1/4 scale V8, first project.

[dsage]

Keith:

Sorry I missed the obvious gap burned from the PTFE seal.
It doesn't surprise me that it burned up since you have it right in the combustion chamber and plastic isn't much good at getting rid of heat. You'll need to have the O-ring out some distance from the bore. Not sure of scale but, could a ring be put in groove centered of the top edge of the liner.? That's about where it should be so it has metal on both sides. It could perhaps be put out in a new groove milled in the junction of the liner and block. If the ring was large enough in diameter it would seal to both pieces. Might be iffy though. If it doesn't seal properly compression might tunnel under the ring (like it did with the PTFE), and get behind the liner.
As Luc mentioned a 1/16th O-ring should technically be large enough to take the compression. I usually allow for about 10-15% crush.
Whatever you do, your going to have to fill the old step with something or you'll have a great deal of compression reduction. Maybe just a steel ring like the PTFE ring to take up space. Not sure how you would hold it in place from rattling around.

As mentioned, a copper head gasket might be the best idea. Same problem filling the step though. In that case the copper gasket might hold the step filler in place for you too.

I think you mentioned waaaay back that the heads are not water cooled so a teflon (or other insulating gasket) would be out in order to get good heat transfer from the head to the block.


Hmmm

Sage

 

[LongRat]

I like the idea of the annealed copper sheet. Conducts well too.
In your position Keith I would go with that but simply make replacements for your current Teflon washers out of the annealed copper. It would then just be a case of swapping parts out, rather than any more machine work on your block. If it doesn't work out you can always try something else later.

 

[dsage]

Keith:

I received a response concerning the carbs with pulse pumps.
You may have heard of this Gentleman. His name is Ron Colonna and he is one of the Masters over here on this side of the pond (in the USA). http://www.ronsmodelengines.com/
You'll see the Challenger V8 under the Gasoline engines section.

<quoted sections from his email>

.....I have a diaphragm carb on my (Challenger) V-8, but it is a Tiliston from a weed whacker that I picked up as scrap from a guy that used to fix them. Walboro carbs are about the same.....

....The bore on this carb is around 5/8", way too big for a four stroke of the size of the Challenger....

...I had to put a reducing bushing in the bore. (to reduce it to) 1/4" bore ...

....There are several ways you can go with the internal fuel pump. This pump needs to be pulsed. The two stroke engine from which it came produces pressure pulses in the crankcase in its normal cycle. So a tube from the crankcase to the inlet of the pump serves to pulse the pump just fine. On a four stroke V-8, the crankcase pressure remains almost constant because as one piston is going up another is coming down and the pulsation s cancel. The diaphragm carb needs the fuel to be under pressure to function. The vacuum at the Venturi acts on the diaphragm to open the fuel monitoring needle. If there is no pressure on the other side of the needle, no fuel will flow into the engine. Paul Knapp built a small diaphragm pump that runs off the cam in the distributor to pulse the internal pump in the carb. This pump was made to look like the vacuum advance diaphragm controlled system attached to the early engines in full size. I simply built a fuel tank with a pressure pump that works like the Coleman lanterns. I put in fuel, seal the tank with a screw in stopper, and pressurize the tank with a few pumps. The pressure remains fairly constant throughout until the tank is empty. The west coast guys use an electric fuel pump to supply the pressure. A gravity feed will not work because the pressure is too low. Also, since a two stroke as used on a chainsaw or weed whacker moves so much air, it produces a high vacuum at the Venturi which as I stated, serves to open the fuel needle. The airflow though the V-8 is much less, so the needle will barely open or not open at all as carb is supplied. The diaphragm works against a spring which holds the needle closed when the engine is stopped and not producing a vacuum. For four stroke use, this spring needs to be shortened a bit to put less pressure on the diaphragm for the vacuum to overcome. As you can see, with a standard two stroke diaphragm carb, a whole can of worms is opened when you decide to use one. Having said that, there are carbs of this type made for small bore two strokes, that will work as is. The key is small bore. A carb from a chain saw or weed eater needs to be modified, but one from a model airplane engine for example would not need any reworking.

