Carb tuning anyone?

[mwizard]

I am running out of things to fiddle with on my car(well not really), so I am looking at carb jetting. I picked up a 2nd carb and am wondering what to jet it to as an experiment at a test and tune day. I am currently running primary/ secondary fuel 170/175, air 175/170. This may be a bit rich on the low end and lean on the top from the results of a dyno session last spring. What are others running?
Ok, maybe I should just go get my car out of storage and park it in the driveway for a few days. It would have helped if I had not missed the first event of the Calif season last wkend due to illness.
Mark

[Albatross]

I don't understand why you would be deviating from your dyno setting. Makes no sense unless the dyno setting(s) were accomplished on an unusually cool day and you were competing on a hot day in which case you might run the engine a little leaner. The carb is capable of minor changes on its own. Work on car handling.

[mwizard]

I don't understand why you would be deviating from your dyno setting. Makes no sense unless the dyno setting(s) were accomplished on an unusually cool day and you were competing on a hot day in which case you might run the engine a little leaner. The carb is capable of minor changes on its own. Work on car handling.

I got 3 pulls on the dyno at a demo day and only had time to adjust timing(for an excellent result). The dyno guys are not Weber carb experts. I have many jets to play with and have increased the primary air one size and the 2ndary fuel one size to compensate for what I found in the attached chart from that event.

I started this thread to get more educated about how the carbs work. I have several books and so far they are not very helpful. I need a nice summary and some more data points.
Mark

[Pop Chevy]

Might help to know what engine you are talking about.

[mwizard]

Might help to know what engine you are talking about.

Kent 1600, Stock with the standard 32/36 progressive weber carb.

[Will Velkoff]

I've never done any dyno tuning on a carb'd car, only fuel injected. But on injected motors, my AFR is basically a flat line within reason. I see on your AFR curve it leans out quite a bit as it runs through the powerband...is that normal on a carb'd car? Or should it still follow suit and be relatively flat through the powerband?

[Neil_Roberts]

One of my fundamental principles is:

Don't guess when you can know.

If you have an O2 sensor and a data logger, you can develop your jetting during a test day at the track. If not, go buy some more dyno time. Max power is usually at an air/fuel ratio of about 12.5. For cars that do a lot of idling, it's a good idea to run lean at idle to prevent plug fouling.

[mwizard]

I've never done any dyno tuning on a carb'd car, only fuel injected. But on injected motors, my AFR is basically a flat line within reason. I see on your AFR curve it leans out quite a bit as it runs through the powerband...is that normal on a carb'd car? Or should it still follow suit and be relatively flat through the powerband?

Will, Thanks for the response. You are seeing what I see. There is no good reason for such a big dip in the 3300 range and then a gradual leaning out passed the optimal 13.3 to 1 on the way to too lean at the high end. The big dip might be the 2nd throat kicking in which I, in my limited knowledge, do not know how to modify. Here is the dyno chart and you can see a flattening out at that point in the curve.
Mark

[mwizard]

One of my fundamental principles is:

Don't guess when you can know.

If you have an O2 sensor and a data logger, you can develop your jetting during a test day at the track. If not, go buy some more dyno time. Max power is usually at an air/fuel ratio of about 12.5. For cars that do a lot of idling, it's a good idea to run lean at idle to prevent plug fouling.

Thanks, Neil. I am looking for a Weber expert with a dyno locally.
Mark

[Art Smith]

