HIT 18 PSI TODAY!!!!

[dyoung14]

This is going off the "6.9 Liter Diesel Engine Update" series of pdf's that were scanned from a Ford publication printed in October 1985. I don't remember who scanned these in (might have been Travis or Heath?), but it seems like I'm the only one who saved a copy and remembers that this publication exists I'd take screen shots of the pages and post them as images or simply post the .pdf files, but I'm worried about getting in trouble over copyright issues.

According to this publication, the differences between the A engine and the B engine are in the piston, precombustion chamber, cylinder head, and exhaust valve. The compression ratio on the A engine is 20.7:1, which was upped to 21.5:1 on the B engine.

The pistons are different in the following ways:

• A engine piston height (top of the piston to center of the pin) is 1.9435 inches.
• B engine piston height is 1.9465 inches.
• A engine piston combustion chamber cavity is 4.5 cubic centimeters
• B engine piston combustion chamber cavity is 6.6 cubic centimeters

The combustion chamber cavity was altered to match the different precombustion chamber.

The precombustion chambers are different in the following ways:

• A engine precups have an overall volume of 21.89 cubic centimeters
• B engine precups have an overall volume of 17.43 cubic centimeters
• A engine precup throats are 0.270" wide
• B engine precup throats are 0.245" wide (which necessitated the design differences in the combustion cavity)
• A engine precup top opening (where it goes into the head) inside diameter's 1.240"
• B engine precup top opening inside diameter's 1.144" (outside diameter's the same)

If I'm reading this correctly, the heads were altered in that the precombustion chamber that's not part of the insert (i.e. where the glow plug and injector tip go in) was reduced in size, but it doesn't specify how much. The exhaust valve on the B engine was made slightly thinner (from 0.020" to 0.017") to clear the taller piston.

Hope this helps a bit...I really wish I could post the .pdf file

So if we used B model pistons that have a bigger chamber in them, with the A model heads with the bigger prechamber an even lower compression ratio without having to do much?

 

[dyoung14]

Warden, wasn't it the 7.3 IDI that had 21.5:1 and the B model 6.9 that had 22.5:1?

I think it was 20thou that was the safe limit to mill the stock pistons down if I remember right. Although there are pistons available that move the pin higher up by 10 thou in case of having to resurface the deck.

Sure wish I could see boost numbers like that....

This boost is with an unknown injecion pump that has my high RPM spring in it, its the same pump thats always had the spring in it, at low RPM if i floor it with no boost it rolls some pretty serious coal, after the boost passes 8-10 psi i cant really see any smoke anymore, and it will hold 15-18PSI all the way past 4000 rpm, it dont really de fuel all that much in the high RPM that i can tell i think sometimes i notce it loose 2 PSI if i take it all the way to 4300 RPM, today once again i hit 18PSI of boost comming home, this time instead of letting out part of the way up the mountain i held it all the way and actually got over 85 MPH comming up a 7% grade

 

[david85]

I don't care who you are, that is crazy fast right there!

 

[BlueBreadTruck]

wow, If I stay in the pedal on a long grade, my egts get very high, fast. I usually have to let off after a bit to keep it under 1100/1000 deg.....it hits that and keeps climbing. I have my inj pump maxed out though. how high is yours turned up? I wish I could keep my egs low enough on long climbs...I was debating turning down my screw some.

 

[dyoung14]

wow, If I stay in the pedal on a long grade, my egts get very high, fast. I usually have to let off after a bit to keep it under 1100/1000 deg.....it hits that and keeps climbing. I have my inj pump maxed out though. how high is yours turned up? I wish I could keep my egs low enough on long climbs...I was debating turning down my screw some.

