The History of My Car & Engine

from Rally Bashing Hack to Weekend Racer

STAGE 7 - G200 Blow Through Turbo - Locked Dizzy

So after rebuild #3 of the G200, (more cracked pistons), and having built 2 G160's before that, I was getting pretty quick at engine rebuilds.

I got some good advice about the ignition timing, and decided the easiest way to control it was to lock the distributor.  I pulled the dizzy apart, removed the springs and weights, and MIG welded the mechanical advance solid.  

I set it at 18degrees static timing, any more timing advance and the starter motor had trouble cranking over the engine, especially when hot.  I knew the engine would make more power with more ignition advance like around 25deg, which would have been safe at my boost levels, but modifying the mechanical advance mechanism was not part of my vocabulary yet. 

I thought I would try out the 38DGMS Weber that I has used previously on the 1600, I know it would flow more and should make more power.  I did a G-Tech run at the drag strip and recorded a best of 14.6 sec at 160km/h on 11psi of boost.  The problem was traction with my tyres.  I know that if I could get traction by using a set of slicks, I might have cracked a 13 second quarter. 

I found fuel economy with the 38DGMS to be pretty bad, as the carby is synchronous (both butterflies open at the same time, just like a 2 barrel Holley).  I decided a progressive carb like my 32/36 would be better suited, as I could keep the primary jetting fairly stock, for good off-boost economy, and richen up the secondary circuit to provide a rich air/fuel mixture under boost.  

I fitted an oxy sensor to the turbo dump pipe, and built a Dick Smith mixture meter to check the mixtures throughout the rev range.  A narrow band oxy sensor is not the best thing for checking an air/fuel ratio in the 12:1 region, but it can give an indication.  Especially if you lean out, it will show you are getting close to the stoic value of 14.7, which is bad!

So I increased the boost a bit more, and for whatever reason, probably low octane fuel or compressor outlet temps being too high, I got detonation and blew the motor again.  To be more specific, I broke the pistons at the ring lands.  

Remember I hadn't been to a dyno yet, hadn't had mixtures checked, and wasn't intercooling.  So I might have been a bit lean under full boost, and the compressor outlet temps would have been getting pretty hot.  A combination that will cause detonation.

I was becoming a regular at the local machine shop where I bought my pistons.  As I did all the work myself except fit the pistons to the rods, each time I blew a motor, I was only up for pistons, rings, and gaskets.  I didn't always need to fit new bearings, and sometimes I didn't have to buy a set of pistons, as I could make a set of 4 good pistons from the remains of the last few engines.  So each build would only cost me a few hundred $$ for gaskets, rings, oil, etc.

Check out STAGE 8 - G200 Blow Through Turbo - Dyno Tuning & Water/Methanol Injection.