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Not sure for a TVR engine as they tend to differ from RR units? Google?
Rover SD1 VDP EFI
Tech Edge 2Y
As you are using an Edelbrock 500 carb you may or may not be using the vac advance system, also the 500 has two vac ports, one is timed and the other is not. (One see's vacuum at idle, the other does not).
Personally I think that you should use the non-port BUT the issue can be that your vac canister might not work too well with the vac signal that the 500 carb is putting out. I don't know if your setup uses a Lucas dizzy but I do know that you can get at least 4 different vac canisters for the Lucas dizzy, they all look the same but you can translate the numbers stamped on them to various features of the canister such as the vac required to start the diaphragm moving and the vac required to get full movement. One of the figures also gives the number of dizzy degrees of advance that the canister gives.
I did find a useful page http://chevellestuff.net/tech/articles/ ... ifold.htm explaining that timed vacuum was developed for non emission controlled engines a a way of burning the excess hydrocarbons for the emissions test and that manifold vacuum is the way to go.
All starting to make sense now so looking forward to having a play. Will double check the notch on my pulley is actually at TDC. I have just printed out some timing strips so will use them to set it to 32 degrees full mechanical advance and then experiment with the vacuum advance after that.
BTW the timed port was developed to reduce emissions, the idea is that the timing is retarded at tickover and this helps clean up certain emissions, it's not all that great for anything else but at least it does allow the use of a vac system when the throttle is opened more than it is at idle.
Some of the info on the web about using vac advance with the Edelbrock carb may have been posted up by me! I have had a change of mind over this, I think that if you can get the right canister then it a good idea to use the vac advance system on the non-timed port. If your engine pulls let say 14" on tickover you need a vac canister that is all in with 2" less vacuum (12"). If the canister needs 13-14" to be all in then you will find that the timing bounces in and out on tickover which is no good. If the canister is all in at say 10" then you might find that the setup does not back off the timing quick enough when you start to place a reasonable amount of load on the engine.
Really looking forward to getting stuck in now and hopefully getting some good gains. We had the Trike on the rolling road at the end of last year and the operator said my engine sounded flat dispite been pretty much brand new with only around 6k miles on it.
He had a play with the dissy but left the vacuum advance on so it's anyone's guess what it's set to now. It made 161bhp at the rear wheels running through a ZF4HP22 auto box. The lamba readings were good so confident the fuelings right.
I have setup several engines using the following method, it works well!...
THE IDLE TIMING FIGURE...
1. You need to find out what timing figure your engine wants at idle, the cam fitted to your engine has a large bearing on this, my engine needs a timing figure in the high 20's! The easy way to obtain this figure is to disconnect the vac system so that it does not mess up your test, set the timing to some arbitrary figure such as 10 degrees. (Basically a low-ish figure). Slowly twist the dizzy whilst the engine is running and listen to how the engine responds, if the idle speed starts to increase (which it will) then it likes what you are doing. Keep advancing the timing until there is no further increase in RPM. You may have to sneak up on the timing a few times in order to hone in on the required figure, listening to the engine carefully and using a vac gauge is important here . Make a note of the figure, lets say it ends up being 18 degrees. Also note the vac figure as this is important, you may have to reduce the tickover speed but then note the vac level, lets say it is 16"
WORKING OUT WHAT THE VAC SYSTEM CAN GIVE
2. Reset the timing back to 10 degrees then plug the vac canister into the non-timed port, the timing should increase if the vac canister is working and suitable, lets say that it goes to 26 degrees, this means that your vac canister can supply 16 degree of advance. The vac canisters actually have a code on them it will be something like 5 17 8, in this example it means that the canister will start to move when it receives 5" of vacuum, it is fully deployed at 17" of vacuum and it can supply 8 degrees (at the dizzy) of timing, 16 at the crank. In this case this vac canister would not be suitable for your engine. You would want one that is fully deployed at around 14". I actually have a load of canisters kicking about, from memory there are 4 or 5 different ones, the figure of 5 17 8 was taken from the first canister that I laid my hand on this morning. Hopefully your canister is suitable, lets say that it adds 16 degrees, we know from the first test that you engine wants 18 degrees at idle which means that you would either set the timing to 2 degrees with the canister not connected and then connect it up or you could connect the canister to the non-timed port and set the timing to 18 degree. (The static timing would still be 2 degrees). Rover actually specify quite low static timing figures and this is because they want you to set it up with the vac system disconnected, you then connect it to a non-timed port on the SU carb.
