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I have a 383 stroke motor. with .555 lift cam and 1.6 rollers. Victor Jr. intake with a Holley 750 with mech secondarys. 1 inch spacer under carb. Does 72-73 jets sound about right for the jets.(I ain't no carb guru) Compression is 11.1 .
The plugs are NGK -UR5 V power. Are these hot enough or should I run something a little hotter. When I pulled a plug it was black. I should say that I have been starting the car in garage and it has been idled alot due to a lot of changes in wiring and other stuff that I had to see if it was working or not.
What would be a normal gap for plugs of this type. Spark comes from an MSD 6A box, Msd distand 8.5 wires. Hope I 'm not rambling here I have been out of this a while and getting my feet wet again.
thanks
Lance:confused:
 

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I run a 357 ci engine and have a bigger cam, same intake and carb with no spacer and comp. is 12.5 to 1 and at 4,000-6,000 density alt. I am running 73-82. So a lot will depend on what air you are in but I don't think you will be way off with them.
 

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If you have a power valve in both sides you are close to Stan D settings. If you remove the power valve in the secondaries for drag racing you need to add 6 to 10 numbers of jet depending on who you talk to so Stan D is 9 numbers high, which fits. The altitude making a jet difference necessary is very open for debate IMO. RM just had an article in a racing publication about how they did not change jets in the P/S engines when they went to Denver because altitude has no effect on jetting.
 

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Altitude has no affect on jettying? Who wrote that article and please expand on that. If I run the same jetting at our altitude that I do at sea level my car wont run because it loads up on fuel so bad in the shut down. It will literally just die as soon as I let off the throttle from being loaded up.

That is interesting and someone would have to go a long way to prove to me that jetting is not affected by altitude since it directly affects air density and thus fuel to air mixture.

Things are certainly different for pro stock is some ways but jetting would have to change with altitude but what do I know.
 

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If you have the time and money, putting the car on a chassis dyno with a decent operator would be a big help on getting everything set right. If theat is not realistic, one thing to try is take the car to the track, put fresh plugs in it and make a pass. As soon as your run is over, find a safe place to pull off on the return road and pull a couple of plugs to see how they look. If they look fat, put the next higher heat range in and do the same on the next pass.
This will help you determine if you are really rich or just to much low rpm running. Possibly one of your local racers may be able to help you get your car dialed in as they know your local conditions better than any of us do.

As far as altitude not affecting jetting, I must agree with Stan. We both run at the same "home" track where you are lucky if the adjusted altitude is 4000ft. I made the mistake of running the same jets on a pass at Woodburn which is ~500ft and ended up aluminum specks on the plugs. Needless to say, had to put larger jets and new plugs in.
 

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This is the article I referred to but it's not the only one I have read that said the same thing. I also said that jetting was open to debate. I use to always rejet but today I never do. Run Bristol with the same jets I would run at Darlington and the exhaust temps are the same.



As I write these words, we’re loading our Speedco Pontiac Grand Am into the transporter for the annual trek to Denver. Preparing to race at a mile above sea level brings home the reality of racing under adverse conditions. While we make many adjustments for Bandimere Speedway, one of the items that is definitely not on our to-do list is to change carburetor jets.

Some racers just about wear out the threads on their carburetors trying to adjust for altitude and weather conditions. The truth is that you really can’t compensate for bad air by changing jets. When it comes to carburetor-equipped racing engines, you can’t fight Mother Nature.

It is a misconception that you must lean out a carburetor at high altitude. The fact is that a properly tuned engine will use the same jets in Denver as it does at sea level.

So why do cars run so much slower in bad air than they do in good air? The obvious answer is that the engine is making less power. When an engine is tested on a dyno, a correction factor is applied to the raw numbers to adjust the observed power to standard conditions. This allows us to compare the dyno test results that are made at different times of the year and under very different conditions. But when you are running a car down a race track, the correction factor is irrelevant. The only power that is available to accelerate the car is the engine’s actual output at that particular moment in time. If the engine is producing more or less power than it would at standard conditions, that’s what you’ve got to work with.

In this age of digital everything, carburetors have an undeserved reputation as low-tech devices. In fact, a racing carburetor is a very ingenious system. A carburetor responds to differential pressure, and therefore it self-compensates for changes in barometric pressure. The gas in the float bowl is always subject to the prevailing atmospheric pressure; the jets deliver fuel in proportion to the differential between the pressure in the float bowl and the pressure in the induction system. So when the barometric pressure falls, as it does so dramatically in Denver, there is less pressure differential and therefore fuel flow is reduced accordingly.

