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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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 Posted: Mon Feb 02, 2009 11:05 pm Post subject: Cam timing monkey buisiness |
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Hey all,
A while ago I was planning a hefty block skim so I bought a vernier cam pulley to re-index the cam, however that has now been shelved as it was stupid (As you all told me at the time  ) However I did fit the cam pulley and decided to try moving the timing a bit yesterday.
The results are seat of the pants and no real distance but I'll keep playing about see what happens. Remember I'm on EDIS so the spark timing is identical to how it was, this is altering the stock cam timing only, not the maps/sparks.
The cam was moved +/- 5 degrees either way (Max on the vernier) just to see what happend.
Initial impressions:
-5
The engine idled fine, but there was the odd cough on hard acceleration and it seemed to have less low/mid tourque but was happier higher up the revs. I assume this is advanced tiing, inlets open later but scavenging better?
+5
The engine idled fine again but the engine is now MUCH smoother throughout the rev range. Seems to have better low range but definately poorer mid/high end power, I assume this is inlets open earlier and better cylinder fill?
I have left it a +5 for now as the engine was so much smoother, I'll probably trim it back to +2.5 see if I can keep the smoothness but gain a bit of power and retune the VE maps.
Any ideas about spark mapping to suit?
Rich
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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Martijnus
Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands
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| Posted: Mon Feb 02, 2009 11:15 pm Post subject: |
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any theories why cam timing should affect spark? The only thing I can think of is a better filling of the cilinders, more fuel to burn, slower burning rate, more advance.
that's the only thing I can think of, but what I try to say is that it could be so that the spark mapping should be the same for the altered cam timing.
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"Rule: Turbo's make torque, and torque makes fun." (C. Bell)
924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment  |
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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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| Posted: Tue Feb 03, 2009 12:40 am Post subject: |
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i was thinking the same, with the exhausts open early then maybe an early spark to burn it for as long as possible, with the exhaust open later then a retarded spark would allow for max compression before sparks.
Normally retardig the cam would retard the ignition as the dizzy is cam driven.
What are the specs on the Franco cam gear??
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Tue Feb 03, 2009 12:51 am Post subject: |
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| Rich H wrote: | | What are the specs on the Franco cam gear?? |
What do you mean by specs?
There were at least four varieties that I am aware of:- -0°/+10°
- -2°/+8°
- -3°/+7°
- a unit with two key ways that would allow re-indexing the cam gear to provide both the -2°/+8° and the -3°/+7° options in a single unit
The Franco gear I have is the "re-indexable" version. Here's a related thread: http://www.924board.org/viewtopic.php?t=25531
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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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| Posted: Tue Feb 03, 2009 1:07 am Post subject: |
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spot on that is what I meant, so the range is similar (10 degrees, but clearly mine is centered around 0.
More reading to do methinks...
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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ideola
Joined: 01 Oct 2004
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Location: Spring Lake MI
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| Posted: Tue Feb 03, 2009 1:22 am Post subject: |
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Just to be clear (for other readers), the Franco gear is variable as opposed to adjustable like the gear you're currently using. The stock spring and counterweight setup causes the timing to be at full advance at 3500 RPM, so it will run at the first number from idle, and will gradually advance based on RPM up to the second number where it will be full in at 3500 RPM and above.
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Rich H
Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK
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| Posted: Tue Feb 03, 2009 2:05 am Post subject: |
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I know, but worth clarifying, ta.
I'm hoping by fine tuning the cam I can gain a little low/mid range, retain the smoothness trading a bit of high range. I'm more interested in liberating a bit more acceleration rather than absolute performance. In the UK I'll never get up to significant speeds, I like driving too much to risk being banned!
A decent 4-2-1 exhaust would be good to I suppose... 
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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ideola
Joined: 01 Oct 2004
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Location: Spring Lake MI
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| Posted: Tue Feb 03, 2009 2:14 am Post subject: |
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So would a stroker 
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Rich H
Joined: 10 Jun 2007
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Location: Preston, Lancs, UK
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| Posted: Tue Feb 03, 2009 2:18 am Post subject: |
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To start though new rings on number 4 would help...
