Total Spark Advance

[krewzer]

Does the distributor used determine the total advance, or does the engine? I have a 1970, using either a '73 modified to limit total advance, or a '75 with built in total advance. I've swapped them back and forth. Not happy with either. No vacuum leaks. Haven't modified the power valve on the 38/38 yet. Other specs: 2L pistons, Chevy big valve head that has 1.84 and 1.50 valves, unshrouded and mildly ported with Pontiac 400 springs. Spring loads are as per tech mod. specs. found on this site. OR-66 hyd. cam, intake matched to carb base, sprint exh. man., 2" exhaust, pertronics, Getrag 5sp. Installing UR flywheel and s-10 clutch this week. Haven't noticed detonation even when total advance goes into the mid 40's. So, I'm lost as to what determines the total advance. If I put my '70 dist. back in, and set timing to allow 50 max., is that different from any other dist. allowing that much advance. Sorry for seeming like such a dumb s--t, as I have learned so much from this site. I know I'm in the final tuning stages, but can't seem to dial in the punch. She's super powerful in the upper rpms, but doesn't have much juice in the first 2 gears.
Thanks for any guidance,
Wes

[krewzer]

Dumb question?

[opelwasp]

Sounds like you may have the timing retarded. It was a dog in 1 and 2, spun the dissy advanced a few degrees made sure it didn't ping and holy crap it screams now. Advance till it pings then back it off a tad.

It may also be that you have a spring broken in the dissys, had this one too. When I would check the advance it would be way too high. The odds that you have to with broken springs? Hey, they are 30+ year old cars.

[wrench459]

yes and no
It just goes to prove you can have all the right parts
but if you cant tune the rascal its a lead sled.
pull all of the hoses off of the '70 dist. plug them up with what ever you have
lying around golf tees,what ever
put 15 degrees base timing
and have fun from there playing around from then on
every engine is different some likes more timing advance some don't
it all depends on the head efficiency

[krewzer]

No broken springs. '73 modified dist. and really cherry '75 are both in good shape. The '70 is crusty. I'll try to get it to ping and dial it back. I was just curious if the '70 engine was able to handle more advance than a later , low compression engine.
Thanks for the reponses

[nobody]

just a small tuning note, a 38dgas is really flat on the low end under 2.4l and almost impossible to fix for 2.0 or 1.9

[opelwasp]

Thanks Dave, you are right. I forgot what my GT was like when I had the 38 DGS on my 1.9. 1 and 2 sucked, it bogged quite badly, I could never get it to improve. My only solution was to build a bigger motor. I remember RalleyBob telling me that was one of the downsides of having a 38 on the 1.9

[kent]

Read thru Tru-crafts posts about tuning the 38 he dealt with a similar prob. I think the "stop watch don't lie" post has most of it. Kent

[nobody]

careful there, first is altitude and second is the motor in general. tuning carbs isn't a one size fits all kinda deal. Tuning in El paso is totally different from sea level. rejetting there proved it just to get out of town.

[Gordy]

Could it be valve timing being off a bit due to milling the head that is hindering the low end performance? I've got the same valves being put in my head with flat top 1.9 pistons and a mild lift cam .438 intake (at the valve) and .426 exhaust at the valve. How much did you need to cut your pistons for valve clearance? How much clearance is suggested between valve and piston?

[Washout]

I've built almost the same exact engine that Krewser has and installed an EDIS ignition controlled by Megasquirt. Right now I've got the total advance limited to 36 degrees and I still get some pinging using 93 octane gas. That seems like a rather low advance to me. Is there anything else that could cause detonation? I'm running a 32/36 Weber until I get the fuel injection finished.

[GTJIM]

34 to 36 degrees is about as much mechanical advance that a CIH Opel motor will tolerate on pump gas.

'Vacuum advance" has little or no influence on wide-open-throttle total advance as there is very little vacuum under these conditions - especially with the larger 38DGAS carb. The vacuum advance really only operates on closed throttle where there is enough vacuum to operate it - this extra advance helps burn the more rarified fuel/air mixes and improves economy.

