Drag Race Jetting Theory Question

scott riley

scott riley

Member
Joined
Dec 7, 2008
Messages
129
Age
53
Location
South Carolina
I've read a lot about jetting for drag racing, and a number of you experienced racers suggest increasing pilot jet and air jet, richening the low end. My question is why is this better? I don't yet see the logic behind this theory. Why would you ever want air/fuel mixutres to be overly rich? In my case, I'll be running (mostly) 1/4 mile asphalt again this year.

Thanks again,
Scott
 

Increasing pilot jet size richens the low-end as you already know. The reason being that in drag racing you are going from a low throttle position to wide-open instantly. Air being very light accelerates into the engine immediately while the fuel being heavier can't keep up and the end result is a lean spot on takeoff. The lack of an accelerator pump where additional fuel is injected with throttle opening requires additional fuel from the idle circuit (slight over richness at idle) to help cover up the lean spot and eliminate any potential bogging or reduced power due to the lack of fuel.

I do not believe the air jet should need replacing in most cases. If you do change it, be aware that going larger will LEAN the engine. The opposite of the pilot (fuel) jet.

Hope this helps. ;)!
 
Very good explaination! Thank you. Makes a lot of sense now. The next question I have is, how do you know how rich to go on the pilot? What symptoms or signs should I be looking for either too rich or too lean? I don't recall experiencing a bog on launch last summer. Is it just a matter of testing different pilots on the same day and see what gives the best launch (i.E. 60' and/or 330' times).
Thanks,
Scott
 
Macheater said:
Increasing pilot jet size richens the low-end as you already know. The reason being that in drag racing you are going from a low throttle position to wide-open instantly. Air being very light accelerates into the engine immediately while the fuel being heavier can't keep up and the end result is a lean spot on takeoff. The lack of an accelerator pump where additional fuel is injected with throttle opening requires additional fuel from the idle circuit (slight over richness at idle) to help cover up the lean spot and eliminate any potential bogging or reduced power due to the lack of fuel.

I do not believe the air jet should need replacing in most cases. If you do change it, be aware that going larger will LEAN the engine. The opposite of the pilot (fuel) jet.

Hope this helps. ;)!
Click to expand...
;)! nice. 3:16 (yammie tony)
 
SCOTT DON,T FALL IN LOVE WITH LEANING MAIN JETS DOWN TO FARR BUT YES INCREASE PILOTS OR CHANGE NEEDLE POSITION SLIGHTLY. FOR 1/4 MILE, YAMMIE TRIPLES LOVE FUEL AND DON,T LIKE TO BE LEAN. 3:16 (yammie tony)
 
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Tony, here's my set up on the last day I ran last 2 times I ran last year:
MJ: 137.5
PJ 47.5
AJ 0.9
AS 1 1/4
w-w-w
89a-10 w/2.4 inner, 4.5g in tip
Red @ 80
Staright 45 yamaha helix
24/38 gearing
Fresh 93 oct Mobile gas
Engagement 4400, launch RPM 4000
Ait temp 75 or so, Baro 30.4mmHg
1.4's 60'
10.42-10.50 @ 120-121.
Sled wt 718 Lbs w/rider
RPM's mostly around 8500 for majority of run.
Cylinders have been ported, not sure how much.
The last run, it burned the PTO piston on exhaust side, no damage to cyl, and just knocked the edge off the Mag piston on exhaust side just before the finish line. I can see on my video of the tach, the RPM's just started to drop (not noraml) a split second before my wife let off the throttle. Also this was the most "loaded" the motor was, in terms of clutching/gearing all last year. So, I'm thinking at this point 140 MJ is probably as lean as I will go. Your thoughts?
 
HAVE A GOOD SET-UP HERE BUT ALOT OF STOCK STUFF. WHAT CAUSED YOUR BURN DOWN. YOU WILL IMPROVE 60 f.t. IF YA RAISE ENGAGEMENT SOME AND SHIT CLUTCHES FASTER. (muti- angle helix. ). 3:16 (yammie tony)
 
I feel the burn down was a combination of things. It also was the second run using BR9EYA plugs instead of the BR9ECS. I had good experiences with those in the past and wanted to try them in th SRX. I believe they make a more complete burn (i.e. more HP ..... but only when enough fuel is present as I learned the hard way). I think the combination of clutching, gearing, jetting, and spark plugs put me over the edge at the top end of the track. Again, that day was the most laoded the motor was, and as I understand it, the more you load the motoro, the more fuel you need to give it.
 
