what is correct a/f number when nitrous hit off brake?

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Re: what is correct a/f number when nitrous hit off brake?

#16 Post by John_Heard » Thu Sep 17, 2009 11:07 am

572 BBC wrote:this is your above paragraph.when u say 6.38 and 6.22 n/f i didnt know what it meant.but i think i know what your saying about trying to shoot for same air fuel ratio weather if its 175 shot or a 375 shot.
Yes, at least for a starting point when stepping up your tune up it should get you close. You'll also need to take more timing out with the bigger jets don't forget..
572 BBC wrote:so what your saying is when spraying lean it out till your car slows down,but be cautious of timing(ground strap radius is where i want heat line i know that).so if i do that i wont be in danger of aluminum specs of piston being burnt onto porcelin or any other damage then?
If you get it lean enough it will start losing MPH, but when the fuel ring disappears from down inside the plug I start getting a bit nervous and stop there. Some people like to run a plug pure white with no fuel ring - that makes me nervous. A light gray ring at the bottom of the plug is what I like to see. At that point you would want to start paying attention to what your AFR is showing also. Remember as you add timing to a plug with a light gray ring, it's going to heat up and get leaner - you'll want to add fuel when you add timing if you're at that point.
572 BBC wrote:i highly recommend this site for people to check out your da man john!
Thanks - I don't believe I'm the "man", there are quite a few out there a lot sharper than I am at nitrous. I just like to try and help folks out when I can.
572 BBC wrote:also are nitrous jets created equal? like i said i have a big shot kit. my friend said he has every size jet for $5 a piece but not nos brand.if i could get nos brand ones at a comparable price id rather get brand named ones...what are thoughts on it?thank you!!!
I've never really studied how accurate nitrous / fuel jets brands are. I've always flowed my own stuff to learn how much fuel/nitrous they move and go from there. There is, from what I've heard, quite a variance in quality with jets. I don't think stainless jets are worth the extra cost... I think that's just a gimmick - there is no way you'll ever have any wear that would be measurable in a brass jet. Again Dave Koehler is a great source for some nice jets at a reasonable price. He makes his own and has perfected the machining needed to make them very accurate, they are also laser etched (not stamped which can distort the hole).

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Re: what is correct a/f number when nitrous hit off brake?

#17 Post by 572 BBC » Thu Sep 17, 2009 6:33 pm

thanks again!

so best to get motor tuned up first so it mphs the best then go to nitrous,this i understand,but would i tune up the n/a with the racing gas and colder plugs?ive got 110 octane in er right now and motor is only figured out at 10.2:1 compression with 286/296@.050 on a 112 lobe sep(motor builder choose it. now that im into researchin stuff wished it was about 10 degrees at .050 less with target operating goal of 4000 to 7000 or justa tick over)....

so what your saying is when a guy is first starting to find his nitrous tune up,retard it plenty(maybe like 4 degrees for every 100 hp gain),and at first focus on getting the fuel right for the nitrous load.then when its good focus on getting heat in arc of ground strap....or can u do both at same time?

im such a noob!!! :oops:

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Re: what is correct a/f number when nitrous hit off brake?

#18 Post by John_Heard » Thu Sep 17, 2009 8:35 pm

Sure, it makes sense to get the car running good on the motor only, find what it wants for the carb jetting and timing, etc. then start playing with the bottle. I believe I would use the same fuel that you're going to spray with, but you may want to use hotter plugs motor only.

4 degrees per 100 is a ball park guesstimate at best, but with your low compression engine you'll be safe there on those small shots. It won't hurt at all to take a couple extra out on that small shot then add some back in if the plugs look like they want it. Sure you can make several changes at the same time, but it's going to be in your best interest to make small changes and study how they effect the spark plug, ET and MPH as you go along. Avoid making big jumps in timing and fuel settings until you feel comfortable with this deal.

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Re: what is correct a/f number when nitrous hit off brake?

#19 Post by 572 BBC » Thu Sep 17, 2009 10:22 pm

is there truth to only being able to get one reading off of a spark plugs when new to get accurate reading (mainly strap for timing im referencing)then need another new set?

actually when i had car at dyno session,with a 260 shot the strap turned color after the curve but before the weld.too much timing im guessin?

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Re: what is correct a/f number when nitrous hit off brake?

#20 Post by John_Heard » Fri Sep 18, 2009 6:37 am

It's best to use a fresh set to get a nice read on them. But once you get the tune up happy you don't need to be swapping in new ones all the time. On some of the guys cars I help with we just put one or two new ones in when making an adjustment - but that's after you get a feel for if you have some holes that are hotter/colder leaner/richer than others and have a good idea which ones you need to watch more closely.

The car on the dyno - if you can see a mark close to the body after the bend on the strap yes it might have too much timing in it. Heat marks on straps are sometimes hard to see - that can be due to the fact there isn't really one there yet and there's still not enough heat in it yet. The NGK's have a coating on them that burns off as the plug gets to temp, once that burns off you start to see some indications on the strap. If you're still seeing some of the plating in places, like on the plug base it's still cold. Also plug heat range makes a bit of a difference.

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Re: what is correct a/f number when nitrous hit off brake?

#21 Post by 572 BBC » Fri Sep 18, 2009 7:44 pm

dart told me to use 5671-7 on motor 8 up to about 225 shot and 9 for 250 and higher....im running 355cnc darts

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Re: what is correct a/f number when nitrous hit off brake?

#22 Post by John_Heard » Sat Sep 19, 2009 10:55 am

Hey they are getting better! Dart used to tell everyone to run a -10 for everything :scratch:

At 250 and higher you may want to go ahead and use -10's, they also make a -11 for a really cold plug.

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Re: what is correct a/f number when nitrous hit off brake?

#23 Post by 572 BBC » Sat Sep 19, 2009 11:25 am

LAST TIME I TIMED MY CAR I SET IT AT 36(LOCKED DIS) AND WHEN ON DYNO I SET RETARD TO 9 DEGREES...REMEMBER 10:1 MOTOR COMPRESSION...SO I FIGURED THE STRAP SHOULD NOT OF HAD SUCH A HEAT MARK ON STRAP(NEAR WELD)....

SO NOW IM CURIOUS BECAUSE NOS TELLS PEOPLE TO RETARD 2 DEGREES FOR EVERY 50 SHOT SO WITH A 260(91 NITROUS PILL) WITH THE 5671-9 IN MAYBE I NEED TO GET EITHER A COLDER PLUG(10'S)OR RETARD IT A FEW MORE DEGREES EH WITH THE 260(91PILL)SHOT EH??? I KNOW SOME DONT GO BY THE 2 DEGREES RULE-SOME ONLY LIKE 1.5 ONLY...WHAT U THINK?THX AGAIN FOR PATIENCE JOHN!

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Re: what is correct a/f number when nitrous hit off brake?

#24 Post by John_Heard » Sat Sep 19, 2009 12:00 pm

The -10's will take a little heat out of it and probably move the mark some. Worth a shot. Also you want to do this on the track, things will likely be a little different with the loading on the engine, etc. If you take timing out of it and it speeds up MPH :mrgreen: , then it liked it. You can also look at what fuel you're using. Higher octane fuel burns slower and can take more timing.

On another note, the Density Altitude, water grains / humidity needs to be considered. Moisture in the air will also slow down the burn.

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