<end quote>

I'm sure you've probably already decided what direction you're going and perhaps none of the above is going to be of much use to you.

But I include it because it is all excellent information for everyone.

After all that IMHO it might be best to go with a Walbro without the pump and use an electric pump.

Sage

 

[keith5700]

Thank you all. I would like to use this Walbro type carb if I can, although once I'd sorted out the electrics the small rc carb seemed to rev well. It got to about 6000 rpm when the Chevy ignition module expired. I will get the Walbro stripped and see if I can identify the spring in question. It would be easier to just pressurise the fuel supply and then that's another variable sorted.
Walbro bore is 11mm. Engine is 19 bore x 18 stroke, from memory anyway.

I will have to try the copper sealing ring too. I'm not sure how much 10 x m3 cap screws can compress 4 x copper rings, but I'm sure it won't be as much as 5 thou'. I can't put a normal o ring at the top as I have another o ring in the sidewall of the liners, near the top, to seal the water system.

Steve, I need to have ally to ally contact with the head and block to cool the heads, since I had to take out the water jackets out of the head, as Sage has mentioned.
I'm having a few days off the frustrating stuff now and making a small supply of mini AN fittings, ready for piping the oil pump up.
I've also ordered some good old reliable points, so we'll see if I can blow them up!
Cheers.

 

[dsage]

Keith:

Not sure how high you want to rev the engine. I know it's impressive to rev it up for show but don't push your luck. Better it survive for a while than throw a rod, float a valve into a piston or otherwise break something. A small carb is good insurance against over-revving accidents.
Another piece of advise - don't deccelerate to quickly either (i.e slam the throttle shut). Again - sounds cool but flywheel inertial has been known to break things as well.


BTW if you insist on going to points AND you want to rev it up, consider a dual point distributor (like the big boys used to use back in the day). Twice the headaches but will give you a better spark at high RPM.s (longer dwell).

Sage

 

[mayhugh1]

I'm building a 9 cylinder radial and am planning to use a Walbro on it. Here is what I've done, though I haven't yet tested it. (I've finished all the parts for the engine and am now assemblying it.) I made an adapter between my intake manifold and the Walbro that also included a "carb bowl". This 'bowl' has an input line running into it fed from a fuel pump and a spring-loaded pop-off requlator built into it inside that feeds pressurized fuel to the carb. The pressure regulator is set at 5 psi and the excess gas in the bowl is returned through an outlet tube to the fuel tank. I made an electric fuel pump by robbing the internals of an RC model fuel-filler that I bought at a local hobby store for $12. I also modified the carb by removing the two check valve flaps under the the carb's pump cover, but I don't think this is necessary because it turned out that 5 psi was enough to push the fuel through them. Like I said, it isn't yet tested, but it looks pretty good. Since my carb doesn't have a choke or a priming system, I may have to prime it for initial starting by pushing down slightly on the diaphragm with a toothpick with the fuel pump running. I drill a tiny hole in the center of the diaphragm cover for this. - Terry

 

[dsage]

not if your free running, the ONLY way (or two) to stop an over rev
#1 a rev limeter
#2 a tune pipe witha steep angle on the baffle
ooppssss
#3 your right footRof}Rof}Rof}

LUC:
Hmmm. I'll take your word for it. I would have thought a small carb wide open BUT unable to deliver enough air and fuel would be the same as a larger carb with the throttle only partly open. So what you're saying is that with a little carb wide open the engine will eventually rev out of control ?



Hey Terry:
Nice to "see" you again. 4 cylinders of the Howell wasn't enough for you . Have you posted any progress on your 9 cylinder on line? I'd like to see your (usual) great work.
PM me some time.