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I have so far kept the engagement "low" for ease of stagging while my wife got used to the procedure of racing. I know I'm loosing some there.
 
THE MORE YOU LOAD THE MOTOR THE FASTER YOU WILL GO UNTIL YOUR RPM DROPS. BUT LIKE YOU FOUND, WATCH JETTING. SOUND LIKE YOU HAVE A GOOD SLED HERE. 3:16 (yammie tony)
 
u will improve ur time with a 52/44 helix it shifts much faster than ur straight 45 like tony says change ur helix to multi angle
 
extremelyfastmax4 said:
u will improve ur time with a 52/44 helix it shifts much faster than ur straight 45 like tony says change ur helix to multi angle
Click to expand...



not sure on this one ......
 
Stock Yamaha lowered as much as possible, upper axles modified to work with 10.6" track, and rear axle modified with 3 stock wheels in between rails to work with 10.6" track. The only aftermarket parts on the entire sled are the asphalt skis and track.
 
The rich pilot also helps cool the piston at the end of the 1/4 run. We found they burned the easy with lean pilot. Go rich and keep chopping the throttle at the end of the run
 
I dont see any mention of where the needles are set up at?. If you are running stock needle spec and your mains lowered with stock pilot thats why you burned it down. Spark plugs are still a 9 heat range and didnt cause it, you do get slightly more heat transfer to piston from the eya then the ecs but its neglible.
1320ft is a long run without richening up the needles some and /or use of a bigger pilot. On asphalt, youll find you can even better the 60ft time by loading the motor down to the point it sounds like its bogging, which will result in better 60ft times. Sled will just roll forward and not spin the track.

Every 1/10th you take off the 60ft time is 2/10th off the et.

I would use a bigger start on the helix, somewhere around 50-51 and keep same finish angle as you have now(45). I dont really see much wrong with mains if you use a 45-47.5 pilot jet, fuel screws at 1.5 turns out and put the needles at 3.5-4. You will find that added fuel will equal mph gain.

your doing good, just need a little fine tune in the carbs and bump the helix a bit.(you want it to only be at 8000-rpm thru the 60ft on asphalt. Low engagement speed is good also.) bigger start on helix will drop start rpm,create loading from slight overshift, just "control" the shift with secondary spring tension as too much wil kill the topend mph. The looser the secondary spring the more mph you will get up top, but you still have to hang onto the belt from the start.
Now youll need to play with the settings on twist. dont be afraid to use the red secondary spring.
 
You're right, Don. I haven't mentioned the needles because (shame on me) I haven't checked them. I believe when I put this motor/sled back together (10 years ago) from race trim to trail trim for the previous owner, I put it back to completely stock settings and would have done the needles then. I am assuming they are at stock setting now (but will check/set before racing this year) and haven't considered them because I didn't think a change would make much difference for drag racing since you are at WOT the whole time. I am not running stock PJ (42.5), but had been running 47.5's for the last 2 days (7 runs) of last season.

I'll trust you on the spark plug as my "good experience" with the EYA's was in comparison to ES's in a 2000 SX700R (EYA's good for 3 thenths on asphalt) ten years ago.

I have already been thinking I want the RPM's around 8000 for a bit after launch, just haven't had enough runs to get there. For most of the runs we made last year (we ran a total of 31 passes), I was running a 53/47 with green @ 60. I changed to the 45/red @ 80 when I changed from 8DN-20's w/5g in each hole to the 89a-10's to get back to a "baseline" on the secondary with the (much) different weights.

I guess a question I have now is how does needle setting affect fuel flow at WOT? As I understand, the needles are completely out of the jet needle at WOT.

Thanks again for everyone's input. It awesome that we have a place to discuss all this important stuff!

Scott
 
hey scott, thinking about maybe setting up the viper for asphalt this summer to play around. probably just run oxford though. dont have the time to head down to epping. you running OPS at all?
 
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