose that exact number because it was roughly in the middle of .60 and .68, which were the BSFC figures for a turbo application.

To answer your other questions, the car will mainly run on pump gas for the first couple of years. I do plan upgrading to a front mount and don't plan on any type of water/alky injection. If, in the future, I decide to take this car past the 375hp mark then maybe I will start considering c16 and/or alky injection.

Getting back to the fuel pump, according to the math I did a Walbro 190 should be ok. Have you, or anyone else that is reading this ran a similar set up and attained those HP numbers? I plan on rewiring the pump as well.

Thanks for the help. As for injectors, it looks like I'll be going for the 750's

 

[My91TalonTSi]

your fine with 550cc on 375whp....but yea, if your planning bigger hp in the future, go with some 750ccs I suppose.

I see you are putting down a little over 300whp on your 550's. What kind of IDC numbers are you seeing? Would the 255 fuel pump and AFPR effect these numbers when compared to a 190lph pump?

 

[tstkl]

and are they whp or bhp, ### that says 1000cc injectors are good for 500 hp, and I know plenty of people who have done more than 500 whp on less than 1000cc injectors.... so my guess is that they are either wrong, or the .65 figure used for those equations is off.

 

[My91TalonTSi]

and are they whp or bhp, ### that says 1000cc injectors are good for 500 hp, and I know plenty of people who have done more than 500 whp on less than 1000cc injectors.... so my guess is that they are either wrong, or the .65 figure used for those equations is off.

What does the ### mean?

Regarding the .65 figure, here is more information on that:

http://www.rceng.com/technical.htm

They are saying that turbo applications will use .60lbs of fuel per hour or more, hence the .65 used in my and Steve's equation.

I do see what you are talking about though. If you do the same math I did for a car with 500hp it show's you need 1000cc injectors.

 

[steve]

so my guess is that they are either wrong, or the .65 figure used for those equations is off.

The BSFC varies depending on the car. I picked a worst case number of what was recommended for typical turbo cars. Your free to pick a different factor.
How much HP per injector depends on how far your willing to push the IDC.
None of these number are wheel HP, I doubt RC has loss numbers for either DSM drivetrain.

Steve

 

[My91TalonTSi]

The BSFC varies depending on the car. I picked a worst case number of what was recommended for typical turbo cars. Your free to pick a different factor.
How much HP per injector depends on how far your willing to push the IDC.
None of these number are wheel HP, I doubt RC has loss numbers for either DSM drivetrain.

Steve

I like the idea of figuring the BSFC a bit higher, it leaves room for error or more power down the road. As for the IDC, I feel safe around 85% and that's what I used when I did the math.

I still would like to touch on the fuel pump. Am I safe running 750cc injectors at 85% IDC with a 190LPH pump? The math says yes, but I would like to have real world examples where people have done this.

Thanks for the responce.

Cheers.

 

[steve]

Keep in mind that the 190 lph pump doesn't flow 190 lph at 70psi at 12v.
You need to decide what your base fuel pressure will be and what you max boost pressure is that will give you a max rail pressure. Due to pressure drops in the system the pressure at the pump will be higher and therefore the flow rate lower. If you exceed the relief valve setting the flow rate will drop like a rock.

Since you didn't derate the injectors for the 1G lower base pressure we'll assume your running 43.5 psi base and 20 psi boost for a rail pressure of 63.5 psi at max boost.
The 190 lph pump according to RRE only delivers 145.11 lph at 58 psi on 12.5v and only 162.78 lph at 14v. The flow rates only drop from there as pressure rises. I think you need more pump.
http://www.roadraceengineering.com/fuelpumpflowrates.htm

Steve

 

[My91TalonTSi]

I think you may be right.

I was really hoping to avoid going with the 255 because of the need to get an AFPR with it. Like I said earlier, I'm a novice tuner and feel that it would be one more thing that could give me a problem.

Other then setting my base fuel pressure (36-38psi), is there any other adjustments I'll need to make on the regulator? After doing some searches, it looks like most people just set the base pressure and leave it be. The only instances of making adjustments were to compensate for a lack of injector, in that case they bumped up the pressure a bit.

Thanks for answering my questions Steve. You've been very helpful.

By the way, I see you rebuild/repair ECU's. I may need your services one day.

 

[steve]

IOther then setting my base fuel pressure (36-38psi), is there any other adjustments I'll need to make on the regulator? After doing some searches, it looks like most people just set the base pressure and leave it be. The only instances of making adjustments were to compensate for a lack of injector, in that case they bumped up the pressure a bit.

Most people are just going to set the regulator to the base pressure and forget it but you can increase the pressure to get a little more out of your injectors or drop the pressure if you too rich across the board.

Steve

 

[selmerguy]

You wont have any problems with the afpr. Very easy install and very easy to work with. Just turn the swrew tighter for my fp and vise versa. Too easy.

 

[deletrius]

I think you may be right.

I was really hoping to avoid going with the 255 because of the need to get an AFPR with it. Like I said earlier, I'm a novice tuner and feel that it would be one more thing that could give me a problem.

Other then setting my base fuel pressure (36-38psi), is there any other adjustments I'll need to make on the regulator? After doing some searches, it looks like most people just set the base pressure and leave it be. The only instances of making adjustments were to compensate for a lack of injector, in that case they bumped up the pressure a bit.

Thanks for answering my questions Steve. You've been very helpful.

By the way, I see you rebuild/repair ECU's. I may need your services one day.

Bah you dont need an AFPR with the 255, there are plenty of 500+whp DSMs in my area that are running the 255HP without an AFPR. I dont have one and the 255 doesnt cause me any troubles.

 

[larsrya8]

Bah you dont need an AFPR with the 255, there are plenty of 500+whp DSMs in my area that are running the 255HP without an AFPR. I dont have one and the 255 doesnt cause me any troubles.

The AFPR upgrade doesn't make the car go faster... it helps when you are not injecting a lot of fuel (like idle and light cruising around town). LTFT Lo will help a bit at idle, but you will end up fouling out spark plugs faster than usual if you daily drive your DSM. Not rewiring your 255 will help a bit too. I ran my car with the stock FPR for a week before it fouled out my BPR7ES plugs. That was the motivation for me to get the AFPR on the car.

I guess an AFPR falls under the "do it right" category. My 255HP and AFPR came as a combo... go figure.

 

[92awddsm]

Bah you dont need an AFPR with the 255, there are plenty of 500+whp DSMs in my area that are running the 255HP without an AFPR. I dont have one and the 255 doesnt cause me any troubles.

Why does your profile say that you have a 190? How many of those 500hp cars drive around daily in stop and go traffic?

I have run a 255 with and without an afpr and there is a difference. Idle and cruising with the stock fpr made for rich conditions and poor fuel mileage. When I swapped in the afpr, normal cruising went from being a pia to being enjoyable.