350 SBC timing

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ok here goes again...... as you all may know i'm building an 81 monte carlo, 350 SBC .060 over bore, vortec heads, 750 cfm manual choke edelbrock carb, HEI distributor with eccel super coil, 8mm wires, th375 trans with shift kit, 2700 stall, and other goodies that dont need to be mentioned. i am having problems with my timing. i lost my cam card and i dont know how much to advance the timing. the specs are as follows.

Brand: Crane Cams
Product Line: Crane Energizer Camshafts
Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: 2,200-5,200 RPM
Intake Duration at 050 inch Lift: 228
Exhaust Duration at 050 inch Lift: 228
Duration at 050 inch Lift: 228 int./228 exh.
Advertised Intake Duration: 284
Advertised Exhaust Duration: 284
Advertised Duration: 284 int./284 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.480 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.480 in.
Valve Lift with Factory Rocker Arm Ratio: 0.480 int./0.480 exh. lift
Lobe Separation (degrees): 112
Intake Valve Lash: 0.000 in.
Exhaust Valve Lash: 0.000 in.
Computer Controlled Compatible: No
Grind Number: 284 H12

hopefully someone can help me. i've asked everyone i know and nobody knows how to use this info to figure it out. TIA!!!!!!!!!

posted by  glagon1979

Well it aint that easy. Your combustion chamber and piston design, plug location. squish height, quench area, rod/stroke ratio, swirl,etc are also determinants.

Your cam is 7/41 51/3 @ 0.05 so you will come on cam about 3700 rpm. Your stally is in the ballpark based on this.

So applying a rule of thumb you want peak BMEP at about 13° ATDC. You want the flame to travel from the spark to the pot wall in that time. So at say 3700 and assuming your Vortech head is a goodun and will give you an aggregate flame speed of say 30 m/s and your bore is about 4", with the spark 1" from the pot wall:

4-1 = 3" = 76.2mm (0.0762m)
@ 30 m/s time to travel is 0.0762/30 = 2.54 millisec
@ 3700 rpm = 123.3 rads/sec = 0.045045 millisec/deg
2.54/0.045045 =56.4°
56.4 -13 = 43.4° BTDC

Having said that, the internet experts will tell you around 10-12° static and total 36°, because they don't know how to tune against det.

posted by  Wally

I can follow the math, but I can not figure out why you substract 13 from the total....:banghead: This may be a dumb question, but I'm sure others would like to know and are just afraid to ask, And remember I am a bodyman not a mechanic, (but also happen to have a degree in electronic engineering)

Thanks in advance.

posted by  corbett_auto

because in my preamble I nominated BMEP as 13° ATDC; spark occurs BTDC.

posted by  Wally

where did these numbers come from?



what, exactly, is the rule of thumb here?

i'm not challenging you, i'd just like to know. never had any formal automotive training...

posted by  dodger65

i'd like to know as well. im sure i was informed about 10 years ago, but drugs and booze have tainted my memory.

BTW, ty wally

posted by  glagon1979

Ah now we are entering the realm of the the old timers and tuners.:wink2:

I didn't think members on this site were all that interested in getting into the nitty gritty of how a car ticks, so I'm a little suprised at the interest.

The cam events are from the Crane site, so that was fairly easy.

The inlet closing angle @ 50 thous is a good indicator of what rpm the engine will come on cam. And you guys always wondered why the 0.050" reference, well this is one of the uses. This is pretty much confirmed by Glagon's own data 2200 - 5200 = median 3700.

The 13° +- is a pretty close approximation of where desirable peak BMEP tends to be with common geometry.

The stally, well that really should below peak torque by a few hundred revs to achieve lockup anyway, but for street it pays to be conservative for a myriad of reasons. The final drive ratio would need to match of course.

posted by  Wally

ohhhhhh... those were the degree btdc and atdc of the valve events... your shorthand was a little confusing...

posted by  dodger65

no they were the cam events. Valve events are different.:wink2:

but yes its normal presentation for us old codgers:

7/41 51/3

duration:

7+180+41 = 228° in
51+180+3 = 234° exh

Centreline:

228/2 - 7 = 107° in
234/2 - 3 = 114° exh
cam is advanced (114-107)/2 = 3.5° (tells me they wanted to maintain some of that lost low end power, probably for automatic tranny)

Lobe separation

(107 + 114)/2=110.5° ( torque will be a little peaky, but strong in the midrange).

From this we can say it is a hot cam (not track or race) that will require;

a static compression ratio of around 9.5 up to 11.0;
multi angle valve seat cut;
tuned extractors and large (preferably twin) exhaust;
definitely a recurved dizzy;
shouldn't need much port work.

It will be lopey at idle which should be good for street cred.

posted by  Wally

Oh and if it was a jap spec cam then based on Glagons figures it would be a 71/71 cam.

posted by  Wally

you know what i was saying, ya damned aussie!!!! :laughing: :wink2:

posted by  dodger65

:mrgreen:

I like your style dodger... you have a sense of humour in an otherwise beige world..

posted by  Wally

thanks--i try...:thumbs:

posted by  dodger65

i found out what it should be from the people at summit and wally was pretty close. here is what they said,



im gonna give it a go this weekend. hopefully i'll be doing burnouts by sunday.

posted by  glagon1979

If you are changing the springs, can you humour me and put a slightly stiffer one in and tell me how it performs?

posted by  Wally

i did buy a kit for my dizzy but i didnt plan on messing with it this weekend. maybe since you ask i will and see what happens. thanks for all the help wally, you da man:thumbs:

posted by  glagon1979

now this thread makes me smile. :mrgreen:

posted by  67Coronet383

i totally forgot about this thread.

update: i didnt get to mess with it like i planned to that weekend but i eventually got to it. (having a 16 month old daughter kinda soaks up your time) it solved some of my problem, i still have a dead spot when you mash the throttle. i have a feeling the vacuum advance in my HEI dist is malfunctioning. ive tuned my carb in as good as i canwithout messing with the jets. thanks for all the help guys, im sure i'll be asking for more help before this actually hits the road! :thumbs: :thumbs:

posted by  glagon1979

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