You don’t have to go to Bandimere to experience the effects of thin air. Even if you don’t travel, the changes in your race car’s performance at your local track from February to August will be substantial. On a typical summer day with 90-degree heat, the relative altitude can easily approach 4,000 feet. The unfortunate fact is that there is very little you can do to regain the missing horsepower by tuning the engine. While you might have a zero correction factor in January, it’s common to see a correction factor approaching eight percent in the summertime – and in Denver, we see 22 percent!

The harsh truth is that you’ve got a car with less horsepower in the summer, so you must figure out how to race it. What can you work on? You can work on the car – the torque convertor or clutch, the transmission ratios, the rearend gears, the tires, and the chassis – to work around the power deficiency. That’s really what we do in Pro Stock, and that’s why you seldom see Pro racers working on engines at the track aside from routine maintenance. We simply take the power we’ve got and try to make our cars use it as efficiently as possible.

In general, drag racers tend to be more engine-oriented than racers in other forms of motorsports. Perhaps that is because we spend relatively little time on the track compared to oval-track and road racing drivers. In my infrequent visits to NASCAR events, I find that most teams regard the engine as a small variable at the track because their engines are developed and tested at the shop. They spend the majority of their track time adjusting the chassis and working on suspension setups. I’ve seen competent teams qualify their Busch Series cars faster than a Nextel Cup car – despite the fact that the Busch Series engines have about 110 horsepower less than the Cup engines!

That simply shows how important the chassis setup is in circle track racing – and points out that drag racers could benefit from spending more time on chassis adjustments and less time on carburetor jets when the weather and altitude conditions are bad. The best place to work on an engine is on a dyno; the best place to work on a car is at a race track. As a professional engine builder, that’s a difficult statement for me to make, but I think it’s the truth.

In reality, the first 1/8th mile pretty well determines a drag race car’s elapsed time. If you’re competing in “go-fast” class such as a Quick 32 or Top Sportsman eliminator, the setup that made you fast in February isn’t going to make you a winner in August. When the relative altitude has changed 3,000 or 4,000 feet, you’re not going to be successful using the same convertor and the same gear ratios that you used in winter.

My recommendation for running in bad air is to target the engine’s rpm range and then make the necessary changes that will allow the engine to achieve that range. A racing engine’s peak power and torque is fixed by the intake manifold’s runner length, the airflow capacity of the ports, the camshaft timing and other factors. The engine is going to perform at its best when it runs at the speed that it was intended to run. Therefore if you go to a high-altitude track, or if you encounter high relative altitude conditions, you have to gear the car to allow the engine to reach its optimum rpm. When you’re missing 300 rpm at the top end due to a change in weather, you must work on regaining the engine speed to maximize performance. Maybe it’s a set of shorter rear tires (if the available traction permits), or perhaps a numerically higher rearend ratio. You still won’t run as fast as you did under ideal conditions, but you will close the gap.

Most of us race to go fast. We love to see a good number on the scoreboard. When the conditions are bad, the e.t.s aren’t as satisfying to our egos, but we can use those times to work on the race car and to learn how it responds. Challenge yourself to regain as much performance as the conditions will allow. The payoff will come when the “good air” returns because your race car will be faster and you will be smarter. Gaining an understanding of how a car responds to different conditions will make you a more formidable racer.
 

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This is what is cool about racing, you can learn something new every day if you open you mind to the potential. So I will have to read this article and educate myself on this even more.

I must say that my mind is working extremely hard to process this, but that is a good thing. Hope this doesn't detract from me getting my work done today.

I guess the one thing for me is I run a set of jets for spring & fall and re-jet for summer and this being the only time I do change jets, except if going to sea level. I don't jet to try to get the car to speed up, since as you mentioned it wont, but I do jet to try to make the car more consistent. If I leave big jets in the car, and I have a swing of temp going down later in rounds I may get a spike in e.t. that is harder to account for. With smaller jets in the summer, I don't see nearly the significant changes.

I think it is a really good point that we can't jet to create hp that isn't going to be there, and yes the ego has cost many a racer in the end.
Thanks for your response and the abundance of info you threw out there. I always appreciate it when people are willing to share their knowledge. I think too many people don't want to share info for fear of losing a competitive edge they may have on someone that doesn't have the knowlege.
 

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So RM, do you have a cool weather/warm weather tune up as far as jetting goes? My understanding is that a car that runs rich, will be more inconsistent than one that runs more lean, but not too lean. Is there a way to compensate for this with timing or what? I'm very interested in this topic and I'm all for not having to change jets, etc. My chassis won't require the changes a pro stock car would, nor do I have the $$ to buy new gears, converter, etc.
 

