timing and compression ratio

[benno440]

is there a method or a tool (besides a timing light) to get you timing perfect, i may be asking a dumb question but i am a bit lost in the tming department.
also how do i work out my compression, hopefully i have given enough information.

engine 440-505" stroker 4.350 bore, 4.25 stroke

stealth heads ported 80cc closed chamber with quench surface

cometic MLS gasket 4.410 bore .40 compressed thickness

flat pistons with dual valve reliefs 27cc, block decked to have piston arrive at top of block (pretty sure its at .00", Is this feasable???) as i have not got piston in blcok yet but am pretty sure thats what the machinist said.


is there any more information i need to give to work out how to properly set my timing?

thanks in advance

 

[RustyRatRod]

What's the compression height of the piston?

If all of the info you gave is correct, you should have 9.849:1. Seems a little low for aluminum heads. I wouldn't attempt timing advice without camshaft specs.

 

[Hemi Rebel]

If you want your timing as perfect as you can get it I would suggest a crank trigger set up. Ideally a computer controlled ignition system with individual coils for each cylinder and magnetic pickups on the crank and cam to tell the computer when to fire each coil would give you the most accurate timing. That and timing gears instead of a chain. Realistically speaking, a crank trigger system and a high quality dizzy will be just fine.

 

[D575]

If you want your timing as perfect as you can get it I would suggest a crank trigger set up. Ideally a computer controlled ignition system with individual coils for each cylinder and magnetic pickups on the crank and cam to tell the computer when to fire each coil would give you the most accurate timing. That and timing gears instead of a chain. Realistically speaking, a crank trigger system and a high quality dizzy will be just fine.

Ha Ha , I'm sure that helped.

 

[Hemi Rebel]

Ha Ha , I'm sure that helped.

I hope so. Its not that uncommon. Ever seen these.

 

[RustyRatRod]

Yeah, a crank trigger would definitely be the most stable.....and that IS what the OP asked.

 

[charger360]

really need a bit more info for a real answer cam specs for sure a bit more info on the build would help a lot as well.

 

[67 B-body]

Some of the Ol' timers would use a vacuum gauge connected to a manifold vac source, and advance base timing to achieve max vacuum reading. Then reworking the Distributor to correct "limit" the total advance to something around 37 to 42 deg.
Myself,,,, I start with something that provides good idle, then drive it... how it reacts and performs dictates how much I peter with it after that.... Typically dialing in more advance shows gains in throttle response and torque off idle up to a point in which it stops performing better..... Once you get this best performance figured out on the low end of RPM,,, verify the high RPM doesn't take the advance too far out.... Mopar dist's use a T for the weights to ride in and the rotor pushes on top of this piece,,,, limiting the size of the windows machined in the T will limit total mechanical advance at high RPM. If you are using vac advance, you can purchase an adjustable vac adv can to control that as well....
Hope I've help!

 

[RustyRatRod]

Actually, you just described how "old timers" might do it, without a gauge. Some even without a light. ....and there's nothing wrong with that. But, nine times outta ten you end up with too much mechanical advance. On a low compression motor, that will usually fly. But on something on the edge, you can get into detonation very quickly at higher RPM and not hear it with the noise of the engine. That would suck.

Some of the Ol' timers would use a vacuum gauge connected to a manifold vac source, and advance base timing to achieve max vacuum reading. Then reworking the Distributor to correct "limit" the total advance to something around 37 to 42 deg.
Myself,,,, I start with something that provides good idle, then drive it... how it reacts and performs dictates how much I peter with it after that.... Typically dialing in more advance shows gains in throttle response and torque off idle up to a point in which it stops performing better..... Once you get this best performance figured out on the low end of RPM,,, verify the high RPM doesn't take the advance too far out.... Mopar dist's use a T for the weights to ride in and the rotor pushes on top of this piece,,,, limiting the size of the windows machined in the T will limit total mechanical advance at high RPM. If you are using vac advance, you can purchase an adjustable vac adv can to control that as well....
Hope I've help!

 

[Meep-Meep]

I believe the OP's question is how to optimize a timing curve for a particular combo rather than a method used to check it. The mag offset stuff is used by the professional shops with the big engine analyses but you can do just fine with a timing light IF you accurately locate TDC.

If you are running a radical cam you may want the initial to be around 12-16 or possibly more. Total advance should fall in the 35 deg range and will depend on how efficient your chamber is, fuel used, mixture strength and probably some other factors I left out. For street driving you will want a vacuum advance and contrary to some opinions here I like the ported port. Add about 8 deg vac under light load conditions only so you will want to check your vacuum signal out on the road.

