efi questions
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From: sasebeo, Japan
Car: 84 trans am
Engine: 350
Transmission: 700R4
efi questions
I have a few questions about efi.
I have a 10.5:1 383
with a hsr, afr street eliminator 195 heads
i was wondering what size of injectors should i get? and could i use the stock trottle body or go with a different one?
I have a 10.5:1 383
with a hsr, afr street eliminator 195 heads
i was wondering what size of injectors should i get? and could i use the stock trottle body or go with a different one?
Joined: Jul 1999
Posts: 12,100
Likes: 127
From: SALEM, NH
Car: '88 Formula
Engine: LC9
Transmission: 4L60E
Axle/Gears: 3.89 9"
Re: efi questions
stock TB should be fine for a naturally aspirated motor.
I know some guys say to keep the injectors under 80% duty cycle, but it's a made up number with no technical supporting argument. The injector does not know that it's open 90% of the time in a window of time vs 80% of the time.
-- Joe
Member
Joined: Oct 2005
Posts: 175
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From: CA
Car: 86 Corvette 383
Engine: 383 LPE build
Transmission: 4+3... T56 soon enough
Axle/Gears: 3.07
Re: efi questions
The 80% is based upon leaving you room to grow, no?
Sounds like a bit of sales BS if you already have 25lb/hr and they are telling you to go to 30lb/hr just to stay at 80% DC.
fwiw I'm making 380+rwhp with ls1 injectors (25lb/hr) at 43.5 psi.
Sounds like a bit of sales BS if you already have 25lb/hr and they are telling you to go to 30lb/hr just to stay at 80% DC.
fwiw I'm making 380+rwhp with ls1 injectors (25lb/hr) at 43.5 psi.
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Joined: Mar 2008
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From: Where winter comes from
Car: 85 vette
Engine: 427 SHP block,AFR 210,MiniRam
Transmission: TKO 600
Axle/Gears: 3:07
Re: efi questions
From rbracing-rsr.com
" Injectors will go static at about a 92% duty cycle and should be sized for an 80% duty cycle to leave some "wiggle" room.
" Injectors will go static at about a 92% duty cycle and should be sized for an 80% duty cycle to leave some "wiggle" room.
Beyond the 85% figure horsepower can actually drop as the pintles or discs in the injectors are essentially "floating", trying to open and close before they can complete each event. At this point the injectors actually put out less fuel until they are fully pulsed static and then the final output goes to 90% of their rated capacity. Injectors should not be pulsed beyond an 85% duty cycle.
Injectors are rated when shorted open @ 43.5 psi (3 Bar). The injectors will, when pulsed, only achieve 90% of this figure."
Some people size and run their injectors at 100% duty cycle at WOT with no problems.
Since an engine can only spend short periods at WOT there is no danger in running 100% duty cycle affecting the life span of the injector.
There is a real danger of having problems tuning at idle if way too big of injectors are bought.
Since I was buying new injectors anyways I sized my injectors for 70% duty cycle because I wanted to guaranty I would be able to run rich enough to make maximum horsepower and yet have a wide enough pulse width at idle. And also to leave me with a little wiggle room if I make some changes in the future to add a few extra horsepower.
Injectors are rated when shorted open @ 43.5 psi (3 Bar). The injectors will, when pulsed, only achieve 90% of this figure."
Some people size and run their injectors at 100% duty cycle at WOT with no problems.
Since an engine can only spend short periods at WOT there is no danger in running 100% duty cycle affecting the life span of the injector.
There is a real danger of having problems tuning at idle if way too big of injectors are bought.
Since I was buying new injectors anyways I sized my injectors for 70% duty cycle because I wanted to guaranty I would be able to run rich enough to make maximum horsepower and yet have a wide enough pulse width at idle. And also to leave me with a little wiggle room if I make some changes in the future to add a few extra horsepower.
Joined: Jul 1999
Posts: 12,100
Likes: 127
From: SALEM, NH
Car: '88 Formula
Engine: LC9
Transmission: 4L60E
Axle/Gears: 3.89 9"
Re: efi questions
At 3,000 RPM an injector open 10ms is at 50% duty cycle.
At 6,000 RPM an injector open 10ms is at 100% duty cycle.
Which number do you think is more important. The mystical and magical duty cycle, or the actual amount of time open?
People read stuff, repost, repost and so on until it's just what everyone accepts. Nobody really thinks about it.
While there is some merit to the belief that some poorly built injectors might become erratic being closed and reopened within a very small window of time, because it defeats the mechanical ability of the injector to fully close and reopen it seems to only hold true on really old OE type injectors.
Most cars with big cam that needs big injectors at high RPM, also run pig rich at idle because the minimum pulsewidth ability of most injectors. Coupled with a batch fire system, that loads all 8 cylinders with fuel every DRP..
