Tips for Drag Racing Tuning Prep?

[blaxmax]

I believe the '85 models were different internal exhaust and changed in '86 to comply with DOT.

 

[Lotsokids]

What was your reason for cutting the crossover tube?

Thought I would improve the sound (like cammed). No big difference. But now if I have trouble with a cylinder firing, I'll be able to know immediately which cylinder is the problem. I've had problems with #3 in the past, but really settled down with the Cobras installed. That's strange to me.

Yes, I think those are stock head pipes.

 

[Dorney]

Ok.....do you remember high school geometry....and how you said that you would never use it in "real" life.....well now you will.

On the Vmax, the needle doesnt sit down into the main jet, but it fits close enough that it causes an obstruction.

Now these are made up numbers....strictly used for illustration purposes. So, you have a main jet, and it has an inside diameter of 4 (see geometry) and the needle fits down in there, and it has an outside area of 2, so that gives you an effective area of 2....for fuel to flow thru. But lets say that you have a bigger jet, and a fatter needle. The inside area of the jet is 5, and the outside diameter of the needle is 3, so it actually gives you the same area to flow fuel, like the other jet. An area of 2.

Something else to think about.....that is only at a certain RPM. At a higher rpm, the engine needs more fuel, so at higher rpm, the needle gets pulled out more, and so will allow more fuel to go thru the main jets.

This is a simplistic view of it....because there are different circuits, and different parts of the carb control fuel at different times....but this should help you to understand how needles work, and below there is a pic that shows the 4 main needles that you can use. Stock, stage 1, stage 7 and factory pro. Thats why I was curious what SHAPE were the needles in the sigma kit.

Next question.....have you ever dealt with the carbs on cars? They will flow a set about of air all of the time. A carb rated for 600 will flow 600 cfm all of the time (again a simplified view). The carbs on a Vmax are called CV carbs....constant velocity carbs. So they will always try to match the amount of fuel, with the amount of air flowing thru them.

Here is where the exhaust comes into play. An aftermarket exhaust flows more air out....right? And because of that....it will inturn PULL more air in, and with the extra air.....it will pull more more fuel with it as well...which makes you TOO RICH. Thats why with an aftermarket exhaust...to DROP to 147's to lean the carbs back out. It looks like you have cobras....which are just slip ons. No performance increase. So drop to the 150's....maybe 147's if you need to. With an aftermarket exhaust.....you would definitely need to drop to the 147's...they flow so much better.

Now then......020 spacer. Unless your running a stage 7 kit with aftermarket pod air filters.....or Seans muscle jet kit, you can really only use the stage 1 needles, or stock needles. (most people dont like the stage 1 needles.....remember when I said throw the directions out the window?) So.....with the stock needles, and an aftermarket exhaust...you need just a bit more fuel when the needles start coming in, so you can do that by raising them .020, and give yourself a bit more fuel in the midrange.

For Vmax purposes, you either have a stock exhaust, slip ons (slip on mufflers....like I said....no performance increase) or a full aftermarket exhaust.

Hope this helps.

Hi Eric
So are you saying that these carbs don't have a cfm rating because every carb has a max cfm flow. Also if these carbs were that over size what good would v-boast be or even more so stage 7. As far as constant velocity goes that just makes up for not having a ( not sure about spelling) excellirator pump which CV I feel makes for a good constant throttle response.

Dave

 

[Traumahawk]

Hi Eric
So are you saying that these carbs don't have a cfm rating because every carb has a max cfm flow. Also if these carbs were that over size what good would v-boast be or even more so stage 7. As far as constant velocity goes that just makes up for not having a ( not sure about spelling) excellirator pump which CV I feel makes for a good constant throttle response.

Dave

I believe that these carbs are 35 mm. The whole point of Vboost is that instead of an intake pulse from 1 carb....it comes from 2. That way you are using a smaller amount or air down lower in the RPM range, and more air in the upper RPM range. Before vboost was invited....it was either or. You either had smaller carbs that worked well on the street and would starve the engine in the upper RPM range, or bigger carbs that did well up top....but didnt have good velocity down low for the street. Vboost gives the best of both worlds. Sean has dynod a Vmax with vboost open all of the time, and lost torque/hp thru the midrange.

There is a huge difference between the velocity of the air flowing thru a carb, and the amount of air (volume). The higher velocity is basically more streetable.

They basically flow the amount of air, that the engine can use. A carb that doesnt have a specific flow rating isnt new. The predator carbs have been out for years on cars....they just arent that popular.

The last question....I dont know. I know that some mikuni's have accelerator pumps.

 

[one2dmax]

No accelerator pump on these.

Also, the "true 4-4 exhaust" is not helping power in the slightest. You lose all benefits from scavenging.

 

[Dorney]

I believe that these carbs are 35 mm. The whole point of Vboost is that instead of an intake pulse from 1 carb....it comes from 2. That way you are using a smaller amount or air down lower in the RPM range, and more air in the upper RPM range. Before vboost was invited....it was either or. You either had smaller carbs that worked well on the street and would starve the engine in the upper RPM range, or bigger carbs that did well up top....but didnt have good velocity down low for the street. Vboost gives the best of both worlds. Sean has dynod a Vmax with vboost open all of the time, and lost torque/hp thru the midrange.

There is a huge difference between the velocity of the air flowing thru a carb, and the amount of air (volume). The higher velocity is basically more streetable.

They basically flow the amount of air, that the engine can use. A carb that doesnt have a specific flow rating isnt new. The predator carbs have been out for years on cars....they just arent that popular.

The last question....I dont know. I know that some mikuni's have accelerator pumps.

I hadn't looked at a Predator Carb In a long time so I had to refresh my memory but cfm rating is 390 min 950 max.

Yes velocity is good for the street because it gives you good throttle response. That's just what the CV carb does by keeping the vacuum up all the time.

Volume is for the higher RPM range & max HP which is great for racing. At that point you'd be much better off getting rid of the hole intake system & put something on like a flat slide kit.

That's what makes the CV carbs along with the V-Boost a good setup for the street by linking both worlds together.

 

[sdt354]

No accelerator pump on these.

Also, the "true 4-4 exhaust" is not helping power in the slightest. You lose all benefits from scavenging.

It's amazing how the CV's compensate for no accelerator pump. Spring tension on the slides plays a big part in throttle response. I tried a pair of stage 7 springs I took off a wreck and the throttle became too twitchy to ride in traffic. Come to find out they were clipped even shorter than normal stage 7's. I went to a pair I took out of a set of carbs I found. They were in between OEM and stage 7 length. Perfect for me. I've tied back different numbers of coils in the past with success too. Three being the best.

 

[blaxmax]

It's amazing how the CV's compensate for no accelerator pump. Spring tension on the slides plays a big part in throttle response. I tried a pair of stage 7 springs I took off a wreck and the throttle became too twitchy to ride in traffic. Come to find out they were clipped even shorter than normal stage 7's. I went to a pair I took out of a set of carbs I found. They were in between OEM and stage 7 length. Perfect for me. I've tied back different numbers of coils in the past with success too. Three being the best.

I was going to ask you about shortening the springs.
Do you know the exact length in inches and what method do you use for tying?
Thanks.