Engine and Transmission Tech

Hi,

I have a 3.5L Rover V8 in my 1981 TR8 (the original engine). It has a few mods, most notably for this topic a Crower 50231 cam which has advertised specs of:

intake: Duration 270 Lift .450
exhaust: Duration 276 Lift .477

The heads have had some work and larger valves installed, and the compression is around 10:1. I'm controlling it with a Megasquirt 3.

While I've had it installed for a few years, I'm still slowly tuning it. Right now, it runs really well, but my milage is really poor (11-12 mpg). While I do enjoy the gas pedal, I think it could do better, and I'm focusing my attention on the ignition timing right now.

From a couple of sources, I can get a few guidelines:

3.5 liter Rover, cam duration of 260-270 total timing 36-38 @ 4000 RPM
3.5 liter Rover, cam duration of 270-280 total timing 34-36 @ 3500 RPM

And another source, more generic, says for a wedge combustion chamber (and gives the Rover V8 as the example), 36-38 max advance (no RPM range)

So I think my cam lands in-between the to first lines. Anyway, stating this in terms of a distributor, I believe these timing numbers are the "mechanical" advance, and that any "vacuum" advance would be on top of that. Is that a correct assumption? Assuming that is, here is what I'm thinking of doing to update my table.

1. Leave everything below 1000 RPM alone (for now at least)
2. On the 3800 RPM column, set timing to say 36 (kind of split the two recommendations)
3. Interpolate the rows between 1000 and 3800 RPM, set everythign above 3800 to 36 degrees

So that should get me something like the mechanical advance of a distributor. Then I need to add in vacuum advance. I've seem recomendations of 10-16 degrees max, but over which ranges should I apply that? What I'm thinking is:

4. At some kPA (say 35), add 10 degrees to each cell above 1000 RPM
5. Add 10 degrees to every cell below
6. Interpolate between 35 kPA and something below WOT (say 85 kPA?)

That should then emulate vacuum advance. Any suggestions on what MAP values I should use?

7. Smooth out any rough edges

Does that sound like a reasonable approach? That would result in a timing map with around 5 more degrees along the top (WOT), and up to 10 degrees in the cruise area, which I'm hoping would help my fuel economy. I eventually want to have the car tuned on a dyno. but in the mean time, having the ignition map closer ought to save me some dyno time (money).

Any thoughts or comment welcome!

Thanks,
Darrell

Have you compared yours to Curtis Jacobson? He ran a 215 so displacement would be the similar with his 0.030" over bore. Cam specs are Kenne-Bell camshaft (model 1XA, 0.462" lift, 260 degree duration, 110 degree lobe center). He listed compression at 10.1:1.

His tables are shown at:

[www.britishv8.org]

Curtis's looks more conservative at WOT and High Speed.



Edited 1 time(s). Last edit at 11/08/2022 05:15PM by mstemp.

Thanks, I may have used that as part of creating my initial map, I don't really remember. I'd like to understand the guidelines I found, maybe they do include "vacuum advance"?

Here is a picture of my installation, too.




Edited 1 time(s). Last edit at 11/08/2022 05:52PM by TR8Driver.

I am going thru a similar program although my Rover engine is quite larger in capacity. I have had it on chassis dyno to tune the higher revs. The load sites from 5500 rpm up are yet to be sorted because the dyno operator was reluctant to go that high. My engine spec is not perhaps the 100 % correct for my driving style (e.g. solid cam) but it works OK.

Now, you mentioned that your mileage is poor. How do you drive the car, at higher revs ? Mine is a road car and will do most of the driving in typical traffic flow so mostly in 5th or 6th gear at pretty low revs. So, what I have now done is logging the data for maybe 15 minutes, then checking data i.e. the revs and load (kPa) and adding more advance to these sites where the engine typically runs. Then take another run and check if it feels better or not. It is also possible that you can slightly decrease the fuelling at the same load sites as well.

So, check your rpm/KPa while driving at steady speed and add advance around there. Maybe not 10 degrees, I would take smaller steps. Each engine is different but this should work just fine. Same for max advance, something like 33-35 degrees should be enough, again depending what sort of fuel you have available.

But, if the driving style is heavy right foot type, then perhaps the only way to help mileage is lighter touch of pedal :-).

