Is cam timing different for offset gudgeon pin?

[JonnyWowsers]

Hi, I'm currently building an A series engine using non A series pistons, the bore size is compatible as is the compression height. The main difference is that the gudgeon pin is offset by 1.5mm. I will be running a full race cam which I time in to the exact degree with an independent boss type adjustable cam gear. My question is- is the required cam timing still the same?
ie. If the cam manufacturer says the inlet should be fully open 106 degrees past TDC is this still correct?? .. Any advice (preferably in lamers terms) would be greatly appreciated..

[99hjhm]

No exactly the same.

BTW the 106 figure they come to is totally random so do some research first if you can to what actually works. If you do have full race cam and not a super high compression ratio, it can be wise to advance the cam slightly. Many also advance the cam a couple of degrees to allow for chain stretch. Only use a IWIS timing chain, only ones that are any good.

Edited by 99hjhm on Monday 18th February 22:03

Edited by 99hjhm on Monday 18th February 22:05

[Stan Weiss]

First know to know in which direction you are installing the offset (toward the major or minor thrust). In one case the piston will be ATDC before the crank journal is and in the other case the piston will be ATDC after the crank journal has been. What bore, stroke and rod length?

Stan

[227bhp]

Stan Weiss said:

First know to know in which direction you are installing the offset (toward the major or minor thrust). In one case the piston will be ATDC before the crank journal is and in the other case the piston will be ATDC after the crank journal has been. What bore, stroke and rod length?

Stan

You would only offset the pin one way

[Boosted LS1]

You could fit the pistons the wrong way if the crowns weren't marked front or didn't have valve reliefs, perhaps.

[Inline__engine]

227bhp said:

You would only offset the pin one way

if you wanted an engine to be quieter you'd move the offset one way, if you wanted maximum power you'd offset the other way

[99hjhm]

JonnyWowsers said:

Any advice (preferably in lamers terms) would be greatly appreciated..

[227bhp]

Inline__engine said:

227bhp said:

You would only offset the pin one way

if you wanted an engine to be quieter you'd move the offset one way, if you wanted maximum power you'd offset the other way

You usually either go central or offset just one way - towards the thrust side. I've never seen an engine with the offset on the 'wrong' side, but am prepared to be educated, it probably depends on crank offset
It doesn't alter the max power or torque figure one iota.

[Stan Weiss]

A few things of interest.

With pin offset the displacement of the engine will change

Depending on the direction of the offset you either have greater than or less than 180 degrees from TDC to BDC.

Many years ago when in the stock class one had to run the stock pistons, installing the pistons with the offset reversed did show a HP increase.

Stan


Bore_=_84.0_Stroke_=_72.0___Rod_Length_=_125.0_RPM_=_6500
Wrist_Pin_Offset_=_0.0
Rotation_Time_of_crank_per_degree_in_Milliseconds0.0256410
Rotation_Time_of_crank_per_rev_in_Milliseconds___9.2307692
Crankshaft_rev's_per_Second______________________108.3333333
Engine_Size_____97.3957_CI___1596.029_cc

---

Bore_=_84.0_Stroke_=_72.0___Rod_Length_=_125.0_RPM_=_6500
Wrist_Pin_Offset_=_1.5
Rotation_Time_of_crank_per_degree_in_Milliseconds0.0256410
Rotation_Time_of_crank_per_rev_in_Milliseconds___9.2307692
Crankshaft_rev's_per_Second______________________108.3333333
Actual_Piston_Stroke_72.00565361793
Cylinder_Volume_24.350837_CI____399.038731_cc
Engine_Size_____97.403350_CI___1596.154926_cc
Crank_Angle_Piston_TDC_-0.53381933334__Piston_BDC_179.03429507215

---

Bore_=_84.0_Stroke_=_72.0___Rod_Length_=_125.0_RPM_=_6500
Wrist_Pin_Offset_=_-1.5
Rotation_Time_of_crank_per_degree_in_Milliseconds0.0256410
Rotation_Time_of_crank_per_rev_in_Milliseconds___9.2307692
Crankshaft_rev's_per_Second______________________108.3333333
Actual_Piston_Stroke_72.00565361793
Cylinder_Volume_24.350837_CI____399.038731_cc
Engine_Size_____97.403350_CI___1596.154926_cc
Crank_Angle_Piston_TDC_0.53381933334__Piston_BDC_180.96570492785

[HJG]

Inline__engine said:

if you wanted an engine to be quieter you'd move the offset one way, if you wanted maximum power you'd offset the other way

You'd have to offset the pin a huge amount (usually a geometrically impossible amount) to gain an appreciable advantage in reduced rod angle on the power stroke.
This is usually accomplished by an offset crank pin relative the cylinder centreline.

