Carb Bore VS Intake Rotor Timing.

[Joe Warren]

We were having a dicsussion at Atlanta about (Big bore carbs ) vs intake rotor timing. I have found that you are much better to modify the existing carb and open the duration up on the intake rotor if you want more fuel in the engine for performance. Big bore carbs only guarantee more air & they change the operating range of the engine to mid - upper rpm. You will loose a ton of throttle response. In the pass i have seen stk engines that had BIG BORE carbs on engines that Had a intake duration so wide it made the engine a Poor Sucker.. This happens when you open the intake rotor too early just a few degrees ABDC and the engine cannot began to make case vacuum before it opens.... Most of these engines you would have to sick you finger in the carb just to start it. Most of these same engines had either a Early opening ( cannot make case vacuum before the rotor opens) or a Very late ATDC closing intake rotor number ( engine has already began to make postive case volume and the intake rotor is stlll open). Both make for a Poor Sucker engines. Now that is Not to say that the other mfgers are not sometimes very consertive on the bore they supply?? Yesterdays 45 engines came with carb sizes- .390 .430 today you see 45 Engines with .450.470 .500 .510..... Yesterdays 67 engine came with .410.430. Today you see .470-.550.... Todays RS91-101 are just barely in the .510 bore size and they are a much larger volume pump. Food for thought:: being we have a barrel valve for variations of throttle speeds. You never benifit from a BIG BORE unless it is Wide open And that is when the velocity at the venturi is the lowest?? & cutting the Spary bar 1/2 off in the stk carb increases area in the bore!!

 

[Joe Warren]

Many records have been set with motors that have the intake rotor timing number optimized and a barley larger that stk carb. I`am just saying that a BIG BORE carb is not always the answer to going faster. There is a lot to be gained in getting the intake rotor timings correct. So the engine can make good vacuum. Todays race engine we let the prop be the governor of the engines abilty to turn high RPM. I know if my engine is whaing Rpm before it comes off the corner iam going to add more cup...... And yes charles thinking of the center openings in the spary bar comes from his older days of building automotive carbs.... Thermoquad carbs ran these same degsined carb spray bars??

 

[dwilfong]

I have opened up the stock .510 carb on my 1.01 to.700 wide and .510 tall. also run my intake rotor at BDC open and 65ATDC close 245 total. This set up still throttles real good. the eng starts most of the time with just putting the igniter on it.

Also remember my video of this eng with NO rotor starting right up and throttled good on the bench.

Once the eng is up to full song the pipe will pull all you need to run the eng. I still say the piston moving up and down at 26000 RPM is out of the picture as far as what is pulled thew the eng.

This is the bases for what I do to my engs.

David

 

[Joe Warren]

David you say opening the intake valve at BBDC?? Dont know if this is helping? the intake stroke has not began. I agree it widens the duration number. Are you trying to add RPM by this mod? from what I have seen about 230 total duration you already have a radical engnie hard to tune and race. Some of my best engine have been under 220 total

 

[dwilfong]

I cut my rotor so it is at the opening of my window with the piston pushed down to the bottom. By this time the pipe is drawing the cylinder down and starting to pull from the case. If the pipe and header has the taper in it from the start it will draw the cylinder down. the pressure difference from case to port to cylinder will be lowered with the right lead in on the pipe. the expansion of the ex into the pipe by the front cone set on the pipe is what scavenges the eng more than the piston. Take the piston out of the picture and look at the eng in these terms as to the flow thew it. it will give a different picture of the whole process. This is what I see in my head. All I can say is cut a rotor and try it. Rotors are cheep. what do you have to lose. One tank of fuel and a $35 disk. David

 

[Joe Warren]

I cut my rotor so it is at the opening of my window with the piston pushed down to the bottom. By this time the pipe is drawing the cylinder down and starting to pull from the case. If the pipe and header has the taper in it from the start it will draw the cylinder down. the pressure difference from case to port to cylinder will be lowered with the right lead in on the pipe. the expansion of the ex into the pipe by the front cone set on the pipe is what scavenges the eng more than the piston. Take the piston out of the picture and look at the eng in these terms as to the flow thew it. it will give a different picture of the whole process. This is what I see in my head. All I can say is cut a rotor and try it. Rotors are cheep. what do you have to lose. One tank of fuel and a $35 disk. David

Believe me when I tell you I have a pile that have been cut. But I have never been to BDC openings or BBDC openings. I would aslo think this would limit your ATDC number too.

 

[Joe Warren]

I cut my rotor so it is at the opening of my window with the piston pushed down to the bottom. By this time the pipe is drawing the cylinder down and starting to pull from the case. If the pipe and header has the taper in it from the start it will draw the cylinder down. the pressure difference from case to port to cylinder will be lowered with the right lead in on the pipe. the expansion of the ex into the pipe by the front cone set on the pipe is what scavenges the eng more than the piston. Take the piston out of the picture and look at the eng in these terms as to the flow thew it. it will give a different picture of the whole process. This is what I see in my head. All I can say is cut a rotor and try it. Rotors are cheep. what do you have to lose. One tank of fuel and a $35 disk. David

Believe me when I tell you I have a pile that have been cut. But I have never been to BDC openings or BBDC openings. I would aslo think this would limit your ATDC number too. But I`am always open for suggestions!!