My pump is also maxxed,

And david85 yes it was very fast, i actually thought i wasnt going to be able to hold onto it around the curves, but needless to say it was fun then when i got to the top and let off my idle oil pressure fell off to pretty much 0 and my oil pressure at 2000RPM was about 10-15 after getting on it so hard for so long, so im pretty sire the EGT's were threw the roof

 

[NMB2]

I was doing 80+ up a grade like that last night with pedal left, and egt's under 850. I can hit 18psi with the w/g hooked up. Less than 3200rpm

 

[Clydesdale]

dyoung.. you should come over to the yahoo side and chat with me about those springs you speak of... [email protected] is the Yahoo addy.

 

[Diesel JD]

I have read elsewhere that all 6.9s were 20.7:1 and all 7.3s were 21.5:1 but I tend to trust the sources here more than those. It looks like with respect to the piston you could chop .020" off of the Mahle pistons with the piston in hole machined .010" higher on the slug and get a total reduction of .030" so you'd be where the A motors were with the pistons, but the heads and precups would be more of a problem. I've heard here that the 6.9 style precups(let alone the A model type) are NLA in favor of the 7.3 style so you'd need a working set of 83 or early 84 heads that you trust on your truck to get the full effect. Might be the best solution for a mild build.

 

[fcdrifter13]

When you hit 21 let me know, the turbo on my truck didnt like it much and egts started to climb. This was with a maxes out IP with the timing slightly advanced than what it should have been and measured after the IC probably 24 pre because my IC is a big restriction way to small for the volume of air. All on stock 94 parts aswell, unless the PO studded it.

 

[The Warden]

So if we used B model pistons that have a bigger chamber in them, with the A model heads with the bigger prechamber an even lower compression ratio without having to do much?

Given that the overall B piston is taller than the A piston, my guess is you wouldn't really gain anything, and could potentially have issues with things not burning completely with the cavity not being optimized for the precup throat. But, I'm not an expert at building race engines...maybe the math would come out to a lower overall CR, but again, I would be concerned with burn/efficiency issues.

And I just have to say this for the record...maxing out your pump and pushing boost, RPM, and vehicle speed uphill like that without a pyro is almost literally playing with fire. I'm quite thankful that I won't be paying your repair bill...

 

[dyoung14]

I was doing 80+ up a grade like that last night with pedal left, and egt's under 850. I can hit 18psi with the w/g hooked up. Less than 3200rpm

Yea but the differnce is my engine is making this max boost with a stock pump thats worn out and stock injectors, and i hit max boost around 2800-3000, but your supose to have some bad ass DPS pump and stage 1 injectors but in reality there not making much more boost than my stock worn out stuff

 

[dyoung14]

dyoung.. you should come over to the yahoo side and chat with me about those springs you speak of... [email protected] is the Yahoo addy.

Send me a pm i dont mess with that yahoo crap

 

[NMB2]

Yea but the differnce is my engine is making this max boost with a stock pump thats worn out and stock injectors, and i hit max boost around 2800-3000, but your supose to have some bad ass DPS pump and stage 1 injectors but in reality there not making much more boost than my stock worn out stuff

, right.

 

[dyoung14]

Given that the overall B piston is taller than the A piston, my guess is you wouldn't really gain anything, and could potentially have issues with things not burning completely with the cavity not being optimized for the precup throat. But, I'm not an expert at building race engines...maybe the math would come out to a lower overall CR, but again, I would be concerned with burn/efficiency issues.

And I just have to say this for the record...maxing out your pump and pushing boost, RPM, and vehicle speed uphill like that without a pyro is almost literally playing with fire. I'm quite thankful that I won't be paying your repair bill...

Yea i know im playing with fire but dont i always? and this engine really aint worth knowing the EGT

 

[Diesel JD]

It's yours and you can do whatever you want with it and no one should attack you for that, but most of us as people that love mechanical stuff are really concerned about the results of your tests. I hate to see engines get scattered, not saying you will, from what you tell us that engine is pretty tired, it may die of something else first. Then again I haven't read all your threads so maybe you're trying to see how far you can push it, in that case I'll speak for myself and say you've proven that they can take a lot more than we all thought.