Jetting for purple plate

[Stunner Racing Engines]

What's there to decide on? Real life experience and years of 4 cycle racing and engine building have already proven the statements above. I see no need to continue the discussion. I've learned my lesson. I'll keep quiet and let you answer all the jetting questions.

 

[Gearhead]

.059 to .060 jet for a purple plate on meth is spot on .

 

[alvin l nunley]

I'm sometimes amazed at how casually jetting is approached in the 4 cycle world. And only in the 4 cycle world of karting! Everybody else in the world takes it quite seriously.

If you've ever been around a dyno, you might know something about correction factors. The correction factors are used to adjust the horsepower calculations to match changes in the ambient air pressure, barometric pressure, and to a lesser degree, humidity. If you have ever used correction factors, you must be aware of the effect they have on the numbers gathered from the dyno run. A rise, or a drop, in these readings can have a pronounced effect on the horsepower numbers.

There is a gauge, called an air density gauge, that will show you, through the day, how much these correction factors change. Air density is; barometric pressure as it is effected by temperature. If the barometric pressure remains constant, the air density will change as the temperature goes up and down. If the air temperature goes up, the air density goes down, and vice versa. If the temperature remains constant, the air density will change as the barometric pressure goes up and down. Just the opposite of the air temperature, the air density will go up as the barometric pressure goes up, and vice versa.

From Longacre; a 1% change in air density would call for a 1% change in jetting. The difference in area between a .039" and a .040" jet is about 5%. The difference in area between a .020" and a .021" jet is about 10.1%. This shows you that you can't swap high-speed jets and get the same percentage effect as swapping low speed Jets.

Now I've been told that the single cylinder 4 cycle engine is not affected by air density changes nearly as much as, say a V-8. I don't see how this can be true!! If that were true, dyno software would not require correction factors for these small engines. Every good dyno program in the world has places to enter the current temperature and barometric pressure. Most even have a place to enter the current relative humidity.

The point is; this should show you how important air density is. This should also show you how important it is to have precision reamed jets with calibrated flowrates. This should also show you how important it is to have jets in between full sizes.

Not to be condescending I assure you, but it amazes me how much debate there is on the advantages, or disadvantages, of different gear sets, with the same ratios, yet the need for jetting differently, when the air density changes, is just dismissed, by some, as not necessary.

And my apologies to the newcomers, as this might just be way over your head. LOL

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)

 

[rebsfan4]

Al, we have been using programs on dynos for many many years now that call for correction factors. It's all common knowledge. These things you speak of are not new and they are not a mystery to the "4 cycle world of karting". It's all part of the mandated format that is used that gets one to the experience based knowledge that Brian and Stunner make reference to.

 

[alvin l nunley]

Al, we have been using programs on dynos for many many years now that call for correction factors.

that's good!! And of course you'll notice from my post that I never said you didn't, in fact I did say that most dynos have correction factors in the software.

So tell me why you don't take that knowledge to the track!! Why are people still drilling jets. Why do people tell me that air density is not important. And most of all; tell me why people get so insulting when I suggested it.

You've seen the difference, (I'm only guessing), what a change in jetting, to match the conditions, can make.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)

 

[rebsfan4]

So tell me why you don't take that knowledge to the track!!

I do.

Why are people still drilling jets.

Not sure other than too cheap to buy a complete set of jets that are sized already

Why do people tell me that air density is not important.

You have to realize that the few who you argue with constantly on this do not speak for the masses. Plenty of people use air density readings.

And most of all; tell me why people get so insulting when I suggested it.

If I'm being honest here, I'd have to say it's because of your redundant insistence that people are lacking in the knowledge of the subject and it's use

You've seen the difference, (I'm only guessing), what a change in jetting, to match the conditions, can make.

Yes sir I have. I've also seen from dyno testing that the small increments you commonly speak of in a jetting change are not that critical when you can get the same result with a simply twist of the air/fuel mixture screw.

 

[CarlsonMotorsports]

Who are all these people who don't take it seriously? Who use drill bits?

I can see racers not being overly concerned about air density or the proper way to ream a jet, but every reputable builder I know understands the importance.
Most racers simply buy their engines from a professional builder. I doubt that many racers take air density seriously or make up their own jets. They simply rely on their builder (to whom they've paid good money) to make sure that they have the right jet installed on the dyno and assume that it was reamed properly.

Can performance be gained by monitoring density altitude and changing jets accordingly? Absolutely! DO most racers do this? No. "Why?" you might ask. They don't have the knowledge, tools, data, or desire I suppose - I really can't answer for why others don't take this seriously. My suggestion to racers is to get a decent air density gauge (affordable) and send it to your builder to place in the dyno room with your engine when it is being pulled. Record the number and use that as a baseline. Computech has a real nice gauge that we use (for checking DA at the dyno cell and tuning at the track).
Now, even with the data, most racers don't have the capability to change jetting in very small increments. .0005" (1/2 thou.) increments are as small as I feel comfortable offering, and I'm the only one I've ever heard of selling 1/2 thou jet sizes. About the only people I sell them to is other engine builders and a few real serious racers. Al, you're talking about needing even smaller amounts - to which the return on investment becomes very arguable. Being that most racers use CHT only (some classes egt is illegal), you'll never get an accurate look at combustion temperatures (real time anyway) using cht alone. Making a jetting change going by only CHT is a crap shoot at best - 50-50 chance you'll improve your engine's performance or hurt it. Most racers are not willing to take that chance and rightfully so.

I think engine builders understand the importance of proper jetting....racers may or may not. Some racers simply want to be told what jet should be in their engine for their specific application. They may not be concerned with the "why." Afterall, this is a beginners forum.
Maybe a separate thread on engine tuning via air density and egt would be a good idea.
Just a thought.