To get a really good idea on where to set your initial timing after the engine is running I have done the following: Set the throttle idle screw on the carb about 1.5 turns from the blades lightly seated in the bores. This should uncover about half the transfer slot or maybe a little less. Even up to two turns is OK but too many turns will provide too much air for idling and cause dieseling on shut off. This assumes a single carb. From there you would adjust the idle speed using the distributor and watching the timing to make sure you fall into something reasonable RPM and timing wise. Your idle speed should fall in the 700 - 1000 range with maybe 15 BTDC. If you end up with like 30 deg initial to get 900 RPM, something is probably wrong and you need to investigate. Also the vacuum reading in neutral should be I'm guessing 12" Hg for a mild cam or if you have a more radical one about 8". Anything unusually low needs investigation. The engine should feel pretty crisp when you snap the throttle, and if that's the case and all the settings are in the range as outlined above, that will be good enough to drive around and do more tuning. From there you would optimize the curve and vac advance and it's best that you do it at the track or on a chassis dyno.

But before you get to this point it would be a good idea to accurately measure your compression ratio. Get a burette -

http://www.amazon.com/ACRYLIC-BURET-PTFE-STOPCOCK-100/dp/B0018BX3ZO/ref=pd_sbs_indust_3

and do some CC-ing. For the cylinder and piston you can TDC the piston then set the gasket on the cylinder and cover with a piece of glass and fill the volume. A compressed gasket is best but if you don't have that then figure slightly higher compression like .1 point for the end result. Pop #1 piston in the bore with no rings and seal with grease. Then find TDC with an indicator and correct the factory marks as required while you are there. Put the head gasket on covered by the the piece of glass and sealing everything with grease but leaving a tiny gap to fill the volume. Do the same with the head, but don't use the gasket twice! Add the numbers together and that is your unswept volume.

Going through the above steps will give you an actual compression figure and then when you measure your cranking compression you will have two pieces of solid data you can provide the tuning gurus along with cam specs, etc..

 

[99ss]

maybe a dwell tac?

 

[benno440]

i think the crank trigger is abit over my head thnks for the info though.

rusty i gave the wrong piston specs sorry, it is flat top with valve reliefs 4.5cc not 27 lol. hopefully this will up the comp a bit.

here you go guys fullspecs of my engine hopefully can help with the timing where i should start at, and to let you all know the cam is degreed in so she is spot on.

*440 - 505 .30 over
*icon 4.5cc pistons
*performance 7.100 h beam rods
*4.25 stroke K1 crank forged
*lunati hyd cam
Advertised Duration (Int/Exh): 300/305Duration @ .050 (Int/Exh): 250/255
Gross Valve Lift (Int/Exh): .540/.540
LSA/ICL: 108/104
Valve Lash (Int/Exh): Hyd/Hyd
RPM Range: 3000-6800

*fluidampner
*comp cams double roller chain
*comp cams hyd lifters and magnum pushrods
*edelbrock dual quad with twin 500cfm eddies
* stealth ported heads 80cc hughes rockers
*msd ignition
* 1 7/8 tti headers into 3" x pipe with electric cutouts


also take into account this is not a racecar more of a showcar and street cruiser, as i have spent to much money to break anything at the track which has resulted in me being poor and cant afford to replace lol.
had to buy even part from teh states and the freight killed me lol.

that being said i just want to set it up right and last a long time
thanks guys

 

[RustyRatRod]

Yeah that helps. 11.956:1 lol

 

[benno440]

that looks a bit better cheers bloke lol

 

[RustyRatRod]

Give me your intake closing point and I can calculate your dynamic compression ratio too.

 

[benno440]

rusty what compression calculator did you use to get 12.1? everyone i try keeps giving me 6.75 to 1, which cant be right

tell me if this sounds right i enter:

bore size: 4.350
stroke: 4.250
gasket size: .40
gasket bore: 4.410
chamber volume: 80cc
deck height: 0.0
piston: flat top 4.5cc

 

[Meep-Meep]

Based on what you put down this is what I get:

Basic volume formula in CI: Bore squared x .7854 x length. Then x number of cylinders for engine displacement.

Gasket = .611 CI (.611 x 16.3872) = 10 cc

V chamber, total = 10cc + 80cc + 4.5cc = 94.5cc. Assuming 4.5cc piston is a dish.

Displacement one cylinder: 63.16 CI = 1035 cc. 1035 cc + 94.5 cc = 1129.5 cc / 94.5 cc = 11.95:1 CR. Dynamic compression takes the intake valve closing point into account. The later the closing point the lower the dynamic CR.

 

[dwayne venton]

the most recent mopar action mag feb. 2013 issue deals with what you need to exact science .happy tuning