Help Please tuning guys, rally guys 1200NZ bottom end ST head

[dstevenson]

Having been watching your forum for a while I know one of you experts on here will know all the answers !!! :cool:

I am building a new engine for a 1200 nova class for stage rallying (for my 12 year old son !!). Must post up some pics as it is a nice car and a super class we run in - what junior rallying/motorsport is all about to get them started young. :thumb:

Currently running basically a blue printed 1200ST but it needs a freshen so we are building a complete new engine, hopefully a little better but don't want to cheat....too much, lol

We are boring a 1200nz (spi) block to max std o'size (1mm allowed), new pistons, lightening, balancing, etc.

I know the 1200 carb head is much better than the restricted spi head but wondering is there any compression loss when using the carb head on the injection block as the nz block has much smaller bore (72mm) compared to the st block (77.8mm).

I had considered boring and fitting the 1400NZ pistons (77.8mm) but that would be cheating !! We know others are doing it but don't really subscribe to the "...if you cant' beat 'em join 'em..." mentality. They will get caught and our day will come!! ;)

Also plan to use the 1400nv cam as I know this is a much better cam but was wondering if there are any piston clearance issues with this combination.

Basically help from anyone who has done this before, knows about how successful it is, or problems we should be aware of before we end up doing something very silly.....and expensive, lol

Thanks Guys for a super forum :thumb:

[Adam]

The 1400nv cam will be fine on std valves etc.
Altho you will have to watch the dizzy drive plate(1.2 have a SLOT drive) whereas the 1.4 cam has a pin drive.
So might need a dizzy change.

I think compression ratio will drop with a smaller bore.
Might be better getting a 1200carb(12st) bottom end.

What inlet setup etc are you using? The std 1.2 carb and inlet?

[dstevenson]

The 1400nv cam will be fine on std valves etc.
Altho you will have to watch the dizzy drive plate(1.2 have a SLOT drive) whereas the 1.4 cam has a pin drive.
So might need a dizzy change.

I think compression ratio will drop with a smaller bore.
Might be better getting a 1200carb(12st) bottom end.

What inlet setup etc are you using? The std 1.2 carb and inlet?

Thanks for the reply. :thumb:

Have to use standard carb & inlet (P'burg 1B) from the 12ST but have fitted larger jets which improves fuelling. Actually fitted a weber 32/34 DMTL on 1300 inlet to try it and did not find much more power but improved response. However, not allowed to use it in the class rules so back to the P'burg

Should be OK with cam drive as I still have the 12NZ injection dizzy (pin drive), ECU, etc. which appears to work well.

It is the block/head combination possibly giving a compression loss as you say that worries me most. That's why I wanted to know if anyone has tried it.

Thanks for your help.

[garyc]

The best thing to do is measure the CR. Most race engines that I know deck the block so the top of the pistons are level with the top of the block. I'd try and get the CR as high as possible as thats where all your power gains will come. Use some kind of additive if you are worried about detonation.

[mikey14sr]

Are you not allowed to use a different single choke carb, Say a weber 32tlp as found on the 1 litre nova?

[dstevenson]

Are you not allowed to use a different single choke carb, Say a weber 32tlp as found on the 1 litre nova?

No class rules dictate it must be a carb as fitted to a production 1200 so no twin choke 2E P'burg, Weber 32/34 DMTL, etc.

[dstevenson]

The best thing to do is measure the CR. Most race engines that I know deck the block so the top of the pistons are level with the top of the block. I'd try and get the CR as high as possible as thats where all your power gains will come. Use some kind of additive if you are worried about detonation.

Sounds about right to me. :thumb:

When we get the new bottom end we will check that all out during the rebuild and machine head/block as necessary to get CR right. just thought someone might have done it before and could say "...just do this...or that...", but probably not for a 1200 as most of you on here will probably ditch it in favour of something with more cc - easier and cheaper than trying to get the absolute max out a small cc motor - but that's all part of the challenge I guess. lol

[Chunky Lover]

had the same task with a one litre nova in autograss its amazing the ingenuity ppl think of!

[garyc]

Oh yes, & dont forget to get 3 angle cuts done on your valve seats!