Weber carbs - reversion plate

[Dutch]

Ray-sneaking pics of FE's in to a SBF discussion is a likeable form of thread drift!

Couldn't help myself :thumbsup:
The thought that something in front of the exhaust valve would have an effect on the exhaust note - can't imagine how that could happen.

 

[Jac Mac]

Couldn't help myself :thumbsup:
The thought that something in front of the exhaust valve would have an effect on the exhaust note - can't imagine how that could happen.

So are you suggesting that if we shut off the fuel supply the exhaust note will not change ????

Hope Randy uses the 180° exhausts on his next dyno session....

 

[Mike]

Here's the thing about the internet. You can come across as a jerk or a jackass and not even know it which I may be doing. When I say nuanced, I mean the finer details in addition to the 180 exhaust. The tone of an engine is much more than just the exhaust so yes, how an engine sounds is as much to do with exhaust configuration as it is induction and valve train noise. I was hoping to discuss the finer details about tuning and sound, not argue over cross over exhaust and what happens after the exhaust valve which seems like a given. This thread is about Webers and reversion and I posted here because I'm interested in hearing a car at idle that has Webers and a cam with at least 112 degree LCA. Randy, if you have anything that would be great. Thanks!

 

[Dutch]

So are you suggesting that if we shut off the fuel supply the exhaust note will not change ????

I wasn’t expecting such a “spirited” discussion and hope I’m adding to what is learned here.
My comment about what happens in front of the exhaust valve is that everything that is heard out of the exhaust has occurred after the intake valve has closed. An optimum air-fuel mixture needs to be present to build combustion pressures – how that mixture was created doesn’t seem like it would affect the sound out the pipes.
Then again I could be wrong . . .

 

[Mike]

Wouldn't it also be affected by what happens before the intake valve has closed because the induction cycle and overlap is responsible for what is entering the cylinder to then be combusted? I think there is a lot more to how a car sounds than any single thing. 180 headers or a cross plane crank will definitely be the foundation but there are several other components that will have as drastic of an effect on over sound as well.

 

[Dutch]

There is a point during the overlap of the valves where both the intake and exhaust valves are open and the exhaust valve closes just as the fuel/air from the intake valve reaches it. The cross ram manifolds the early Chrysler engines had – the optional dual quad manifold seen on the early Z-28’s were designed to take advantage of the energy from the velocity of the incoming charge – very much like supercharging. That would affect combustion pressure and the character/sound of an engine.

 

[Mike]

Right but we're talking GT40s and Webers.

 

[Jac Mac]

Right MIKE... Here is the 'hot oil'...
Your motor to make the 'right' noise.

289 crank (steel) if you can find one, use 5.315 rods with this.
or
302 crank ( steel), use the popular 5.400" rod with this.

SVO or Dart 8.200 block with whatever bore size you like 4.125' would be my pick.

Flat top pistons to match rod/stroke combo of choice.

Aim for 11/1 compression if you can get gas to suit.

Cam- Ford Le Mans C7FE-6250-A [ 108.5° L/C, 318°/304° @ 0.0125"/0.0155" ]

Heads I am tempted to suggest staying with good iron heads, but the alloy stuff helps with compression on todays gas.

Exhaust primaries 36" long, 1.625" od for smaller combo[4" bore], 1.75" for larger 4.125" bore.

Light flywheel/clutch/damper-balancer units & preferably internal balance.

That should do it, needs a really switched on engine builder who is up to speed with all the tricks to keep your flat tappet cam alive and with the better roller rockers available today the top end should be good for 7500 or more, the bottom end will be good for 8000+ with a dry sump setup.

 

[Randy V]

Hope Randy uses the 180° exhausts on his next dyno session....

We had the headers there but unfortunately they would not fit the dyno and the water brake assembly was too tall.. We had thought aout reversing them, but threw in the towel when we realized the logistics were just too extreme...

Mike, I spent a couple hours last night looking through raw movie footage and found a number of other interesting clips but none of the engine idling.. I may still have the tapes, but that will take a bit more digging as I've moved my residence since then.. There was another member here who had the same cam. I don't recall which one though.

 

[colin artus]

The IDA/Le Mans cam combo definitely has the 'ripping calico' sound but is no good on the road ( in 289 form). Sub 3500 rpm runs quite rough, oils plugs, pops, bangs, spits etc. 4000rpm on up simpkly glorious!

 

[Mike]

Right MIKE... Here is the 'hot oil'...
Your motor to make the 'right' noise.

289 crank (steel) if you can find one, use 5.315 rods with this.
or
302 crank ( steel), use the popular 5.400" rod with this.

SVO or Dart 8.200 block with whatever bore size you like 4.125' would be my pick.

Flat top pistons to match rod/stroke combo of choice.

Aim for 11/1 compression if you can get gas to suit.

Cam- Ford Le Mans C7FE-6250-A [ 108.5° L/C, 318°/304° @ 0.0125"/0.0155" ]

Heads I am tempted to suggest staying with good iron heads, but the alloy stuff helps with compression on todays gas.

Exhaust primaries 36" long, 1.625" od for smaller combo[4" bore], 1.75" for larger 4.125" bore.

Light flywheel/clutch/damper-balancer units & preferably internal balance.

That should do it, needs a really switched on engine builder who is up to speed with all the tricks to keep your flat tappet cam alive and with the better roller rockers available today the top end should be good for 7500 or more, the bottom end will be good for 8000+ with a dry sump setup.