-- Joe
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Joined: Oct 2005
Posts: 175
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From: CA
Car: 86 Corvette 383
Engine: 383 LPE build
Transmission: 4+3... T56 soon enough
Axle/Gears: 3.07
Re: efi questions
Yeah, good info. I might end up re-tuning mine at a higher pressure as it runs ~92% DC on the top end and I'd rather run the right fuel pressure for the injectors and hopefully get a little bit better spray. I completely agree though. 90% of the time smaller injectors are probably better.
Junior Member
Joined: Mar 2008
Posts: 73
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From: Where winter comes from
Car: 85 vette
Engine: 427 SHP block,AFR 210,MiniRam
Transmission: TKO 600
Axle/Gears: 3:07
Re: efi questions
As long as you are not near minimum pulse width at idle what advantages do smaller injectors offer?
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Joined: Jul 1999
Posts: 12,100
Likes: 127
From: SALEM, NH
Car: '88 Formula
Engine: LC9
Transmission: 4L60E
Axle/Gears: 3.89 9"
Re: efi questions
1) Keeping the injector reliable at idle, meaning pulsewidths that BOTH the ECM and the injector can deliver without error
2) Keeping the fuel pressure within a reasonable range. This is very important and often overlooked. I've had detailed conversations with Rich at Cruzin Performance about injectors on the flow bench with low pressures. I've always run between 50-55psi to keep the spray pattern good.
If the injectors are so big that you can't maintain both #1 and #2, you failed.
-- Joe
Joined: Sep 2003
Posts: 25,895
Likes: 429
From: Pittsburgh PA
Car: 89 Iroc-z
Engine: 555 BBC Turbo
Transmission: TH400
Axle/Gears: MWC 9” 3.00
Re: efi questions
I agree with the above but for what its worth, good tuning SHOULD beable to handle any size injector you want. I have a mild 401 motor with 80lb/hr injectors that idles just fine so far. Calculated duty cycle is like 40-50% I think it was. I left it large because I plan to push alot more out of it and wanted to be sure I had enough fuel.
Good quality injectors can handle low pulse widths and still deliver accurate fueling for motors that dont really need large injectors.
My 383 I went with 42 lb injectors but they were more than likely way overkill. I do not have any duty cycle data for that motor but I cant imagine it being that much since it was a 400whp motor. I just wanted to make sure I didnt sell myself short and left room for growth. Plus there was a big sale on these so I jumped on it.
Most 383's should be fine with 30-36 lb injectors. Even some high pressure 24's may work ok but keep duty cycle up.
Good quality injectors can handle low pulse widths and still deliver accurate fueling for motors that dont really need large injectors.
My 383 I went with 42 lb injectors but they were more than likely way overkill. I do not have any duty cycle data for that motor but I cant imagine it being that much since it was a 400whp motor. I just wanted to make sure I didnt sell myself short and left room for growth. Plus there was a big sale on these so I jumped on it.
Most 383's should be fine with 30-36 lb injectors. Even some high pressure 24's may work ok but keep duty cycle up.
Member
Joined: Oct 2005
Posts: 175
Likes: 0
From: CA
Car: 86 Corvette 383
Engine: 383 LPE build
Transmission: 4+3... T56 soon enough
Axle/Gears: 3.07
Re: efi questions
I agree with both of you, good to hear you are not having issues tuning in low DCs like that.
Joe, I'd like to hear more about this 50-55psi. I know LS injectors are designed for 58psi, is this the optimal psi for fuel spray with most of the injectors our cars use?
Joe, I'd like to hear more about this 50-55psi. I know LS injectors are designed for 58psi, is this the optimal psi for fuel spray with most of the injectors our cars use?
Joined: Jul 1999
Posts: 12,100
Likes: 127
From: SALEM, NH
Car: '88 Formula
Engine: LC9
Transmission: 4L60E
Axle/Gears: 3.89 9"
Re: efi questions
I agree with the above but for what its worth, good tuning SHOULD beable to handle any size injector you want. I have a mild 401 motor with 80lb/hr injectors that idles just fine so far. Calculated duty cycle is like 40-50% I think it was. I left it large because I plan to push alot more out of it and wanted to be sure I had enough fuel.
Good quality injectors can handle low pulse widths and still deliver accurate fueling for motors that dont really need large injectors.
My 383 I went with 42 lb injectors but they were more than likely way overkill. I do not have any duty cycle data for that motor but I cant imagine it being that much since it was a 400whp motor. I just wanted to make sure I didnt sell myself short and left room for growth. Plus there was a big sale on these so I jumped on it.
Most 383's should be fine with 30-36 lb injectors. Even some high pressure 24's may work ok but keep duty cycle up.
Good quality injectors can handle low pulse widths and still deliver accurate fueling for motors that dont really need large injectors.
My 383 I went with 42 lb injectors but they were more than likely way overkill. I do not have any duty cycle data for that motor but I cant imagine it being that much since it was a 400whp motor. I just wanted to make sure I didnt sell myself short and left room for growth. Plus there was a big sale on these so I jumped on it.
Most 383's should be fine with 30-36 lb injectors. Even some high pressure 24's may work ok but keep duty cycle up.
