Help with timing table

[riche]

I have been reading and snooping around the internet looking for information to help me make the right decisions in adjusting my timing table. I have a vortec headed older 350 block bored 30 over with flat tops. The cam is a howards cam in my signature. http://www.summitracing.com/parts/hrs-110951-08

My goal right now is to try to adjust my cruising area of the table for economy. I ran across some info on saying that older small blocks could handle as much as 50 deg or more in timing under light load cruising. Then here recently I ran across the 98 vortec table on your site and in the table range I would like to tune (70 kpa, 2500 rpm - 373 gears) it has 30 degrees so I am really scratching my head right now trying to establish some limits so I do not make a bad mistake. But is there something I am missing here about the vortec table?

I have been trying to to understand distributors in order to make a sound decision. And I was thinking that if I had a regular distributor and assumed it had 34 deg total and initial timing and a vacuum advance of 10 deg. I would have 44 total at cruising (2500 rpm). But I look at the table and I see around 30 deg. Is a 98 truck set of 0 degree or is there an initial timing I should add?
Or is simple bc my theoretical distributor has not advanced all the way @ 2500 rpm?
Would appreciate any advice or conversation on this topic.

Thanks,

[steveo]

when playing with timing in a particular cell (assuming you're starting with timing that could use more advance)

first, you'll increase timing and vacuum will increase.

you'll increase it more and at some point, vacuum will start to drop off. this is *probably 1-2 degrees past* the point of optimal.

you'll increase it more, and it will start to run rough. it'll surge and be 'not so smooth'. this is because the fuel is burning too far into the compression stroke, and you're getting a bit of downward force when the piston is supposed to be moving up. but it's not quite knock yet...

increase it a bit more and you'll get pinging and knocking, which is really bad and noticeable.

in very low load ranges, it'll take a ton of timing without reaching the pinging point, but there's a very wide range between 'it doesn't make more vacuum' and 'runs like crap'.

the idea is to provide enough advance to combust all the available fuel and no more. every engine is different.

more efficient (and hotter) combustion chambers do not require as much advance to completely combust the fuel, whereas cold running aluminum heads which are also efficient, obviously in a different way, like more advance.

also the bigger your cam, the more sensitive to cam surge from too much timing advance. big built engines don't use a lot of timing advance, to smooth them out.

too much advance in very high vacuum areas can also deaden engine braking, which makes the car much less fun, imo.

thing is you don't need to 'tune' your entire tuning map, since timing maps are curved. you just need to tune a couple of points and hand-smooth the rest.

start by tuning your most common cruising cell. lets say you cruise on a flat road at 2000rpm and that's 35kpa. do a log and determine MAP. then try adding 3-4 degrees there (you wont hurt it at that low load). does vacuum increase? ok then keep going.

one thing about that too, you always have to remember, since you're adding vacuum, you're pushing your timing 'cursor' into a totally different cell! so dont just keep tuning on the same cell.... and keep your timing map nice and smooth

[steveo]

oh also old vac distributors are a great reference point but keep in mind factory carbs don't ever run at stoich, they run on the rich side. so guess why they need all that extra advance?

.... to burn all that extra fuel off!

so just 'cause a carb n points motor runs 50 degrees doesn't mean you should

[Fast355]

oh also old vac distributors are a great reference point but keep in mind factory carbs don't ever run at stoich, they run on the rich side. so guess why they need all that extra advance?

.... to burn all that extra fuel off!

so just 'cause a carb n points motor runs 50 degrees doesn't mean you should

Just FYI, in my experience non feedback carbs actually run LEANER than stoich at cruising speeds. My factory Q-Jet on the 1983 G20 Van would cruise at 16.5-17:1 at light throttle down the highway and only richen up with moderate throttle.

I had one of those blinky LED afr gauges back in the day and swapped TBI to Carb to TBI numerous times on the same engine. Even a "performance jetted" Edelbrock 1405 would peg the meter lean and cause the LED to disappear lean until the throttle was opened.

[steveo]

you might be right; i've only had a couple carbed smallblocks and they were both factory rich (probably 12.5:1 judging by the plugs)

but lean cruise AFRs need more timing too.... a perfect afr doesnt need as much advance to burn completely

[riche]

Thanks for the advice steveo, Got another question though. It seems like my engine is a little high in the kpa values. Like you just mentioned 35kpa @ 2000 rpm. My motor is more like 60kpa @ 2000rpm. Do you believe that is normal i.e. my cam or something contributing to this higher kpa?

