Ignition timing for the low comp engine on boost ??

mgbv8 · Post by **mgbv8** » Sun Jun 09, 2013 9:01 pm

Hi folks!

Some of you know that I've been tinkering with the GM 4/71 on the Rover V8 for just over a year now. On the old engine I was using the rule of thumb of retarding the ign by 1 degree for each pound of boost. This worked just fine. 12 psi of boost = 12 degree's of retard. that was on the stock 9.35:1 CR.

But the new build is now down to 7.5:1 CR.

I've done 2 days testing at the Pod and found she runs ok at a stock timing of 32 degree's max advance. At idle she now prefers another 7 degree's to get a smooth tickover so 17 degree's at 900-1000 rpm.

With stock timing she ran 10.9 seconds. With 2 degree's more advance she ran 10.7 seconds.

I'm sort of running in the dark at the moment with trying to get the most from the boost. I have noticed that the charger does not generate the amount of boost that it used to on the old engine. At 5000 rpm the old higher comp engine would make 13-13.5 psi of boost. On the new low comp lump I can only make 11.5 psi at 5800 rpm. I understand why this is happening so thats not an issue.

Once I gwet the engine maxxed out on boost i will want to turn the nitrous back on and then re tune back to nitrous use. But in the meantime I'm wondering if there is any rule of thumb for ign timing on the lower CR engine??
I'm happy to experiment but I wont have time for any dyno sessions in the next 2 months to fine tune. So i need to do all I can trackside at the moment.

Perry

Post by **Rossco** » Mon Jun 10, 2013 10:57 am

Perry

Number of conflicting factors here in determining ignition and/or gas settings

Are you running an intercooler? You're going to be generating a lot of turbulent air and as a consequence a lot of heat

With the low compression what is she like off-boost and when does the boost come on?

If the old engine at 9.35:1 worked why have you reduced the compression on the new engine?

Normally having turned the compression ratio down you'd turn the boost up but your setup appears not to allow you to do that?

Are you going to run gas to plug the boost lag gap and turn it off when the boost come on?

kiwicar · Post by **kiwicar** » Mon Jun 10, 2013 12:08 pm

Hi
"Are you going to run gas to plug the boost lag gap and turn it off when the boost come on?"
No, from past experience he will keep adding NOs until one of the following happens. . .
a/ he can't get any more to go into the engine and it starts comming back out of the carb.
b/ he gets into the 9s repeatadly. . . .
c/ the tyres can't take any more and something else has to be tried
d/ something goes bang (please not again)

My money is on b/ followed shortly by either a/ or possably c/

Best regards
Mike

mgbv8 · Post by **mgbv8** » Mon Jun 10, 2013 3:40 pm

Ha Ha!!
Mike has finally learnt how I tune engines

Rossco!
Are you running an intercooler? You're going to be generating a lot of turbulent air and as a consequence a lot of heat
No intercooler. They are available but way out of my price range I think. Carb sits on top of blower.

With the low compression what is she like off-boost and when does the boost come on?
I can drive it around the pits with no issues at low throttle before it makes boost. Its nice and smooth. Boost comes in within a second of me flooring it off the line. Boost builds with rpm and is maxxed out in 1st gear, from then I keep my foot hard in and change gear at full throttle.

If the old engine at 9.35:1 worked why have you reduced the compression on the new engine?
I broke that engine and decided it would be nice to lower the CR so I can shove more charge into it. This was the perfect time to do it.

Normally having turned the compression ratio down you'd turn the boost up but your setup appears not to allow you to do that?
The blower is making a bit less boost pressure because I assume its flowing more through the engine. I can increase the boost with a pulley change but I dont want to over rev the charger.

Are you going to run gas to plug the boost lag gap and turn it off when the boost come on?
There is no boost lag, just a bit less boost pressure. The idea was always to add nitrous for assistance and charge cooling. I can control the gas as I want really. on the old engine I used to hit it with 75hp off the line and bring in the next 75 shot progressively over 1.5 seconds to be at full power for the end of 1st gear.

The rule of thumb I have been using on stock compression was 1 degree per pound of boost + another degree of retard per 50 shot of gas.
So when i set up the old engine I was running 16 degree's all in. I tweaked this on the dyno to find the sweet spot of 19 degree's max advance with 13psi and a 150 shot of gas.

If i set it to 10 degree's at idle its lumpy. 17 degree's at idle is sweet. I'm wondering if this 7 degree's of advance would work at the top end as well?
I can adjust the max advance limit by adjusting the plate in the dissy while still having what ever idle timing I want. Then i have 7 degree's of adjustment each way on the MSD remote.

I'm out again this sunday at North Weald to do some runs with more timing added back in. I can dial in 1 degree at a time from inside the cabin so I can pretty much hot lap without having to get the strobe out and re time, then it can cool down in the queue. I've set the timing to give me 17 degree's at idle with 30 degree's all in. And i still have 7 degree's of + or - before I have to tweak the dissy again.

Post by **Rossco** » Mon Jun 10, 2013 3:58 pm

Perry

You're running 2psi less therefore making around 13% less power. Additionally the lower compression ratio is also going to have a negative effect due to you not being able to produce higher boost than before.

Timing is not only dependent on boost and compression ratio but also on brake mean effective pressure in the cylinder. There are some calculators out there that will do the maths for you so I'd run the figures for the old 9.35 engine and do a comparison with the figures for the new engine.

http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

Figures around 200 - 300 are about right for a blower motor. Anything higher is going to blow itself apart.

I would expect the mean cylinder pressure to be lower on the new engine which should mean you can run more timing, however lets not forget that every engine has a max advance which is also dependent on head design etc. So it just might be possible that your previous optimums are as good as it gets even for this engine.