Hi Jukka,

Thanks for the reply! I know that some of my low economy is due to having fun, but even when I'm crusing on the freeway (60-70 MPH, 5th gear) for the majority of the time, I still only get 11-12 MPG. Fueling there is a bit lean of stochiometric. So yeah, I could focus on just those areas of the table, but I'm thinking my table could use more timing across the board as well.

Thanks,
Darrell

I feel like you shouldn't need 38deg WOT, and I'm sure if you get it on a chassis dyno that will show as too much. Remember the key is to run as little advance as is needed to make the power, not to throw all the advance in there and dial it back.

First question - are you 100% sure that your timing is spot on by using an indictor / piston stop to verify the TDC point?

If you are keen on economy at midrange, then establish out which cells are in play at the light throttle cruise points you're concerned about (go out and drive to figure it out) and add a few degrees in those areas and pull fuel out to 14.7 ish (assuming you have the ability to do that).
Don't worry about anything other than WOT at the higher throttle / RPM ranges, you never hang out in these areas for long enough (other than WOT) to care


Getting the advance just right in these light-throttle areas is a little hard to do on they dyno, but the key is to drive it and figure out what TPS and MAP readings you see at which speed sites, and then home in on those areas on the dyno. Then you can hold the speed steady on the dyno, and add advance while both listening for knock, and watching the torque. It can be easier to pull the advance away initially (to 26 deg or something like that) then tap in advance, watching the torque increase one tap at a time; once it starts to plateau, you're getting closer, and once a tap adds no more torque, thats the point you want to be at.

The real key, is finding a chassis dyno operator who will 1) Follow your request and do this, or 2) allow you to work with them, maybe them driving the car and dyno, and you driving the keyboard.



Edited 2 time(s). Last edit at 11/09/2022 10:32AM by turbodave.

Thanks, Dave. Yes, I've checked my TDC using a piston stop and it is correct.

I'm already running a bit leaner than 14.7 in the cruise area, which is why I've turned my attention to timing. So I'll play around in those areas only for now.

"I'm already running a bit leaner than 14.7 in the cruise area"

Wow. 11-12 US mpg? That's terrible.

36 degrees all in is the usual. 32-33 degrees may be optimal, not counting vacuum advance if using manifold vacuum.

Darrell,

To me your timing table doesn't look out of the ordinary. Just a thought, have you tried adding slightly more fuel? It is possible to actually decrease fuel mileage by going leaner. This is because if the engine is to lean, it can loose efficiency which in turn requires more throttle input to achieve the same power output.

You can test this by monitoring your MAP (engine vacuum) readings at a given RPM/engine load, while adding fuel through your VE table. If engine vacuum increases when adding fuel, then you know that your heading in the right direction. Often times it's easy to get hung up on trying to run at 14.7, when in reality many older engines prefer a touch more fuel.

Bill

Thanks again everyone for the input. Looking back on my MPG tracking, I was getting about 16 MPG with the same engine when I was using a FiTech TBI and distributor, but with the Megasquirt, sequential injection, individual coil, I'm betting about 12 MPG. The engine is smoother and more responsive in the current setup, so I'm surprised at the difference in economy. Target AFRs are similar (the FiTech only has a 3x3 target table).

Anyway, I've gone ahead and added 2 degrees of timing in the cruise area, then blended that in. In one test drive, I can't notice any change, so that seems good. It will take a few tanks of fuel to know if the fuel economy improves. This time of year, I may not get that until spring. So we'll see in a few months.

Set yourself up with a dyno session! You'll be amazed what you can do in a short hour with a good operator.

Hi Dave,

I'm hoping to get back. I had a short session this fall, but the tuner decided to stop when he saw some smoke. He thought it was an oil leak, though my theory is now that the rubber cap on the drain of my oil separator fell onto the exhaust. I can't find any burnt remains on the exhaust, but the cap is missing, and its location would have it fall right onto the header. Anyway, my big winter project is to make sure it wasn't an oil leak.

Before he stopped (and really did any tuning), he said I was getting 230-ish torque and 220-ish HP so I'm pretty happy with that. Unfortunately there isn't any graph since he stopped before the pull completed.

-Darrell