I have descovered that the stock 89 bin's timing is not very close to my 400.It seems like my 400 would like more timing sooner and have hair less total timing.Does anybody have a larger displacement engine.If so what is your timing like rpm vs load and wot?I would like to know if a 400 likes more or less timing (total,part throttle,idle)than a typical sbc.

 

A lot will have to do with the heads you are running. My L98 does not like much timing at all (just as Bruce predicted). Going past 28-29* will result in audible ping. This is the "Total Effective Spark Advance after deducting Knock Retard". To do my tests and to determine the real Effective Total Spark Advnce, I limited my max Knock Retard permitted in WOT to only 1*, so the knock sensor was not skewing my test of Total Spark Advance.

This is something I do not recommend people try unless they are prepared to suffer the consequences.

But Formula5, I think you are going to find the choice of heads has a lot to do with the max spark advance your engine can take - not counting the "individual differences" from engine to engine, even when spec'd identical. That is one of the reasons we say that even between "identical engines" that one "optimal eprom" will not be "optimal" for the another "identical" engien.

 

Glenn,

Can you go into total "effective spark advance" a little more? Here is my situation:

I burnt two chips one night before I went to a test and tune session. Chips were identical, except for a reduction in timing in the Main Spark Advance vs. Timing vs. Load table (MAF car) above 3800 RPM (roughly 5% decrease). Ran both chips and picked up 1.4 mph in trap speed with the decrease. I'm assuming that this means I probably I could lower the spark curve.

I'm using the ARAP spark table as the basis for my table. This table shows some values up to 48.5 for advance. How is this related to total "effective SA"? I think I know the answer, but my mind is a little fuzzy tonight .

I'm going to search on TESA here in a minute to try and figure it out myself (I seem to remember you and Grumpy discussing this in another post).

Sparks a flyin'

 

I have 1971 400 heads,I think they're 72cc or 74cc.My pistons is flat top,I know that effects timing.It's just been years since I studied flame travel and timing.What effects in general do you think stock heads,f/t pistons,large displacement has on timing.

 

And people laughed when I said I got by far the best performance at 28d BTDC max timing. (54cc chamber AL heads).

YES, ALLLLLL Tooooo Often folks fall into the trap of more timing has to be better. When in fact the opposite is true. You want to run the least amount of timing CONSISTENT with best performance. Depending on the code if your running 2d too much timing and you trip the Knock sensor, you'll be taking 3-4d of timing out. There is some hystersis in the timing correction for knock.

 

One thing is I'm not running a Knock sensor.So is the knock retard tables not working,or sould I take the retard out?I'm not running a Knock sensor bc I can't find one to match my 400's harmonics.The sensors I tried were too sensitive(I tried tape and all).I do have a good deal of time tuning old carb,mech hei setups,so I know how far to go.Right now the way I've been tuning the main spark table and backing off the wot spark table.I'm pretty sure that at 208 lv ,the spark advance stays the same to max load value.Then bring the wot table up a bit at a time.That seems to work but stock spark (lv8 of 208)@ 1600-2400rpm is 22.1d,@2800-4000rpm is 20d.It just seems it would be better to scale the timing like 18d,18.5d,19d,19.5d.Like a mech distributor.any thoughts?

 

?

 

So exactly what harmonic range are you looking for?.
If you were to research things some you'd see the range is guiet versatile. If you're triggering the K/S the best thing is to thoughly research what noises your engine is generating, and then look at changing the dampening of the sensor's mounting. The sensor does a much better job at tip-in, and trace detonation then most people give it credit for, and you can tune into a range where the tip in is ridiculously bad, and still not trigger the knock sensor.
If you were to look into the Marine SBC vortec applications you'd see how GM handles the issue.

 

Formula,

Bruce is asking the right question. I was running into a knock sensor retard problem when I first installed my roller cam into an 84 Xfire motor. In fact, even today, the knock counts go up after a downshift. Although the motor was running lean, the real problem came from the 84 sensor picking up the noise generated by a roller chain. I switched to a post 87 konck sensor and ESC module designed to work with roller cam and chain. No problem since then. I have the TFS 23d heads with 64 cc chambers and using the stock pistons. Something around 10.3:1 compression. On a dyno I tried several curves ranging from 34d to 38d and coming in between 2800 and 3600rpm. The TFS heads apparently like 38d TSA. I got virtually no spark counts suggesting the ability to increase TSA but looks like I got a good increase in power and torque switching from 36 to 38d TSA. The full advance currently comes in at 3600rpm.