[Fast355]

Thanks for the advice steveo, Got another question though. It seems like my engine is a little high in the kpa values. Like you just mentioned 35kpa @ 2000 rpm. My motor is more like 60kpa @ 2000rpm. Do you believe that is normal i.e. my cam or something contributing to this higher kpa?

He just tunes 2 door V8 powered sports cars. Those of us tuning box like trucks/vans/suvs will see different loading than a light, aerodynamic car. I run about 50-70 KPA at 2,200 rpm @ 70 mph and that drops to 40-60 KPA at 2,600 RPM @ 85 mph.

[steveo]

yep, cams that make more high end power produce poor vacuum.

i just looked at your specs. it's a fairly large duration but low lift cam. i wouldn't expect a ton of cruising vacuum.

you shouldn't even be driving it at 2000 rpm. with that cam you should cruise at least 2500rpm. notice you also have a very narrow timing map, half your timing map is just decel range now.

since you're running vortec iron heads (designed for low end power only) with high rpm operating cam (an odd choice, since now you have weak bottom end and poor flow on the top end), you'll probably find throwing timing at your fairly narrow operating range doesn't help a lot.

no offense but welcome to the 'i put too much cam in it' club

[riche]

I hear you definitely got what I was asking for there, I was looking for a low lift cam (bc vortec head) but wanted a good sound. (Close overlap). Guess I sacrificed to much in the other departments. I still can do one hell of a burnout though...
Could you try to explain to me a little more about the narrow timing map comment? You were able to determine that from the cam?

[steveo]

just like i said, since the timing map is rpm vs vacuum, and your cam makes way less vacuum than a normal one and operates at higher rpm, you only get like 1/3rd of the timing map in realistic operating range. its good enough, just not as much fine tuning as you'd probably expect looking at the graph.

[Roadknee]

Vortec heads flow very well for a production head and will pull past 6000 rpm and make north of 400 hp on a 355 with the right cam. Your cam is light on lift but should still pull well to 5800 or so. The vortec chambers are very efficient and I doubt you'll need more than 40 degrees anywhere. 30 is pretty good for 2500 rpm and 70 kpa. You'll get best economy running just to the lean side of misfire with that stealthram

[riche]

It does make sense that you say I should not be cruising at 2000 rpm with that cam. B/c I was trying to explain to someone the other day that it doesn't sound like it is running smooth till after 2000 rpm. When I picked it I thought I was making up for less lift by adding duration. Wasn't really worried about making power on the bottom end b/c all that would make me do is spin the tires more. But from what I gather from yall is vortec heads like the lift and not the duration/lobe overlap?

[riche]

I apologize for asking this question, I probably should understand it better and I think I do but here it goes.
If kpa is atmospheric pressure, and I understand that vacuum is measured in inches. They are to different things yet vacuum (as the engine is changing rpm) is directly affecting the atmospheric pressure correct?

So when you say my cam makes less vacuum and has a small rpm window it is more like saying I just eliminated a portion of the map bc my cam will not change or make vacuum at certain rpms?

I think I understand what you are saying I rarely see my engine in the lower portion of the table unless like you say I let off the gas during decel. Just want to make sure I understand why.

[Roadknee]

But from what I gather from yall is vortec heads like the lift and not the duration/lobe overlap?

Vortecs in stock form don't like a lot of lift because they don't have enough valve seal to retainer clearance. I have one set in my shop and there is only 0.500" clearance. It's recommended to run 0.060" clearance at max lift, so my heads shouldn't run more than about 0.440" lift unless I have the guides machined down.

[Fast355]

Vortecs in stock form don't like a lot of lift because they don't have enough valve seal to retainer clearance. I have one set in my shop and there is only 0.500" clearance. It's recommended to run 0.060" clearance at max lift, so my heads shouldn't run more than about 0.440" lift unless I have the guides machined down.

Seems that GM was not very consistant with this. I have a set of 906s from an Express built in 11/96 and the retainer to seal clearance was .530". I have a set of 062s off a crate engine built in 2006 that is more than that. That being said I have seen a set that had less, much less, down around .480". I have seen heads have variances valve to valve as well. So long story short check all 16 for proper clearance before assuming anything.