Wolfplace I am trying to read this dyno sheet, and I want to know if I need to change my jets on the carburetor. The engine was dyno with the stock jets (Demon 650 Mechanical secondaries),which according to Barry Grant stock Main jets are 70 primary and 78 for secondary and no modifications whatsoever was done to the carburetor, which I find weird. Any how the engine made great power but since the addition of 3.73 gears, and a pro billet MSD distributor, I think I can make more power considering no tunning was done. Other engines making the similar power and torque all run 750 cfm carburetors either from Holley or Demon. Since you run a dynometer I was hoping you can help me out in this matter. I want to rejet the carburetor but not sure what to increase it with? You can access the dyno sheet on link below

Dyno results (http://www.sacchevelles.com/TC%20DYNO%20RESULTS.doc)

I am no pro at this by any means but when they dynoed my motor they jetted the carb according to the exhaust gas temperature. It seems to make sense, and they not only do it for a living but it was Pat Musi the IHRA racer so ill belive him and his crew.
Good luck Dane

There's no way to tell for cetain whether the engine wants either more or less fuel from what's in those tables. Next time you get it dyno'ed, have them use an exhaust gas analyzer, so you can have a fuel mixture reading. Water temp, fuel pressure, and all that are not of much help for tuning, as long as they're within a range of reason.

It would help to know what size motor you have and what's in it, to guess at whether a 650 carb is appropriate. That's too small for anything bigger then a 350 in most cases. Although, that's what it looks like this graph is, is a 350 on an engine (not chassis) dyno, since it reaches the carb's rated flow at about 6000 RPM like a 650 would do on a 350, according to the vacuum.

Gears and distributors won't really affect carb jetting requirements very much. If they affect it at all, generally they will make the engine want a little less fuel, not more.

Why do you want to "rejet the carburetor"? What makes you think it needs it?

The main thing to look at is the BSFC numbers. You didn't say what your combo is, but IF those numbers are correct, they are very low, much more typical of a very high compression race engine. Your HP and torque CURVES look pretty decent so I would guess you aren't way too rich or lean, although the BSFC #'s would indicate lean for a pump gas engine.

I use wide band A/F 90% of the time and have never been run astray.

I would expect BSFC numbers around .45-.5 for a pump gas engine.

So I question the accuracy of the BSFC numbers which isn't an uncommmon issue in my experience.

Everyone has an opinion so I'll go ahead and give you mine, too. For carb sizing you want as small a carburetor as you can have so long as it meets the air/fuel requirements of the engine (assuming a street application here). This was you're likely to have a reasonable venturi size which while keep the velocity of the a/f mixture up and ensure good mixing. One way to tell if a carb is too small is to get a vacuum reading under full load with wide open throttle. The vacuum reading you take under such a condition should be 0" or very close to 0. As others have pointed out there is no real way to determine jetting requirements from the information you've provided. When I dyno tune my engines (chassis dyno) I use a wideband 02 sensor to measure the air/fuel mixture. The closer you can get to the engine the better, and I've had better results with heated sensors when taking readings from the tail pipe. You need to provide more information about your combo (intake, heads, compression, etc...) to get any guesses about where the jetting should be. I'm surprised that your engine builder and/or dyno operator didn't try different jetting while the engine was on the dyno. There's a lot of work involved setting up an engine on an engine dyno and it seems pointless not to power tune it once it's there.

Originally posted by doggy69:
I am no pro at this by any means but when they dynoed my motor they jetted the carb according to the exhaust gas temperature. It seems to make sense, and they not only do it for a living but it was Pat Musi the IHRA racer so ill belive him and his crew.
Good luck Dane What exhaust gas temperature did they strive for and how did they measure it?

