Kevin.payne.15 Posted January 22, 2017 Report Share Posted January 22, 2017 Hi all. I took the GT6 to. A. Rolling road to get tuned and was somewhat disappointed to find poor performance being attributed to low engine compression despite having it rebuilt 2 yrs ago The concern is that I may have a 2000 engine rather than a GT6 Engine block number is me63527he and head number is 218225 Any idea what engine this comes from? Also it has domed Pistons that look like they stand proud of the surface so how do I measure the capacity please ? Also what's a decent compression ratio to go for on a lead free conversion? Thanks Kevin Link to comment Share on other sites More sharing options...
Kevin.payne.15 Posted January 22, 2017 Author Report Share Posted January 22, 2017 Ok so following another thread then I've determined that it's a2000 mk2 engine I have So any idea how I work out how to measure domes etc then how much to skim off pls Thanks Kevin Link to comment Share on other sites More sharing options...
steveweblin Posted January 22, 2017 Report Share Posted January 22, 2017 If you had the engine rebuilt I would talk to the rebuilder to find out the spec. What ever the block and head were originally if the engine has had work done on it it likely to be different now. Link to comment Share on other sites More sharing options...
steveweblin Posted January 22, 2017 Report Share Posted January 22, 2017 Why not ask the firm doing the skimming? Link to comment Share on other sites More sharing options...
Pete Lewis Posted January 22, 2017 Report Share Posted January 22, 2017 Not looked up on the data but late mk 2 2000 had a very flat cam pair of grott SU HS4 and lower bhp (88) than any other 2ltr spec Ive got one, doubt I will mess around these days but it will never set the world on fire pete Link to comment Share on other sites More sharing options...
Kevin.payne.15 Posted January 22, 2017 Author Report Share Posted January 22, 2017 Why skim. The compression was low but not leaking so it's basically just a low compression engine for which the performance was unspectacular to say the least Engine rebuilder happy to skim but wanted some volumes and as the engine is in the car it's pretty difficult now to get it to him. Hence my desire to do measurements myself I know for instance that the bore is oversized and can measure diameter but domed Pistons more of a challenge Kevin Link to comment Share on other sites More sharing options...
Kevin.payne.15 Posted January 22, 2017 Author Report Share Posted January 22, 2017 Also. Just remembered. He found that the bottom end had recently been rebuilt so only replaced bearings and concentrated on head but didn't measure compression at that stage Yep 80 bhp is about all I get Kevin Link to comment Share on other sites More sharing options...
AidanT Posted January 22, 2017 Report Share Posted January 22, 2017 My GT6 runs with a mk2 2000 but the engine number ends with a HC. It has standard gt6 ancillaries though and when tested a couple of years ago on a rolling road gave 96 bhp. Not sure if my bhp is achieved from the different set up or the engine being HC Aidan Link to comment Share on other sites More sharing options...
steveweblin Posted January 22, 2017 Report Share Posted January 22, 2017 To measure the capacity of a cylinder with a dome just use a syringe as you would for the head. Run the piston down the bore an exact known distance, then seal up the piston/bore gap with a thin smear of Vaseline. Fit a clear acrylic plastic cover to the bore that has a small drilled hole, and start squirting.... Knowing the volume of the space = height of space x area of space. The volume of the dome is equal to the volume just calculated - the volume squirted in Link to comment Share on other sites More sharing options...
Pete Lewis Posted January 23, 2017 Report Share Posted January 23, 2017 And look at the grooves in the cam front bearing to identify whats fitted the later 2ltr vitesse vintage was reckoned to be the best before emmisions started to de tune things No point in skimming if the cams a 1974 yr and as flat as a pancake pete Link to comment Share on other sites More sharing options...
Clive Posted January 23, 2017 Report Share Posted January 23, 2017 What is worrying me (surely not!) is that the last mk3 GT6 had domed pistons and that head casting. Maybe they were set at a higher CR at the factory. According to this http://triumph2000register.co.uk/?page_id=183 yours "should" have a CR of approx 8.8:1, I thought the laste GT6 may be 9:1? so naot a million miles out. However,a skim to give 9.5 (or even a touch more, say 9.75) and a mk2 vitesse cam would bring the engine to life! Re dome size they are 8.66cc (From http://www.totallytriumph.net/spitfire/skimming_your_head.shtml ) Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 23, 2017 Report Share Posted January 23, 2017 Your Engine is from a 2000 Saloon cir cur 1974. It should have domed pistons and the head 218225 which is common with the 2500PI saloon. The only difference with your engine and the engine fitted to the late GT6 Mk3, from KE10,001 was the camshaft, 306785. The Vitesse Mk2/GT6 Mk2 is the best camshaft to use, 308778. The CR is given as 8.75:1 for your engine and 9:1 is stated for the late GT6, basically the same. Though if someone has fitted flat top pistons and not domed top then the CR will be around 7.5:1. I wonder what the compression reading are for each cylinder as for the 8.75:1 they should be 150-160 PSI. If the engine is low compression the figure would be around 140 PSI. Dave Link to comment Share on other sites More sharing options...