Using an unmodified distributor on a modified motor defies logic - hi-perf motors usually require more initial advance and a quicker advance curve.

With the 'standard' initial advance of "0" degrees there has to be 34 to 36 crankshaft degrees mechanical advance in the distributor counter weights.
But with 10 degrees initial the distributor must be reduced accordingly to 24/26 degrees to limit the full advance (initial plus mechanical) to 34/36 degrees. More initial advance means more mechanical advance MUST be removed from the distributor. There is a thread about this modification on here:

http://www.opelgt.com/forums/1b-igni...stributor.html

Quicker advance is also discussed and is achieved using a pair of light springs on the counter-weights instead of the stock light and heavy pair. After-market Chevy V8 distributor spring kits can also be a good source of various weight springs.

The '75 distributor has about 25 degrees of mechanical advance built in stock so can be used with 10 degrees of initial advance. However it has vacuum retard for emission control so is not ideal in stock setup for a performance motor.

Your 38DGAS equipped motor needs as much initial advance and quicker advance to the 34/36 degree maximum mechanical advance you can build into one of your distributors.

I am still waiting for the local distributor 'wizzard' to plough through his 6-month back-log of repairs to get my '75 distributor run up on his machine to check its advance curve ..... may have to keep my eyes open for an old dissy machine to buy!

[madhatterpdc]

Always use a timing light!

I've had to set the timing by ear before and would swear the car was "running great", then when I got to the timing light out, I was off by as much as ten degrees, dialed that out and it ran even better.

[yellaopelgt]

How do you tell what year the dist is?? Is there some marking somwhere that says(75)???
joe

[nobody]

Only way to be absolutely sure is to pull it straight off an unmolested 75 motor. Check the motor number to be sure it was or is a fuel injected one first.
Hold on, Joe, go and compare them and tell us. The one in your car vs the crate one.

[Lindsay]

Originally Posted by yellaopelgt How do you tell what year the dist is?? Is there some marking somwhere that says(75)??? joe

Opel Distributor Specifications - Powered by OpelGT.com
1975 Bosch P/N 0 231 170 140 = No Data Shown?
Looking at chart, it is slightly confusing to myself. Think information is missing for later distributors.

http://www.opelgt.com/forums/1b-igni...stributor.html
OK - Think this thread fills in some missing distributor info for 1968 to 1975 distributors.

[nobody]

It took me a minute or two to remember. 75 has a plastic cover over the other vacuum pot space and nylon bushings in the lower springs.

[Washout]

Since my ignition is totally programable this is the advance curve I am currently running. I've dropped the total advance by 1 degree to see if that has any effect on the detonation. If not I'll keep dropping it until the pinging is gone. I would like some of you who have much more experience in timing that I to review my advance curve and see if it needs some tweeks. Thanks
This is based on 100 hPa (1000 ft above sea level).

RPM advance in degrees
700 12.8
900 15.8
1200 18.0
1500 20.6
2000 25.2
2600 31.5
3100 34.0
3700 34.0
4300 - 6000 35.0

[GTJIM]

Looks a bit quick at the lower revs and does ont reach full advance soon enough to me .....

The leap from 700 rpm (12.8) to 900 rpm (15.8) is a bit steep and also the subsequent changes to 2,000 rpm. Check where in the rev range detonation is occuring and reduce the steepness of the advance curve in that range.

Advance should all be in by around 3,000 - 3,500 rpm at the beginning of the peak of the torque curve.

[Washout]

I did some adjusting as recommended, what effect should altitude have on timing? The progam I'm using uses different timing based on hPa pressures as referenced by altitude. How much timing difference should there be between sea level and 1000 feet?

[GTJIM]

1,000 ft should not have much more effect upon advance than the natural variations in air pressure, due to cyclone/anti-cyclone activity, at sea level.

