Improving compression ratio.

[GT6boy]

If there is one lesson to be learn't from trying 'improve' your car it must be this: research, research and then research some more!
Got a very interesting thread going on the dark side currently, which I hope some here will find very interesting :). http://www.sideways-technologies.co.uk/forum/Blah.pl?m-1234209983/
In summary, my 2 litre flat pistoned GT6 was an amalgam of parts from disparate sources. Not in anyway standard, so there-in lies the rub: never assume anything!
Once back on the road after its rebuild, it always had a torque spike. Put this down to TH5 cam.
However, the learning curve has since sequentially highlighted a number of contributory factors, and then finally with the major underlying cause coming to light.

Contributory factors:
Cam incorrectly timed in. Since timed in correctly.
Incorrect distributor springs/weights. Now Megajolt fitted so infinitely tuneable at home.
Phoenix 6-3-1 having uneven primaries.  :-/
Incorrect valves clearances- now set at .017" hot. Not helped by poor after market rockers.

Single obvious cause:
Gasflowed/skimmed head was giving way to low CR!
I had a 2.5S head skimmed from 3.475" to just under the mk2 GT6 depth of 3.300, leavung it at 3.298".
However, at the time not knowing enough about these things, I failed to get a proper understanding of what combustion chamber volume I wanted the work to leave me with.
Turns out after whipping the head off and doing some maths, my actual CR was 8.3/1 :'(!
That would explain the relative lack of performance!(it was still putting out 120bhp on the RR mind 8). The cylinders were spread across 145-150psi.

Last week I have had Sideways Dave doing the restoratative skimming work on my head and he has now bought it up to 9.1/1 now, but because of the large combustion chamber volume and a degree of core shift in the head, I wont be able to raise it any more by further head skimming due to the danger of breaking into the waterways.
Dave has done a marvellous job with this head 8). Talk about attention to detail! I cannot wait to get the car back on the road to quantify the results.

So the moral of the story is make sure you know exactly what the spec of your car is/was, so you know what point you are starting from. Do it once and do it properly!

[Lord Sorbington]

That was a good thread.  I'll be interested to see how much power you gain from having that work done.

[GT6boy]

I'm keen to drive it to say the least! I'd also like to put it back on a RR to see the difference as the torque curve was very pronounced.

[esxefi]

at 8.3/1 cr you could have stuck a supercharger on it. 8)

[GT6boy]

Hmmmm, that occurred to me as well..... That would be a whole new level of expense to do it properly :-/. The whole point of this exercise was achieve the sum of the current parts :).  

[Richard B]

Yes I don't think I would start with a 2500S (3.475) head as basis for a 2000.

Would you have been better off finding a 3.300 or 3.400 head and strarting again?

[GT6boy]

At the time, there was a school of thought that the 2.5S was the best flowing head of the 6 cylinder cars, as they left the factory. Recent research brings this into question.
This particular head has a degree of 'core shift'. The vast bulk of the waterway holes give the impression there is enough scope for another .040" skim, but in the area of one waterway hole, it is painfully thin.
As you suggest Richard, knowing what I know now, I would opt for starting with a 3.300 head which when initially inspected wouldn't have just such 'hidden' gems awaiting to be discovered part way through its 'go-faster' work.... :(

[Glenn]

Out of interest - can anyone tell me where the notion originated that the 2500S head was the best flowing head?  What is the current thinking - sounds like not everyone agrees.

Has anyone got info on how to do "work" to improve the flow of a standard head?  Just, I've recently bought some grinding stones and wouldn't mind having a go!

[99176]

Easily addressed when you 'freshen' the engine Bruce.
Enlarged bores and decked pistons and you'll soon be worrying that the CR is too high.
You're just planning ahead is all.

[esxefi]

Rust_Spot wrote:

Has anyone got info on how to do "work" to improve the flow of a standard head?  Just, I've recently bought some grinding stones and wouldn't mind having a go!

what are you going to use them in?don't bother with an ordinary drill as the speed is way to low and it will take years.you need a die grinder either air or electric powered.

a carbide burr works really well and will quickly burst through to the waterways if your not paying attention.

really only need to clean up the port surfaces,match manifolds to head ports and gasket holes,careful work  around the valve seat,ie, blending in the radii to the seat and remove any mismatch as a result of coreshift.smooth out the sharp edges in the chamber.

[GT6boy]

99176 wrote:

Easily addressed when you 'freshen' the engine Bruce.
Enlarged bores and decked pistons and you'll soon be worrying that the CR is too high.
You're just planning ahead is all.

