Metro Turbo Head
Discussion
The standard C.R. for the Metro Turbo is c.9.4:1 which is quite high for even a L.P. turbo the 4.5 psi., which is standard. To go to 7 psi boost, which is desired by the owner, does require lowering the C.R. to about 8.5:1 and fitting a customised distributor. If this is not done, even with a specially curved non-standard distributor, it runs a high risk of detonation and piston failure. The engine in question was re-built by an alleged 'engine builder' and had a geometrical c.r. of 10.45:1 (I know 'cause I just measured it accurately!). The ignition was not modified in any way and the boost was left as standard. It 'blew-up' after 4 miles. In fact, it blew up as soon as any turbo boost came on. The pistons, Hepolite 21253, not really suitable for a turbo application anyway, are destroyed.
I now have some Omega pistons to lower the c.r back to about 9.5:1, but must get each chamber volume out to about 24.5 cc from its current 21 cc in order to get down to 8.5:1. It was 21.4 cc, but I can see that the head was skimmed before the last build (4 miles ago!).
I now have some Omega pistons to lower the c.r back to about 9.5:1, but must get each chamber volume out to about 24.5 cc from its current 21 cc in order to get down to 8.5:1. It was 21.4 cc, but I can see that the head was skimmed before the last build (4 miles ago!).
Pete - I'm sure you are maximising the amount material removal / valve deshrouding with respect to the head gasket line, removal of the beak is the next bit to go. Only way you'll get the CC.
If my memory serves me right the beak is removed on really highly boosted A-series motors from V-max and similar as it overheats locally and causes pre-ignition.
Out of curiosity - What piston dish cc value on the Omega's?
If my memory serves me right the beak is removed on really highly boosted A-series motors from V-max and similar as it overheats locally and causes pre-ignition.
Out of curiosity - What piston dish cc value on the Omega's?
Edited by FWDRacer on Tuesday 4th November 09:24
The pistons ae Omega 1015 with a dish of 10 cc. They also sit 0.034" lower in the bore than the Hepolites, which gives another 3.4 cc. With a BK450 gasket, the gasket volume is 4.0 cc so the head needs to be 24.5 cc to give just over 8.5:1, which I feel is about right for about 7 psi max boost.
I reckon I can get almost that amount out be removing the beak, de-shrouding to the max. The only problem is that my air compressor is playing up as of last evening, but I have a guy coming to fix it, hopefully tomorrow. Then it's 'grind away' time.
I just can't believe that anyone building an engine and charging for the work would build a turbo unit with 10.45:1 comp. ratio.
Thanks for all the advice,
Peter
I reckon I can get almost that amount out be removing the beak, de-shrouding to the max. The only problem is that my air compressor is playing up as of last evening, but I have a guy coming to fix it, hopefully tomorrow. Then it's 'grind away' time.
I just can't believe that anyone building an engine and charging for the work would build a turbo unit with 10.45:1 comp. ratio.
Thanks for all the advice,
Peter
Pete,
Someone will need to confirm it, but I believe the casting is slightly thicker on the turbo heads to allow the extra machining.
As you probably know they run sodium filled Exhaust valves, hence the thicker stem.
The metro ran a 4.5psi wastegate, but used a bleed valve to achieve 7psi with the electronic controller. The problem wasn't due the engine not being able to handle it, but the gearbox. With too much torque at low rpm.
Someone will need to confirm it, but I believe the casting is slightly thicker on the turbo heads to allow the extra machining.
As you probably know they run sodium filled Exhaust valves, hence the thicker stem.
The metro ran a 4.5psi wastegate, but used a bleed valve to achieve 7psi with the electronic controller. The problem wasn't due the engine not being able to handle it, but the gearbox. With too much torque at low rpm.
Andy,
I have heard that the heads were the same as any other 1275, thinner with larger waterways to help with cooling and thicker to provide more strength. In the mist of time, who knows? However, I do intend to sort out this engine so that it runs well, won't 'blow-up' and will give reasonable power. My customer doesn't want to go to the expense of a 4-pin diff, so I'm fitting a new comp. diff pin (thanks, Rich) with new planets, thrusts, etc.
A 'custom-curved' Aldon dizzy to match the final spec and a boost limit of 7 psi should do the trick.
By the way, I love your original 'S' rockers.
Peter
I have heard that the heads were the same as any other 1275, thinner with larger waterways to help with cooling and thicker to provide more strength. In the mist of time, who knows? However, I do intend to sort out this engine so that it runs well, won't 'blow-up' and will give reasonable power. My customer doesn't want to go to the expense of a 4-pin diff, so I'm fitting a new comp. diff pin (thanks, Rich) with new planets, thrusts, etc.
