Thicker head gasket chain too short???

[POKTROKT]

22 hours ago, tzei said:

That is not much. What it was with old gasket?

Used to be an E21 head, decked 0.5mm and with domed pistons probably about 9.3

Edited August 5, 2018 by POKTROKT

[tzei]

5 hours ago, POKTROKT said:

probably about 9.3

This is whereabouts i would like it to be. Use thinner gasket solves both?

[POKTROKT]

19 hours ago, FB73tii said:

I just went through this. A few years ago I had a valve to piston clearance problem (valve pockets were not deep enough) and the band-aid solution was a .120" (3.05mm) Cometic MLS. It was tight but the chain went on, the head had been shaved (don't have the measurement now), and all works fine. Fast forward to another engine I had built for me and it turned out there was a piston-head clearance problem. I thought "great, I'll just slap another .120 gasket in there and be on my way."

 

Well, that was not the case. The timing chain (new, as were both sprockets) was too short by a lot and some research suggested somehow a link was hiding down in the front cover (i.e., turn the engine over both ways and maybe it will free up) as Toby mentioned above. That didn't work. I pulled the front cover, and somehow thought I could modify the tension rail to create more clearance. Well, I finally got the chain to fit, only to realize the rail now interfered with the front cover--it just was not going to work even with tension piston removed (I slotted the bottom of the rail to move it in, and also tried grinding off some rubber).

 

Finally I realized the engine builder probably used a full height head. The engine was never designed to use a much thicker head gasket with a full height head. Now a winter project is to pull the pistons and get them shaved. Compression was 13.8:1, so it is fine with me to cut the pistons--want it more like 12.5:1 for this engine, which will run a Schrick 336 cam. --Fred

 

 

Thanks all for the feedback! 

 

This is great information, the head is an E12 and with the tensioner out and prying it open it is still about 0.5mm too short. 

 

Wow, 3mm and 13.8:1! those pistons must be crazy domed, race fuel?

 

Looks like I should have milled the pistons while they were out, at this point I'd like to slap a stock height gasket and button it up so I can drive it the rest of the season, with a full height head it will put it closer to 9:1.  

[POKTROKT]

According to this CR calculator http://www.not2fast.com/turbo/compression/compression.shtml#chords

 

Old CR with milled head and 1.4 MLS HG: 9.1:1

New CR with 2.5mm MLS HG: 7.9:1

New CR with 1.4mm MLS HG: 8.7:1  borderline for a turbo at 7-8psi

[POKTROKT]

Never posted an update on this.  

Ended up using a 1.4mm thick MLS headgasket.  Reshaped the combustion chamber slightly to remove some volume by removing a sliver of material around the exhaust valve. 

Motor is running great at 8psi of boost with no pre-ignition. 

 

[Preyupy]

There is a reason BMW opened up the combustion chambers AND used DISHED pistons to get the compression ratio down on the Turbo’s. 

[Einspritz]

11 hours ago, POKTROKT said:

Motor is running great at 8psi of boost with no pre-ignition. 

 

Are you using a knock sensor either alone or in conjunction with a Lambda sensor and timing retard?

 

If so, did you calculate the knock frequencies to make sure the sensor (Hz) is appropriate?

 

I observe knock on my system while I cannot hear it, which changes with the type of fuel I am using. it helps to know this as it is integral to your tuning under full throttle / full boost / full load.

 

 

[POKTROKT]

14 hours ago, Preyupy said:

There is a reason BMW opened up the combustion chambers AND used DISHED pistons to get the compression ratio down on the Turbo’s. 

condescending post! is this typical of you @Preyupy ? 

 

13 hours ago, Einspritz said:

 

Are you using a knock sensor either alone or in conjunction with a Lambda sensor and timing retard?

 

If so, did you calculate the knock frequencies to make sure the sensor (Hz) is appropriate?

 

I observe knock on my system while I cannot hear it, which changes with the type of fuel I am using. it helps to know this as it is integral to your tuning under full throttle / full boost / full load.

 

This is a blowthru system therefore no knock sensor.  It is on MSD boost timing control with a timing retard knob.  My comment above was in comparison to how it was running before.  Granted that my hearing is very good I may not be catching every uncontrolled ignition event.  The car runs at 6.5psi, i cranked it to 8psi to see if I could get it to give me an audible knock, nothing yet on 94 octane pump fuel. Effective compression ratio at 8psi is about 12 to 1, i'll bring it down to 6.5psi and just drive it. 

Edited February 13, 2019 by POKTROKT
Corrected base spring boost pressure to 6.5psi

[POKTROKT]

This is my setup 

[Preyupy]

Not at all, it just seems like a bandaid on a knife wound. Why not put flat top pistons in it? You get much better combustion control without having the flame front have to work around the dome.  With all the cost and work to build all the systems on the outside of the engine it seems a bit strange to start off with a high compression engine.  

 

I know what a custom MLS gasket goes for ( I have about 8 of them on the shelf for M10 & M30 engines).  Build a flat top piston bottom end and sell off the 9.5:1 short block to someone that is running NA. 

 

You got it working and that is what matters. 

[POKTROKT]

3 hours ago, Preyupy said:

Not at all, it just seems like a bandaid on a knife wound. Why not put flat top pistons in it? You get much better combustion control without having the flame front have to work around the dome.  With all the cost and work to build all the systems on the outside of the engine it seems a bit strange to start off with a high compression engine.  

 

I know what a custom MLS gasket goes for ( I have about 8 of them on the shelf for M10 & M30 engines).  Build a flat top piston bottom end and sell off the 9.5:1 short block to someone that is running NA. 

 

You got it working and that is what matters. 

I don't disagree.  I don't claim to be a master engine builder, this is my first turbo engine and have learned quite a bit including that the CR is not ideal.  Without going too deep into it, there are a multitude of reasons why I didn't use flat or dished pistons.  For one the engine came out of a 320i race car with the cartech kit, forged pistons, etc.  it didn't seem like a stretch to use the same pistons. The dome is not very pronounced and was hoping a thicker gasket would give me the CR i was after. 

Today I measure 8.5 to 1 ratio, left quench areas to help combustion/swirl and seems to work fine for now.  You're right however, I will build another bottom end in the future with the proper pistons. 

Edited February 13, 2019 by POKTROKT

[Simeon]

5 hours ago, POKTROKT said:

This is my setup 

 

What kind of carb are you blowing through under that plenum?

[POKTROKT]

Dual 44mm Mikunis - running 6.5 psi

This vintage ad rates it at 210hp