Yes, you're right there Keith. The +.030" pistons are sitting .019" below presently, so putting that right would give me 9.5/1. Just a bit of a bummer to have to strip the engine to achieve that- was hoping the head-work alone would 'release the beast!'
I can't wait to get the head back on to drive the car, I feel the increase from 8.3/1 to 9.1/1 should make a noticeable improvement in itself :). That allied to other tidy work in the ports will all add to the potentia for improvement.
Bring on the Trackdays ;D!

[99176]

Doing Cadwell in it Bruce? I'll wait for ya !!

[GT6boy]

99176 wrote:

Doing Cadwell in it Bruce? I'll wait for ya !!

You betcha Keith ;D! I'd like to get a picture of ours car together incidentally. 'Ang on- I'll wait for ya!?! We'll see about that sunshine!!!! What CR are you running- yours and Tim's car seemed pretty comparable performance wise.

[99176]

Can't remember numbers but I know I agonised for ages when building it.
You work it out: 40thou oversize, 8thou pop up. 35cc chambers (standard gasket).
Mild cam, so a good percentage is being used !

[99176]

Edit:   5thou pop up.(Fat finger typing).

[GT6boy]

Right then, well I work that out as 9.8/1 CR. Oh what I wouldn't give for that! It appears you will indeed still be waiting for me!!
I'd be interested to see what your cylinder compression tests readings might be?

[Tim Bancroft]

See you both at Cadwell then!

[99176]

Waiting for you as well then Tim ??

[Tim Bancroft]

How's your gear lever now, Keith?

Bugger type 9 in place-damm!

[GT6boy]

Head back on and torqued down. Managed to improve the car from its wheezy 8.3/1 CR to 9.1/1 before we ran out of material to remove due to core shift and the combustion chamber being too enlarged( :().
Car runs noticeably better now, very bright in comparison to before on pick-up and has a better torque spread- really good improvement on useful torque for slowly traversing hills etc. Very much a case of money well spent. I'd love to get the car up to 10/1, but that wont happen with this 2.5S head.
Anyone got an old mk2 GT6 head they don't want?

[GT6boy]

Incidentally, I got some of the allen key headed sockets to fix the Phoenix 6-3-1 to the head. Talk about making life easier- wish i'd done it years ago! Thanks for the tip Tim :).

[Richard B]

GT6boy wrote:

Anyone got an old mk2 GT6 head they don't want?

Take some pennys to Stoneleigh?

Vitesse MkII is the same.

I suspect a GT6 MkIII pre dome pistons or a MkII Saloon of the same ilk (ie pre ME50000) will be useable.

[Deleted User]

Looks like Mr Sideways has done a great job!!!
He should advertise his services :)

[Dale]

GT6boy wrote:

  Managed to improve the car from its wheezy 8.3/1 CR to 9.1/1......................  
  I'd love to get the car up to 10/1,

Might just be worth saying a brief word here about the importance of 'Effective CR' as opposed to simple numerical CR.

If you have a short duration cam such as the standard (rubbish!) 2500S type then the valves close way earlier than with say a TH5.

This means that the piston is coming up and compressing the mixture with the valves fully closed for much longer with the S cam than with the TH5.

This in turn means that even with a fairly low numerical CR you can still get plenty of torque, albeit very low down (a well driven S will beat most other similar cars away from the lights - even if it is towing a caravan, but only up to about 30 mph - where's the fun in that!), and can also suffer epic pinking to boot.

Conversely if you're fitting a 'proper lumpy' bumpstick with long duration and, oh yeah there's overlap to consider too, you can afford to increase the numerical CR much more than if you're using a cam designed to give more torque in the middle ranges and less out and out oomph at the top end.

Basically it depends what you're building the engine to do and as has undoubtedly been said here and elsewhere before, the importance of matching the head to the cam should not be underestimated.

Similarly you don't want to go 'hogging out' too much material from the head if you're not building a right little screamer as all you'll succeed in doing is losing all your bottom end torque, having something which bogs down off the line and will likely never be able to run cleanly at lower revs due to the velocity of the intake charge being too slow - as when you over-carb an engine.

Complicated innit? If anyone knows the perfect answer please let me know!   ;)

[GT6boy]

Good points Dale- ta for the allen key sockets by the way!
My disappointment with this head was that I gave the guy who did the original gas-flowing/skimming and unleaded work on the 2.5S head, all the exact info about the intended useage. This included cam profile, carb set-up, exhaust system and even that it was a +.030" re-bore with the pistons sitting below the deck .018" :(. You live and learn.