Thanks, couple of questions. Not finding much info about what pistons to use with a 3" stroke and 5.40" rods. Regarding the cam. I don't really care about the driveability but I do care about burning the car down. Is the reversion bad enough with that narrow LCA that you will have fuel vapor cloud floating around in the bonnet and is the fire danger truly something to consider or is it more just poor driveability?

ripping calico

Never heard that before and not even sure what it means but I think that's what I'm after

 

[Scott Calabro]

Mike,

ROSS manufactures stocking pistons for use with a 5.4 rod in both 3 inch (302) and 2.87 inch (289) strokes. P/N's 80545, 80546 (302) & 99506, 99826 (289).

I used the 99506 pistons with 5.4 rods and a little Holman & Moody C30Z cam (289 Hi-Po). My dyno runs were filmed and showed a reversion cloud on top of 44 IDF's.

The H&M Le Mans grind would be sick with a 289 but you would have to run 11/1 or better plus straight AvGas or race gas. Crower makes a Le Mans grind as well.

Of course, open megaphones would be the crowning touch.

The risk of fire always adds to the excitement of the drive too!

I never had a fire though.

Cheers,
Scott

 

[Jac Mac]

Mike, I will send you an email later with part numbers and stuff related to this.

 

[Randy V]

If I were inclined to run a cam with the Lemans Spec duration, I would also be inclined to run a ventilated rear wind screen like this one on 1083...

You stand a much better chance of driving a bomb on the street than you would the track..

Also - Make very good friends with your local Weber tuner if you want to get it and keep it in top spec with jetting..

 

[Mike]

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?

 

[Randy V]

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

A couple of little variables called barometric pressure and relative humidity... Sucks but its true.. Best bet is to have them tuned at the altitude and time of year that you'll be doing most of your driving.

 

[Mike]

I would tune them for altitude for sure in Colorado and its a summer only car but I also doubt I am going to care if they were slightly off if the pressure was 815mb vs 840mb. I run my sled with static jetting and it's a 16:1 straight race gas engine that has to run from -20f to 32f at elevations from 9500 up to 14,000. I purposely run it a little fat so I don't burn it down when it's really cold and I'm down low coming off the truck. I personally think don't want to jet to be right on the line.

 

[Jac Mac]

With regard to reversion, most if not all engines running webers will have a point in the rev range where reversion/stand-off will occur, as long as the engine is running correctly this will not cause a fire until other factors create a condition for it to happen. Some of those factors could be a mechanical issue like wrong valve springs, bent pushrods/valves, ignition issues, wrong timing curve, cross firing in leads etc.
I look at pics of cars and many don't seal off that area around the carbs from the engine bay, this area was designed to operate as a large plenum chamber for carb air, to leave it completly open to the engine bay is to invite the hot air up into the carb intakes, hot air & vaporising fuel is a potential bomb and hot air wont improve your HP levels.

Mike I sent your info via PM.

 

[Ian Anderson]

With regard to reversion, most if not all engines running webers will have a point in the rev range where reversion/stand-off will occur, as long as the engine is running correctly this will not cause a fire until other factors create a condition for it to happen. Some of those factors could be a mechanical issue like wrong valve springs, bent pushrods/valves, ignition issues, wrong timing curve, cross firing in leads etc.
I look at pics of cars and many don't seal off that area around the carbs from the engine bay, this area was designed to operate as a large plenum chamber for carb air, to leave it completly open to the engine bay is to invite the hot air up into the carb intakes, hot air & vaporising fuel is a potential bomb and hot air wont improve your HP levels.

Mike I sent your info via PM.

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

 

[Randy V]

With regard to reversion, most if not all engines running webers will have a point in the rev range where reversion/stand-off will occur, as long as the engine is running correctly this will not cause a fire until other factors create a condition for it to happen. Some of those factors could be a mechanical issue like wrong valve springs, bent pushrods/valves, ignition issues, wrong timing curve, cross firing in leads etc.

I look at pics of cars and many don't seal off that area around the carbs from the engine bay, this area was designed to operate as a large plenum chamber for carb air, to leave it completly open to the engine bay is to invite the hot air up into the carb intakes, hot air & vaporising fuel is a potential bomb and hot air wont improve your HP levels.

Splitting your post into two paragraphs for clarity..

Para-1 Good point and very true... But another potential ignition hazard is the potential backfire through the carbs caused by a lean condition. IR intakes don't have much of an issue in loading up unspent fuel so that one is out..

Para-2 Good point as well and this is truly a double-edged sword isn't it? Seal up the plenum to reduce heat being introduced to the carbs and you get more of the cool air from the side inlets. At the same time, you create a semi-contained space where fuel vapors can collect, thereby creating a potential bomb. It could be argued that leaving the plenum open rather than sealing it, could provide a means of ventilation of these unspent gasses.. Considering the side inlets would provide a positive pressure point, the gasses would then be ushered into the engine bay and out the vents above the exhaust or out the tail..

Still, all in all, the low-speed drivability of the cams with narrow lobe centers is something that a lot of people just don't take into consideration. Few things are worse than a car that makes you look like a fool as it bucks, chugs and stalls out right in front of the crowd at cars and coffee... There are band-aids you can throw at it, but they all have their price and are never really satisfying.