decreasing compression ratio in 3B (2 Viewers)

[slammed41]

greetings,

well, after spinning a big end bearing in my turbo'd 3B, i'm wondering about options before i do it again to the replacement engine! now, i don't know for sure that the spun bearing was directly attributable to having a turbo installed on a non-turbo engine, but i would certainly like to reduce the possibility.

i'm guessing that lowering the compression ratio (and maybe turning the wastegate down a bit!) would allow the engine to tolerate the extra pressure. the 13B-T has (i think) an 18:1 ratio while a 3B has 20:1.

it seems there may be different options, but i'm not versed in the affairs of the machining world. i'm guessing i could do one or more of the following:

1) install pistons from a 13B-T. will they fit into a 3B? if so, can i just use the pistons or do i need to use the con rods as well?

2) shave the existing 3B piston tops. is there enough meat to do so?

3) machine the head to create more volume. can enough metal be taken out to affect enough reduction in compression?

i'm not even considering a thicker head gasket as i've read of too many horror stories when going that route.

i would appreciate any "real world experience" stories and comments, particularly from engine builders.

cheers,

roy

 

[iggi]

given the number of high mileage 3B's with turbos the spun bearing doesn't seem to be a common issue.
How many kilometer's on your motor?

 

[slammed41]

iggi,

the actual kilometers is unknown, but somewhere between 400K and 450K would be my best guess. the turbo has been on the engine for one year, so it didn't take long to go boom.

i may never know the cause of the spun bearing...i'm simply wanting to be cautious with the next engine! the replacement engine that i now have sitting on a pallet has unknown kilometers on it, too, although the PO tells me it has under 300K.

i was running 15 lbs. of boost with the fuel turned up. now that i'm used to that kind of power difference from stock, i'm loath to lower the boost! i want to avoid eating engines (gets costly) so, i thought if i could reduce the compression ratio, i'd still be able to run a higher boost level. also, i am considering the use of a water-to-air intercooler to reduce heat buildup and egt levels.

of course, budget comes into play here, so the sky is NOT the limit in terms of potential cures.

r

 

[gerg]

IMO if your turbo was running 15 lbs boost it just finished off an already screwed bearing. I dont feel it would do this by itself. That boost combined with way too advanced timing might though.

I too am looking to reduce my compression but havent done the math as far as how much to take off. I got a quote from a shop to cnc mill my pistons 2mm down and maintain the valve recesses for 200$. There is alot of meat on the crown to handle this. I would for sure get them ceramic coated while your at it. You will experience a harder time with cold starts.

Another route could be to cc the combustion volume in the head and port out just enough. Porting heads is its own black art though. I have yet to find even some pictures of ported deshrouded diesel heads let alone equasions on how to do it.
Since your motor is out of the truck I would for sure get the bearings moly coated.
Some thoughts
g

 

[Radd Cruisers]

You cannot install DI pistons into an IDI engine... they are not the same thing at all.

Compression ratio was not likely the issue, low oil pressure or a tired bearing was.

Given that your engine was old, spinning a bearing was bad luck, fix it and move on.

There are LOTs of used 3B parts around if you need a crank or anything else.


~John

 

[slammed41]

thanks, gerg. i'll talk with a few machine shops and see what can be done on the head.

thanks, john. so, it appears that 13B or 13B-T pistons will NOT work in a 3B.

and, it seems that the consensus so far is that bearing failure is not necessarily related to high compression. i thought higher cylinder pressure may have stressed a bearing more than usual. who knows? it could have been the oil pump crapping out or the bearing deep-sixing itself prematurely. i haven't done a full autopsy, simply dropped the pan to see the damage to the crank and big end of the con rod.

i'm happy to "move on" and get the truck up and running again, yet i am now a little cautious and i'm hoping to have similar performance with increased longevity potential.

rebuilding my current engine is cost prohibitive for me. if it was just repairing what was damaged when the bearing spun, it wouldn't be so bad. my guess is, however, that once i dig into the engine, i'll find all sorts of things to rebuild or replace while i'm in there and i've heard figures of $5K and up for a proper job.

so, it's going to be a used engine replacing the blown one. i'd like this one to last a little longer, so i'm looking for ways to help it along. i figured reduced compression must help, although getting a few numbers down on the ratio might be costly, too.

what are the culprits involved in reduced longevity (other than poor maintenance)? i'm imagining that heat is a big factor. perhaps an external oil cooler will help, as well as an intercooler.

any comments from long term turbo'd 3B owners would be appreciated.

 

[VTCDN]

...................

 

[gerg]

I doubt that a custom head gasket would be wise as the current head gasket is isolated from combustion pressure due to the liner seating on the head. This is a huge advantage that makes the 3B gasket last much longer and resist high combustion pressure. If you had a thicker gasket it would be exposed to the combustion pressure not allowing the liner to seat, lowering the compression ratio yes but at a huge price. I doubt that thicker gasket would last very long at all.

Look at what cummins guys do to keep their gaskets from blowing out. One of the things is to "fire ring" or "o" ring the head. Machining in a metal o ring into the block and head to do exactly what our little old 3Bs have done since the 70s.
g

 

[Radd Cruisers]

You can probably pick up a rusted out BJ60 (1983-on with 5 speed and PS) for pretty cheap, and with low reasonably low mileage.