 

Full advance all in by 3600rpm?Thats what I want to do but the stock curve does'nt advance fully until 5000rpm.Wouls'nt that be a bad thing?My engine is externally balanced and has eniterly different harmonics.If you could suggest a paticular knock sensor,grumpy.I have no way of testing my harmonic range.

 

Hey Formula5,

I am running a VERY similar combo to you right now. I have a 406 in my 88 GTA with a stock TPI setup on it right now and an Edelbrock Hi-Flo base bolted to a set of stock 1970 400 heads.

I had Fastchips burn me a chip so I could get the combo running and it runs pretty well. However, I also am not using a knock sensor for some of the same reasons that you are. I had them disable it from the start because I have a pretty good idea what the total spark advance should be with a 400 (32-34) and I also know that they like more initial advance with the curve coming in completely before 3,000.

I also have a SR now and a set of alum L98 cylinder heads for my combo. My other concern is that when I bolt these heads on, my compression will be about 10.8 to 1. You can understand why I would rather use a bit less timing overall with this combo

So, do you really need a knock sensor for tuning? I would rather err on the side of safety without my knock sensor and trim the timing back a bit. I am not quite sure what to do about this and since I am going to start burning my own chips soon, this is an issue.

Anyone else have some suggestions?

 

I would try the knock sensor from a preOBDII LT1. Those are some noisy bastards, or at least the ones I have encountered are and the one in my driveway is no exception. Maybe it has something to do with all those gears in the front cover but then again the valvetrain is kinda loud too. The point is to get one on there as not having a knock sensor is not utilizing the power of the ecm. It's not that you can't live without it because you can but why bother burning proms for that matter.

 

Right now the sensors I tried would detect knocks like all the time.I am from the old school ,the good old days of jets and recurve kits.I did'nt have the luxury of an o2 sensor ,scan tools and knock sensor.I have made some very strong running pontiac 400 t/a's.And never had major problems.Point bieng that w/out a knock sensor I'm light years ahead in tuning,as far as engine management vs non ecm .But the 93-94 lt1 sensor is a good idea.Grumpy what do you think about having the total timing come in by 3600 rpm?

 

I'm not Bruce, but generally you want most of your spark advance around peak TQ. On my basically stock L98, I have most of spark advance kicked in by 3,200 rpm on my engine and then I slowly add another .8* in .4* increments at 4,000 and 4,400 rpm (close to peak HP) in the Main Spark Table while adding a bit more fuel.

 

I'm just confused,The stock table for wot vs rpm has 8.1d advance @4800rpm and 6d at 3X00rpm.What am I missing here?

 

?

 

This is the amount of timing that's added to the timing from the main spark advance table while you're in Power Enrichment (aka: WOT).

 

Thanks for your reply,it seems like people here have a problem following up long post.Ok my 89 stock bin has the table "advance in wot vs rpm(in pe)".That table reads
2.1d@400rpm
3.9d@1200rpm
4.9d@2000rpm
5.6d@3200rpm
8.1deg@4800rpm,.The timing jumps 2.5 deg @ 4800rpm.Thats adding advance past 3600rmp.The "main spark vs rpm vs load" table at "208 lv8" is
20 deg from400-800rpm,
19.0d@1000rpm,
16.2d@1200rpm,
19.0d@1400rpm,
22.1d@1600rpm to 2400rpm,
20d from 2800rpm to 4000rpm,
22.5d@4400rpm,
23.2d@4800rpm
Ok @4800 rpm in I will get 23.2deg + 8.1deg(for pe) .On the base table the advance gets higher past 3x00rpm.My other problem is the advance drops @1200 rpm then stays the same until 2400rpm.Then is the same from 2800-4000rpm.That just does,nt seem right.I am still confused in stl

 

Yep, you'll have a little over 31 degrees of timing at/above 4800 rpm at WOT, unless your LV8 drops below 208. Next time you make a run (either on the street or at the track), take a print copy of your spark advance table with you and mark on it where you're at for various RPM/LV8 points. With my old 305 engine, I never got above 200 LV8, and the curve it took on the spark advance table looked like a sideways bell-shape, making it difficult to accurately change the WOT timing. Now with my 355, I'm off the table as soon as I hit the gas, and don't go below 208 LV8 until I'm above 5000 rpm or so.

AFA the timing drop at the lower revs, you'll probably never hit that RPM/LV8 area since the converter's stall speed is 1500 rpm. Add the main spark advance and WOT timing values together at different points (interpolate between the RPM levels) to see what your total timing is.

 

My load value goes past 208 when I hit the gas .I guess the lv8 for 208 and up uses the advance value of 208.So should I put the total advance in by what rpm?As you can see the stock chip is not in by 3X00 rpm.

