IDI - effect of reducing compression ratio on hp & mpg

[craiggroombridge]

Wow I been away too long, where to start....

C/R's gas well established between 8:1 and 12:1.
is more better NO the higher c/rs in race appalcations are for the long bleed ( duration) of the camshafts on the intake lobe I have good experance with this, my last engine for my Bonniville (small block chevy) was a 400 ci with 9:1 comp and i have a dyno sheet for 550 hp. 580 tq.
at 4300 and 3200 rpm

on diesel from 15:1 to 24:1
the higher on pre-comp only this is for starting issues the more movement in air means more heat generated,
Di engines are usally at 17:1 or 18:1
again more is not better.

on this note i finshed with my injection pump experament with results:
stock idi no turbo 1992
untouched 1600 psi breaking  55mpg
reset to  1850 psi 58 mpg
set to turbo 2250 psi 53 mpg with black smoke and low top end
timing at n/a settings .088 mm

changed timing to turbo 1.00 mm
untouched at 1600 59 mpg
reset to 1850  61 mpg
turbo 2250 53 mpg with black smoke and sputter
so
with a stock engine i found with 1.00 mm stroke on the pump and 1850 breaking the best mpg hope this helps

Craig

[Black Smokin' Diesel]

on this note i finshed with my injection pump experament with results:
stock idi no turbo 1992
untouched 1600 psi breaking  55mpg
reset to  1850 psi 58 mpg
set to turbo 2250 psi 53 mpg with black smoke and low top end
timing at n/a settings .088 mm

changed timing to turbo 1.00 mm
untouched at 1600 59 mpg
reset to 1850  61 mpg
turbo 2250 53 mpg with black smoke and sputter
so
with a stock engine i found with 1.00 mm stroke on the pump and 1850 breaking the best mpg hope this helps

Craig

It's really nice of you to share such info, thanks!!!
Those psi figures are the injector's breaking pressure right? Do you think we'd get similar results on a turbocharged engine?

Reason I'm asking is because I have 135bar mercedes injectors (1958psi) timed to TD specs on my 1.6TD.

[Mark(The Miser)UK]

on this note i finshed with my injection pump experament with results:
stock idi no turbo 1992
untouched 1600 psi breaking  55mpg
reset to  1850 psi 58 mpg
set to turbo 2250 psi 53 mpg with black smoke and low top end
timing at n/a settings .088 mm

changed timing to turbo 1.00 mm
untouched at 1600 59 mpg
reset to 1850  61 mpg
turbo 2250 53 mpg with black smoke and sputter
so
with a stock engine i found with 1.00 mm stroke on the pump and 1850 breaking the best mpg hope this helps.

Craig

[/size]

Good contribution Craig :!:

The 1600psi is only 108bar. All else the same it looks like peak mpg might occur around 1700psi or 115bar. From the results it may be 65 to 70mpg. Factor in a freeflow exhaust and then a lowered compression and then 80mpg might be hit. :shock:

[craiggroombridge]

yes breaking psi and this engine has 440,000 kms and not sure of the current state of the rings on the pistons but it started better at 1.00 mm on timing then it did at .088 mm

[xud9te]

Greg,

Did you find a way of altering the compression ratio without machining anything yet?  

For the purpose of tests on mpg?

[jtanguay]

maybe that's why my MPG is suffering with 160 bar injectors... have you tried running 1.05mm injection timing for turbo injectors??? i'm currently running 1.06mm or so...

i wonder if there is any validity in VW upping the turbocharged injectors breaking pressure to 155 bar, to ensure a safe fuel atomization??? has anyone tried N/A injectors on a TD???

[saurkraut]

Yes I have put 135 bar injectors in a TD.  With out changing anything else: smoother idle, measurabley less power on top end, no appreciable difference in fuel economy.

[Mark(The Miser)UK]

Greg,

Did you find a way of altering the compression ratio without machining anything yet?  

For the purpose of tests on mpg?

For testing it has to be varied combinations of head gaskets. We're not talking 100's of miles... just say 20miles, fuelling from a measured 'baby bottle' in the engine compartment.

