CR calculation GT6 Mk2

[hurricane36]

HI I hope to undertake my engine building in a few months time so have been checking the compression ratio so that I could adjust it if required. Could I have any comments as to the calculations correctness or not as the case might be.

1998 / 6 = 333cc 

Bore with a + 0.020" (0.508mm)   = 75.208mm  radius 75.208/2 = 37.604mm

Stroke 76mm   

compressed head gasket thickness  = 3.5cc

non decked volume = 2.7cc

swept volume 3.1416 x (37.604 x 37.604) x 76 = 337.6234cc

combustion chamber volume (actual arrived by via pippette method) = 40cc

CR calc

(337.6234 +3.5 + 2.7 + 40) / 40 =  9.596 to 1

I was expecting a combustion chamber capacity in the 35 to 38cc range?

Thanks Kevin

 

 

[glang]

Dont the non decked and head gasket volumes also have to be added to the divisor in the final equation? I would have thought they add to the volume left when the piston has moved its full travel which would give a comp ratio of 8.3

[hurricane36]

yes you are correct  I thought something was incorrect thanks obvious when you think about it thanks kevin

[glang]

I see online some sites also include the volume around the piston down to the piston ring which would lower your ratio slightly more away from the book value of 9.25. I take it youll be skimming but whats the plan?

[hurricane36]

Yes I have taken that on board.

Now next question does anyone know if the GT6 dome pistons have the same dimension from the gudgeon pin to the top of the piston at the circumference as the flat top pistons?

I ask this as I was going to use dome top pistons and an alternative cylinder head to adjust the compression ratio.

However if they are shorter to accommodate the dome of 8.6cc to keep the capacity of the engine at 1998cc then the calculations will be the same.

If they are not shorter the engine capacity of the Mk3 GT6 was 1998 - (6 x 8.6) = 1946.4cc

Or am I talking a load of rubbish

Kevin

[RobPearce]

The dome-top pistons don't affect the "capacity" as that's displacement, so governed by bore and stroke only. Compression ratios and combustion chamber sizes don't come into it.

However, the dimension from gudgeon pin to circumference is the same. The dome protrudes into the combustion chamber to allow the use of the deep 2.5L head without getting a stupidly low compression, just as you were thinking of doing.

[hurricane36]

Whats the plan you ask? Well I have owned Triumphs over 50 years and decided that I should use up the special parts I had acquired on a special as my last Triumph so to speak. The car is a Vincent Hurricane (probably the last original to be built as I had had the body a long time!) based on a GT6 Mk2.  Phase 1 of the project is finished (iv been in the workshop over this year staying safe!) and once the restrictions are lifted and I have my jabs I hope to use it over the summer and sort out any issues. Phase 2 is to build a 2ltr Lucas PI engine for it. Again I have all the parts and a few heads to chose from, one being from a 2500S which I understand is the best flowing in standard form, however it is 3.475" in height so getting down to 3.300" for a CR of 9.5 :1 is possible but I understand there is sometimes a problem of core movement that occurred during casting  and I could land up in the water jacket apart from a load of swarf in the workshop. I was therefore considering the possibility of using dome pistons that were fitted to the late GT6 MK3 but this might not be possible too as the head would still need to be machined down and I could land up with no clearance in the head for the dome of the piston. Failing that 3 SUs or Webbers but I really fancy building a 2ltr PI and trying to sort this keeps me busy during these current problems. Sorry I cannot work out how to add a picture to the text showing the car -I'm a valve and steam man!!!!

Kevin

[glang]

Wow cant help much there except that I would have thought the GT6 engine/head with skim is a good basis as it has the bigger valves which Im not sure all the later heads had.

Also I believe 3 SUs is not a good idea as our engines should be thought of as 2 x 3 cylinder units so best is 2 single choke carbs or 3 Webbers with a choke for each cylinder... 

[thescrapman]

I was hoping to have started helping Lewis run his 2-litre PI engine in by now, but circumstances are against us.

Once things free up, I will see if he is available for a trip out.

[hurricane36]

Hi Colin, hopefully we can get together at the Hurdle Makers again soon? Do you know what cylinder head and pistons Lewis has used? and if you fancy a run out in April/May when I can leave home and meet up I'll put the kettle on and you can have a look at the Hurricane and my set up.

Keep safe Kevin   

[hurricane36]

HI, would anyone have a dome top piston I could borrow or they could let me know the diameter of the dome at its widest point and the height of the dome? 

Also the cc for the volume at the top of the piston in an un-decked 2ltr engine along with the cc of a compressed head gasket.

Thanks in anticipation. Kevin