 

Yep, is uses the advance value at 208. Where your total advance maxes out depends on what your engine likes (to paraphrase BruceP, aka Grumpy). In the stock timing for my old 305, the timing advance continued to climb up to 40 degrees at 5000 rpm and never hit detonation, but that's with the original inefficient heads. With my 355 and Trick Flow heads, I have it all in by 3600 rpm (engine doesn't drop below 4000 rpm when drag-racing), but then increase it another 1-2 degrees above 4800 rpm.

 

Greg,

Were your TFS heads the Twisted Wedge design of the 23d heads. I have the 23d on my Xfire and had similar results with a total of 38d advance in by 3600rpm. Any sooner than that didn't seem to make any difference. Was that your experience?

 

They're the 23-degree design. Due to the 11.1 compression, my total timing is around 26-30 degrees, depending on how rich I run it and how cool it is outside (can run a little more timing when it's cold outside). I first had the timing all in by something like 2400 or 2800 rpm, but the engine was knocking at those revs, so I keep it low until 2800 or 3200 rpm.

 

I'll throw in my expereinces with a similar setup(s). I'm no chip burner yet but I think you will find this relevant anyway.

Did a 350 to 400 swap in my brother's 87 GTA this summer. Reused the stock knock sensor and have no problems with knock retard. It will register as little as 8 spark counts in 1/2 hour of driving with the advance set to our soncervative "street" setting. I'm talking about loosening the distributor and setting initial timing with the timing wire disconnected. More counts when we kick it up 4* for racing. We are using a flat-tappet cam in the 400 unlike the (noisier) roller cam from his original 350. This motor runs about 10:1 compression with aluminum TFS heads. That's been our expereince with that.

Now here's the stuff I DO really know something about: On the many 400 motors I have worked on I gerenally find what you have found- they want more timing down low, but slightly less total timing at WOT. 32* with iron heads at WOT is about all they need or will tolerate. With your stock 400 heads (~75cc chamber actual measured) and flattops you will be a smidge over 10:1 compression. With the lousy design of the old emissions heads you will NOT be able to run very much advance on pump gas at that kind of compression ratio. That 32* I mentioned earlier will become more like 28-30 ABSOLUTE MAX before you start to get audible knock. You will be able to take it up to the full 32*, however, if you run race gas in it.

As for the advance RATE of that I'll just say that I can get improvements in torque by getting the advance "all in" as low as 2200 RPM on a non-computer controlled ignition! Yes, 2200!! This is for DRAGSTRIP ONLY use. No vacuum advance used. The torque increases are nothing short of eye-opening. Versus a slow advance (in by 4000, let's say) I can drop 3-4 tenths just by increasing advance RATE.

For a street motor I shoot for the curve to be "all in" by 2800-3000 RPMs as a compromise. This is because I run vacuum (load compensating) advance on the street which will over-advance the timing at lower RPMs if used with a super-fast RPM related advance curve. At the dragstrip you are VERY quickly revving through 1st gear at WOT so the engine can take a lot of advance very quickly. On the street where loads are more constant you can't do that without encountering detonation. So you have to compromise and slow the RPM-related advance down. I'd say that 3000 RPMs would be a good RPM to shoot for to get the RPM-related advance "all in" if you are talking about a car that is street driven. If you are RACING ONLY I don't see why you couldn't burn a RACE ONLY chip with the advance all in by 2200-2400 (and set all load compenating advance tables all to 0*).

Back to the "street" chip- once you do that you'll have to back down the load-compensating advance in a similar way I limit the travel of the vacuum advance cansiter on a mechanical distributor, or you'll be over-advanced at lower RPMs. On a mechanical distiributor I use no more than 12* of additional vacuum advance. But mechanical systems are no where near as flexible or accurate as your ECM's programming. You could probably put in more timing that that under various conditions but I don't know how to advise you specifically beyond that.

Hope there's something useful in all that for you!

 

Damon,that is what I have wanted to hear.I had to reread your post to soak up all the useful info.I just wanted to make sure it was ok to have the total advance in by 3X00 rpm.Sounds like you have alot of exp with spark advance.Everyone else has gave great info too.I can feel how much advance my car likes.But wanted to here from people with 400sbc exp.Non ecm controled info Damen gave is most interesting.Having a race chip w/ the advance in by 2500 rpm is something I will have to check out.One thing I was wondering,my heads are 1971.A)Would'nt that make them pre emission heads?B)Do you really think(know)that with flat tops I have 10:1 comp?