Green fiat 128 1100 1976

any air filter assembly from a very long list of Fiats that use a similar carb could be used on a 128, as there is plenty of under bonnet clearance. the tallest and most voluminous filter housing is from a Lancia Beta,

using the stock 2 barrel progressive carb is going to limit the maximum power output of the engine, at the very very best you'll see high 90;s at the wheels, as thats all the air the carb will flow... on a european spec 1800 / 2 litre lancia it feeds the engine with enough air to make roughly 115/120hp at the crankshaft.

It also kinda limits your choice of cam / compression ratio in the engine you're building, as a very long duration cam won't have high enough manifold vacuum to properly atomise and draw fuel at idle / low revs... and at high revs the air speed wil be high enough to not allow the air to make the turn into the manifold runners very well.

SteveC
 
Are you going to put that carb on that "race" motor that you're rebuilding? Or is this for the motor in the car already? like something to drive while you build that motor? I agree with Steve that if it's the "race" motor, that 2 barrel carb is going to be a major restriction compared to the rest of the engine spec...
 
The stock carb might not be optimum for a race engine but it will work just fine. Folks (including me) have been using them for many years. :)
 
Yes that would be the carb going on the race motor and the last owner put down 120 hp to the wheels and if i remeber correctly he used this carb as this is the inlet manifold he gave me with it
What other carbs would you recommend for maximum power?
I dont know what cam it specifically has but im gonna visit him soon so i will ask if he remembers
 
MikeHynes said:
The stock carb might not be optimum for a race engine but it will work just fine. Folks (including me) have been using them for many years. :)
Click to expand...
I'm running one too, and I'm seeing around the H.P. figures that Steve quoted.. But my motor is fairly stock... slightly higher compression, slightly hotter cam. But the spec of that engine that he's building is a good bit beyond stock. Forged oversized pistons, larger valves, hot cam... Seems like a waste of high end components if you're going to strangle it with a small, not very high flow carb ....
 
I would think a pair of Weber downdraft DCNF 40's would go well with that setup...... I'm putting 4 flat slide Yoshimura bike carbs on my 2 litre that I'm building...
 
T.G said:
Yes that would be the carb going on the race motor and the last owner put down 120 hp to the wheels and if i remeber correctly he used this carb as this is the inlet manifold he gave me with it
What other carbs would you recommend for maximum power?
I dont know what cam it specifically has but im gonna visit him soon so i will ask if he remembers
Click to expand...
yet again, it's not about the gross HP number you can achieve, if it's a road car with standard gear ratios you want to be focused much more on the torque output.

And personally I would be very sceptical about that HP number...120 at the wheels would be above 135 at the flywheel, and I know that carby will just not flow enough air to get there.

Maybe he did have 120 at the wheels, but not with that manifold and carb combination... I'd hazard a guess that sometime in the last `15 years while the cars been sitting he's sold the dual carbs and extractors the engine had ... which is why the engine had no inlet / exhaust attached when you found it.

SteveC
 
Lowtechprime said:
I would think a pair of Weber downdraft DCNF 40's would go well with that setup...... I'm putting 4 flat slide Yoshimura bike carbs on my 2 litre that I'm building...
Click to expand...
How do you get 2000cc of swept volume? the longest stroke stock crank (78.4mm) and 87 bore is 1860cc give or take...
 
fiatfactory said:
yet again, it's not about the gross HP number you can achieve, if it's a road car with standard gear ratios you want to be focused much more on the torque output.

And personally I would be very sceptical about that HP number...120 at the wheels would be above 135 at the flywheel, and I know that carby will just not flow enough air to get there.

Maybe he did have 120 at the wheels, but not with that manifold and carb combination... I'd hazard a guess that sometime in the last `15 years while the cars been sitting he's sold the dual carbs and extractors the engine had ... which is why the engine had no inlet / exhaust attached when you found it.

SteveC
Click to expand...
Ah yes that could be very true indeed
The engine was compleetly disconnected from everything it was just sitting whith the motormounts on because he was always planning on refreshing the engine just because he had been racing with it for 3 years but he never came to it because of health problems thats why the car is so rotted now and he decided to sell the engine to me because i have been coming there here and then to ask about te cars and his racing history because thats very intresting so im the only one who he would sell to
But what carb / manifold would you suggest?
 
T.G said:
But what carb / manifold would you suggest?
Click to expand...
It will always come down to dollars - or euro in your case - that determines what you'll do.