On the bright side I expect it'll take a bigger shot of giggle gas.

Just a suggestion but why not fit a wideband O2 sensor such as an Innovate. At least you'll have real time data as the cylinder heads launch into orbit.

kiwicar · Post by **kiwicar** » Mon Jun 10, 2013 4:03 pm

Hi Perry
can you bring in the timming more slowley, you said earlier that you had it all in by about 3k revs (at least I think that is what you said) as the boost builds with revs I would be inclined to aim to set it for the 17 at idle as it seems to like that and then try bringing in the full advance closer to peak torque, say about 28 degrees by 2500 revs and say 34 or even 36 by you get to 4500 or so I think with the lower cr but better cylinder filling that I think you are getting you need to give it more advance as it revs out to keep it pulling but not too much in the mid range, not because of pinking as I dont think it is at risk but just to avoid too much advance too early pushing the piston back.
Best regards
Mike

bigaldart · Post by **bigaldart** » Mon Jun 10, 2013 5:23 pm

If this is race only then why have an advance curve? Just lock it out, bringing it in slower is pointless and only hurts throttle response. Then add retard when you bring on the gas. Overspeeding the blower with a GMC on petrol is impossible, you will drive over the crank first. Airflow suffers when rotor speed gets to around 13,000 rpm, of course the charge gets hotter but airflow doesn't suffer. Run it on alky and you can use the potential, well as much as the crank will take anyway. Old time blown gas racers would run up to 30 psi on race gas with 7.5 or 8/1 compression, all forged internals though, I would think 20 is practical for you, keep timing lower and run it fat, especially on launch, lots of idle fuel a good black blast when you hit the throttle and rich all the way down, just not smoking rich. When we ran a mix of gas and alky we ran around 22-28 degrees on a 3.5, well rich and it was quick and responsive. BTW Industrial alky we just bought cost just over 50p a litre

Alan

mgbv8 · Post by **mgbv8** » Mon Jun 10, 2013 5:58 pm

bigaldart wrote:If this is race only then why have an advance curve? Just lock it out, bringing it in slower is pointless and only hurts throttle response. Then add retard when you bring on the gas. Overspeeding the blower with a GMC on petrol is impossible, you will drive over the crank first. Airflow suffers when rotor speed gets to around 13,000 rpm, of course the charge gets hotter but airflow doesn't suffer. Run it on alky and you can use the potential, well as much as the crank will take anyway. Old time blown gas racers would run up to 30 psi on race gas with 7.5 or 8/1 compression, all forged internals though, I would think 20 is practical for you, keep timing lower and run it fat, especially on launch, lots of idle fuel a good black blast when you hit the throttle and rich all the way down, just not smoking rich. When we ran a mix of gas and alky we ran around 22-28 degrees on a 3.5, well rich and it was quick and responsive. BTW Industrial alky we just bought cost just over 50p a litre

Alan

Thanks Alan!
I thought that as I go to high rpm off the line and dont back off to shift that the timing would be ok coming in all at once. Thats what I've done from day one.

I dont want to over run the blower because I dont want to hurt it. Nige wasnt keen on making me another pulley when i mentioned this to him a while back. And it wasnt cheap so I want it to last

On petrol only I've got AFR at mid 11's on full throttle from the top of 1st gear on a run. Thats where it seems to like it. It goes rich when I stage at around 1800 rpm as I'm on part throttle then with the drive train under load on the foot brake. I do get a black puff as I leave the line and then its stays at the same AFR right to the finish line.

I've jetted the dominator with 85's in the primary plate and 88's in the secondary plate and this seems to be just right for this build.

By Alky do you mean Methanol?

What are your thoughts on using race gas instead of petrol?

As the original ign timing would have been about 32 degree's at 9.35:1 Should I keep it here on the lower CR and back it off as I add nitrous, or should I add a bit more in on petrol only until I max it out and then back off for nitrous use?

I'm enjoying learning about this new engine and its potential.

I think I should try and make some time for a dyno session eh ! Thats the only way I'm going to know what the torque and power is doing isnt it?

Keep the thoughts coming chaps. I've got just under 4 weeks to be ready for the first 9 second attempt on 7th July.

Perry

bigaldart · Post by **bigaldart** » Mon Jun 10, 2013 8:16 pm

Hi Perry, as we ran the car we found the more fuel we put in it the faster it went, and the less advance is needed, with a blower with Petrol in the mix it is very different from a N/A car. It sounds like you are in the right area, best power is not always where max advance is, with a Rover I have always found that more advance will help off the line but it sounds like the engine is fighting itself on the big end and it usually flattens out a little. I would certainly run race gas if you want to go fast, we ended up running super unleaded 50/50 with methanol and then 3% acetone by volume to keep it mixed, carb ended up like swiss cheese though. From memory jets were around 4mm in the end, 50cc accelerator pumps on an 850 DP, also drilled out idle ports in metering block. Race gas would have been as good and probably more consistent, but nowhere near as much fun for playing mind games with competitors. Plenty fuel at the hit is the key to running petrol with a blower though, you need to keep it out of detonation then as that is the highest load at the lowest rpm, once it starts to detonate you will never get enough fuel in it to stop it. Once it gets through the low rpm phase it can tolerate more boost without detonating. You would be surprised at the A/F ratio and timing used on a pro stock engine, 16-1 plus compression is getting to blower territory, they run more fuel and less timing than you think, a rich mixture burns faster as the droplets are closer together, hence less timing.
We are shooting for eights having run a 9.13 with a bad top end misfire, 120 in 5.8 at the eighth but only 141 at the top end.

Alan

mgbv8 · Post by **mgbv8** » Wed Jun 12, 2013 5:13 pm

Cool!

I'll drop some more fuel in and leave timing as is for the first run on Sunday.