What if your timing is at 7.5% instead of 15%?

[DHort]

Does it make the car go slower?
Does it make the car burn gas faster?
Does it make the car harder to start?
Does it affect the amount of advance you can do with the spark lever?

The world wants to know.

[Allan]

I must be out of this world! Who'd have thunk it?

Allan from down under.

[Moxie26]

DHort .... TIMING IS MEASURED IN DEGREES, NOT PERCENTAGE

[Mike Silbert]

Dave,

Camshaft design is a science I leave to ones with more experience than me but I do know some things.
Massive numbers of tests, time, and money has been spent on this but it is extremely difficult to find real head to head testing.
To scratch the surface of Model T cams see this: https://mtfctulsa.com/Cams/index.html
EVERY camshaft design is a compromise between many conflicting factors to produce the best profile for a certain setup at a certain RPM range.
I prefer use the experience and advice of ones with more experience than me.
I talk to owners of cars and ask them their setup and watch their performance to determine what I do to my setups. And my setups are sometimes different depending upon the intended use of the vehicle.

There are a lot of things that matter and it is not just timing.
Here are a few design factors and I am sure there are a lot more.
Desired RPM range, Desired engine smoothness, Desired fuel economy and emissions, Desired Power, Desired valvetrain life, Lift, Maximum size that will actually fit in the engine, Overlap, Ramp angles and profile, Intake open and close angle, Exhaust open and close angle, Size / type of the lifter, Valve spring strength, Spring bind height, Weight of the valve train, Size of the valves, Angle of valve seats, Size of the engine (Cubic Inches), Type of engine (flat, F, overhead) / flow path design, Exhaust system restriction, Carb size / flow design, Normally aspirated vs turbo vs charger, and more.
All of this is based on the theories that two objects cannot occupy the same space at the same time and the metal parts and gasses take time and energy to start and stop at a maximum rate and top speed.
The engine RPMs change but the maximum acceleration rate does not - This is also why we advance the ignition timing as the speed increases.
Some modern engines use variable valve timing and advance the cam as engine speed increases showing how important cam timing is.
Some factors are very important like it has to fit and some not so much until you cross a line, think valve float or bind damage.
Some factors are very difficult to change like port design and the head gasket shadowing the valves.
The camshaft is only one part of a balanced system that ALL needs to work together. The biggest bang for the buck is to fix the most restrictive portion of the system and improve that. Think, a massive cam will not fix a restrictive carburetor or one that is not efficient at the desired flow rate.
Also think it is all a balanced system and all has to work together.

Advancing the camshaft will generally improve High RPM performance but will reduce Low RPM performance, what speed do you want to run your engine.
Advancing the camshaft will burn more fuel if you decrease the engine efficiency, cause parasitic losses, or just run harder (faster). The fuel burn rate is determined by engine efficiency and how much air/fuel is pushed through. The throttle position and load is the biggest variable.
Advancing the camshaft may effect ease of starting if it makes it inefficient or has the valves open at the wrong times like intake open when it sparks at starting speed. Briggs and Stratton engines have an easy starting feature to adjust the valve timing to make them easier to start.

Ignition is directly effected by camshaft timing BUT when you time the engine ignition it negates and resets it.
The amount of ignition advance is fixed mechanically in a Model T Ford by lever lengths and wear conditions of all the joints.
The total ignition advance angle to piston will change if the base (starting) timing is changed or the lever lengths or the slop in the joints changes.
You have almost complete control over ignition timing with the left lever on the column so this is a less important factor.
Put the lever where it is most happy and have a nice drive.
Does bad ignition timing make the car slower - Yes
Does bad ignition timing make the car burn gas faster - Yes. Anything that makes the car less efficient will make it burn more gas.
Does bad ignition timing make it harder to start - Sometimes. It is tolerant until it crosses the TDC line or is way too late.
I set my spark timing @ 1/8" down (after TDC) lever up on T's and A's (same as the 3rd notch method) with the lever all the way up. I also verify that I get full timing advance without excess slop, binding, or wear.

It is degrees and not precent.
You also did not specify camshaft or ignition timing, your last question implies cam timing.

Part of the world might want to know but I would like to see real life head to head tests and numerical data with various setups.
More tests like the Tulsa Cam Project expanded.
The effects of cam timing on a SB Chevy are next to useless on a Model T.

[TXGOAT2]

Ideally, valve timing, lift, and duration would be variable according to engine speed and load. Camless engines may be in the future, with valves operated by some sort of solenoids or hydraulic actuators under computer control.

[TMiller6]

I’m guessing the original poster opted for the percent symbol because the superscript “o” doesn’t lend itself to a lot of keyboards.

[TXGOAT2]

I use "deg."

[DHort]

Moxie

It was 1AM when I typed it, so I guess % was on my mind. Must have been thinking of tariffs.

Mike, thank you for all that info.

[Jerry VanOoteghem]

Moxie

It was 1AM when I typed it, so I guess % was on my mind. Must have been thinking of tariffs.

Mike, thank you for all that info.

I think we all knew what you meant Dave