Rejetting carbs

Russ, I expect you're right...and your weber tuning abilities exceed mine. From a very practical perspective I've found that there's a fair number of carb'd 308s running around that seem to have non-matching webers ie. different sequence/size of progression holes drilled in the throats in one or two or three of the four webers. This makes tuning very difficult! A guy I know had exactly that situation and was changing out jets and etubes like crazy (he didn't notice he had non-matching carbs) and it was driving him nuts as the car just wouldn't run right on any combination (for good reason)!

 

How could that happen?

 

Somebody replacing a single dash-numbered carb for a specific car with a replacement -12 or one from another car.

These carbs are much like radial tires - all of the set must match. E.g., if you can't find a matching set of OE, then you have to use four of the -12 replacements.

 

OK, so I finally got the car onto a Dyno.
Here attached are the graphs.
To remind you that it is an early carbed 76 GTB. P6 Cams, supposedly Borgo Hi comp pistons etc. (haven't been down that far so can't confirm)
No restrictions in the air box.
Aimed well and truely for the track.
HP (although in KW on the graph) comes out @ 206 HP which isn't too bad. The torque is good @ 216.
The AF ratio however is a disaster. I know that it is cranky around 2600 and you can see that on the graph.
I thought it would be running too rich and I was right.
The other major dip in the AF ratio @ 5200 you can't actually feel at all, Nevertheless it's pretty bloody rich.
Note that the torque dips when the AF goes too rich.
As discussed previously I have 36mm Venturi, F36 Emulsion, 140 mains, 170 a/c, 55 idle Thats how the car was delivered to me.
And I was pondering going to F24's and maybe F6.
Would this tack be a folly now as I am already too rich? or do I go to F24's or F6 and go to a smaller main and larger A/C etc?
The dyno figures above with 40 degrees advance and the dyno was a hub mounted dyno.
Give me your thoughts.

Steve
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The first thing I would do is get a set of 34mm venturis; In a street car with non ported heads, 36 is a bit big. To fully use the 34s, youll still can spin pretty high.
I agree with F24
The 55 idle are probably ok with those cams
Empirically would go with a`135 or 140 main and start with a 200 a/c
These are just my opinion - and al contingint on 34mm venturis and 24 ets

 

+1 with what Russ said.

I am confused. How is it making more torque than bhp? I believe the formula employed by a dyno is: torque x rpm / 5252.

 

Per that calculation you are at 248.36 bhp at the hubs which is about 285ish at the flywheel. That thing must move!

Are you not getting any detonation with 40 degrees total advance?

A vid of this engine in action would really be neat

 

By my calcs the 153.7 KW equals 206 HP at the wheels.
I believe that calculated losses for a hub driven dyno are less (makes sense) than a typical rolling road dyno.
Maybe in the order of 12% loss instead of maybe 18% for a rolling road.
How do you get to the figure of 285?

No detonation at all.

Never thought about videoing the dyno, but once i was there and she was @ 8,000 in an enclosed room, then I thought that a video camera would be a good thing. I will have my video of the car on the track hopefully finished this week. 7 laps, 22 mins, heaps of camera angles, even a version for your I-phone if you want.

Steve

 

After seeing the pretty terrible AF ratio results I started thinking about the accuracy of the jets in the carbs.
Everything is as deleivered to me, but many times you see people trying to work out their carbs only to find that jets have been drilled etc or every carb doesn't have exactly the same jets. Emulsion tubes drilled etc to try and match another tube.
I will pull all of them this weekend and verify that all are as it would seem.

Maybe a lot of the problem is in the idle circuit and leaning that out may point it in the right direction?

Steve

 

Does anyone have a set of F24's or 34mm venturi's they want to sell?


Steve

 

I calculated that number by multiplying your lbs torque x the rpm that torque is generated at then divided by 5252. I have yet to see a 308 engine with more torque than bhp so you confused us all quite a bit with that one. I figured 12% drivetrain loss as well due to the hub dyno. The only other P6 cam engine I have seen a dyno sheet for was with 32mm venturi and it yielded 277bhp with 216lbs torque at the flywheel. Your 216lbs of torque at the hubs seems to be consistent with somewhere around 280bhp+ at the flywheel according to that rough estimation.

I don't think I would lean out the idle circuit. It looks like the main circuit is just kicking in too early. Trying F24 tubes with 200 a/c and 135 mains would be a good start. Also, how is your ignition advance at that rpm where it is very rich? Any idea how many degrees it is getting there?

 

And yes, you would be my personal hero if you posted a good vid of this engine in action!

 

how funny...I just found 12 x 34mm venturis from my old boxer!