JohnD Posted January 23, 2017 Report Share Posted January 23, 2017 What has not been mentioned above is why Triumph used domed pistons. They used cylinder heads from the same castings for both 2L and 2.5L engines. Both engines had similar compression ratios, +/-1, and at TDC their pistons rose to lie flat on the top of the block (more or less!). But each piston sweeps 333cc and 417cc respectively, so the combustion chamber of the 2L had to be 84cc smaller, or else the CR would be much less. So much more metal had to be removed from the face of the casting to achieve the CR for 2L Some unknown person in production engineering realised that a dome on the piston would make no difference at all to the swept volume but would occupy that extra space in the chamber. The volume of the chamber in the head could be the same for both capacities and Triumph could use the same machined castings for both 2L and 2,5L engines! This saved time and money, for the small expediture of a changed piston design. Brilliant! So, your domed-piston 2000 head has the same thickness as a 2.5l engine, much more than previous models, and has plenty of 'meat' to skim to achieve a higher CR. 10:1 would be a good target, and Clive has already pointed you to the best article on the 'Net about this (IMHO, of course!) John PS I will be most grateful if someone will tell me the figure they get for the dome volume, by measurement. I have arrogantly used that value ever since I calculated it. J 1 Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 23, 2017 Report Share Posted January 23, 2017 John, Its not relevant in this case to state why. By the way the 2 litre has a CR of around 9:1 and the 2.5 litre 9.5:1 when using the same block and head. If the engine is a standard 1974 2000 Saloon type then its only difference with the late GT6 Mk3 is the camshaft. Dave Link to comment Share on other sites More sharing options...
Clive Posted January 23, 2017 Report Share Posted January 23, 2017 I am intrigued why the saloon has a slightly lower CR than the GT6 when the head/block/pistons are supposed to be the same. As I mentioned above, maybe the GT6 got a skim/slightly thinner head at the factory. the "sportier" (not exactly a race cam, but more a mild road cam) used in the mk2 vitesse and mk2/earlier rotoflex mk3 will need a higher CR as "sportier" cams lower the dynamic CR. Not sure I would go for 10:1, but 9.75 would definitely be safe. And .25 on CR is significant. Certainly would be noticeable to many drivers ( the ones who enjoy the sporty characteristics of these engines anyway) Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 23, 2017 Report Share Posted January 23, 2017 Clive, The head is the same on both the 2000 and the late GT6, not skimmed. Why a difference CR was quoted, who knows. The things that reduces the performance by a small amount on the late GT6 compared with the earlier Mk3 and Mk2 were the change to - 1/ A cam camshaft with less overlap but with a higher lift. 2/ Carbs with weaker needles and its now difficult to get richer needles. A change to the Vitesse/GT6 Mk2 carbs and cam works well on this engine. Even just changing the carbs results in a much better response from the engine. Though I should say that changing to the Mk2 carbs does mean reverting back the the smiths valve breather set up. The CR is not really the problems with the domed piston engine. Dave Link to comment Share on other sites More sharing options...
