Hi,
I'm thinking about putting my stock 69 L36 on a chassis dyno to help me square away my carb tuning. I have no knowledge of the different types of dyno's there are and which one would be best for my application. I'm looking for a recommendation here. Since I want to run it on the dyno primarily for carb tuning I want A/F mixture readings from idle to WOT. The dyno sheets that I've seen posted on this forum show A/F readings starting at 3K. Is it possible to read A/F through the entire range? I thought I'd arm myself with a little knowledge before I went any further with my dyno shopping. Any good dyno tuning sites would be much appreciated.
Thanks,
Warren

 

I’ve only tested on Dynojets so I can’t speak on other chassis dynos, although I suspect they have similar capabilities and options.

In order to tune your carb on a dyno as quickly as possible, the facility should have a wide band O2 sensor. This can either be attached directly to your header collector or exhaust pipe just aft of your exhaust manifolds if you have an O2 bung. Otherwise they just stick the probe in the end of your tail pipe.

I’ll assume yours is a Holley carburetor. If other than a Holley I think the circuits are similar, although how they are actuated and adjusted are different. You have 2 primary fuel enrichment circuits, the idle and main circuit. You want to tune the idle circuit for low rpm, part throttle cruising and the main circuit for WOT. The Dynojet allows you to tune both circuits. Target your main circuit for an A/F ratio of somewhere between 12.5 – 13.5:1 ; the idle circuit to about a 14 – 15:1 ratio. The Main circuit A/F ratio is primarily adjusted by jetting, and can be further refined w/ air bleed adjustments (usually not required). The idle circuit is primarily adjusted w/ the idle mixture screws and is further refined by adjustments in idle feed and idle air bleed orifice size.

A few checks you should do before going to the dyno:

• Timing -- make sure you have approx 36 degrees total advance. Lars and others have posted some excellent information on this topic.
• Fully open throttles – I can’t tell you how many times I’ve witnessed carburetors that did not open fully at WOT. Have a friend mat the gas pedal (engine off of course) and check to make sure the secondaries are fully opening. If you have vacuum secondaries, this will have to be checked during a WOT pull at the dyno (viewed safely from the side)
• Closed throttle position – the primary throttle blades should uncover approx. .040” of the transfer slot as viewed from beneath the carb
• Fuel level – should be just at bottom of site plug threads
• Fuel pressure – ensure you have 3.5 – 8 psi throughout rpm range. Target 6 psi

Lars has posted some great information about some of his more common findings during his countless TFB tours, and has offered some straightforward advice on carb and timing tuning. Lars knows of what he speaks – you can’t go wrong following his recommendations.

Depending on the dyno facility you are using, you probably want to being along a few hand tools tools and gaskets:

• Timing light
• Carburetor bowl gaskets
• Metering block or metering plate gaskets
• Jets
• Vacuum diaphragm springs (if equipped w/ vacuum secondaries)

Adjusting your carb to factory settings will ensure the dyno is used to confirm and fine tune the settings.

Good luck,

Mark

 

Very good advice above

I might ad the main reason you see the dyno runs starting at or near 3000RPM is because most cars tend to hesitate or bog slightly below that and this will have an adverse effect on the dyno readout as well if could actually have some affect on the entire curve.

Actually I have seen most start in the 2500RPM range. THe reason for this is exactly what you are there for, a poorly tuned carb/motor and the fact that most prople are there to see peak HP and TQ numbers anyway. A properly tuned motor will pull from low RPM, my dyno pull started at 2000RPM, an experienced dyno operator can tell where the motor will pull from.

You can see your idle and cruise A/F by asking them to record it a idle then on the dyno the operator can use a steady 2000 RPM for a minute or so to determine cruise A/F

 

Thanks for the replies. The carb is an Edelbrock 1901 Q-jet. When I freshened up the motor 1500 miles ago I sent the carb and distributor to Lars to be tuned. I have a basic understanding of carb tuning and timing from articles I've read by the many experts on this forum and from the very patient Lars himself. Timing is at 36 total, mechanical advance all in by 2900. I've got spare rods and jets and gaskets. I want to be sure the car is set up for peak performance and longivety. When I mention dyno to my friends they look at me like I'm crazy. Is chassis dyno tuning that detrimental? What transmission gear is the test conducted in? I have an M-20 with a 3.08 diff.
Warren

 

The reason many dyno graph start at 3000+ rpm is that they have automatics and if you go full throttle below that rpm they downshift. Since you have a manual you can dyno from whatever rpm you want (and the engine tolerates). I have some dyno runs where I started at 1500 rpm.

You always dyno in direct (1:1) gear. For your Muncie, that is 4th.

Dyno tuning is not detrimental. There is less load than flooring it on the highway. Be aware that since the load is less (you're accelerating a 1600 lbs drum on a dynojet) you will end up too lean and with too much timing. Once you find the max power on the dyno you should jet up 1-2 sizes and make sure timing is no more than 36 degrees.

The most common dyno is the Dynojet. Another one that is getting more popular is the Mustang dyno. They are very different animals. The mustang allows the operator to hold the motor at a steady rpm simulating cruise, climbing a hill or WOT. The Dynojet is inertia only, meaning you floor it and the dyno measures how long it takes to accelerate the 1600 lbs drum. Dynojet does not let you do steady state load.

The dynojet will produce numbers about 10% higher than a mustang dyno.

 

Be aware that since the load is less (you're accelerating a 1600 lbs drum on a dynojet) you will end up too lean and with too much timing. Once you find the max power on the dyno you should jet up 1-2 sizes and make sure timing is no more than 36 degrees.

Zwede,

If carb tuning is the reason I want to dyno tune should I look for a Mustang type dyno? Or do I use the Dynojet and shoot for a slightly richer than normal A/F mixture while on the dyno? Should I expect the dyno operator to know this?
Warren

 

Since your main reason for going to the dyno is to tune the carb I think your best bet is a Mustang dyno. You won't get the bragging numbers the dynojet gives, but you will be able to tune part throttle as well as WOT.

A dyno operator should know all this, but many don't. You can lease a dyno with very little money down so many of these dyno shops are short lived entities...