1.2 Tuning - Realisticly, sensible thread!!

[pyromaniac_yeti]

The car;

Corsa 1.2 SPi

Bad start already, but with the apparent lack of cheap nova's and the headaches i had trying to use the last one for work...figured i'd have a crack at the next one up as a bit of a work hack.

Situation;

The corsa's started loosing a little coolant & burning a little bit of oil. The idle also isnt as smooth as it once was, probbably due to being thrashed half to death by myself. It's just not as quick as my old 1200 nova! Probbably due to loosing 10bhp in a fancy injection system & cat. Engines on its way, but it's not terminal as of yet.

So;

Teddy's offered me his 1200 from his nova as he's looking for a bigger lump to go in it. The engine has low miles, doesnt knock, bang or rattle - HOWEVER...It's got a knackered head gasket.

Therefore;

The 1200 carbed engine from the nova will fit straight into the corsa with minimal amounts of wiring alterations. The carbed engine should have the round ports on the head - and from memory that's what gained the carb engine most of it's extra horses. I've also managed to persuade him to chuck in the cam from a 1.3SR he's had knocking about. From what i've researched the carb engine nets me 10bhp extra, and the SR cam adds a further 5 to that, or thereabouts. Assuming its all setup propperly & depending on wear etc.

The head has got to come off to change the gasket, at which point i'll deal with the timing belt & waterpump, and clean the head/valves up a bit. Whilst it's off anyway it leaves the ideal oppertunity to fit the replacement cam.

So;

The head will need a skim, as it seems false economy to try slapping it all back together with a potentially warped head...and here come the questions. Back on the 1.2 8v tuning guide, various people went on about increasing the compression. If i've got to have the head skimmed anyway, its not likely to cost anymore to have a bit extra skimmed off...However nobody actually mentioned any figures. Was this ever worked out? If not, how do i go about calculating it - are there any good guides online for this sort of thing? What would be an ideal C/R to be aiming for on a fast road engine?

I've also got a lot of bike bits knocking about including a bank of carbs somewhere, fabricating a manifold isnt an issue - however would there be any real gains from doing this, i recall the 1.3SR nova's had a twin choke carb rather than the single choke on the 1.2's and that this was supposed to offer an increase in power, however nobody could quote a figure. Fully aware of the fuel pressure issues, would probbably ditch the in tank pump & fit an inline electric pump from a bike on the firewall or simmilar. Or would OEM manifold modified to suit a twin choke of suitable size from the scrappy be a better choice?

And lastly, exhaust manifold. They're pretty expensive. I dont feel like paying tons for a fancy tubular one to extract 2bhp from a 1.2 engine. However, if i'm fabricating an inlet manifold for bike carbs i wouldnt mind having a crack at replacing the crappy cast factory manifold. This would also allow me to do away with the corsa downpipe & thus the cat. I know you can buy collectors for DIY fabrication projects online, however theres a set formula for tube diameters & primary/secondary lengths to get the best from an engine. Has anyone ever actually purchased one of the fancy manifolds for the small engines & if so could they possibly post up a picture w/dimensions on it??

Whilst gas flowing the head would be nice, would make good gains etc - its far beyond me, i dont know what i'm doing, i dont have a flow bench, and im not shelling out £300+ to get someone else to do it for me. The heads port shape alone is enough to be considered an upgrade from what i currently have!


Looking for around 75bhp & something to do with my evenings & spare time for the next month or two...Any constructive help/pointers would be appreciated, however i will point out that this is a for the hell of it project, more of a technical excercise than anything else :thumb:

Many thanks

Iain

[pottersrebel]

the only thing i can comment on is if your going to the trouble of uprating the engine and skimming the head then also consider skimming the block, both the head and block can warp so worth doing both if within your budget, also while your having the head skimmed get the flywheel lightened it should make it more rev happy

[mowgli]

1. the 1.2 carb head only needs skimming if there is surface damage.
2. the reason it makes 55hp is that it is a revvy unit & not some long stroke, small bore emissions designed crapper like the spi is
3. the sr cam is exactly the same as the std 1300 & it might gain you 2hp at most.
4. fit a 4-2-1 manifold from any of the corsa 8v range. it will be an obvious improvement over the 4-1 nova pipe.

but the most important thing is this.. you are always chasing the odd hp here & there with a 1.2, and a std 1400 bottom end will be a massive improvement over the std 1.2 bottom end. and if you simply get a 1.4 bottom end & fit all the 1.2 top end, including single choke carb, you will be getting about 80-85hp without any tuning parts(apart from the factory 4-2-1 that is).

you will have to spend on rigging up a decent method of getting fuel from the tank, as the spi pump will be too powerful for a carb, so some sort of regulator will be needed.