 

This is NEVER a wrong course of action. Excellent move.

As to leaning the idle circuit: It's something I like to do, but remember you are running high overlap P-6s with poor vacuum signal at idle which requires a bit larger idles. If anything, skosh down to 53 - Normal cams can get by with 50s, so I am guessing here P-6s may get by with 53 or 55. Remember everytime you go down a bit on idle, you have to do something to bring the main in earlier. The 34mm venturis will do alot to help you do that, without going to an overly rich main.

Good work!!
Best
rt

 

How handy!
Pete are you willing to part with 8 of them for a fair price if need be?

Steve

 

From approx 5,000 onwards it max's out @ 40 degrees.

I am wondering now if I should do some preliminary tweaking of the jets etc on my own with a gas analyser before going back to the dyno.
You guys use an LM-1 don't you? What do they cost and where can I source one from?

Steve

 

Ya know, I have absolutely no use for them, but there's a sentimental attachment. A friend of mine thinks he has 8 though. If you have no luck, let me know and I'll ask him. We're an Australia, which doesn't help much.

 

I got mine on Ebay through the US.It was cheaper than buying it locally...

 

Hey I'm only in Sydney! not that far away.

Steve

 

lol

oooooops....i didn't realise!

 

I just bought F-24s from Pierce for $27 each - they have them, don't know about the 34s, should be readily available.

hth,
chris

 

Well, the incredibly short story is yes, the smaller ventris have a higher velocity

Smaller venturis have increased signal and are much more responsive (read: "drivable") than larger ones. The larger venturis will ultimately flow more air --IF-- the engine and head can flow more air. That is you can over-carburrate a car with too large venturis that the engine can't generate enough of a suction to use properly.

In a normal 308 - and all Ferraris in my experince - all of the components are pretty well matched to each other: head, cam and carb (40DCNF, 32mm venturi) giving a good low range drivability and providing progressively more air up to near redline. Now, say you wanted to spin it a bit faster to take advantage of P-6 cams. Now you must determine between the head and the carb which is the flow restriction (ideally you would upgrade both, but...). For the sake of whatever, we'll just say that the head actually can flow a bit more at higher rpm (it can), so you can increase the flow through the carb with a bigger venturi. What you loose is some of the low range respnsiveness, because of the lower velocity through the wider venturi and the lower "suck" at low rpms due to the high overlap, high rpm P-6 cam.

There is a Weber chart at home (I'm travelling) that perhaps someone can post relating cylinder size, rpm and appropriate venturi size.

 

+1 what Russ said.

Another way to explain how a larger/smaller venturi must be matched with the flow rate of the engine is to think of the venturi as a restriction in the flow which creates a low pressure point allowing for the fuel entering into the throat to be more easily atomized. The smaller the venturi, the faster the flow at the maximum restriction point and the lower the air pressure allowing for better atomization of fuel. Better atomization = bigger bang in the cylinder. To illustrate, gasoline laying in a pool on the ground is certainly flammable but it's not highly explosive like gasoline which has been atomized (broken down into very small droplets with much greater surface area allowing for faster combustion/combination with air/O2).

If you put large venturi on an engine with only modest displacement (like a 308) then you don't get much accelation of the flow through the venturi and therefore only a modest/minimal drop in pressure which prevents the fuel from fully atomizing. This is a different (but related) concept to the AF ratio. Even if you have a perfect AF ratio you'll get poor performance if you have poor atomization due to incorrect venturi sizing.

The reason why too-large venturi is most apparent at low rpm ("she lost some on the bottom end there for some reason Bubba") is because it's at low rpm that the delta between the optimal and the actual flow rates and pressure drop is proportionally the greatest.

I'd recommend trying the 34s as well.

 

OK, thanks guys for the info.
With all respect to the guys @ Maranello back in the 70's when these cars were made and indeed when the Le mans car was born, what tools did they have at their disposal?
Nowadays we have Dyno's (did they have them then?) portable AF ratio meters, 4 gas analysers etc etc.
Certainly they had many resources and heaps of people and time for trial and error, but I wonder what techniques they had in those days to resolve issues like this?
With the theory that they determinded 36mm venturi's and the F6 ET's, I wonder how correct it was?
Were they just trying to get as much fuel and flow into the engine? but did they have the tools to correctly measure the changes they were making?

Russ I do have that Weber grpah also and will try and dig it out.
From memory it does show that 34 to 36 is good for the cylinder size of the 308 at around 7500 to 8500.

Maybe a smaller venturi, (faster flow) might be a way to remedy some of the reversion you get @ lower rpm's?

Steve