The Dyno was an engine dyno, not chassis dyno just to clear things up. The engine was broken in while on the dyno, so each pull showed a increase in power both in horsepower and torque, I assume that was the rings and the rest of the engine started seating. Anyhow the only modification made at the time of the dyno was timing, which we found optimal at 35 total, this was with an old petronix conversion kit. I assume now that I have a MSD that the timing and spark will be more efficent therefore possible resulting in less timing needed. The race shop did add a carburetor 1 inch spacer to see how much increase, the engine made 2+ horsepower and torque. My engine combo is as followed.
The car makes great power but when I get on her hard it sounds like she wants more gas ;) "Feed me Semore Feed me all not long"

Engine: 350 bored .60
pistons: keith black heupertonic(spelling) pistons
Heads: full Port, compcam 1.6 full roller rockers, 2.02 valves, 76cc (flowed 260 cfm, not at what lift)
Intake: Edelbrock Air_Gap RMP
Carburetor: Speed Demon 650 (double squirter) Primary jets 70, secondary jets 78
Cam:CompComs XE 274 installed on a 106
Timing: current total at 35, but have to check what initial is?
Distributor: MSD Pro Billet
Headers: Flowtech 1 5/8

CR 9.251

Spawn,
Sorry, but I cannot tell you what to do with jetting with what you have there.
What is the "pressure" "PSI" numbers column? If they are fuel 3.5 is a bit low.
The BSFC numbers would indicate it MAY want more fuel but I have seen a lot of Demon carbs that have excellent numbers compared to Holley's that usually tend to go richer on the top.
Still mid threes are extremely good,,,almost to good smile.gif looks a little lean.
You need Lambda numbers or A/F numbers to give you an indication of jetting.
A note on A/F numbers, they are calculated in the software by dividing the air by the fuel & can be off quite a bit especially in a sweep or acceleration test. They will be very good in a step test.
Not a huge deal if the dyno operator knows about how far off they tend to be.
Someone mentioned exhaust temps. I do not jet to exhaust temps unless they are way out.
There is just no "right" exhaust temp for different combos. It is effected by ignition timing & cam timing as much as by fuel.
In a restricted engine I see exhaust temps that a closing on 1800 degrees & first thought to most is it is way lean. Fact is it has gone way rich & fuel is burning in the pipe!! Same deal with way lean, the temps can go down in the pipe, not enough fuel for good combustion.
They are great for getting the cylinders more equal & if you have a baseline for your combo to call "avarage"
The 650 Demon is probably not to small for your engine although it is always nice to try a little bigger to see what happens.
Demon does not flow their carbs the same as Holley according to them & I have found a 750 demon acts closer to say an 830-850 Holley.
Ideally no vacuum would be nice but I don't see it very often 1-1.5" is pretty fair.
I almost always jet to best power in a performance engine within reason so it would have been nice if they had gone up about 4 jet sizes just to see what happened.

I don't know who actually did your dyno but Rex has an excellent reputation & has been doing this as long as I have.
I used to race at Sacramento with him a "few" :D years ago when Pro Gas was in it's infancy before I started driving the TAD car for Santos
Tell him I said Hi when you see him. ;)

Mike Rex is a great guy his son Glen has been doing a lot of the family business since Rex wife passed away last summer. Bob Bunker (spelling) actually built my engine. Bob and (Old) Ed dyno my engine on the dyno stand, they were very happy with the power the little small block made.

As far as the carburetor I was thinking about going to a 750 cfm, but I now I want to look at buying a Holley jet kit and tunning it that way. I think I might have access to a chassis dyno here in Sacramento, I have heard that is one of the better way to tune a car

Originally posted by Spawn22:
Bob and (Old) Ed dyno my engine on the dyno stand, they were very happy with the power the little small block made.That is some pretty good power there. smile.gif What heads do you have?

Rod the heads are a stock casting GM off a GM truck, not sure what year but I can get the casting number off the build sheet if you want. The real magic on the heads where done by two guys who port heads, one guy from (Nick)West Coast Induction the other (Craig) from Rex Hutchison race shop. The ports look really good and clean, they openned here up and she breaths really good!

Yeah, I'd be curious to know if it's not too much trouble. I wouldn't want you going out into this Arctic weather to check under the hood or anything. Oh, nevermind, I see you live in California. :( It's less than 5 deg. here right now. :eek:

Plus I don't drive the chevelle to work, she is a weekend car. So I will give the information when I get home.