[Mark(The Miser)UK]

Yes I have put 135 bar injectors in a TD.  With out changing anything else: smoother idle, measurabley less power on top end, no appreciable difference in fuel economy.

I wonder if 1mm on TD pump is wrong with n/a pressures. Andrew has his 99 cents dynamic tester to confirm this :wink:  Smoother idle sounds like a great advantage.

[saurkraut]

I guess its time to re-read the SAE paper on the TD motor and see why the 155 bar pressure was used on the TDs.  Using the 135 bar injectors with less advance may work better than the same injectors at 1mm, but are they working better than the correct injectors for the engine, with the pump set to the specified pump timing.

I didn't leave the 135 bar injectors in very long.  It was a non-availability thing for the TD injectors.  I still have the 135 bar injectors sealed in a plastic bag.  If i get bored, I'll get GTD nozzles and recalibrate them.

i don't concider a smooth idle a desirable thing, at the expense of power.  As long as all four are fireing at idle, I'm ok with that.  My engine spends very little time at idle.

[jtanguay]

hmmm smoother idle... sounds to me like what the TDI's do with their dynamic timing

[Mark(The Miser)UK]

I guess its time to re-read the SAE paper on the TD motor and see why the 155 bar pressure was used on the TDs.  Using the 135 bar injectors with less advance may work better than the same injectors at 1mm, but are they working better than the correct injectors for the engine, with the pump set to the specified pump timing.

I didn't leave the 135 bar injectors in very long.  It was a non-availability thing for the TD injectors.  I still have the 135 bar injectors sealed in a plastic bag.  If i get bored, I'll get GTD nozzles and recalibrate them.

i don't concider a smooth idle a desirable thing, at the expense of power.  As long as all four are fireing at idle, I'm ok with that.  My engine spends very little time at idle.

I see what you mean about desirability of power... However
1) Standard power standard TD bars isn't maximum power available.
So could more power be tuned in to the n/a bar TD setup you had?

2) Smoother idling means less mounting wear probably.

3) n/a pressures mean less wear on pump and less work done on compressing.

4) Whats the physics of changing bars downwards?
 i) larger droplets?
 ii) Is it the same volume of diesel squirted slower?
 iii) Earlier or later start of burning?
 iiii) Longer burn doing more work on the crank?

5) I dunno.... :roll:

[xud9te]

Mr. the Miser,

Who mentioned anything about the length of the test, or over what period?  I am not sure i see the point of your remark...

I have an idea for reducing Cr without changing headgaskets, that can be changed to suit.  

Also remember thicker headgaskets will destroy the quenching effect, not desirable.

[saurkraut]

"I see what you mean about desirability of power... However
1) Standard power standard TD bars isn't maximum power available.
So could more power be tuned in to the n/a bar TD setup you had?"



Like I said, we need to look at the SAE white paper and see what the reason for increasing the break pressure was.  I don't think the VW engineers were stupid and overlooked a very obvious performance enhancement.  The only screw up I'll assign to them is the pea shooter exhauste.

I can put forth an educated guess why the idle was better, more advance at idle.

As for my own ride, its deffinately making more power than stock.  It has: full 2.5" exhauste, GTD nozzles that i calibrated to 155 bar, closed waste gate, boost pin at the highest fueling, and diddled with governor.  Full blast boost is about 22 psi while the EGT occasionally tops out above 1300F.  It has embarassed many a V8 gasser and quite a few rice boys.    The only change I did was to put the 135 bar injectors in and the power measurabley dropped.  IE its top speed was significantly lower over a known distance.  But i had a great idle.

I have a hard copy somewhere of the TD SAE paper.  I'll try to dig it out and look at the break pressure thing and see if they disclose the reason.  There is an outside chance that they did not put a reason why they increased the break pressure because of proprietary reasons.

[jtanguay]

sounds to me like the timing should be around 1.05mm and maybe a little more timing advance in the upper rpm range??? (with the 135 bar injectors..)