Question about OBS injection

Copracr

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On the early 90's u-hauls I used to work on they had a big mechanical fuel pump in the middle of the valley (used to call it a spider pump). Why aren't these as good as the cummins motors from the same era (talking reliability and power potential wise here)?

I'm not real familiar with either motor other than R+R parts someone else told me to do.
 

Bobby@cp

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It doesnt pump enough volume of fuel..... it barely keeps up with fueling when you add a chip.
 

backwoodsboy

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I'm assuming you're referring to the old IDI 7.3 engines?
For one there are In-Direct-Injected (IDI) so fuel delivery isn't as controllable.
Second there are naturally aspirated, or turbo-charged but in limited boost applications. Little/no boost=less viable air for combustion=less fuel can be delivered (or at least burned effectively).
Being In-Direct-Injected they also have a pre-chamber for combustion, which is restrictive and doesn't lend itself to performance applications.
Don't have a ton of knowledge on the IDI platform, but that it what comes to mind off the bat.
 

Copracr

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So low fuel pressure due to being port injection, not direct injection? and with that comes less control/efficiency?

Are the old cummins mechanical DI then?
 

backwoodsboy

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So low fuel pressure due to being port injection, not direct injection? and with that comes less control/efficiency?

Are the old cummins mechanical DI then?

Yes, 12v Cummins would be mechanical direct inject.
To put it into perspective, the 6.9/7.3IDI had an injector 'pop' pressure of roughly 1,900 psi. 12V Cummins have a 'pop' pressure of roughly 3,800 psi. 7.3PSD have a 'pop' pressure of 3,500 psi all the way up to 26,000 psi at the upper end of the spectrum dependent on tuning and other variables.
Greater injection pressure->greater atomization->more efficient burn.
Also have the issue of IDI's being naturally aspirated, so there is a limited quantity of air available regardless of what quantity of fuel can be injected.
 

Fordguy100

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That is pop pressure, not peak pressure. That said, DB pumps have fairly low peak injection pressures.

There are a couple of fella's who can tune a pump to put out more fuel. Up to 180cc's.
 

backwoodsboy

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Yeah, I realize there are more factors at play than 'pop' pressure, but I'd probably need a spread-sheet, couple pie charts, bar graph, box plot, and a histogram to put them all together.
Since the OP is a HEUI guy I figured I'd keep it as simple as possible a focus on one element.
 

Copracr

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So what prevents you guys from having more pressure? Couldn't a hi-po pump be made for it? then add a super turbo and show those 12v's what for?
 

rustbucket

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Not hardly, the 6.9/7.3 idi's also had lower compression and the heads didnt flow worth crap iirc. By the time performance street diesels really hit the scene the 7.3l di was already out so no one really sunk much money into R&D to get more out of the IDI motor. I put head gaskets on one that had a ATas bolt on turbo kit. Made 8psi of boost and i could still out run on foot haha
 

backwoodsboy

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Not hardly, the 6.9/7.3 idi's also had lower compression and the heads didnt flow worth crap iirc. By the time performance street diesels really hit the scene the 7.3l di was already out so no one really sunk much money into R&D to get more out of the IDI motor. I put head gaskets on one that had a ATas bolt on turbo kit. Made 8psi of boost and i could still out run on foot haha

IDI's actually had higher compression ratios than the DI 7.3. Something like 21.5:1 for an IDI vs 17.5:1 for a 7.3 Powerstroke. This made them less suitable for high boost applications needed to make power. Hypermax had a mean IDI back in the day, and I believe it was running reduced compression in the neighborhood of 15:1.
Same engine was also running an inline P-Pump, custom injectors, twin turbo's, and had some extensive head work done. Other than that, there isn't much info on the engine. There's some speculation that it eliminated the pre-chamber (that caused the stock heads to flow like chit) which in effect no longer made it an IDI.
 

rustbucket

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I stand corrected there was one IDI that could throwdown. The higher compression would explain the 8psi of boost that ATS kit made, prolly blow the HG's faster then it did.
 

Fordguy100

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I honestly wouldnt want to take an IDI and do anything more than a hot pump, turbo, cam, and thats about it. Nobody really knows how stout they are at that point or beyond.

VE cummins also have a hard time making big power, most swap to a P-pump. Guess rotary pumps have their downfalls.
 

OldschoolPSD

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The issue wasn't really the pump so much as it was the overall ****ty design of the injection system with the pre cups and all that garbage. I'm sure if you wanted you could get decent fuel out of that little stanadyne pump but that turd motor wouldnt know what to do with it...
 

Fordguy100

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Although it isnt the best design for making power, its not the end of the world. The IDI will never be a high HP engine without a ton of money thrown at them. Right now, if I was to invest $2500-3000 I would be able to lay down what the other two guys are (350hp and 750ft lbs). Thats not out of this world, but it would be neat.

The nice thing about low HP Diesel's, is the usually outlast almost everything out there.
 

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