Thread: Carburator Jetting Advise

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I have a 383 Stroker in my 55 Chevy and I'm pleased with how it's running. Altough, I drive it everyday, I do take it to the strip every now and then and have some fun. It was recently dyno'ed (before a good tune up and carburator adjustment - secondary's not opening completly) with 315 @ the wheel and later I turned a 13.5 in the quarter. A guy at the track told me I could drop a few more seconds and increase the HP by rejetting the carburator. Later he gave me some larger jets for my Demon 750 carburator. He made it sound like my carburator should have been an 850 or jetted. Does this make sense and what are the advantages/disadvanages of increasing the jet size.

PS - I should mention the guy at the track is my 18 year old son who built the engine for me a few years ago while in high school. I've learned he'll tell me anything to get more HP and spend my money. He also gave me a broucher from Demon after the dyno stating a 850 on an engine with that HP. Jet's were cheaper for him to buy sinse I wasn't spending a dime at the time for another carburator.

http://www.geocities.com/fpayne55

Thanks for confirming some of my thoughts, my sons a bit quick sometimes to change things and I'm not as quick with the kowledge to disagree, but I'm learning. Altough his done very well building this engine. I think he can pick up more HP and time with experience on the track. He has made 9 runs on a strip on three times (twice very far from home so he did push it to hard). The times have come down from 15.7 to the 13.5 with knowledge and minor things. He thinks the car should be at 12.9-13.1 based upon the HP and weight. I feel he can hit 13.1 with knowledge on tuning and getting it to lockup a bit better. He could also change the gears but I won't let him due to the highway driving we do in the car.

Smog tests are a good way to test a carburetor. Your goal is to get as high a CO2 percentage as possible. High CO means a rich mixture. High HC also means a rich mixture or worn rings. High 02 percentage means a lean mixture. High NOx means high combustion temps, which is a good thing. If you like, I can post some links to mixture/bi-product graphs.

I think my 3.73's are a good all around gear. I get over 12mpg on 6050 gvwr truck while driving at 65 mph without overdrive with a 350.

6050 gvwr is great because the 95db max sound limit only applies to vehicles under 6000 lb gvwr. Not that my truck is louder than 95 db

Thanks, that would be great. It will give me something to keep him busy on this weekend when he comes home from college.

We are going to be doing a similar project on large marine diesels. One of the professors here got $30,000 worth of various sensors for testing emissions. We're going to put them on a small CAT hoooked up to a generator to tune the engine and get minumum emissions without reducing its output and efficiency. Then, we'll do the same thing on our 12,500 hp large diesels. I am getting involved because I don't want him doing anything that would reduce efficiency like lowering compression did on 70's cars. Then again, I don't see what you can do on a diesel to reduce emissions. You can't adjust mixture on a diesel and compression temps need to be high.

Since he built the engine 18 months ago, it's been in several areas. We did change the gear from the first runs. First time he ran it was on Power Tour in 2002 with a TB350, bad timing (it was detinating at around 4000 RPM), 2.?? gears in Columus OH. SInce then and on the latest runs we have added a posi unit, 4 Llink, MSD 6D, 3.43 gears, 700R4 built for me by a friend with GM for racing/street, and had that same friend check the timing that day. He still feels like the distributor isn't positioned exsacly correct (something about a few knoches off). The car improves with each run. This last time the first pass we blow out a great deal of smoke (cruisin too much) and then dropped with each run until the 13.5. Other than those things the enginee has been the same. The 60' times are at work for this lats time on the strip.

As far as optimising the engine, spending a little time (and money) on the chassis Dyno my be worth your time.

This will take out a lot of variables that you find on the track and you can play with it until you find the optimum jetting and timing. Be sure to note the temp, humidity and air pressure on the day you test it. Major changes in these 3 variables when you are at the track may require that you make MINOR jet changes when you actually run the car.

Your basically at the "test and tune" stage and from the sounds of it you've made the big improvements already (15.7 to 13.5). At this point I think one of the main things is to let your son work on his driving and get the car really consistant so you know that the difference in time slips are due to any changes you make and not the way the car is driven. At that point start making changes ONE at time and see how the car reacts. If you start doing 2 or 3 between each run you may do an improvement in one area that is cancelled out by a change in another.

Good luck, it sounds like you and your son are having fun.

Thanks, Mike. Yes we are having fun. Taking the 55 and my 71 Nova to Turkey Run in Daytona this week. A friend gave us the use of two condo's and off we go to several days of fun and sun. My 70 year old mother is riding down as co-pilot in the Nova with my 15 year old son. She wanted some time with the grandkid's and Isaac want to show her his new ride. We purchased it eariler this year when he turned 15 and if he graduates college it becomes his for life. If not it becomes mine. I have the same deal on a 64 Ford F250 my oldest is restoring, but it's been in a frame off restoration for the past year. Hopfully it will be going to the shows next Fall. Thanks again for the advise