40 replies to this topic

[Spider]

Hi Everyone

I have a std/std CAM 6232 crank for my GTM engine, my question is, is it ok to regrind to +10 on a race engine? as I have to use 1 1/4" SU's I can't see it revving much past 7000 rpm. I read somewhere that a 10 thou regrind has a negligible affect on the strength, is that true? Also on the same theme, is a centre strap necessary on a A+ block?

 

Ta

S

 

For Competition Engines I've always ground the cranks - even new ones.

 

When having it ground, asked for it to be Indexed (or corrected depending on their terminology) and Phased. Some Cranks are very good here and some are way out. In regards to size, if you are looking to get every last 1/10th HP from the engine, I used to have the pins ground to -0.040" on the smaller journal sizes.

 

Indexing it will correct the stroke of each Crank Pin and Phasing it will ensure that the Crank Pins are exactly 0⁰ and 180⁰ (as applicable) to each other.

 

I would have all the spinning parts balanced and I would advocate against a centre main strap or any fancy steel cap - these engine just don't need them.

 

If you are contemplating wedging the crank, have that does before any of the grinding operations. I would suggest it it a good idea here, not just to lighten the crank but - if done right - extend it's life.

[grizzler73]

Thanks Moke Spider, that is valuable advise. Can I ask what the reason behind the taking down of the small journals to -.0040 is for? I'm guessing to reduce the weight of that area?

[carbon]

Reducing the journal diameter will reduce frictional losses.

 

Not a lot, but as Moke Spider says 'if you are looking to get every last 1/10th HP from the engine'...

[Spider]

It reduces weight and that almost immeasurable amount of 'friction' (or more accurately - drag).

 

By reducing weight - in the right places (which is just about anywhere) - it usually pushes the natural harmonic frequency of the crank and bottom end up in the spectrum and - hopefully !!! - outside of the rev range that you'll be spinning at.

 

What I'm on about here is Harmonics that are made in any engine and transmitted in to the crank when running. These harmonics are the 'forces' that want to tear the crank apart. At some RPMs they almost cancel out and have near no effect, but at others (and on load), they 'add up' and can become destructive.

 

This GIF shows what's going on in a crank

 

 

Just about anything you can do to lighten the bottom end will usually also extend the bottom end and in particular, crank life.

[carbon]

I like your illustration of the crank at 6,500rpm.

 

Shows why the harmonic damper is ultra critical for crank at high rpm...

[Spider]

Shows why the harmonic damper is ultra critical for crank at high rpm...

 

YES !!!!!

 

(",,,,It's only a 'pulley' ,,,,,,"         )

[Earwax]

Further to the dampener importance:

 

 

I am trying to understand whether these are all super scientific or just scattergun in effectiveness. , and secondly in race application, with all the other modifications happening ( lighter flywheel, bladed crank, lighter rods pistons etc - although all dynamically balanced) do they render the dampener less effective at say that crucual rev range.  

 

 

At the moment , i have used good old Cooper S dampeners and a MED dampener with apparent success up to 8500 revs and sadly the occasional 9000 over revs on heel and toe - but for a road car i think the big weight of a KAD type would be suited also....  but is it luck , i don't know ?  I know in super development categories ( eg NASCAR) or popular (CHEVY V 8) lots of scientific work has gone into these exact variables, But what about the poor old mini with only 3 mains?

 

Anyone with experience , or knowledge of the testing prior to producction of mini crank dampeners, i would appreciate comment .....sorry to OP about this side track

[nicklouse]

i could have said Balancing is more important that a centre strap. which it is.

 

a comment on the commercially available centre straps many are just not worth it poor materials and poor finishing.

 

I just like them as a belt and braces approach for a engine that does not hold high revs for a prolonged time. a circuit car is a different kettle of fish.

[grizzler73]

No no,side track away! that was my next question. I have heard good things about the MED damper, that was going to be my choice.

[Dusky]

Chevies only have 5 main bearings, so were not off too bad
The fluid filled dampers sound like they are the best. But recently (2 days ago) I Saw someone with a fluid damper where the fluid just solidified after X years of use. Another problem arrises...

[hhhh]

Well as long as the fluid solidified while the engine was running at worst vibrational RPM, it's all good.

[Spider]

Ideally, the Damper should be tuned to the whole engine bottom end, more so with the 'Metalstic' steel / rubber types that most of our cars came with, however the fluid types are much more 'universal' in their application.

 

This graph may give some indication as to just what that pulley (harmonic damper) really does and how effective it can be

 

 

 

A is without and B is with a correctly tuned Damper.

[Earwax]

great graph spider,..   so do we assume good old BMC had the original metal/rubber ones 'tuned' and the aftermarket ones newly tuned by damper specialists?  i have yet to see one of these sorts of graphs come with the Aftermarket ones ( even if confidential due to the proprietary nature of the work , it would make me feel better if it said it was designed in conjunction with XXYYYY etc company

 

By thje way , each time i see the Trademark / watermark of the airborne moke, i ask myself is it a smile on the driver's face or the gritted teeth of  terror when realising his run up was a bit too hot for this dune..

[Spider]

great graph spider,..   so do we assume good old BMC had the original metal/rubber ones 'tuned' and the aftermarket ones newly tuned by damper specialists?  i have yet to see one of these sorts of graphs come with the Aftermarket ones ( even if confidential due to the proprietary nature of the work , it would make me feel better if it said it was designed in conjunction with XXYYYY etc company

 

By thje way , each time i see the Trademark / watermark of the airborne moke, i ask myself is it a smile on the driver's face or the gritted teeth of  terror when realising his run up was a bit too hot for this dune..

 

Cheers.

 

I think it fair to say that BMC did them well and appropriate for the engines they produced. Clearly too, with what was going out their doors, in those states of tune, it wasn't an overly critical component or aspect as essentially, the same Damper was fitted from the 998 Cooper Engines though to the 1275's (noting the early Coopers S's had a different damper), so I draw from this that the bottom ends are in some respects, over-engineered. That's also clear that we can soup them up and that take it well.

 

I think it could be a minefield for after-market types and the equipment needed to test for this is rather elaborate and expensive, so just how they are being designed, I'm unsure, but I'm also of little doubt there's some clever people among them. I wouldn't consider my self any kind of guru in this aspect of engines, however it is an aspect I understand and appreciate needs to be addressed. eg, I am aware that 1275 Cranks don't last too well with either a failed Damper or without one altogether.

 

I know the Romac ones are very good and I am led to believe those from KAD are also good.

 

The Flying Moke - It's a brilliant Photo for sure. There's a story behind it which is being worked on, just don't hold your breath!!

[timmy850]

So it it worth reusing an old crank damper? I have a couple but some have slight degradation to the rubber. Can the rubber be replaced with some new sikaflex or similar?