Dave Sage

 

[dsage]

Not to get too far off topic here but how does an engine ever idle?
The throttle plate is just restricting the air through the carb (and therefore the amount of fuel it introduces into the air) and therefore the speed. Why isn't reducing the maximum air due to too small a carb the same thing. I can't figure out why an engine with the throttle held slightly open and not altered doesn't run-away.

BTW. Every model engine I've ever seen just sits on the bench running. We don't normally put loads on them - other than their own friction. So why don't they all rev out of control?

I need to understand this.

Anybody else want to weigh in here?

Sage

 

[Cogsy]

I'm with you on this one Sage. If the carb is small enough, it will restrict the RPM of the engine to a low range, even when wide open. Of course, such a small carb wouldn't deliver anywhere near enough power to run a proper load, but as you say, model engines don't normally have any load on them.

All I can think of is that a small carb that limited the max RPM may not have appropriate throttle response. I know if I had a sweet little V8 like this, I'd want to be able to blip the throttle and have the engine roar instantly...

 

[dsage]

Re: Blipping the throttle.
I'm with you on that one, That is very cool to hear an engine with instant throttle response.
My V8 has too small of a carb (sized for a V4) and you're right, it accelerates pretty slowly and only revs to about 5500. Not very impressive. I was considering it a safety feature that it would not over rev. (My original point). I stretched Jerry Howell's V4 to a V8 and considering the 12 piece built-up crank I'm am concerned with it's durability.
I guess to be impressive one needs to have a light foot and be careful as Luc suggested.
I hope Keith gets it sorted out so we can all hear an impressive performance. It sure looks impressive.

Sage

 

[keith5700]

Luc, you're just confusing people again. Maybe you could start your own thread for your engineering theories? 5250rpm isn't a magic number, it's just where the hp and torque curves cross each other on an engine printout. It will be the same for a lawn mower and an F1 engine, but nothing to do with whether an engine will rev out of control with no load.

The limiting factor for revs will probably be the valves bouncing, or more likely, floating, which will be related to the valve spring load. The carb inlet size wouldn't normally restrict a full sized engine from revving under no load, as the bore will have been sized to allow full revs under maximum load. So under no load the carb is way too big to restrict rpm.

In my case, with a 5mm carb then under no load it probably will rev till the valves bounce, but the carb will be far too small if I asked the engine to produce, say, 3bhp at 6000rpm.

If I put a 2mm bore carb on there then full throttle might be 1500rpm at a guess, so the carb is restricting the rpm, as it does when a larger carb is at pert throttle.

 

[mu38&Bg#]

Keith, its great to see progress on your engine.

Re: carb size. Like Sage suggests there is no difference between a closed throttle and a small carb. If the carb is small enough to limit no load RPM to 5000 or 8000 RPM it will never run any faster without load. It would be the same to remove the throttle plate and install a restictor plate with a .250" hole or whatever. If you never intend to run a load, use a small carb, if you will do dyno testing size it accordingly. Too large a carb might leads to poor vacuum signal and poor carb performance. Walbro carbs can be tricky to pick because they are individually tuned for the application by Walbro. if you get too far out of the intended volume flow range, performance will be poor(not tunable via the needles).

Re: Walbro fuel pumps. The built in Walbro fuel pumps in W series carbs make about 5 PSI. If you feed fuel under pressure to the fuel inlet of one of these fuel will flow through and the fuel pump need not be connected to a pulsed pressure( two stroke crankcase or single cylinder four stroke intake). In the RC hobby world we have Cline regulators which work with a pressurized fuel tank using a check valve on the exhaust or crankcase. These are simply the metering portion of the Walbro carb which you add to an existing carb. These are great for their application. Some commercial radial RC engines use a crank driven fuel pump to pump through the stock Walbro carb. A pulse connection to a sinlge intake port may not provide enough pressure because the runners are connected to each other via the plenum(rather large volume compared to a single cylinder engine).