Mark-

when it comes to the Weber on your FFord engine, the most important thing to recognize is it should not be thought of as a single carburetor but rather two venturi's that have individual fuel jets, air jets, and dump tubes. the next thing to understand is the intake manifold runners are very different for the outer two cylinders and the inner two cylinders. the mounting of the Weber results in one venturi "mostly" servicing one pair of cylinders and the other venturi "mostly" servicing the other pair of cylinders. it's my opinion that making a single measurement, ie: A/F or O2, to measure two different quantities is not likely to get you the right or optimum answer. A/F and O2 sensors measure some unknown weighted average of what's going on in the inner pair of cylinders and the outer pair of cylinders. two lean cylinders and two rich cylinders look just fine to a single sensor and that's the problem in my view. exhaust gas temperature (EGT's) sensors (usually K-type thermocouples) in each cylinder's exhaust is the only way to get the right answer on jetting and more importantly the union of fuel, jetting, ignition timing, and atmospheric conditions (temperature, humidity, and barometric pressure). EGT's should be mounted as close to exhaust port face of the head as possible; all four EGT's must be mounted in the same position the same distance from the head; and all four EGT's must protrude into the ID of the exhaust the same distance. it would be a mistake to assume the exhaust gas flow is uniform on the ID of your Kent's exhaust. matching your Weber, intake manifold, head, AND the exhaust from your car on your engine builder's dyno is time well spent.

Art
artesmith@earthlink.net

[HayesCages]

It may help to know if your carb has been race modified or is stock as well, i.e.: are there little brass tubes that have been added to the ID of of the throat?

[mwizard]

It may help to know if your carb has been race modified or is stock as well, i.e.: are there little brass tubes that have been added to the ID of of the throat?

Lawrence, It is an Ivey preped carb and has the brass tubes. I just talked to Gron Parry, a local carb tuner, about working on the tune and he said Ivey bends the tubes to get at each cylinder. Pretty amazing. I will settle for close to perfection.
Mark

[mwizard]

I sent my dyno charts to the local Weber tuner and he said I had a pretty good curve already, so I guess I just get to work on the nut behind the wheel.
Mark

[TimH]

IOW Jay got it about right again...

I keep running out of excuses for being so slow.

[bob darcey]

I suggest that you keep thinking about this because, a.) 95 HP is nowhere near enough, and b.) your A/F curve says it's too lean in the RPM range you should be worrying about. Don't know where the "optimal 13.3 to 1" info comes from but as someone said, 12.5 to 12.7 is a more common target for power. Don't know what level of prep your motor is in, but a decent 1600 in legal FF prep should make around 110 HP. If you're running a bone-stock cylinder head and Pinto pistons, 95 might be about right. Your jetting doesn't sound too lean but the results are, so maybe float level adjustment, clean out the dump tubes, etc.

I agree with Art, EGT's are very helpful. I use a two-channel on-board system from Westach (http://www.westach.com/products/DUAL...TURE/index.php) with one probe in #1 exhaust and one in #3, generally shoot for 1350 F.

[TimH]

Isn't that 95 based on axle torque?

[Neil_Roberts]

I've seen an 87% gearbox efficiency claim for a Mk9. With that, 110 hp at the crankshaft produces 95.7 hp at the diff output flanges. On a chassis dyno, there are some additional power losses from tire sidewall flex, tread slip, and the CV joints. Also, because a chassis dyno is not a constant-rpm device, the rotary inertia of the flywheel reduces the measured power output. So, there is no cause for alarm here.

The total area under the power vs RPM graph is really what matters, and there are a couple of things that can be done to improve a Kent for autox. Although I don't remember the numbers, less cam retard and larger valve lash settings both improve mid-range power. I have looked through my notes from long ago, but couldn't find the numbers. I'll look again and post the numbers here if I find them.

[mwizard]

I've seen an 87% gearbox efficiency claim for a Mk9. With that, 110 hp at the crankshaft produces 95.7 hp at the diff output flanges. On a chassis dyno, there are some additional power losses from tire sidewall flex, tread slip, and the CV joints. Also, because a chassis dyno is not a constant-rpm device, the rotary inertia of the flywheel reduces the measured power output. So, there is no cause for alarm here.

The total area under the power vs RPM graph is really what matters, and there are a couple of things that can be done to improve a Kent for autox. Although I don't remember the numbers, less cam retard and larger valve lash settings both improve mid-range power. I have looked through my notes from long ago, but couldn't find the numbers. I'll look again and post the numbers here if I find them.

Excellent news Neil. Nice to know my motor is close to max. I have been gathering parts for a new motor, but now there is no rush. The dyno I used was a hub unit so one less friction loss area.
I look forward to more info.
Mark

[bob darcey]

OK, dumb assumption on my part, but you're still too lean. And you would like the EGT setup.

