Increasing Compression On Mi16 Engines

[SweetBadger 94]

I had to take the head off my MI the other week to do the stem seals, so I decided to look into how best to increase the compression ratio to go with an uprated inlet cam...

 

I think that the only options discussed on here so far to increase the MI compression are:

 

- install new pistons (very expensive)

- skim the block & liners (expensive)

- fitting a copper head gasket (cheap but risky if liner protrusion is not spot on & requires 3yr maintenance)

 

There is another option which is a bit more pricey than the copper gasket option but gives none of the associated issues provided it's installed properly:

 

I've just fitted a 0.68mm thick cometic MLS head gasket to my 1.9MI; the gasket is not normally stocked or listed in any of the UK cometic suppliers catalogs, but is available on special order. Part number is C4225-027, I got it delivered from Burton Power in about 3 weeks for about £90, it's 0.027" thick or 0.68mm. Compression now reads 230psi across all cylinders, approx 11:1 compression ratio.

 

Can definitely confirm that petert's advice about the benefits of increasing the compression on this engine is spot on - I run a 526 catcams inlet camshaft and the engine was a bit peaky on standard compression; very flat in low to mid revs. It drives soooo much better with the new gasket and increased compression! Way more mid range torque and a much more even power delivery.

[petert 588]

Great info. Well done. Don't forget you've closed up the valve-piston clearance as well.

[James_m 0]

Sounds like a nice cost effective modification, nice find. Does it go harder at the top end as well?

[SweetBadger 94]

Yep, pulls harder right to the limiter too...

 

Think this might be in part due to the timing being retarded slightly, as I had to take up the extra slack in the cambelt with the rear tensioner.

[GilesW 41]

Interesting info. Seems liek this woudl complement one of petert's stage 1 cams then?

 

Afraid I'm not familuar with the XU9J4 specs. How have you calculated the 11:1 CR?

[SweetBadger 94]

I didn't calculate the new compression ratio myself, the info on compression increase due to a thinner gasket is here:

 

http://forum.205gtidrivers.com/index.php?showtopic=70496&st=10

 

Quite easy to calculate though; info here: http://www.rbracing-rsr.com/compstaticcalc.html