I like DCNF pattern carbs on the sohc, but they can be expensive to buy. The chinese make copies of all sorts of Webers, but I'm yet to see a copy of a DCNF. You can buy IDF and DCOE copies very cheaply, but the quality and finish leaves a bit to be desired, caveat emptor.

There is always the option of a single simultaneous weber, DCNF and IDF manifolds for one carby are out there if you look.

After the expense of buying a carb (or pair of carbs) and a manifold then there will be the expense of setting it up correctly for your application.

Properly tuning / balancing twin Webers isn't something many people are good at, sure they can get them to run - but more often than not they are a long way from optimal. If you've never tuned a pair of twin carbs before it can be a bit of a steep learning curve.

A single simultaneous dual throat carb will be a lot easier to set up and half the expense in jetting components. But many people even find this difficult to get right, and not just your average lay person, I'm talking people who are trained mechanics, have a read of a few threads here, here and here. These days I think any mechanic who's under the age of 45 would have very little experience with carburettors.

Bike carbs are another possibility, but apart from needing a bespoke manifold, then there's the whole issue of tuning them properly to suit... they may come from a similar capacity engine, but a bike engine will turn a LOT more revs... at low air speed they give all sort of tuning issues, and finding someone who knows how to tune the particular type of bike carb you use is even more difficult than finding someone well versed in Weber tuning.

For me the best bang for buck option is a single DCNF pattern carb, I've put one on top of a modified stock manifold for the lemons racer I'm building, see it here

SteveC
 
Lowtechprime said:
I'm running one too, and I'm seeing around the H.P. figures that Steve quoted.. But my motor is fairly stock... slightly higher compression, slightly hotter cam. But the spec of that engine that he's building is a good bit beyond stock. Forged oversized pistons, larger valves, hot cam... Seems like a waste of high end components if you're going to strangle it with a small, not very high flow carb ....
Click to expand...
If you can run dual carbs, or a larger carb, etc. you should.

But sometimes you have to use a standard carb. At least that's what the SCCA required for G Production class X1/9s when they were first classified - many years ago. Many racers built engines with high compression, big cams, forged pistons, etc. Yes, they were limited to stock size valves, and a limited overbore, but cams and compression were pretty much open. Those hot rod engines ran pretty well, and some still do. ;)

I don't know what the highest HP that was achieved back then, but I have dyno sheets for some "top tier" engines, that shows 120 HP, with a 32DMTR. But I'm sure the stock type carb was a limiting factor (just like the stock size valves).
 
yes but an engine / carburettor requirement in a race car that has rolling starts, never sees less than 6k and is mostly at WOT during a 20 minute sprint race is a totally different requirement for a street car that needs to idle, has standing starts, needs reasonable pull at lower revs and isn't driven at WOT for just 20 minutes at a time...
 
Racecars need to be able to take off from a standstill. They also need to idle. Heck, they even need to be able to back-up. Most of the issues related to using a racecar on the street are related to the gearing, clutch, and camshaft, choices. Not the carb.

I understand there is a difference between race engines and hot street engines, but it sounds like the engine in question is going to be more like a race engine than a street engine. Given all the requirements for a street engine that you list above, I think you make an argument for a smaller carb - idle, pull well from lower revs, etc. :)

I actually think the 32 DMTR is quite capable of working well in a hot street engine, but if given a choice, I would choose to use dual DCNFs, or some kind of FI. That's why one of my '74s has dual 40 DCNFs on it. ;) To sum it up, I think we agree that it's best to have a bigger carb, or two bigger carbs on a hot street engine. I don't think we agree that a 32 DMTR will work on a hot street engine. And that's OK with me. :D
 
I visited the old owner tonight and he said he always used this carb on it which would fit the manifold that i got with the car
He said that because he always ran a smaller carb he used those thin and bigger valves to get more air into the engine ( he showed me a set thats the same as i have and they are very thin compared to the original ones which he also had a set of ) and thats why he could get such high numbers whith this 2 step carb
And yeah its pretty much gonna be a race engine in a car that only has vw mk1 rabbit/mk1 golf front adjustable coilovers which i modifyd and in the rear it just has some stiffer schock asorbers and original leaf spring
im not planning on modifing the rest of the car very much (only time will tell ofc😜)

He also has a jetting kit for that carb which he would let me use for free so i dont have to buy one and would help me tune the carb right if i buy one like that so im really struggeling on what carb im gonna buy
 

Attachments

  • 20230602_203519.jpg
    20230602_203519.jpg
    155.8 KB · Views: 74
  • 20230602_203512.jpg
    20230602_203512.jpg
    153.3 KB · Views: 75
You must log in or register to reply here.
Back
Top