I'll add stroker, new rings and 4-2-1 to the list... 
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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ideola
Joined: 01 Oct 2004
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Location: Spring Lake MI
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| Posted: Tue Feb 03, 2009 2:31 am Post subject: |
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How about a Cometic MLS head gasket while you're at it? We only need a few more commits to get the numbers we need...
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Martijnus
Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands
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| Posted: Tue Feb 03, 2009 6:30 am Post subject: |
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| Rich H wrote: | i was thinking the same, with the exhausts open early then maybe an early spark to burn it for as long as possible, with the exhaust open later then a retarded spark would allow for max compression before sparks.
Normally retardig the cam would retard the ignition as the dizzy is cam driven.
What are the specs on the Franco cam gear?? |
The exhaust opening earlier isn't a reason for an earlier spark.
There is only one optimal moment for the spark (edit, I mean at a given rpm...there's only one optimal moment for maximal volume pressure).
You want maximal volume pressure right after TDC (not sure if it's 1deg after tdc or 10 degrees, I can imagine that 10 degrees would cause more leverage on the crank if you know what I mean).
Anyway, that point doesn't have any relation to the valve timing, that only influences the cylinder filling. So that point is always the same, I think it even is the same at every RPM.. in the same way as on a bicycle... you don't start to push earlier when you're pedalling faster.
But, because the mixture takes some time to burn and reach maximum volume pressure, the spark itself has to occur before TDC, so let's say the fuel needs 4 ms to burn, you should spark 4ms before the optimal crank position which I mentioned is reached. That means when the engine is running faster, that 4ms means more degrees, because the crank turns faster.
long story short... the only thing that would affect your ignition is the time the mixture takes to burn, and that is partly caused by the pressure... so if that pressure is different with the new timing, your ignition needs adjusting, but I think the pressure difference is so little, you shouldn't change anything. And another thing is how precise you're able to set the ignition...but with edis that's no problem 
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)
924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment |
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Rich H
Joined: 10 Jun 2007
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Location: Preston, Lancs, UK
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| Posted: Tue Feb 03, 2009 7:32 am Post subject: |
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No would have done
Thanks Martinjus.
Rich
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1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec |
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RC
Joined: 25 Mar 2007
Posts: 2637
Location: Australia
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| Posted: Tue Feb 03, 2009 8:19 am Post subject: |
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5* +/- is quite a lot of change for a cam Rich. With a single cam (fixed lobe centres) advancing the cam will move the powerband down lower, while retarding it will theoretically benefit the top end. As you`ve discovered. So as expected the + is advance and - is retard. Just try moving it 2*.
Its a compromise either way. You cant increase torque at both extremes by changing cam timing. You know a blower is needed for that!
No experience at all with the Franco gear at all but will happily wager a case of beer (or a pallet) that it in fact RETARDS the cam at higher RPM, 3.5K or whatever.
The only effectively accurate and safe way to set ignition timing is at MBT running under load on a dyno. |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Tue Feb 03, 2009 8:30 am Post subject: |
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| RC wrote: | | No experience at all with the Franco gear at all but will happily wager a case of beer (or a pallet) that it in fact RETARDS the cam at higher RPM, 3.5K or whatever. |
Mea culpa. Roger, of course, is correct. I re-read the owner's manual, and the other thing I wasn't quite right on is that it stays at the + setting UNTIL 3500 RPM, and by 4200, fully retards to the - setting. Sorry for the confusion...that's what I get for relying on my memory. 
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Scorpio
Joined: 05 Jul 2007
Posts: 1957
Location: Brisbane, Australia
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| Posted: Tue Feb 03, 2009 1:14 pm Post subject: |
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So advancing an adjustable cam a few degrees will help the lower end.."UP AND GO" ? what effect would it have on a blown ..EFI version?
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1979 NA
MS1..EFI..
GARRETT T25 TURBO
BILSTEIN SHOCKS
GT BASED CUSTOM BODYKIT
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Think mean think fast
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924 FAQ (Technical)
