Overdrive timing
 

It was determined that my last engine was not burning all of the gas and carboned up the heads and and pistons.

So I would like to ask those have overdrives where is your timing at.

At 70 mph I am turning 2300 rpms. Where should my timing be at that point.

Part throttle most of the time.

Thanks

Greg

Other folks responses should only be concidered a ruff starting point unless someone responds with the exact same combo as yours running and running on the same brand fuel as yours nit just the same octane level, and another big point will be the coolant temp.

Do you use a vacuum Advance? This will increase the Timing especially at part throttle wich makes the fuel burn earlier and more complete...

Assuming you're in the neighborhood of 12 degrees initial advance with 36 all in at around 3000 rpm, what should realistically be seen is about 28* of mechanical advance at that rpm and you should have another 10-12 degrees of vacuum advance in cruise for around 38-40 degrees total advance.

steve25 is absolutely correct though, those figures are only a starting point and assume initial timing of 12 degrees, 24 degrees of mechanical advance and a completely linear mechanical advance curve. That's likely not to be completely accurate itself and your specific engine may want more or less timing depending on the actual load it's seeing at the time.

If you have the right carb setting and the right gear ratio there is no modification needed to the timing. 2300 RPM @ 70 MPH tells me the gear ratio is wrong causing the engine to load up. Think of it like driving a stick in 4th gear and doing 25 MPH. A lot of bad things go on when RPM's are too low for the MPH. Good RPM for 70 MPH should be 2650 - 2800 RPM with an O.D.

My 3 cars are dialed in to run 65-70 at 2000-2200.2 have ODs and one has 2.68s in the back.All are stick shift cars.All iron headed 9.5 CR and all 428-434 CI,all are set up to have 36-38 total all in at 2000.FWIW,Tom

I think it will take a bit of experimenting. Timing and fuel go hand in hand.
I would start by leaning up the cruise mixture.

Low engine rpm and max Torque production for a given rpm under under a motors full throttle peak torque rpm requires as much timing as you can get away with safely, and if you think you can gauge this by the seat of your pants then I have a Bridge I would like to sell you!

If need be I would save up the money needed to get the car on chassis dyno for a half a day and get it done right!

If your car was turning 2300 rpm at 70 mph you have about a 3:08 final rear gear. That is what my 74 Formula is turning with a 4spd trans. My 74 Trans am is turning the same with auto trans.

Ignition timing is important. Also (relatively) easy to adjust. Good starting point. Others have given reasonable advice.

Don't forget VALVE timing, plug heat range, port (and intake manifold/exhaust manifold/header) size/design, fuel curve (carb "jetting", which would include "jets", but also metering rods, idle and high-speed air bleeds, idle restrictions, mixture screws, power valve circuit, secondary opening rate--the whole gamut of carb tuning.)

If this was me--and it isn't, yet, but will be perhaps next summer--I would rather deal with a Q-Jet than a big Holley when cruising at relatively low rpm. That said...I have every expectation that a Holley can be dialed-in to work just fine. If you have a Holley, remember that the main jets are the CRUISE fuel metering, power metering is from the power valve circuit. You may need to enrich the power valve and/or the power valve circuit restrictions, and then reduce the jet size to get a nice, lean cruise and still have the WFO fuel curve correct.

The complicating factor will be if the RPM is low enough that you're still partially on the idle/transfer circuits; and that's going to vary with carb CFM/primary throttle bore size (which is why I'd want a Q-Jet.)

Thanks for all of the knowledge. I get it there are so many things involved with timing.

I would like to know when I read peoples post some say they are all in at 2600 rpm some say 3000 rpm and so on.

For street and highway driving when do you want your timing all in?

Thanks Greg

Get a vacuum gauge. adjust timing for max vac at cruise. record timing with timing lite@ cruise rpm and cruise vacuum by using a myty vac type hand held pump. check to see you don't exceed a max of 38 deg at full mechanical adv.

The fuel/air mixture needs to be ignited at a certain point in the combustion cycle to have cylinder peak pressure at 14-20° ATDC for max effiency.

