Square four Conrod eye to eye centre distance.

[D.Ryan]

Hi,
Dennis calling,
I have a 1950 Mk1 four and I have recently converted to 12 volts and an Accuspark electronic ignition. (Kit no 39) the TorqueSoft program on the net requires the distance between the big and little end centre to centre. Can anyone please advise what the dimension is? ( I didn't measure it when I had the engine apart - duh)
Regards Dennis.

[GuyWalton]

Surely that will be the stroke length which is 78mm . But maybe not .

[chris.shearwood]

Hi Dennis,
I think it is 5.9"
Regards, Chris

[fpassmore]

Stroke is dependant on throw or eccentricity at the crank

Frank

[D.Ryan]

Hi and thank you for the replies.
Perhaps I should have worded my original enquiry better to make it clearer.
I use the aforementioned TorqueSoft - Piston Position - Calculation tool to determine the timing on the engines that I know the centre to centre distance of the Conrod. Once I have the correct dimension I can use trigonometry but this program is very accurate and especially in conjunction with a dial gauge down the plug hole. You can also alternate between crank angle and piston position. I do have a degree disk arrangement for the four that I drive off the front crankshaft nut slot. The unit is completely sealed and I can check both static and dynamic timing. I static time my engine to 8 degrees before TDC and with the 15 degree advance plate and on crank rotation translating to 30 degrees I have a fully advanced timing of 38 degrees. It works well and it's not too advanced.
Regards Dennis

[nevhunter]

You are correct about rod length and crank throw BOTH being required when converting angle to piston position.. It will vary with rod length. Defining TDC from the piston posn requires care also as the piston tilts and flops about. Going about 1' either side and bisecting those two points is preferred for a TDC position to locate the timing disc accurately rather than just trying to Guess the highest point as best one can. Nev.

[D.Ryan]

Hi,
I d agree the dead stop method is the most accurate for TDC and I do have one from a modified plug for that that purpose. A thought has just occurred to me that if I was to use the dead stop in cylinder no 3 and use the dial gauge in cylinder no 1 to split the difference for the TDC would that work? Or am I being to fussy for the sake of it?
No matter what repeatability is the key.
Regards Dennis

[chris.shearwood]

Dennis,
What timing advance does your TorqueSoft program/tool give if you use 5.9 inches as the distance between big and small end centres?
Chris

[david.anderson]

Conrod length is 150.25mm or 5.915”. With a 75mm stroke the rod/stroke ratio is just over 2-1 which is consistent with the singles that also have a rod stroke ratio of slightly over 2-1. This is at the higher end of rod stroke ratios which are generally between 1.6 and 2.0. However there are always exceptions such as the M3 BMW which has a 1.48 rod stroke ratio. Modern engines are generally lower rod stroke ratios.
The longer rod stroke ratio is generally credited with reducing cylinder bore wear due to the lesser rod angle and generally also results in a reduced ignition timing as there is more piston dwell time around tdc. An engine ignition advance of 38 degrees with a long rod stroke ratio is an indicator of a poor combustion chamber shape. An efficient combustion chamber would require much less advance.
David

[D.Ryan]

Hi all,
With a 75mm stroke and David's dimension of 150.25 for the con rod, 8 degrees BTDC translates to .456 mm. Full advance at 38 degrees translates to 9.73 mm BTDC. The program is Torqsoft piston position calculation tool. Google it. It is good fun to play with different angles and piston positions.
Regards Dennis