Keep in mind, some ecm's like the '730 have both injector drivers wired together inside the ECM, so each DRP both banks fire at once. On other ecms, like a '749 you can fire one bank or the other, therefore you can get away with twice the pulse width but delivering the same total fuel.
I.e,
both banks at once firing .5msec per drp over two DRPs.
vs
left firing 1msec on a drp, right firing 1msec on the next drp.
Total fuel on both banks is the same after 2 DRPs.
I've had issues with some brands being stable under 1msec pulse width. The injector simply can't mechanically close fast enough and you end up with a rich condition.
-- Joe
Joined: Jul 1999
Posts: 12,100
Likes: 127
From: SALEM, NH
Car: '88 Formula
Engine: LC9
Transmission: 4L60E
Axle/Gears: 3.89 9"
Re: efi questions
The problem is, when guys crank the fuel pressure way down to deal with idle richness. You get a dribble throughout the entire powerband. Now, on a batch fire TPI type car, most of the time the injector is fired against a closed valve so in theory the fuel should atomize when the valve cracks open, but I'm sure some puddling still exists.
For the visuals and specifics, you should call rich at cruzin performance. Great guy to talk to, and he's flow tested pretty much every injector on the planet at least once.
Keep in mind, I'm the kind of guy that likes to ensure I don't have a problem, rather than simply assuming I shouldn't have a problem. Check twice, make it right.
-- Joe
Junior Member
Joined: Mar 2008
Posts: 73
Likes: 0
From: Where winter comes from
Car: 85 vette
Engine: 427 SHP block,AFR 210,MiniRam
Transmission: TKO 600
Axle/Gears: 3:07
Re: efi questions
Is there not two parts to this discussion, ie initial sizing for a new build or deciding if your existing injectors are big enough?
Honda sizes their injectors for 100 % duty cycle at WOT at sea level.
As long as the engine is getting enough fuel at WOT in the best air you will encounter it does not matter if your DC is 50 %, 80% or 90 +%.
There is nothing to be gained from larger injectors than necessary, especially if you have to lower fuel pressure to compensate.
The 80 % rule seems to only apply in initial sizing so the majority of people get an acceptable injector size.
The mods most people make to their engines after the initial build are usually to increase HP so for the people that do they have a buffer to work with.
My 42 lb injectors at 50 psi are at 4.5 ms pulse width at an AFR of 13.4 - 13.6 at 900 rpm idle, at 50 degree air temp at 900 feet above sea level, batch fire. Mild 427 sbc build. I am not far enough along in the tuning process to know what my DC will be at WOT.
Honda sizes their injectors for 100 % duty cycle at WOT at sea level.
As long as the engine is getting enough fuel at WOT in the best air you will encounter it does not matter if your DC is 50 %, 80% or 90 +%.
There is nothing to be gained from larger injectors than necessary, especially if you have to lower fuel pressure to compensate.
The 80 % rule seems to only apply in initial sizing so the majority of people get an acceptable injector size.
The mods most people make to their engines after the initial build are usually to increase HP so for the people that do they have a buffer to work with.
My 42 lb injectors at 50 psi are at 4.5 ms pulse width at an AFR of 13.4 - 13.6 at 900 rpm idle, at 50 degree air temp at 900 feet above sea level, batch fire. Mild 427 sbc build. I am not far enough along in the tuning process to know what my DC will be at WOT.
Joined: Sep 2003
Posts: 25,895
Likes: 429
From: Pittsburgh PA
Car: 89 Iroc-z
Engine: 555 BBC Turbo
Transmission: TH400
Axle/Gears: MWC 9” 3.00
Re: efi questions
Idle in the 900 rpm range, its around 1.19-1.28 ms base pulse width with air fuel in the mid 14's. '730 ecm, code $59.
Peak at WOT on my last run was about 9.37-9.43 ms at a rich air fuel of mid 10's to 1 with aout 16 psi boost.
So 80's are abit big for 800-850 hp estimated, but they work ok so far. But then again its boost referenced so on 16 psi does that mean my base pressure of 45 psi becomes 61 psi and now the injectors are more like 96's?Idle is stable enough for me, i just need a better alternator as the voltage dies off alittle after running alittle while. I think its all the electronic things running off the main power feed. Need higher output alternator.
Just saying an injector thats too large shouldnt hurt too much. Its better to be slightly larger than smaller.
Peak at WOT on my last run was about 9.37-9.43 ms at a rich air fuel of mid 10's to 1 with aout 16 psi boost.
So 80's are abit big for 800-850 hp estimated, but they work ok so far. But then again its boost referenced so on 16 psi does that mean my base pressure of 45 psi becomes 61 psi and now the injectors are more like 96's?Idle is stable enough for me, i just need a better alternator as the voltage dies off alittle after running alittle while. I think its all the electronic things running off the main power feed. Need higher output alternator.
Just saying an injector thats too large shouldnt hurt too much. Its better to be slightly larger than smaller.
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