[pyromaniac_yeti]

Regarding lightening up the flywheel, how much is too much? Lack of flywheel weight would make the engine easy to stall/loose speed & harder to start. I belive theres a fine balance - but whats deemed acceptable?

As for skimming the block, the headgasket wasnt badly blown so i dont think it'd really need it. I've always known heads to get a skim whilst replacing the head gasket, so pretty much the only reason for doing was for peace of mind.

Mowgli,

Would any 1400 bottom end work, or would it have to be the carbed one? The 1400 head on the 1200 would significantly lower the CR, so doing it the other way round would up it. Has it been done before & would i have any valve clearance/detonation issues?

Fuelling wise i'd planned to completely remove the in tank pump & run with the mechanical cam driven unit on the 1200 head if opting for a car carb. I belive theres a few common VW's that run a 32/34 weber, which was offered as an upgrade on the SR's wasnt it?

[mowgli]

ok, time to put my army helmet on again..... skimming is totally unnecessary on 99% of all cylinder heads. some do, most don't.. but take one to the engineer & he will always say it needs skimming...then charge you money for doing it.

people always go on about warping etc... right, then they tighten the head down with enough force to pull a house over & then start the engine up & let it heat up to nigh on 100 degC.. the block is iron, the head is ally, they expand at different rates then contract at different rates.... skimming is not important since copper gaskets & iron heads fell out of favour in the late 70's.

re flywheel skimming, the 1.2 flywheel is not exactly heavy, and you could skim some from as far out as you dare & not from where the clutch fits, ask stu, its about inertia, but i don't thinnk a road engine should necessarily have it done, from a reliability point of view..

[John]

*runs for cover*

[pyromaniac_yeti]

*runs for cover*

Did i say something bad?? Lol

Guess it'll have to wait on initial inspection & actually getting the bloody thing out of the car at the weekend. I'll post pics & let y'all make a decision based on what you see.

Probbably stick with the 1200 nova bottom end unless i can get a 1400 bottom end for peanuts as per this engine. If one pops up then i can always put the clean head & such on that block instead.

[Stuart]

The 1300 cam will net 5bhp on a healthy 1200. velos did the test many years ago.

Skim the head, gain compression ratio, gain a shade of power. Job done.

[pyromaniac_yeti]

Everyone says skim the head on all the guides, however - how much do i get taken off so as not to have the above mentioned clearance & detonation issues??

[Balley]

They take 60 thou off 1.3 heads for class two grassers

[mowgli]

They take 60 thou off 1.3 heads for class two grassers

and then run them at full revs for a few minutes..

this is for a daily driver.

re the bottom end.. all 1400's will fit. you could even fit a 16v one as the pistons would give better compression.


re the skimming argument. yes skim for raised compression if its the best way to do it, but not simply cos the gasket needs swapping.

[pyromaniac_yeti]

What about an x16xe? I've been offered one from a mk4 astra for not a lot as all the electrics are questionable on the engine. Would just swap that in, but by the time i've bought a new loom, ecu kit & replaced half the sensors it's going to get expensive.

[garyc]

You would be surprised how little skimming the head on a 1300 head does to the static CR. Even with nearly 80 thou of mine and a decked block I was quite surprised on the actual figure that was measured at build time.

[mowgli]

the x16xe needs the later f13 or f15 gearbox too, but the torque you'd get from the xe bottom end with the 1200 head would be quite good.

[pyromaniac_yeti]

Whats the thinking behind the autograss cars running the SU's?

http://i950.photobucket.com/albums/ad345/gsi-tommy/nova037.jpg

http://www.moddedmicras.co.uk/upload/files/25-Picture%20392.jpg

To me that looks like it'd flow poorly, however everyone appears to be doing it. I'm aware of the class 2 restrictor plate rules, but whys the SU the carb of choice? And why setup like that? If they're doing it on the micras too (and the one i've pictured was a pro built engine) then it seems like a lot of aggro to go to to gain nothing - im intersted as to the reasoning behind it all :confused:

[chrisd1986]

Do you need to do any other mods to get the 1200 head on a x16xe block? This would be a interesting setup

[burgo]

You would have to check valve clearances

[Stuart]

An su carb has no butterfly to get in the way, technically a superior design to traditional carbs, but they are freaking massive.