A 'custom-curved' Aldon dizzy to match the final spec and a boost limit of 7 psi should do the trick.
By the way, I love your original 'S' rockers.
Peter
Cooperman said:
Andy,
I have heard that the heads were the same as any other 1275, thinner with larger waterways to help with cooling and thicker to provide more strength. In the mist of time, who knows? However, I do intend to sort out this engine so that it runs well, won't 'blow-up' and will give reasonable power. My customer doesn't want to go to the expense of a 4-pin diff, so I'm fitting a new comp. diff pin (thanks, Rich) with new planets, thrusts, etc.
A 'custom-curved' Aldon dizzy to match the final spec and a boost limit of 7 psi should do the trick.
By the way, I love your original 'S' rockers.
Peter
Wish they were mine, just a random pic I found on the net showing the difference in port area.I have heard that the heads were the same as any other 1275, thinner with larger waterways to help with cooling and thicker to provide more strength. In the mist of time, who knows? However, I do intend to sort out this engine so that it runs well, won't 'blow-up' and will give reasonable power. My customer doesn't want to go to the expense of a 4-pin diff, so I'm fitting a new comp. diff pin (thanks, Rich) with new planets, thrusts, etc.
A 'custom-curved' Aldon dizzy to match the final spec and a boost limit of 7 psi should do the trick.
By the way, I love your original 'S' rockers.
Peter
I've just a 998 city project atm.
I do want my friends engine though 1380 16v Turbo, been on the dyno producing in excess of 300bhp.
Couldn't tell you how reliable that will be and he's still trying to get a gearbox built to handle the power.
But getting the urge for another turbo engined pickup.
The problem is that it already has some play in it so I really have to change it. I once had a pin in my 'S' diff that wore down until the thickness of the pin in the planet wheel area was about 3/16". I don't know why it didn't just snap especially as it was like that for the last few tests of a rally. That's when I first went 'cross-pin'.
I just finished grinding out the combustion chambers and I got a nice 25 cc in each which gives a geometric c.r. of 8.5. I reckon that with a good custom-curved dizzy from Aldon and an intercooler, running 97 octane fuel nit will set-up nicely with 7 psi boost.
I'll put the block onto the box this afternoon and time in the cam. That'll keep me busy for a few hours.
I'll put the block onto the box this afternoon and time in the cam. That'll keep me busy for a few hours.
A bit late now, but it's entirely possible to run 7psi with the stock CR if you ditch the useless clockwork distributor and run a mapped ignition system (it certainly detonates with the stock system running). The addition of an intercooler makes this even safer.
Just FYI, if you can find a set the pistons fitted to the 1275 Metro Van have a bigger dish that gives around 8.5:1 with the stock head and are strong enough to handle 7psi. Just a shame the gearbox has the lifespan of a mayfly!
Just FYI, if you can find a set the pistons fitted to the 1275 Metro Van have a bigger dish that gives around 8.5:1 with the stock head and are strong enough to handle 7psi. Just a shame the gearbox has the lifespan of a mayfly!
Mr2Mike said:
A bit late now, but it's entirely possible to run 7psi with the stock CR if you ditch the useless clockwork distributor and run a mapped ignition system (it certainly detonates with the stock system running). The addition of an intercooler makes this even safer.
Just FYI, if you can find a set the pistons fitted to the 1275 Metro Van have a bigger dish that gives around 8.5:1 with the stock head and are strong enough to handle 7psi. Just a shame the gearbox has the lifespan of a mayfly!
The main reason for 4psi with 7psi bleed was the gearbox reliability, they are still wear much faster than a NA engine.Just FYI, if you can find a set the pistons fitted to the 1275 Metro Van have a bigger dish that gives around 8.5:1 with the stock head and are strong enough to handle 7psi. Just a shame the gearbox has the lifespan of a mayfly!
annodomini2 said:
Mr2Mike said:
A bit late now, but it's entirely possible to run 7psi with the stock CR if you ditch the useless clockwork distributor and run a mapped ignition system (it certainly detonates with the stock system running). The addition of an intercooler makes this even safer.
Just FYI, if you can find a set the pistons fitted to the 1275 Metro Van have a bigger dish that gives around 8.5:1 with the stock head and are strong enough to handle 7psi. Just a shame the gearbox has the lifespan of a mayfly!
The main reason for 4psi with 7psi bleed was the gearbox reliability, they are still wear much faster than a NA engine.Just FYI, if you can find a set the pistons fitted to the 1275 Metro Van have a bigger dish that gives around 8.5:1 with the stock head and are strong enough to handle 7psi. Just a shame the gearbox has the lifespan of a mayfly!
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