2300 rpms is 2300 rpms 50, 70 mph or 100 mph, the mixture needs to be ignited at the same time, if not you loose power.

The centrifugal weights and springs adjust timing per rpms and the vacuum advance adjusts timing depending of load on engine. The higher the load the more the timing retards.

Original Pontiac timing curves is set for a wide range of driving conditions and is prefered as long as you have the carb properly set up to deliver a combustible mixture in ALL areas.
I have found most street Pontiac V8 works well with 10-12° initial + 20-22° for a total of 30-34° at 4000-4400 rpms, add to that 16-20° from the vacuum advance (load compensator).

Lugging an engine at 2300 with full centrifugal advance + whatever vacuum advance is a fine recipe for knocked out copper colored main and rod bearings.

FWIW

Hmmm. How did our engines ever last when speeds were 55MPH, tire size was 26.5, and we had rear gear ratio's that were 2.56, 2.73, and 2.93?

Maybe the 2.56 posi in my brother's '68 Bonneville w/400CI should have destroyed his engine because he was lugging it doing 55MPH on the interstate. What did those factory engineers know anyway?

I guess I better tear down the 360 in my Fury with its 2.73 gearing just to make sure I haven't beat the bearings out of it by lugging it around town. On the highway I cruise at 75MPH, so I guess I am good there, right?

Never heard of such BS, guys. 1,200 RPM in 4th gear would be lugging it. 2,300 RPM is not lugging an engine unless you have such a radical cam the engine didn't come alive until 2,500-3,000 RPM's.

And as pointed out, the factory advance curve generally did not reach total advance to near 4,000 RPM's. Did all those engines suffer from carbon build up on the pistons & heads? Of course they did because that was a normal expectation of the old leaded gas. I don't recall anyone adjusting timing to prevent this as most mechanics/garages simply told you to blow the carbon out of it by hard accelerations a couple times - and you could literally see the black smoke pour out the exhaust and when it cleared up, you were done and the engine ran great again.

There must be a really young crowd on this forum. :doh:

I think other issues are at play here.

That defeats the purpose of OD.

My 71 Ventura spun that rpm with 295 DR 3.73 and T400.

vacuum advance OD
 

I think you have to take the car weight into account also. The change in vacuum readings is an indication of load on the engine. I'm building a '33 Chevy with a 400 Pontiac in it with a 3 speed OD trans, 3.55 rear gear and 28.5 rear tires. In OD the ratio should be about 2.5. It has a mild cam and 9.0 static compression. It should weigh in at about 2800 pounds when finished. I should be cruising at about 2000-2200 rpm. I have not run the engine yet. My distributor is set for 24 centrifugal and 10 initial all in at about 3000. I have the vacuum advance limited to 10 degrees. I expect to have to experiment with the initial, centrifugal and vacuum advance settings after I get it running. I am hoping for lots of low end torque with no expectations of high rpm running. Billk

I agree with you 100%

25 MPH in 4th gear (which I can do - but don't recommend for any distance) is about 1000 RPM. 70 MPH in my '64 GTO is just over 2700 RPM in 4th gear with my original Muncie M20.

I'm not sure how Chief can compare running 2300 RPM at 70 MPH with an OD transmission like running at 25 MPH with a 4spd.

And he wants to run more RPM's than I do at 70 MPH with a 4spd with an OD???

footjoy is running a GTO also - so his overall weight is close to mine. I don't think our engine's are that different either.

If I were to go to an OD tranny, I would want my RPM's somewhere in the low 2000's at 70.

Most cars on the road today are running around 2000 RPM at 70 MPH with an OD drive tranny....

You know this made me stop and think. I had a 40 ford coupe with 350 and th350 with 3:10 gearing. It loved 70-75 miles per hour with 19 mpg. Best dependable hobby car I ever owned.

Some times I try to think to much. Of course you guys know that.

But keep going I liketo learn about these things.

Thanks

Greg

3.73 /.67 gives a 2.49 ratio in OD

my 69 bird has 3.31s with the .67 OD!Tom