Technical Turbo question

[nmrmak]

I have a few questions about the turbos on cinq/seis as it's part ordering time. The plan is to do a 1.2 16v conversion + megasquirt (waiting for the parts) and after that's done, turbo it. Some work has already been done for the car safety and handling, and there will be more to come.

Main question: I've been thinking about it and i probably can't understand it because i'm tired (excuses ) but... if you have a NA engine, then you have vacuum at the throttle body. So when i install a turbo, i'll have positive pressure there (above atmospheric). How will the brake booster work then if it needs vac input? Maybe i got it all wrong, but it's just something that's bothering me now and i want to think things through. One idea is to take vacuum upstream of the turbo, but i don't think i've seen anyone really do that?

Side questions (i could probably do a search for this, but if you know the figures that'd be great):
- what boost is considered "low boost"? I think i read 0.3 bar for the van aaken. AFAIK, that requires no compression ratio work.
- what is considered high boost on a sei?
and finally, what CR do 1.2 16v FIRE engines have when stock?

The idea :idea: is to go for a garrett GT15 and aim for around 120 HP, which i think is quite doable with a 16v engine. What boost do you think i'll have to run for that? I've done some calculations based on what's said on garrett's site, but there are some variables i just don't know and so the boost varies a lot, too much to be called a good approximation.

Thanks for all input in advance, even if you tell me to go do a search Although, i couldn't find anything for the brake booster issue.

EDIT: P.S. Maybe it would be a good idea to update the faq (at https://www.fiatforum.com/cinquecento-seicento/39455-cinq-sei-turbo-faq.html) with the boost info, because that's the first place i looked and i guess it's a common question.

 

[StoneNewt]

In reference to 'cetno turbo conversion low boost should be considered anything where the CR doesn't need to be changed. This level will be around 0.3bar, but the exact level will depend on things like charge temp, how good your fueling is, etc.

What boost levels? Well that depends, my 1242 engine running its current manifold is making the same torque and more power at 1bar than my P75 manifold at 1.3bar. But I'd be expecting to be around the 0.5 to 0.7 bar mark.

 

[allanhelen]

You are on boost for very little of the time, so there is enough vac for the brakes to work, however i would suggest putting a one way valve in the vac line as close to the manifold take off as you can get it.

 

[nmrmak]

Thanks for the one way valve idea, never thought about that
StoneNewt: Are you running a 16v engine? Also, are forged pistons and sodium filled valves a must for 0.5 - 0.7 boost range? The compression on the 16v is probably different, but i won't know the exact figure until i get the engine and measure it, so no idea by how much i'll have to drop it.

 

[StoneNewt]

I'm running a 1242 8v engine & exotic valves aren't needed up in the 190bhp area. Forged pistons are needed in a roundabout way, you need to lower the CR some how & while a decompression plate works it has a negative impact on the combustion chamber characteristics, which aren't great to start with, so I'd go for forged pistons.

 

[allanhelen]

Forged pistons are not a must but a lower compression ratio is, i run up to 1.3 bar on dished none forged pistons. Forged pistons are the way to go for silly power but they do cost an arm and a leg.
(£600 to £700 ex vat)

You need to get down to at least 8.6:1 and with the engine you have chosen the only options you have are custom pistons or a decomp plate (these are not ideal but do the job and ~£100) .

 

[ycming]

all the effort if fitting the 1.2 16v, why not just got the 1.4 16v? then low boost that ?

Ming

 

[nmrmak]

When you say i need to get down to 8.6:1, what boost do you have in mind for that CR?

 

[allanhelen]

I have that CR and run 0.8 to 1bar day in day out and up to 1.3bar for short periods once you get over 0.4bar a lower CR is a must on these engines

 

[StoneNewt]

There's no gain in just low-boosting a 1.4 16v, by the time you've reduced the CR to around 9.5:1 you may as well go for a high boost 1.2 16v which won't give as many problems with excessive amounts of torque for the chassis. Basically the 'cento chassis can't really handle much more than 150lbft of torque & tweaking it doesn't do that much to help it deal with excess torque, it just handles out right power better.

 

[nmrmak]

Thanks, allanhellen!

StoneNewt: I was thinking about the 1.4 engine, but gave up on it because its availability in Serbia is pretty bad and from the conversions i've seen, it would take alot of work to get into the car. I decided to use a 1.2 16v engine because it flows better than an 8v engine, and since my setup is already MPI, i won't need too many additional parts for it. Thanks for the chassis idea, right now i'm running without rear seats and with the middle barrier thing in place (not sure how it's called, but you can see those on seicento vans). The difference between the barrier in and out of the car is literally huge. It's like a different car when it's not in, because it really tightens up the chassis. I can still feel some chassis flex when doing sharp direction changes, so a rollcage is planned too, along with a front top strut (but not sure how much that will help when the cage is in).

Thanks for all the info, i'll do a search on what the stock CR is on a 1,2 16v and figure out by how much i have to reduce it and how i should do it. As i said, i'll try removing some material from the combustion chamber, and if that's not enough, i'll have to use a decomp plate, but i've heard i might have timing problems with that, which probably means i'll have to remove some material from the belt tensioner. I'll see about that when i have the engine.