[Inline__engine]

to some people a few hp in a many hundred matter....

[JonnyWowsers]

Hi. Sorry I not replied earlier been busy n all that.. .. Thanks guys for your help, I was already aware how the offset affects things depending on which way in relation to thrust they are fitted which brings me to my next question.. But first the specs as requested are as follows.. 74.79 boresize, standard 81.28 stroke, rod length 146.25 rod length centres.. I was gonna aim for about 12:1 comp ratio...( should be a tad higher but abit worried about detonation and running hot with there only being. 0.100" between the bores especially the middle of 2 & 3 in between the exhaust valves not helped with being siamese ports..)..

The next question being is the 'increase in power' worth it.. Or am I better off having a smoother potentionally more reliable, longer lasting engine.

Another quick question while I'm on haha.. At what point compression ratio wise might you need to run fuel additives...
👍

[JonnyWowsers]

Back to the cam timing. With the 'not super high' comp ratio and allowing for chain stretch you would say advance it by a couple of degrees then... Where do the cam makers get their 'random' figures from? I would have thought the cams they sell are all tried and tested and been through development etc. Or are you just saying its just a good starting point and depends on engine spec.. I'm not disagreeing with you,
just interested.. 👍

[PeterBurgess]

I remain to be convinced any aftermarket cam manufacturers do empirical testing rather than using some software and experience to produce 'new generation' cams. We have seen many cams designed on software ( not posh OE software) meant to be the dogs boll*cks produce a crap power band and power and even damage valves and springs from wrong acceleration rates.
It is the engine builders who do the real experimenting so it may be wise to go to someone who specialises in building engines like you have to get a cam/s which will work and be given the timing and if required valve clearances to run.

[Boosted LS1]

I'd take an intake of breath if i were you and stick to the simple things in life. :-) It'll be easier.

[Boosted LS1]

PeterBurgess said:

I remain to be convinced any aftermarket cam manufacturers do empirical testing rather than using some software and experience to produce 'new generation' cams. We have seen many cams designed on software ( not posh OE software) meant to be the dogs boll*cks produce a crap power band and power and even damage valves and springs from wrong acceleration rates.
It is the engine builders who do the real experimenting so it may be wise to go to someone who specialises in building engines like you have to get a cam/s which will work and be given the timing and if required valve clearances to run.

Agreed completely. As if cam manufacturers build numerous engines and then book very expensive dyno time. I doubt they go near dyno's let alone build a multitude of engines.

[JonnyWowsers]

The ST 649 cam for example used in the 's' n Longman engines must have been 'developed' back in the day though, they obviously never had computer simulation to develop their cams. Old skool.

[JonnyWowsers]

Any advice on the my post prior to
'back to the cam timing'?

[PeterBurgess]

As I understand it the 649 was developed, probably with lots of dyno feedback as it is an OE cam, for open exhaust use. Mike Garton redeveloped the cam to work on silenced systems, this is probably from his extensive experience (knowing how knowledgeable he is). it was the designated cam for Miglia engines and is called the STR930. This profile in a B engine is very sweet for Sprint and Hillclimb use ( the 649 is called a 770 in the B series).

[227bhp]

I don't think your actual initial cam timing in this respect is important at all. You've got to start somewhere with it so you might as well just set it to whatever is recommended or an educated guess from someone knowledgeable and forget the pin offset. It doesn't matter, because you're unlikely to nail it right from the start. That's where computer modelling comes in and you're unlikely to have the in depth knowledge of your engine to do that.

Your static CR and cam duration are closely linked and the outcome of that combination will have a huge influence on detonation.
The rule is the higher static CR you go the wilder the cam (or intrinsically where IVC ends up) you will use and the DCR will drop. Aim for a DCR of about 8 or 8.5:1 and you'll be ok for pump fuel.
Static CR is only part of the equation, as the name suggests its static, no engine ever made any power stood still.