WORKING OUT WHAT THE MECHANICAL ADVANCE CAN GIVE
3. You also need to know what your mechanical advance system can give, the advance system inside the dizzy has a number stamped on it which is dizzy degrees but I would not trust it!. The best way to find out what the system can give is to set the timing to say 10 degrees with the vac system disconnected, then rev the engine to 3.5-4k RPM and make a note of the timing figure, lets say that it goes to 40 degrees, that must mean that the mechanical system can give you 30 degrees of advance. Whilst you are messing about with this try to find out at what RPM the system has added all of its timing, around 3-3.5K RPM being a good figure in general. The 32 degrees for advance that you have found on the web seems like a reasonable figure for your engine.
SETTING UP THE TIMING
4. You can now set the timing up, connect the vac system to the non-timed port and set it to 18 degrees (2 degrees static). We know that your engine wants 32 all in and that your bob-weights can add 30 degrees, this means that at wide open throttle settings your all in advance would be 2+ 30 which is 32 degrees! Your timing figure at high-ish RPM when the throttle is just cracked open will be 32 + 16 which is 48 degrees, this is no problem at all and will give you good MPG, if you whack the throttle open the timing would back off down to 32 degrees assuming that you have a suitable vac canister!
You can see that in my example the figures 'just happen' to work out nicely, in the real world this might not always be the case, if you are out by 1 or 2 degrees at idle then I would not worry too much but if its all miles out then you are getting into modifying the dizzy by filing bits off it or building bits up with weld. Lucas made lots of different dizzies for the Rover, they all look the same from the outside but they have different mechanical advance figures and different bob-weight springs.
BTW your figure of 160 BHP at the wheels may indicate that that the fueling is OK at WOT (it may not, I have no idea what your engine should put out) but the fuelling at WOT is about 5% of setting up a carb, the WOT fueling is easy to sort out because you just have to select the correct secondary jets. (To get an air fuel ratio AFR of around 12.8:1). In the real work when setting up an Edelbrock carb you need to setup up the idle circuit, the cruise circuit, and the acceleration circuit, these are the circuits that you will be running on 99% of the time, they can be a right pain to setup! Most rolling road operators have no idea about setting up these circuits on an Edelbrock carb, also unless the rolling road can hold the engine at a steady load (most can not) then you can not set up the cruise circuit on the rollers anyway.
Ricardo and others have done may tests on engines, they found that in order to get maximum torque out of an engine under any operating condition maximum cylinder pressure need to occur at around 14-20 degrees ATDC. In order to achieve this the mixture needs to be lit BTDC but the exact ignition point depends on engine speed AND on the cylinder pressure just before the mixture is lit, this second variable has a huge effect on the flame speed, a low value will have a low flame speed. Things that affect the initial cylinder pressure are the compression ratio AND how well the engine is breathing. (Its volumetric efficiency, VE). If the throttle is just cracked open on cruise then the engine will have a very low VE, it is hardly able to take in any fresh charge, it will also be contaminated with exhaust gas and this means that during the compression stoke most of the time the pressure is below atmospheric, its is actually a vacuum, it may only actually go above atmospheric as the piston approaches TDC, even then it will be a fairly low PSI number. This low pressure value means that you end up with a low flame speed which in turn means that you need to fire the mixture much earlier in the compression stroke in order to get maximum pressure on the piston at 14-20 degrees ATDC. Simples!
EDIT...I've just read the gumph in the link properly rather than skim reading it, I'm pleased to see that a lot of what this bloke states matches up with what I think! (With regards to vac advance systems).
BTW...If anyone is still awake you now know why when you increase the CR of an engine you generally would need to retard the timing from standard by a degree or two. (Assuming that the standard timing was the optimum for the standard engine!)
Now when i started it the strobe was showing the the thick white line to be to the left of the pointer(looking at it front on) so well advanced.
I can turn the dissy round so the thick white line is in line with the pointer but much further than that and it starts to sound erratic.
When i rev it the timing starts to advance but then it all started jumping around, this was all with the vacuum advance off and plugged btw. Plugging the vacuum advance back in does'nt seem to make a great deal of difference either.
I have managed to find the dissy that came with the engine but it needs a remote module hence why i swapped it in the first place. It's a 35DM8 and as it came with the engine and is a later unit i'm thinking it might be the way to go.
Will try and find a module for it and see if it performs any better.
Going to refit the original TVR dissy now and expecting a big difference
James 33 wrote: ↑Fri Jan 25, 2019 2:48 pmFound the problem! Took the dissy apart and it seems I'm not the 1st person to delve in there. Vac advance disconnected and so were the bob weights! I could move the rotor arm left and right quite a bit without it springing back. So basically it was just doing what ever it felt like
Going to refit the original TVR dissy now and expecting a big difference
LOL, that's the problem with the sort of stuff that we work with, you never know if a dimwit has been fiddling about with things before you get your hands on it. Before you fit the other dizzy make a note of the numbers on the vac canister, you can de-code them using the info that I posted up in my big long post above! It would be worth checking the numbers on the first dizzy vac cansiter too just in case that one ends up being more suitable.