Originally Posted by Work Horse
Thanks everyone I appreciate the positive comments.
xacoupe time you got some slicks I reckon!
TTNOS8 I'm fitting the nitrous myself, any tips?
I have a separate pump and fuel line for the kit, and will dial the timing back. I'm going to start with a 100hp shot and see how it goes.
Hallam reckon I can run a 150hp shot safely, but they will tune it if I go that far.
brenx I don't fully understand how the double pumper was so much better than the vac sec, but hey I'll take it !!!!
I mean the 2 accelerator pumps on the 650 only operate once off the line, then I just keep my boot in it. The only differance between the carbs @ WOT is 50 cfm.
My 60ft dropped by one tenth, but I dropped nearly 5 tenths by the end of the strip!! I can't fault that improvement, the 650dp is for me.
If this is your first trip down nitrous lane, I might recommend starting with the lowest possible shot configuration available from the system and working up from there. You need to carefully monitor your A/F and ensure that you do not enter into a lean burn situation. Retarding ignition 2*/50 shot is a good baseline for experimenting. A lot of where your exact combination will make the best power and not predetonate will depend a lot on your total combination. Camshaft valve timing, compression ratio, chamber size/shape, piston size/shape will all play an important role in determining where the sweet spot is in your combination, especially once the nitrous begins to flow.
I wouldn't touch the nitrous until I was sure that I had the most out of my baseline combination. (Good place to segue into the carbs)
The reason that you're getting better performance from a 650 DP is likely because of:
Dual metering blocks on the DP probably have a fatter total jetting (especially on the secondary side) with the DP over the VS carbs. If you were monitoring A/F with a wide band O2 sensor, you'd probably find that you were leaning out slightly at the top end...or possibly even more than slightly. If you already own a VS carb, there is a lot that you can do to improve its total performance without buying a new carb.
Very few people know how to properly tune a VS carb. This is one of the reasons why a DP is a bit more commonplace. The first (and probably foremost) benefit is obtained through adding a secondary metering block upgrade kit. This gives you fine control over jetting front and back. The secondaries of a stock VS carb have fixed metering plates. These things are junk for race use. You have to fatten up the primary (bottom end) so much to keep the top end from leaning out that you lose power on both sides of the 4-bbl. Not too many people use proper vacuum secondary spring rates effectively. The best way to go here is to send Holley tech support an email with the complete details of your combination and let them advise you on the baseline configuration of your carburetor. They have seen just about everything you can imagine related to their products and can offer really useful advice 99% of the time.
However, if you're destined for a DP, then you'll find that you probably are getting more performance out of it as a result of its richer mixture and finer ability to tune the secondaries. Without knowing the jetting of each carb when used to produce the results and without looking at the plugs after the shutdown, its too hard to say. Most likely is that you were fat on the bottom and lean on the top.
I've been assuming square bore carbs for the VS. A spread bore is modestly different. Though I sorta doubt that you've been fumbling around with adapter plates at the track during this ordeal.
If you know your jets at both ends of the 650 DP, I'd like to hear them. If you have the carb opened up, take a look at your power valve and let me know if it is stock at 6.5"mg.
Not to be an antagonist, but what does "well setup" mean? One guy's idea of a great setup may be another tenth less than it could perform if properly set up...maybe more...who knows without an idea of what is setup about it.
Let's take the classic case of one carb, two different engines. This is essentially what happens when you have a carb that is working well on one guy's combination and swap it to a different engine. Every combination is different. It makes sense that tuned for one engine is going to mean at least a slight variation for being tuned for the other engine even if the two engines are nearly identical, especially when you factor in the differences in total combination, which affects ET...not just the engine. The same exact engine (except for flexplate/flywheel) in a stick car with a 4.11 gear will need a different tune than an auto car with a 3.50 gear.
My point is to tune the carb to your engine/chassis combination and be sure that it is making the best performance for your combination before getting started with nitrous. You're likely to gain at least another full tenth with just some fine tuning work. It's better to have that 10th on your side before giving it the nitrous so that you get even more benefit out of each shot.
It is possible that you may be leaning out on the top end...especially if you haven't checked it...That could be dangerous if you add nitrous as it will find any holes in your current combination while making them harder to recognize and report while under boost until too late.
Don't get me wrong, I'm not trying to be negative about it at all. I think that you're definitely on the track to finding out what your combination wants. I would only like to be sure that you understand what your combination wants based on the details of it so that you know how each change affects it. If you're not reading your plugs now, for example, I wouldn't expect you to start once the shot goes on. I think that you've been bitten by the same speed bug that owns each of us on here in some way...I'd just add a word of caution and suggest that you take it a bit more slowly and methodically so that you can definitely state (at least to yourself) that you have your combination just right for this condition and now are stepping into a new "unknown" (change) prepared to handle it. Otherwise, you'll likely end up at some point where you're having all sorts of exaggerated problems as a result of mixing nitrous at higher and higher levels (trying to catch and harness that crafty little speed bug!) without fully understanding what goes on as a result of a snapshot of your combination before making a pass.
I don't think that you need to get out a lab coat and a microscope either, but I think that you're entering into a point where at least writing down your car details/changes into a racer's log is a good idea. If you find that you can't fill in very many of the blanks, it is a good sign that you don't know enough about your total combination to really make useful changes to it from a position of knowledge and "planned attack."
Adding the nitrous at this interval sounds a bit more like using a bazooka to take out a post box. I'd rather see you (at least consider) taking careful aim at it from about 500m and popping it off its stand through excellent marksmanship if the analogy works at all. My point in saying this is that nitrous is better known as "too late gas." If there was anything amiss at the start of the pass, by the end of it, it is probably too late and nitrous "ate" it.
Of course, people everywhere are blindly adding shots to their cars and driving the wheels out from under them so don't pay any attention to me if you're having fun and respecting it at the same time. Don't let me be a party stopper. I'd just rather that you find a moment to reflect on it as you go and not as a result of a breakdown that just happens one night at the track. There is definitely a time to just go out and drive it and have fun, but there is also a time, I think, to be relatively "scientific" in your approach to it just like observing an experiment in chemistry class. Take lots of notes and know as much as possible about the world underneath the bonnet. Make assumptions and test those assumptions based on the validated data you get from your experimentation.
We use substantial electronics to record data about each pass we make. We learn about the electronics and we learn about what's going on with the combination. We look at every cylinder's exhaust gas temperature. We look at the oil temperature and pressure at an interval of 1 reading every .001th of a second. We measure engine RPM and driveshaft RPM and counter it all with data from a tri-axis accelerometer. We capture cylinder head temperature (four places, front and back of each head), water temperature (four places, front and rear of the block, waterpump inlet and outlet), transmission fluid temperature, differential temperature and we use heated O2 sensors along with mass air flow sensors along with a relatively humidity/barometric pressure sensor to record data for the entire pass. We analyze the data after each pass and compare it with our expectations for the pass. This is what I call making an "informed decision" when tuning or changing something about the car. The more data that you can capture and effectively/efficiently use, the more you can realize the intended results.
I'm not suggesting that you spend 5000+ on a data logger, but I am trying to recommend using a simple pad and paper racer's logbook to chart your changes. If you're going to the track and you don't have one, you're definitely there to play and just have a good time, not tune the car...IMO.
I hope that I'm not being too overbearing or anything in saying this...