Clive Posted January 23, 2017 Report Share Posted January 23, 2017 I worry about this stuff. I don't know why the factory quotes a different CR, but it does. So it is probably the head (most likely in reality). Anyway, a sportier cam will need a higher CR..... As to carbs, I have an aversion to breathers being piked into the carb body, a surefire way to cause mixture issues. Far better is to use a catch tank, and either vent to atmosphere, or into the air filter. On very high performance engines the breathers are inadequate. Recently saw a race TR5 that had a 1 1/4" breather to over come crankcase pressurisation. That was on an engine build that cost rather more than one might expect, so not an old smoker. Flippin quick too.....(guessing at 200bhp) he left my 150ish bhp spitfire for dead on the straights. I need more power... Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 23, 2017 Report Share Posted January 23, 2017 The reason for reverting to Mk2 carbs is the fact you can't get richer needles for the later carbs and the mixture adjustment is very limited. The Mk2 carbs and needles give a richer mixture right through the RPM range of the engine. Dave Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 23, 2017 Report Share Posted January 23, 2017 I agree with a race cam you need a higher CR. If you are using a catcher tank you also need a crankcase breather. The easy way is to fit an electric fuel pump and use the hole left by the existing pump to bolt a breather pipe to. If this engine has domed pistons then the CR is around 9:1 then no skinning is required as its far off the 9.25:1 of the earlier Mk3. Having had to deal with the same problem a couple of times with cars using this engine changing the the Carbs and Cam back to the earlier version solved problem. Dave Link to comment Share on other sites More sharing options...
JohnD Posted January 23, 2017 Report Share Posted January 23, 2017 John, Its not relevant in this case to state why. By the way the 2 litre has a CR of around 9:1 and the 2.5 litre 9.5:1 when using the same block and head. If the engine is a standard 1974 2000 Saloon type then its only difference with the late GT6 Mk3 is the camshaft. Dave Not relevant? Why is it irrelevant to know the story of the domed pistons, and the reason why? John Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 24, 2017 Report Share Posted January 24, 2017 John, In this case Kevin wanted to know what engine he has as he has been told it is a low compression and that was the reason for the poor performance. We know from the engine number, head number and that it has domed pistons it is not low compression i.e. 7.5:1. The Manufacturers figure for this engine is given as 8.75:1 - 9:1. The earlier GT6 Mk3 had a CR of 9.25:1. From my own experience the lack of performance from this engine is caused by two factors. Firstly, probably mainly, the use of needles that give a lean mixture over the working rev range of the engine. Secondly the change to a cam with less overlap. Though it should be said that the cam in the 2000 saloon engine is the same as the Mk1 GT6 but not the same as the one used in the late GT6 Mk3 which had the same overlap but with greater lift. The CR is not the problem as the whole point of what the manufacturers did to reduce the emissions from the engine was to run on a lean mixture using lean needles and a cam with less overlap. I fully understand your point of view. Dave Link to comment Share on other sites More sharing options...
Clive Posted January 24, 2017 Report Share Posted January 24, 2017 Camshaft profile and CR are linked. Change the cam and it alters the dynamic CR. So use the mk2 type cam and it will effectively reduce the CR even further. (that is why engines with wild cams can run very high CR without damage. Change the cam to a road one and it will cause detonation. I have been there.....and learnt a valuable lesson. Wasn't cheap either.) Of course, fuelling is the other issue. The rolling road *should* have reprofiled the needles for max performance. My 1500 had that done. teh guy took 2 attempts and got a perfect CO reading over the entire rev range. Took him 15 minutes to sort once the car was on the rollers. Car was fantastic after, totally transformed. And next, we haven't covered distributor. That can cause massive power loss ifthe advnace is wrong. Again, needs to be matched to the cam and make sure it is giving full advance. Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 24, 2017 Report Share Posted January 24, 2017 Clive, I agree with all your comments. To get the best out of using a Mk2 cam it is advisable to fit a Mk2 distributor. However, the Mk2 cam works well with the domed piston engine even using the late GT6 Mk3 distributor. Bearing in mind today's fuels its better to have less advance than to much on a road car. I come from a tech background so fully understand that we are dealing with a system and each part is dependent on the other. Its understanding what will improve things and having put into practise in the past got the right results. As interest the late GT6 Mk3 cam may have had less profile but it did have greater lift than either the Mk1 or Mk2 cam. Triumph were trying to make up for the reduction in overlap by increasing the lift. The cam used in the 1500 Spit/Dolomite has the same lift and profile as the, late GT6 Mk3, late TR6 and late 2.5PI Saloon. All these cars got the emission treatment in the 1970's so I am not surprised re-profiling the needles improved things. I agree in the case of the late GT6 re-profiling or richer needles would be the first step in improving the performance. Dave Link to comment Share on other sites More sharing options...
Pete Lewis Posted January 24, 2017 Report Share Posted January 24, 2017 Who said "service with a file" Pete Link to comment Share on other sites More sharing options...
dave.vitesse Posted January 24, 2017 Report Share Posted January 24, 2017 The bloke re-profiling the needles Pete! Dave Link to comment Share on other sites More sharing options...
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