Generally, increasing altitude means that less air is 'pushed' into the cylinder for each induction stroke, the effect of which is a bit like reducing the compression ratio. So a little more advance may be tolerated at higher altitudes ... as long as fuel/air mixture ratios are also adjusted for the changes.

Read a bit about air/fuel ratios and advance at Pike's Peak - if you can find it somewhere on the www as that varies from 4,000 to 15,000 feet altitude!

[krewzer]

Been away on holiday for the last week and want to ask my original question again. I have a 1970, high compression engine. Total advance, according to the FSM, is 50. Even though the engine is built-up, as described in the opening post, will the engine withstand the higher total advance? Or, does the distributor used determine the amount of advance to be used? I have had total advance in the mid 40's, basically looking for ping, but never reached detonation. What are my total advance limitations on this early engine?
Thanks again for revisiting this thread.

[krewzer]

Thanks again for revisiting this thread.

[Washout]

You will have to experiment with the advance to see what your engine will tolerate. I built the same engine you did, same cam, punched out to 2 ltr, big valves, the whole bit. In my case using 93 octane pump gas I can only tolerate 34 degrees total advance, beyond that I get detonation. As stated earlier I'm using an EDIS ignition and programing it with my laptop so it should be pretty accurate. I believe some of the advance referenced in the FSM is based on retarding the advance during deceleration, that may be why the 50 degrees is referenced. That's just my guess though.

[GTJIM]

Originally Posted by krewzer I have a 1970, high compression engine. Total advance, according to the FSM, is 50. .... Or, does the distributor used determine the amount of advance to be used? .... What are my total advance limitations on this early engine? Thanks again for revisiting this thread.

The "Total" advance you are quoting from the FSM is made up of mechanical advance of around 30 to 36 degrees (crankshaft) and vacuum advance of 14 to 20 degrees - depends on the actual specs of the distributor you have as there are variations both between years and between actual distributors.

The only conditions that the motor will "see" 50 degrees of total advance are at above about 4,000 rpm on the over-run when the throttle plate is closed. In this condition the manifold vacuum is high enough to pull vacuum advance and the motor is reving high enough for the full mechanical advance to be all in. The motor tolerates this large advance because at high vacuum the fuel/air charge going into the cylinders is quite rarified (low pressure) even after compression and requires LOTS of advance to burn at all.

Under acceleration or even cruising the throttle plates are partially open and manifold vacuum is not as much and cylinder pressures are higher - ignition advance depends almost entirely upon mechanical advance - about 34 to 36 degrees total advance. The is NO vacuum advance added to the mechanical advance under these circumstances and cylinder pressures are much higher at the end of the compression stroke just before ignition.

Detonation usually only occurs at wide open throttle with too much mechanical advance - which is entirely influenced by initial (set) advance when the distributor is installed and statically 'timed'. Changing initial advance changes the amount of full advance by the same amount.

For instance - standard carb. motors are set at zero degrees initial advance and have about 35 degrees in the mechanical advance. The Fuel Injection distributor is set at 10 degrees initial advance and has only 25 degrees in the mechanical advance for a total of 35 degrees advance.This is ignoring any vacuum advance/retard as it is not relevant at wide open throttle as there is little or no vacuum.

The important measurement is total advance at wide open throttle at 3,500 to 4,000 rpm - this should be in the range of 34 to 36 degrees. If detonation occurs at these settings and under these conditions then you are not running high enough octane fuel or too much compression for the fuel being used. The temporary fix is to reduce initial advance ... and thus total mechanical advance - but his also affects engine performance.

The correct fix is to recurve the distributor to allow initial advance and reduce total advance by limiting the mechanical advance in the distributor - as the 1975 distributor has had done at the factory - or, by ones self, as outlined in other threads on this site.

NONE of the factory distributors (except the 1975 FI unit) are suitable for hi-performance motors without modification to the mechnical advance curve.

HTH