Weber carbs - reversion plate

[Randy V]

Hi

I have heard that if you are in this situation and you get a carb top fire you need to hit the accelerator which will actually suck the flames into the combustion chambers

Is there anything in this?

Or do you just fit a good and refillable fire extinguisher pointed at the Webers?

Ian

Carb-top fire is true, you can suck in the flames. However, you'd have to be pretty quick to suck an explosion into the engine... IE - no, not possible. Once the vapor is ignited, a millesecond later you have a huge bang and hopefully your rear window to the cabin has protected you..

 

[Mike]

Maybe there is a reason Webers fell out of favor to 4bl carbs

 

[Scott Calabro]

Why would you have to fiddle with jetting once you had it dialed in?

Thanks Jac Mac

Scott, do you have any video of your car/engine? Why race gas with 11:1?

Mike,

Go to you tube and search "731dash60". Look at the 289 engine runs. Video 1 shows a 600 Holley on a 180 intake with no cloud at high RPM. Videos #2 & 3 show a reversion cloud, particularly video #3.

The 11/1 ratio was used because I wanted to get my right hand drive/shift, 289 to replicate (289 HiPo had 11/1 also IIRC) as close an experience to driving a GT40 road coupe as I reasonably could. Also my car was a demo, so in my mind having a 13/1 LeMans cammed 289 would have been a turn off to most prospective customers.

I would have thoroughly enjoyed it though

As Randy correctly posted, when running Webers your end up with essentially 8 carbs. As there is no common plenum to mask the effects of changing density/temps, you end up feeling, seeing & hearing those effects more. If you are going to run your setup in an area that will experience drastic changes density altitude and temps, the effects of mixture change will be more dramatic than say running at sea level on a 50 degree day.

It's my feeling that the Le Mans cam will be great if you want that type of experience. The high compression ratio is needed to compensate for the amount of crankshaft duration that both valves are open (overlap) and the resultant loss of cylinder pressure.

My setup ran best in the morning on days with higher barometric pressure. Like your sled, I had my setup slightly on the rich side.

If you do go this route, you will find that oxygenated pump gas will go stale in Webers very quickly. The fuel that is left in the float bowls on shutdown will cook out the ethanol and seemed to become "stale". Running 93 pump, my setup was difficult to start after sitting only a few weeks. Once I switched to AVGAS or Sunoco race gas, that went away. One fall I parked it in the hangar at work and did not turn the key to spring six months later. After running the elec. pumps and pumping the accelerator a few times it fired off like it was only sitting one day

As always YRMV.

If I was building a personal car and not another demo, I would run what you are thinking of doing. The car is a toy anyway, make it the toy you want!

Cheers,
Scott

 

[Mike]

Why are you seeing reversion at high rpm's? I would expect it at low rpm's...

 

[Scott Calabro]

Why are you seeing reversion at high rpm's? I would expect it at low rpm's...

You would think, and in some combinations that may very we'll be true. With my old engine's combination of parts it was obviously more present at high RPM with the load on the engine stabilizing. It is not the best quality film though. On the road I sometimes ran with open stacks and never observed reversion in my Lucas 608 :laugh:

If I understand correctly it has to do your particular combination of parts and time.

This article explains it well;

Race Engine Reversion - Understanding Reversion And How It Affects Power - Circle Track Magazine

 

[Cliff Beer]

Well, more is more......if there's some reversion at low rpm then wouldn't there naturally be more at high rpm? After all, that's more time with inlet and exhaust valves both open, more fuel flowing, throttle plate open more, etc.....

 

[Jac Mac]

Well, more is more......if there's some reversion at low rpm then wouldn't there naturally be more at high rpm? After all, that's more time with inlet and exhaust valves both open, more fuel flowing, throttle plate open more, etc.....

Magic word above is 'Time'
Lets say we have 120° overlap, 1/3 of one revolution.
Whatever 'time' the valves are open for @ 1000 RPM is six times longer than it will be at 6000 RPM.... Cliff can get his bean counter skills out & work the actual numbers for us!

 

[Mike]

Overlap would cause reversion at low RPM but I'm still not clear why you would see reversion when RPMs go up as then overlap begins to help flow. Seems counter intuitive.