[mowgli]

i think its also down to putting a control on the performance too, and i think the original regs would have been set up around minis & then metros, which had the su as std, obviously.

isn't the only way to tune them to machine down the needle??

[garyc]

A motor bike carb is the same design as an SU and they do have a butterfly. Once you understand how they work it is quite easy to tune them\ set up on a rolling road and Mowgli is right the needles are machined (by file or emery) to the right size. The original regs go back to MK1 & 2 escort days and mini's or metro's never raced in class 2. They seem to produce slightly more torque, compared to say a weber, in the low mid range so I guess that is why people use them, why this is I dont know.

[meritlover]

An su carb has no butterfly to get in the way.....

this is not true.

[mowgli]

http://www.mgbexperience.com/article/images/su-hs4-section.gif
let battle commence

[meritlover]

this diagram is very primitive. there are no details of the cross drillings. but if you understand how a venturi works then the principle operation and reason for its beauty becomes obvious. particularly advantageous to those who dont want to change many jets or play with accelleration enrichment plungers and jets.

it is a beautiful design

[Andy]

lol mowgli

http://t1.gstatic.com/images?q=tbn:ANd9GcSHwv8qyeeltx0ovUQ1qH7zNyklEnvtL-qN7YSw7Jrkpxmlh6re

[mowgli]

you're welcome..

[Stuart]

meh, they dont 'need' a butterfly.

[Andy]

meh, they dont 'need' a butterfly.
Why not? im not trolling here btw,im actually interested to know

[Stuart]

Ever played with Nitro RC cars? Same concept.

You have the shaped 'rod' filling up the 'venturi' and then have the throttle action move the rod outwards to open up the 'venturi' to let more air through (and of course fuel). There is technically less of a restriction using it that way than a traditional butterfly setup.

[Andy]

Thanks for that.

[meritlover]

meh, they dont 'need' a butterfly.



this is incorrect. you miss the point of a C.V carb entirely

[meritlover]

and dont seem to appreicate how/why it works like it does.

[pyromaniac_yeti]

Motorcycle CV carbs have a butterfly and a slide, the butterfly opens as a throttle. The slide position depends entirely on engine vaccum thus doing away with any need for an accelerator pump. The carb slide position depends entirely on load, so as such the carb will only ever open up as much as the engine needs nomatter how hamfisted the operator is. Poor explanation, but more or less to the point.

Webers and the like have an accelerator pump system instead, hence ****e fuel economy when you run a 45 DCOE on a mini instead of twin SU's

Now - carb wise

Nova SR pierburg 2e twinchoke & matching manifold VS mk1 Astra 1.6 manifold & GM varajet twinchoke

Which & why?

[meritlover]

Motorcycle CV carbs have a butterfly and a slide, the butterfly opens as a throttle. The slide position depends entirely on engine vaccum thus doing away with any need for an accelerator pump. The carb slide position depends entirely on load, so as such the carb will only ever open up as much as the engine needs nomatter how hamfisted the operator is. Poor explanation, but more or less to the point.

Webers and the like have an accelerator pump system instead, hence ****e fuel economy when you run a 45 DCOE on a mini instead of twin SU's


thank you for restoring my confidence in man kind. A correct answer at last. well written, technically accurate and informative. there was too much false data in the previous posts and a meritlover response woult have resulted in personal attacks, abuse and tears which must always be avoided.

i can tell we are already feeling each other.
xxx

[mowgli]

if you can find a varajet with a 1.3 manifold, you will be very lucky. the performance is better than the pierburg, the economy is about the same, but they are rarer than a bacon sandwich in tel aviv...

i never understood why gm stopped using their own carb & bought in..

[Stuart]

if you can find a varajet with a 1.3 manifold, you will be very lucky. the performance is better than the pierburg, the economy is about the same, but they are rarer than a bacon sandwich in tel aviv...

i never understood why gm stopped using their own carb & bought in..

Costly, and in that era carbs main stream death was on the cards. Just buy in what is mass produced, cheap, reliable enough and isnt going to leave a factory of your own guys out of work when superior injection comes along.

You can still run a slide only carb, it is possible. Ok so I got the SU butterfly bit wrong, meh I can deal with that, I wont cry :p

[mowgli]

they lasted another 6 odd years, and they were still fitting them to astras at the same time as putting pierburgs on novas...

[Stuart]

6 years is 'only' one maybe two model revisions, while injection planning and testing would have been well under way (took 5 years to upgrade Transit to a proper PCM and that was in the time of really fast development)