 

[StoneNewt]

The problem isn't chassis flex so much as basic chassis geometry & traction levels. My sei is caged, braced in the floor area & across the top of the engine linking to the firewall yet still pushing out more than 150lbft of torque makes the car harder & harder to deal with. Around the 170lbft point I didn't get close to WOT near peek torque on anything other than straight, dry roads in high gears.

In other words there was an area of the torque curve I simply didn't use, that's basically wasted output area under the torque curve. Part of my work with the manifold is to move that excess torque to the higher regions of the rev range which are significantly under this torque limit. The result is a car that's more drivable, quicker A to B & has a wider powerband.

I duno if the 16v engines do actually breath better than the 8v items after you've worked over the head. There seems to be very little you can do to improve the 16v head in terms of flow but the 8v head can be made to flow much better at higher rpm.

Timing problems with the 16v units doesn't make sense to me, the way you set the engine up is to get the engine to the mid point of the stroke and lock the cams in place (this may require some crank movement if the last guy didn't do the job properly), you then unbolt the cam pully & move the crank so everything truly is mid-stroke. now if you lengthen the centre to centre of the crank and cam pulley all you do is make sure both items are properly centred & everything is setup again. This isn't the same as a 8v where the cam pulley is rigidly fixed to the cam.

 

[nmrmak]

The timing problem, as far as i understood is that if you increase the crank-cam distance, you'll be stretching the timing belt too much, so my idea was to reduce the stretch by machining the belt tensioner. Again, i can't know this until i have the engine in my garage, as i've never seen a 1.2 16v engine "in real life".

The 16Vs should breathe better than 8Vs by common sense, but i'll be doing some work on the 16v head as well, hopefully reducing the amount of boost i need.

If you have that sort of torque problems, have you tried using road slicks when on dry roads? Maybe using a higher diameter tyre would help too. I used to get wheelspin very easily on stock 155/70/r13s, but now that i have 185/55/r14, there are no accidental wheelspins anymore. I can really feel the lack of torque now that the tyres are 2cm higher overall. Of course, slicks are usually not street legal, but why not try a bigger tyre?

 

[arc]

You are on boost for very little of the time, so there is enough vac for the brakes to work, however i would suggest putting a one way valve in the vac line as close to the manifold take off as you can get it.

There is already a one-way / non-return valve on the front of the servo, hence why there is still vacuum assist with the engine off (till you press the pedal a few times and use it up!)

 

[StoneNewt]

The 16Vs should breathe better than 8Vs by common sense, but i'll be doing some work on the 16v head as well, hopefully reducing the amount of boost i need.

All else being equal, so probably out of the box the 16v head flows better, the problem is it's hard to get any flow improvements out. However the 8v head can be improved & optimised to your flow requirements with little effort.

If you have that sort of torque problems, have you tried using road slicks when on dry roads? Maybe using a higher diameter tyre would help too. I used to get wheelspin very easily on stock 155/70/r13s, but now that i have 185/55/r14, there are no accidental wheelspins anymore. I can really feel the lack of torque now that the tyres are 2cm higher overall. Of course, slicks are usually not street legal, but why not try a bigger tyre?

I'm running 190/570R15 (cut) slicks for trackdays & it's easy enough to get to the point where you're constently lifting off to stop the inside wheel slipping, thus leading to understeer, on high speed corners. Not a problem in its self but as you try and find the limit you're oscillating between engine braking & fairly bad understeer, this often unsettles the car enough to cause unwanted oversteer.

I first though this was bad throttle control but soon discovered that using a lower gear, which means more thrust on the wheel, didn't show this problem. The reaction seems to be a problem with engine/drive train to body relationship problem. On a rolling road we actually measured that for a given thrust level at the wheels a higher torque level by the engine level makes the engine/gearbox shift more on its mounts, even more confusingly once above 140lbft even if the absolute thrust at the wheels is lower the engine can move noticeably more on its mounts.

 

[nmrmak]

Maybe a LSD diff would sort you out, but i see your point. It is after all a small car and was never intended for those torque and power levels. I hope that by turboing mine sensibly i'll keep most of the drivability of the car, especially with a 1.2 engine.

 

[allanhelen]

There is already a one-way / non-return valve on the front of the servo, hence why there is still vacuum assist with the engine off (till you press the pedal a few times and use it up!)

yes they do if it still works but it is only designed to hold it's vacuum against atmospheric pressure not 2x atmospheric. Why i say put one close to the manifold is that on boost you end up presurising the whole vac line and filling it with petrol and this was fuel needed by number 4

 

[nmrmak]

Can that happen on an mpi with no additional injectors?

 

[StoneNewt]

LSD did help but it's still there, now I do run without a front ARB but I have to say that the ARB made understeer far more prominent & problematic.

It won't fill the line up with fuel but it'll still potentially pressurise the servo line. I solved the problem by removing my brake servo

 

[fingers99]

The charge robbing will depend on where you put the servo take off/what intake manifold you use.

Does make some sense to pop another valve in, but I never tried it (and lost a piston).