 

[Jac Mac]

In an IR setup the air/fuel mix does not flow continuously into the cylinder, each time the inlet valve closes flow stops, however the air/fuel mix is still moving and as it stops, it compress's a bit in the inlet runner before bouncing back out again. How far it bounces back out is a function of the speed and size of the venturi's plus all the other stuff at the time-RPM, runner length, valve size, throttle opening, engine load etc... So we effectively have two types of reversion, the low RPM type with low airflow rates where the overlap literally blows the fuel mix back out, and the higher RPM type that gets 'bounced' out........ That's about as simple as I can make it sound & Im sure some boffin somewhere would kick my butt for that terminology.
Its also why a 4bbl has much more going for it since there are 2 or 3 inlet valves open at any given time and a continuous flow demand as a result, the stop/start scenario is still taking place in the individual runners of the manifold, but the plenum in the manifold helps to kill the effect of these. Tom from Germany was in the process of fitting one of the 'new' IDA intakes with what I would call a divorced plenum prior to his accident, but I don't think it ever got fitted to the car, was not on it at time of accident.

 

[Jac Mac]

extra::

In an IR setup the air/fuel mix does not flow continuously into the cylinder, each time the inlet valve closes flow stops, however the air/fuel mix is still moving and as it stops, it compress's a bit in the inlet runner before bouncing back out again. How far it bounces back out is a function of the speed and size of the venturi's plus all the other stuff at the time-rpm, runner length, valve size, throttle opening, engine load etc... So we effectively have two types of reversion, the low rpm type with low airflow rates where the overlap literally blows the fuel mix back out,........Then we have the higher RPM type that gets 'bounced' out!
I need to clarify and add a couple of extra thoughts here... Overlap occurs while the piston is at or either side of tdc, with 'le mans' cam exh valve is still open when the inlet opens @ 52° btdc , exh valve does not close until 42° atdc.... This is where having a longer rod helps... On your 3.00" stroke with the longer rod the amount of piston movement during the overlap phase is quite a bit less than what it would have with a shorter rod, since it hangs around TDC for a bit longer & does not move as far or as quickly it will not tend to pump as much out into the inlet port at low rpm when there is little or no exhaust scavenging taking place.......... That's about as simple as I can make it sound & I am sure some boffin somewhere would kick my butt for that terminology.
Its also why a 4bbl has much more going for it since there are 2 or 3 inlet valves open at any given time and a continuous flow demand as a result, the stop/start scenario is still taking place in the individual runners of the manifold, but the plenum in the manifold helps to kill the effect of these. Tom from germany was in the process of fitting one of the 'new' ida intakes with what i would call a divorced plenum prior to his accident, but i don't think it ever got fitted to the car, was not on it at time of accident.

 

[Mike]

Great explanation thanks

 

[Cliff Beer]

OK, just for the sake of debate.....

I get the inherent benefit of a common plenum with regard to lessening the effect of pulsing in the inlet flow....in other words a more continuous flow demand.... I believe this is partly why a four barrel on my SBF seems less sensitive and smoother running than IDAs....

What cuts the other way...at least with an IDA setup....is that a common plenum would probably diminish the precise metering and atomization of fuel into the individual intake valves and also disturb the very direct flow characteristics of the typical IDA inlet (single short tube, basically). That's just my guess/impression.

How about this? An equalizing /common "tube" plumbed into each IDA inlet downstream of the carb? Would this help to create a more constant demand flow while not dramatically disrupting the atomization and flow characteristics?

hhhhmmmm......

 

[Randy V]

There is at least one manifold I have seen with those characteristics.. While it won't help with fuel distribution to any degree it will give better vacuum reference to each carb as well as other vacuum operated devices.. There are also manifolds where people have just tapped in and run tubing (think 1/4") between each of the runners to a common manifold to be used as a vacuum source for a vacuum advance distributor.

 

[Mike Telow]

I love the sound of this GT40.

https://www.youtube.com/watch?v=M0WNa-K6-QI

 

[Jimmy P]

Mike, my all time favorite! If not mistaken this car was at Amelia a few years back and I enjoyed a nice chat with the owners care-taker. I heard the car coming down the road from its transporter and knew straight away what it was. One of the best things about Amelia (I'm heading there tomorrow) is the fact the transporters park about a 1/2 mile away from the show area so many spectators gather outside just to watch (and hear!) the cars driving to the hotel to get parked.

 

[Randy V]

Nice ...ummmm... Sound!!!!!

http://youtu.be/l4cS2OYtqb8

 

[Scott Calabro]

I'm partial to this one

http://youtu.be/xLOC8lp6MW0

 

[Randy V]

Sounds rather Gnarly Scott!

 

[Jac Mac]

Actually this 'IS' the sound you should all be aiming for...car #95... 4:30>4:45 .

[ame]www.youtube.com/watch?v=u-wkg1io6B8[/ame]

 

[Randy V]

If that doesn't get you a chubby - nothing will... Sweet music from end to end really....