Rip the engine out, install it into your beast, slap on the turbo, crank the boost, and watch your EGTs. Put on some GOOD gauges and monitor the oil pressure, boost, temperature & EGTs.

The difference between direct and indirect pistons is that with IDI the combustion chamber is in the head, with a DI engine the combustion chamber is a big pocket in the piston - for that reason, you cannot swap pistons.

here's a picture of a random DI piston: piston (1)

Anyhow, I am sure you can get your Cruiser up and running again for a moderate amount of cash, and get back to enjoying it in pretty quick order.

~John

 

[VTCDN]

I doubt that a custom head gasket would be wise as the current head gasket is isolated from combustion pressure due to the liner seating on the head. This is a huge advantage that makes the 3B gasket last much longer and resist high combustion pressure. If you had a thicker gasket it would be exposed to the combustion pressure not allowing the liner to seat, lowering the compression ratio yes but at a huge price. I doubt that thicker gasket would last very long at all.

Look at what cummins guys do to keep their gaskets from blowing out. One of the things is to "fire ring" or "o" ring the head. Machining in a metal o ring into the block and head to do exactly what our little old 3Bs have done since the 70s.
g

Better rethink the Liner sealing / seating on the head theory. At least on this 3B engine. Yes some do, But NOT 3B

Squish area is formulated with the diameter and thickness of head gasket is taken into account plus. Now Squish area is the amount of area left when the piston is at TDC and compression ratio is BDC to TDC area forced into squish area.

VT



Pix of Inside a 3B
Poor CanCrusher

 

[gerg]

I disagree. The liner protrudes from the block for a reason. It also leaves a significant seal mark on the head. My machinist, who does high output diesels, mocked my OEM gasket at how weak it was at the cylinder and said that without the liner they would erode rapidly.

You should first try out the cheaper solution of a thicker head gasket with higher boost and see how long it lasts before you recommend it to others.
g

 

[crushers]

sheesh, you guys are missing the point.
if he wants the next engine to last longer then do:
compression test of donor engine
drop the pan and check bearing clearences and install new bearings.
you spun a bearing, it had nothing to do with the turbo or boost. the engine was tired.

actually, i would run some seafoam though the oiling system, drive for 50Km, drain the oil and do again. drain the oil. this will clean out the oil passages.
now drop the pan and replace the rod bearings, very very seldom do the mains show much if any wear.
use quality oil and filter.
pull head and replace the cracked prechambers and install new QUALITY head gasket to factory specs. the compression is not the issue. reinstall head.

now go drive the s*** out of it.

 

[Dougal]

An indirect injection motor like the 3B will be very hard to start in the cold with a lower compression ratio.

 

[gerg]

I agree with the compression not being the issue with the bearing and that it would be harder to start.

That being said I measured the liner protrusion from a block in my shop and it was 0.125mm compared to the 0.131 mm at the cylinder ring I got from a used gasket. Torque the head down and I betcha it would soon become 0.125mm as well.
g

 

[slammed41]

thanks, wayne.

i appreciate that you get the point, and i appreciate the advice. if i interpret correctly, you are suggesting that by cleaning out the oil galleries and replacing the BEBs of the donor engine, i will get longer service from it.

my current engine (with the spun bearing) has had the head off recently...new precups and valve guides...so i ought to be able to swap the head over to the donor engine with the new style factory gasket.

cheers,

r

 

[crushers]

yah, the issue is bearing clearences. if the clearences are too loose then you spin a bearing, it really is that simple.
so clean the oil passages to have fresh clean strong flow of oil to the new bearings.
if your head is good then pull the other and swap as needed.

yah, i am a bit blunt at times.

cheers and good luck on the swap.

 

[VTCDN]

That being said I measured the liner protrusion from a block in my shop and it was 1.25mm compared to the 1.31 mm at the cylinder ring I got from a used gasket. Torque the head down and I betcha it would soon become 1.25mm as well.
g

No your not Blunt but to the point crushers..

I thought I mentioned Oil problems over turbo, but whatever.


In the years that the North American fuel went to crap and price fixing had start to set in Most euro cars required fuel that didn't make the engines "ping" Audi,Renault ,Peugeot ,Volvo,MB,BMW all had thicker head gaskets for changing the compression ratio. I did many @ the dealers .

As with Liner protrusion, pre cup protrusion
Your not a barrel lapping in VW air can .

See FSM ,
We are disusing a 3B Daihatsu (Gesundheit) Cast.



And as One gent said , lower Comp ratio will have a effect on starting.
and lwr ratios is crazy talk for this engine.

VT

 

[gerg]

Granted the manual does say that. I guess I am happy that mine is out of spec and my gasket is not taking the brunt of the cylinder pressure. If lowering the ratio is crazy talk then call me crazy

g

 

[gerg]

I also found this interesting. • View topic - July 2008: Cats and Liner protrusion
.005 inches is .127mm Not far from the .1mm in the book.

g

 

[VTCDN]

OK
...............

VT