[mwizard]

OK, dumb assumption on my part, but you're still too lean. And you would like the EGT setup.

Bob, What is the EGT set up?
Mark

[bob darcey]

http://www.westach.com/products/DUAL...TURE/index.php

I have the 2DA2, the one at the bottom of the page.

[mwizard]

http://www.westach.com/products/DUAL...TURE/index.php

I have the 2DA2, the one at the bottom of the page.

Thank you. It says for use with 48 in type K thermocouples. Where can I get more info on this setup?
Mark

[Art Smith]

Mark-

for autocross, it's my opinion that horsepower isn't the best metric to be using for evaluation of your engine's performance assuming your objective is competition versus participation. to Neil's point, there are a number of compliant things that can improve the mid-range torque of your FFord engine at the expense of top-end power. the area under the rpm-torque curve for a FFord engine is essentially a constant making tuning a zero sum game. thought of another way, nothing is free! the camshaft in most FFord engines is installed retarded with respect to TDC to improve top end power for road racing at the expense of mid-range torque. in addition to four EGT's, three simple ways to improve mid-range torque come to mind that would improve performance in an autocrossing environment: 1.) have your engine builder adjust the camshaft timing to straight-up or advanced with respect to TDC. 2.) use a real 4:2:1 exhaust (ie: not an asymmetric 4:2:1) tuned for someplace in the neighborhood of 5500rpm (your data system will tell you the right number). 3.) use a real 4:2:1 with smaller primary tubes to move the engine's torque peak toward your optimum rpm.

http://www.apexspeed.com/forums/showthread.php?t=56179

Art
artesmith@earthlink.net

[bob darcey]

Same place for gage, T-couples and info: Westach, Sonoma, CA (707) 938-2121

[Dick R.]

Anybody have any typical data for full throttle percentage for autocross and road racing assuming "typical" course/track with fresh tires and dry pavement? I suspect that autocross has a much lower full throttle percentage than road race due to very short "straights" and short gearing limiting the opportunities for full throttle.

I've only done one autocross since installing a throttle position sensor and conditions were less than perfect. However, according to DL1 data in a Race Technology Analysis bar chart full throttle was about 23 percent of the run. Part of the problem (in addition to driver "talent") was car setup which prevented full throttle too often. This is my first priority to improve. FYI looking at longitudinal acceleration will give similar information to full throttle percentage.

Dick
CM 85

[mwizard]

Anybody have any typical data for full throttle percentage for autocross and road racing assuming "typical" course/track with fresh tires and dry pavement? I suspect that autocross has a much lower full throttle percentage than road race due to very short "straights" and short gearing limiting the opportunities for full throttle.

I've only done one autocross since installing a throttle position sensor and conditions were less than perfect. However, according to DL1 data in a Race Technology Analysis bar chart full throttle was about 23 percent of the run. Part of the problem (in addition to driver "talent") was car setup which prevented full throttle too often. This is my first priority to improve. FYI looking at longitudinal acceleration will give similar information to full throttle percentage.

Dick
CM 85

Good info, Dick. I am thinking of spending more time in 3rd gear to stay away from the redline because my top Hp is about 5700 and torque is table top flat from 3K to about 5500.
Mark

[chrisw52]

After reading this thread I see a data acquisition system in my future..

[mwizard]

Chris,
Good Blog post on the wheel bearings. It looks like replacing yours will be an improvement.
Were there any symptoms that alerted you? Will you have the car together for the SCCA event on Feb 10? The previous owners of my car were very thorough in the restoration, so I probably don't have an issue.
Mark

[chrisw52]

the only real warning I had was after I broke the wheel stud. The machinist at the shop showed me the inner casing of the bearing and the grease ( or lack there of ). When I saw the dried up grease, I knew that the other bearings were probably in similar condition.

[mwizard]

the only real warning I had was after I broke the wheel stud. The machinist at the shop showed me the inner casing of the bearing and the grease ( or lack there of ). When I saw the dried up grease, I knew that the other bearings were probably in similar condition.

It is a good idea to let a pro check the car periodically
Mark