Less power after the rebuild - Part two
 

A few weeks ago, I posted that I had less power after my engine rebuild.

I received a lot of replies, voicing concerns with things like the rings seating. One or two posters suggested that I recheck the timing and the air/fuel mixture.

Took the car back to my wrench and there was a problem with the timing. He re-timed it and it runs just like before. (It wasn't running bad before the rebuild, but with a couple of broken head studs, I decided to bite the bullet.) He also found an air intake problem.

Anyhow, I am happy again!

Question: He said the timing was off by 5 degrees. What do you think about that? I'm amazed that this could have made the difference that I felt in the performance.

- Bill

I should add that my air mixture leak was around my new popoff valve. It had to be re-epoxyed to seal it. I wonder how big a difference that could make?

Hmmm, retarded timing and lean mixture due to vacuum leak vs. advanced timing and richer mixture could make a big change. A lot of us with SC's set the max timing advance to 35 degrees at 6K RPM.s, or approx 10-12 at idle. In conjunction, we remove the O2 sensor and richen the mixture. Who says CIS cannot provide *instant* throttle response :)

Well I'm glad you got that sorted out. Enjoy the ride.

Brad: Tried the 35deg (10-12 at idle) and the engine didn't like it at all. Went back to 5deg BTDC with great results. This is an '80 SC USA with SSI's, Flowthrough and no O2 sensor. In my mind, when you go to that much advance, the combustion takes place before the piston makes it over the TDC. ?????

Gunter, did you richen your mixture as well? Sounds like we have fairly similar setups. There can be issues with pre-detonation, if so, that's when to back off the advance a bit. I run 91 octane and have not had any issues. I believe others have run with 87 and have not had issues.

we remove the O2 sensor and richen the mixture.
 

Does this alone richen the mixture enough to run more timing? What other performance tuning can we do? My 82sc does not like very much more timing than 7 deg had the O2 sensor dis con when pluged back in it seems to surge more also behind the speedo was a connector that had been dis con I pluged it back in and now my O2 lamp works like it's suposed to. Thanks for the tips Speedy1

Brad: Yes, even with a rich mixture. 91 octane is good. I may try it again with a new timing light after the winter hibernation. I am not quite clear on the timing lights with "advance setting" feature???I just have a simple strob light now and go by the 30deg??? mark on the pully and guess the rest. I think the extra mark on the pully is 30deg??

There is no "30deg. mark" on the pully, just TDC, 5deg.BTDC, and then the 120deg. marks for valve adjustment. Timing lights w/ advance feature allow you to check *total advance* under load or at a certain RPM, ~34deg. is what you are looking for.

There are several myths regarding ignition timing, pinging/knocking, and performance, not to mention octane. And I almost forgot, lead in fuel. Advancing timing above the optimum spec DOES NOT increase power, in fact just the oposite, my '82 SC, (9.3:1 CR), does not ping w/ 87 octane @ 35deg. total advance @ sea level. Even climbing local mountains under full load, and I beat the shizzle out of my car. (That's what they're designed for). ;)

.....and I should say that I probably waste money by running 91 octane, but I guess it offers some peace of mind.

Denis, I was wondering whos pile of shizzle that was :)

-B

Indeed, timing is everything!

:)

-Wayne

I just recently readjusted my timing to 5 degrees BTDC at idle (it was slightly more avanced). I have never set the timing at 6,000 rpms to get 35 degrees BTDC. After reading this post, It sounds like I would be around 8-10 degrees BTDC if I set the timing at the high RPM's. I have a Euro with 8.6:1 CR and run 87 octane gas. Should I set the timing at idle or 6,000 rpms? Thanks!

Timing is indeed everything. I wish I had known this as a younger man.

Oh, and regarding cars, ignition timing is also VERY important in terms of performance and longevity. Some 3-liters can handle 35 degrees full advance, and some do not tolerate this. Not sure why, but some can and some cannot. Advancing timing until detonation occurs, and then backing off a couple of degrees, is a tried-and-true tuning method.

On my stock smog air pump dual pully the "extra mark" is 35deg.

IMO, different mfg. timing lights may be 2deg difference. 1960s info.

IMO, the exact same sister car may require a slightly different max timing.

"Advancing timing until detonation occurs, and then backing off a couple of degrees, is a tried-and-true tuning method."
--- IMO, Super is correct. It's still a good idea to monitor this setting under extraordinary conditions. ie: desert heat, altitude, spark plugs, etc if the "tried and true" was set at sea level on a rainy cold day, etc.

"above the optimum spec DOES NOT increase power"
--- IMO Speeder is correct. Also enviroment conditions may change, thus "optimum" may change.

IMO, CIS is very altitude change forgiving relative to carbs.

Trivia... EGT can radically change when pinging occurs. One EGT probe in any exhaust tube is big info. CHT won't react fast enough with ocasional pinging.

"Timing is indeed everything"
--- ditto

SET the timing at idle and CHECK its advance at 6000. You don't want to be fiddling in the engine compartment when its revving that high.

All our cars have the 5-degree BTDC mark. Measure the distance between TDC and the five degree mark and make another timing mark to the right of the 5-degree mark using this distance. This will be 10-degrees BTDC. Do this again to create a 15-degree BTDC mark. Take another measurement between TDC and 15-degrees BTDC and make a 30-degree and 35-degree BTDC mark.

Setting idle timing at the 10 BTDC mark is equivalent to 35 degrees at 6000 RPM.

Once you set idle timing at 10, get the RPM's up to 6K and verify its advancing to your 35-degree mark. If not, may have a sticky advance or a leak in the vacuum advance/retard hoses. Total advance (idle to 6K) for our distributors is 25 degrees.

kstylianos

That sounds like a logical approach, the idea of moving the distributor around while the engine is screaming at 6,000 rpms didn't appeal to me so I always did it at idle condtions. Did the 10/30, 15/35 degree relationship come from published data or from experience? Thanks!

John,

Please see edited post above........I initally assumed a 20-degree total dist. advance which is not the case. It's 25-degrees (published)

also see:

http://forums.pelicanparts.com/porsche-911-technical-forum/85355-since-911sc-does-not-have-chip-would-advancing-timing-help.html