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So why do cars run so much slower in bad air than they do in good air? The obvious answer is that the engine is making less power. When an engine is tested on a dyno, a correction factor is applied to the raw numbers to adjust the observed power to standard conditions. This allows us to compare the dyno test results that are made at different times of the year and under very different conditions. But when you are running a car down a race track, the correction factor is irrelevant. The only power that is available to accelerate the car is the engine’s actual output at that particular moment in time. If the engine is producing more or less power than it would at standard conditions, that’s what you’ve got to work with.
Interesting article, but it seems to take an overly simplistic view of air (o2) density concepts.

The thought that was left lingering in my mind after reading this was "please explain why the motor makes less power". Until that is addressed by the techie who wrote the article, I think there's something missing that would lend more credibility to his claims.

Something that changed in the environment is responsible for the loss in engie npower. Is it not possible to compensate for that by changing the fuel settings, which is what people are attempting to do by rejetting?

To simply accept your motor's base tune setup as unchangeable when the outside environment changes seems to be a distinctly "non motor guy" point of view. There is value in tweaking everything else non-motor to use your power effectively, but that does not change the fact that the motor has lost power and could possibly regain some of that by retuning.

Great discussion. I'm just playing devil's advocate here. Don't take my comments as an attack by any means. :)
 

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This is a great topic, but I would have to say that you can't make as much power in 4,000 ft of air as you can at 1,000 ft of air. As I mentioned, the rejetting isn't to try to make the car faster in worse air it is to try to make it more consistent and less tempermental to the changes that can occur in the heat of summer from 1st rnd to final rnd.

If you throw a bunch of fuel at the car with large jets, then if you get a change that allows more air to the system, you will have a significant spike in et that is much harder to account for.
 

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If you track weather conditions you will quickly see that low barometer is slow and high barometer is fast, (over simplified for this conversation). That is due to air pressure. What Dave is saying in the article is that carbs compensate for this and require no jet changes for it. I run my engine pretty lean all the time, never change it. It will go faster if I add jet but I want consistant not max power. My car is very consistant all year round with a leaner set up. If the air temp drops 10 degrees I pretty much know what the car is going to do. When I ran it fat it could do some real crazy stuff in the cool air but I couldn't predict what that might be. I don't claim to know alot but as I stated origionally the subject is open to debate. My carb is not stock and has been custom designed for my motor. Most people can't get their carb man to come over and dyno a motor with their carb like I can. So read the info, consider it, then use the info that seems to make sense and works for your set up. If it makes no sense or doesn't work for you, you are only out a T&T session and a little time. RM
 

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Discussion Starter #13
Man it was my ? that started this and i'm pretty proud. This is some great reading and you guys are great for getting it in writing. I really enjoyed it even if I don't quite get it all. Thanks
Lance
 

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RM,
We are basically in the same mind frame here, I have a 4 size jet spread from spring to summer so if I just split the diff. then in the heat I would be slightly fat and in the cold slightly lean but not in a position that would probably hurt the motor. This way I would never have to rejet but also never see the full potential of the engine and not stroke the ego.

The nice thing about Pro stock is that if your car goes .03 quicker than expected it is awesome. With bracket racing, as you definitly know, if I gain
.03 more than I predicted it can cost me the race, and certainly makes it tougher to drive the stripe.
 

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What are you talking about, I don't see anything about that on my post>>>are you seeing things again? Do you hear voices in your head? lol.
 

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The article is by Reher Morrisons David Reher not me Real McCoy. Same initials but that's all. Real McCoy
 

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2NovaCars, I have a 383, 10.5:1, RPM AirGap, Holley 750 double pump, power valve in front, no powervalve in the secondaries and I'm running 74 front, 88 rear, with 37 squirter in front and 42 squirter in the rear. Plugs look like a text book picture of good combustion after a pass.

You may want to check your idle mixture adjustment setting if your plugs are sooted up badly after idling in the garage a good bit. If you have a 4 corner idle adjustment, run the screws all the way in (gently) then back them off about 3/4 a turn and start it up and warm it up and then adjust them in 1/4 turn increments until you have the best idle.

Good luck!
 

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Discussion Starter #20
Hi Pick
Thanks, I will try that as soon as I get some new plugs . Do the NGK UR5s sound like the right plug . Or are they a little cold? Also they are gapped at .040. is that in the ballpark or is that too much.
Lance
 
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