Re: RC glow engine carbs These are simple carbs like a Mikuni, but even simpler. The main jet is adjustable via the main needle. This must be set at WOT close to where it needs to be to get the low end set right. Any changed on the main needle will influence the low speed needle. The tapered needle closes toward a seat as the throttle is closed, this provides midrange and idle metering. Either the seat or the needle position is adjustable relative to each other, and this adjusts low speed and mid-range mixture simultaneously. The idle needle taper is designed for air fueal ratio of 4-6:1 and with gas needing 12-16:1 to run, they can be tuned with perfect WOT and idle settings, but will result in a fat midrange. The only remedy is a different needle taper, which you can't buy. Often a suitable setting can be found with a slightly rich mid, but carbs taken from car or boat engines should be avoided as they run vary rich air fuel ratios due to high percentage of nitromethane in the fuel.

 

[canadianhorsepower]

Luc, you're just confusing people again. Maybe you could start your own thread for your engineering theories?

wow that was really uncalled for,
but it's a good idea, it would help you getting this thing running

 

[keith5700]

Hi Diesel. So can I assume that the rc carbs would run fairly well if a new needle was made with the same small and large diameters, but the middle was fattened out a bit to lean out the midrange? Or doesn't it work like that?

 

[petertha]

I dont beleive there are many, but here is an example of RC model 4-stroke gas walboro style carb. Sorry I dont know details.
http://www.aerobertics.be/productDetail.php?prod=7247&cat=3&brand=140


Lots of walboro style used on 2-stroke & 2S multi-cylinder variety. Again, sorry I dont know much about how they are plumbed and/or pulsed.

http://www.desertaircraft.com/engines_detail.php?Page=DA-170

 

[mu38&Bg#]

Chances are pretty good if it was from an airplane engine, the RC carb will run just fine. Many glow engines have been converted to gas and spark using stock carbs. If you really wanted to get the tuning just right you could consider making a low speed needle with less taper. Finding the running settings is going to be the tricky part, but if you've managed to get it to run off and prime and run on the carb, you're pretty close. I'd set the main needle 3/4-1 turn out and open the low speed needle incrementally until it fires on the carb. Use a small fuel tank and mount it just below the spraybar.

Peter, the Walbro carbs used on four strokes are no different than the two strokes, except that some have a spring pushing against the pump diaphragm to assist the pressure pulse from the intake. If you use an external pump it doesn't matter. WT-456 is an example of a Walbro used on four stroke engines. It has an acceleration pump and pump spring. We use these on converted glow to gas and spark 20CC single cylinder engines like OS FS-120 making about 2.5HP at 8000RPM. The accel pump allows a slightly leaner low range and still gives a good transition from idle to full throttle. There are engines out there using regular two stroke Walbro.

Greg

 

[keith5700]

OK the points ignition system is in and working. It's a bit crude at the moment as I wanted to see if it works ok, and indeed it does. I can't believe all the time I've wasted with the 2 electronic crapnition systems so far. Now I know it works I need to source some smaller points and fit 2 sets, so I can run each dizzy off it's own set.
The engine started almost straight away, and will keep on revving till I get scared and back it off. At low revs there is a very realistic burble from the silencers, but at high revs the noise from the points is very overpowering. I can sort that out with a slower ramp up and down on the points cam.

I now really do need to get the oil system and cooling system working, before it seizes up!

 

[Gazzaleach]

Well done that man

 

[RManley]

Such an amazing engine with so much detail. I cant wait to hear & see it run (hint)

I still cant get over your exhausts.

Rob.

 

[keith5700]

Update time! This is a short version as the first reply got lost in the ether somewhere.

I've been working on the pipework for the oil system. This includes a fine mesh filter for inside the sump, for the pickup pipe, and an adjustable pressure regulator for the oil return pipe.

This has caused a problem with exhaust smoke, after running the engine with a sumpfull of oil. I have 2 compression rings but no oil ring.
I was hoping to get away without one, but apparently not.
I need to strip it down now and see if I can squeeze a George Britnell style oil ring into the existing pistons.

Random shot of milling out the side pockets of the big wing sump