Ignition Timing for modified engine

[Swallowtail]

Hey everybody,

I just started my rebuild 2.4t engine for the first time. The engine is all original except for a Piston/Cylinder Set with 9.5:1 compression – quiet a step up from the original 7.5:1.

The idle advance for the original engine is 5° ATDC @ 900rpm. I tried this but I did not like the engine sound so I stopped at this point for now. I’m aware that the more important setting is the 35° BTDC @ 6000rpm but this will have to wait until the engine is run in.

Now I’m wondering if
- My distributor is even safe to use with this compression ratio.
- There is a recommended initial setting you guys can give me.

Also the carbs have not been modified until this point. I was planning to run the engine in with the original settings and to the proper tuning after this point.

- Is it safe to do this or am I risking a lean condition or similar?

Thanks!

[Swallowtail]

One more info - the vacuum port on the distributor is not connected and the corresponding ports on the carbs are plugged. I do not know if this is stock on the '73 T(V).

[Mixed76]

"The idle advance for the original engine is 5° ATDC @ 900rpm. I tried this but I did not like the engine sound so I stopped at this point for now."

What does that mean- what sound was it making, and what did you stop? Compression ratio doesn't affect idle timing much - throttle is almost closed so the effective compression ratio is very very low - and should not give pinging at idle. Make sure your cam timing and base timing are correct? Then look at carb tuning?

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[Swallowtail]

Thanks for your input. It looked to me like the right hand side of the engine was running rougher than the left. I might even have heard some pinging or misfires but I'm not sure. I did not notice this before I set up the timing (before it was roughly around 0° or even a bit BTDC on idle).

Cam timing is spot on but you are absolutely correct about the carbs. Right now I just threw them on the way I took them off. I have a vacuum gauge on order and will to the basic setup the next time I start the engine.

Just looking for some basic infos I'm risking anything with the current setup (see questions above).

[Mixed76]

No risk of detonation for timing off by+/- 10deg or a lean mixture at idle.

If you idle it a long time with severely delayed timing you could damage the heads from overheating on the exhaust side.

Are you checking timing with a light or the marks on the distributor?

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[Swallowtail]

That is good to know - thanks a lot!

Right now I have checked with a cheap timing gun and set it as shown on the left of the picture.

I have another timing light coming which shows the revs and where the advance can be set.

[Tyson Schmidt]

Your vacuum pot is not a vacuum advance, but a vacuum retard.

With it connected and functioning, it will retard the timing about 10 degrees. So if you are checking it with the vacuum line disconnected, it should be roughly 5 degrees before TDC.

BTW, that's too high static compression ratio to run with T cams. I would have the cams reground to E, and set them at the retarded end of the spec.

[ed mayo]

You're not running the vacuum retard, just set the timing to 5 BTDC.

[Swallowtail]

Thank you both! That makes absolute sense and I will set it to 5° after TDC. This will also get me into the range where the engine sounded "healthy" to me.

Originally Posted by Tyson Schmidt

BTW, that's too high static compression ratio to run with T cams. I would have the cams reground to E, and set them at the retarded end of the spec.

Interesting. Regrinding would definitely be an option if it proves necessary. I would very much appreciate if you could elaborate why the T cams are not suitable. What are the problems you expect me to run into?

[911 SLANT]

Swallowtail take Tysons advise. He is a really top notch mechanic and engine builder one of the best here. I'm surprised he chimed in. He's like a Unicorn

[zztot]

So is Ed Mayo……..has built more engines than I am old😏

[Swallowtail]

Originally Posted by 911 SLANT

Swallowtail take Tysons advise. He is a really top notch mechanic and engine builder one of the best here. I'm surprised he chimed in. He's like a Unicorn

Oh, don't get me wrong. I have no doubt he is correct. I'd just like to get a better understanding as to why the T cams are not suitible.

Is it "just" a matter of performance or will there be other issues? In other words: Is it okay to run the engine as is and keep this in mind for a future upgrade or shoud this be done immediately.

[Tyson Schmidt]

The reason is that With T cams, the intake valve closes very early in the compression stroke compared to a more aggressive cam profile. So the dynamic compression ratio will be higher, which could lead to detonation in high load situations.

If you look at factory specs, you’ll notice that the T has 7.5/1, E has 8/1, and S has 8.5/1.
The differences being matched to the cam profiles of the different specs.

8.5/1 is about where you’d want it with T cams. 9/1 with E cams, but you can set the cam timing at the low end to reduce the dynamic compression a bit to work safely with your 9.5/1 pistons if you go with the E grind. You may also want to run the ignition timing at 2.2 specs, which would be around 30 degrees at 6k.

Once the motor is broken in, you can do a compression test to get an idea of where you are. If it’s like up in the 190 psi range, your going to have issues. In tge 150 to 175 range should be alright. Maybe you can get better than 91 octane where you are.

[Swallowtail]

Thanks a lot Tyson! That is very valuable advice to me.

Just to make sure I undestood you correctly let me summerize:
With the T cams valve lift at TDC shoud be between 2.4 and 2.8 (I have set it at 2.6).
With the E cams valve lift at TDC shoud be between 2.7 and 3.1 and your recommendation is to set it at the end of the spectrum (3.1).



As to fuel:
I believe getting higher rated fuel is much easier around here compared to the US. On selected fuel stations I can get up to 102 octane ROZ which shoud be equivalent to about 98 octane in the US rating system.

What I can get at every station is the European standard fuel "Super 95" which equates to US Premium "91" and our "Super+ 98" which equates to US 93.

[Swallowtail]

And one more thing - I have just noticed that the valve spring distance is slightly diffent between the cams. Is this something that can be ignored?

[Tyson Schmidt]

Actually to retard the cams you want to be at the lower end of the range, so 2.7mm at overlap. So the intake valve stays open later.

And to be safe, running it with the T cams, I would set your timing at 0 TDC at idle, which should be around 30 BTDC at 6000 to start out with.

Years ago, (Like 30) I built a 2.4T with 2.2T pistons, which was about 9/1 CR. It ran great, but if you gave it too much throttle at too low an rpm, it knocked like hell. That was with stock 2.4T ignition timing. Might have been fine with 2.2T timing.

Your CR is even higher, so definitely run the equivalent of US 93 octane. And ideally, get those cams reground to E.

[Tyson Schmidt]

Originally Posted by Swallowtail

And one more thing - I have just noticed that the valve spring distance is slightly diffent between the cams. Is this something that can be ignored?

It most likely won't be an issue. Your T spec installed dimension is greater than the E spec, so there wont be any coil bind issues. Likely done to stiffen the springs for the 500 rpm higher rev limit of the E. You could add 1mm of shims under the spring seats if you are worried about it.

(The S spec being larger than the others due to the higher valve lift.)

[Swallowtail]

Thank you again Tyson. I really appreciate the very helpful input!

This is my first build so I obviously have a lot to learn.

I will keep my eyes open for a good used set of E cams. If if find nothing in the near future I will have my T cams reground to the E profile.

Should I find a set of S cams do you think that would also be a good option (I would have to check clearance of course)? I could imagine that drivability would be worse than with the E cams.

[Tyson Schmidt]

S cams won’t match your ports.

[Swallowtail]

Makes sense - thanks!