 

[dwilfong]

I have bin messing with the pipe Scott made for me and it will suck the guts out of the case. the needle is over two turns out from where it was with the MACS pipe.

The only problem is it is hard to tune. the volume is large with this pipe and it stows a large mixture load in the pipe then the sharp rear angle slams it all back in.It hits like a sledge hammer. I call it the piston breaker. but it drops off bad after you hit the RPM tune of the pipe.

Some more changes should make it gentle. But it did show me how the right front half will draw all the way to the carb.

 

[Jeff Schindler]

I have bin messing with the pipe Scott made for me and it will suck the guts out of the case. the needle is over two turns out from where it was with the MACS pipe.

The only problem is it is hard to tune. the volume is large with this pipe and it stows a large mixture load in the pipe then the sharp rear angle slams it all back in.It hits like a sledge hammer. I call it the piston breaker. but it drops off bad after you hit the RPM tune of the pipe.

Some more changes should make it gentle. But it did show me how the right front half will draw all the way to the carb.

Can you telescope the pipe and have a servo shorten it on demand? Always thought that would be trick.

 

[julianconde]

jeff they do that in tunnels mr.ernie laflaur does that thru a 4th channel it pushes in and out the pipe on the fly just like 3rd channel needle valves

 

[Joe Warren]

Slipper pipe would be good for Small to Medium engines sizes.

 

[Joe Warren]

I cut my rotor so it is at the opening of my window with the piston pushed down to the bottom. By this time the pipe is drawing the cylinder down and starting to pull from the case. If the pipe and header has the taper in it from the start it will draw the cylinder down. the pressure difference from case to port to cylinder will be lowered with the right lead in on the pipe. the expansion of the ex into the pipe by the front cone set on the pipe is what scavenges the eng more than the piston. Take the piston out of the picture and look at the eng in these terms as to the flow thew it. it will give a different picture of the whole process. This is what I see in my head. All I can say is cut a rotor and try it. Rotors are cheep. what do you have to lose. One tank of fuel and a $35 disk. David

You take the piston out of the picture and it Will not be Running.......

 

[dwilfong]

I cut my rotor so it is at the opening of my window with the piston pushed down to the bottom. By this time the pipe is drawing the cylinder down and starting to pull from the case. If the pipe and header has the taper in it from the start it will draw the cylinder down. the pressure difference from case to port to cylinder will be lowered with the right lead in on the pipe. the expansion of the ex into the pipe by the front cone set on the pipe is what scavenges the eng more than the piston. Take the piston out of the picture and look at the eng in these terms as to the flow thew it. it will give a different picture of the whole process. This is what I see in my head. All I can say is cut a rotor and try it. Rotors are cheep. what do you have to lose. One tank of fuel and a $35 disk. David

You take the piston out of the picture and it Will not be Running.......

Ya I guess you are right is has a little to do with running the eng. :lol:

 

[dwilfong]

I have bin messing with the pipe Scott made for me and it will suck the guts out of the case. the needle is over two turns out from where it was with the MACS pipe.

The only problem is it is hard to tune. the volume is large with this pipe and it stows a large mixture load in the pipe then the sharp rear angle slams it all back in.It hits like a sledge hammer. I call it the piston breaker. but it drops off bad after you hit the RPM tune of the pipe.

Some more changes should make it gentle. But it did show me how the right front half will draw all the way to the carb.

Can you telescope the pipe and have a servo shorten it on demand? Always thought that would be trick.

A two angle rear cone set will do the same thing to a point.

 

[shoboat]

Just need better pistons.

 

[BigChuck]

" It is common experience that disc valve raing engines need slightly larger carburetors than when reed valves are employed."

Pg 460 Chapter 6 subsection 4.2

 

[dwilfong]

Thing about why this is true. how much more vacuum dose it take to open a reed than a open window?

Don't think a pipe will draw thew a reed.

 

[dwilfong]

Just need better pistons.

Thy are being made as we speak. with extra compression distance to adjust the timing with out jacking the sleeve around.

 

[BigChuck]

Thing about why this is true. how much more vacuum dose it take to open a reed than a open window?

Don't think a pipe will draw thew a reed.

Don't need to think...............Everything is explained with equations and solutions: Just dont' feel like hurting my brain to do the trig and calculus.

 

[Joe Warren]

Just need better pistons.

I agree with Mark on this one. if you told anyone that knows anything about engines and how they operate... That we are running 60 Nitro in a engine that has a cast piston..... they would laugh at you....... :lol: :lol: .... try that in your wifes car tomorrow morn.......... the mfgers that have switched to offer billet pistons have done this for a reason.