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Post by scottydugg on Jun 21, 2019 15:51:19 GMT
I've got my 1.8 BP4W head booked in for a skim in a week or so, but in all honesty I'm not sure how much to go for. From what I've read 40 thou keeps it non interference, but can require adjustable cam gears? I've already sunk enough into this "little" project so can anyone confirm what skim I can get away with on standard cam gears, without the engine throwing a fit/running badly. I don't want to Canoe it up
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Post by Dweenimus on Jun 21, 2019 16:03:18 GMT
Sorry for the lack of help but how much is the skim costing? For future reference
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Post by dickie on Jun 21, 2019 17:24:54 GMT
You will gain performance by having higher compression, and lose performance by altering the timing. You really need adjustable timing gears if you skim over the Mazda specifications. I understand that it is possible to re drill the timing wheels or move the pegs in the camshaft.( not an easy job to do correctly ) Offset dowels have been used for some engines. Is your machinist a tuning shop ? if so he should be able to advise you for the best.
It is easy to gain 10.5 -1 compression with a Mk 2.5 Block Swap and use your existing head and sensors. Its a case of how far do you want to go on what budget.
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Post by Zed. on Jun 21, 2019 17:28:54 GMT
What do you want from the car? What do you use the car for? What supporting mods have you done? What do you expect from said mods? (ie, will a skim become a mod or as part of a whole?) if you want a noticable performance increase a skim can work towards this BUT also re-seating the valves will help increase performance by restoring the cylinder heads capabilities to keep the combustion in the cylinder when you skim the head, the camshafts are moved closer to the cylinderblock & the timing belt length between the crankshaft & camshafts is shortened which retards the cam/crank timing. it's debatable if you will notice this retarding with small 'clean-up' skims especially as you can set the 'CAS' to the Mazda setitngs for the ecu to run as designed you can modify the standard camshaft pulleys to 'adjust' for the skim with a file (file the 'slot' that the camshaft's alignment 'pin' locates to allow it to index an ammount clockwise. - important note being that the skim thickness does not relate to the ammount of cam-retard / advance correction so 0.020" skim may be 0.005" to file the slot to re-time.... FOR EXAMPLE!) The above means nothing if the pistons / bores are worn & the cylinder / combustion pressures vent into the sump Rich.
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jon
Chats A Lot
Posts: 270
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Post by jon on Jun 21, 2019 17:40:13 GMT
My two penneth...
Do you care about originality?
If I was refreshing an early mark1 engine and didn't care about keeping it that original I'd find a mk2 head and a mk2.5 (square top) intake manifold. Will flow better than an early head even with a lot of head work done on it, can rev higher, bolts straight on and is cheap and available.
If I had an aftermarket ECU I'd prob go the whole hog and fit the mk2.5 VVT head, but I don't think you list one in your spec...
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Post by wannabe on Jun 21, 2019 20:10:47 GMT
when you skim the head, the camshafts are moved closer to the cylinderblock & the timing belt length between the crankshaft & camshafts is shortened which retards the cam/crank timing. I don't understand this and it makes my brain hurt lol In my head, if the cams and pistons are correctly timed (e.g. no.1 piston at TDC and both inlet and exhaust valves closed on that cylinder) what does it matter how short the belt is between crank pulley and cam pulley? Surely they are now fixed/timed to each other and will just both rotate together, regardless of distance between them?? Or is it that a toothed belt means the cam pulley only has so many positions it can be in in relation to the crank pulley, and if half a tooth is needed to get the cams correct, but it's (obviously) not possible, the cam pulley/cam will be up to half a tooth out? At which point adjustable cam pulleys will enable that half a tooth to be recovered? Which is effectively the expensive version of your DIY adjustment, as below? you can modify the standard camshaft pulleys to 'adjust' for the skim with a file (file the 'slot' that the camshaft's alignment 'pin' locates to allow it to index an ammount clockwise. - important note being that the skim thickness does not relate to the ammount of cam-retard / advance correction so 0.020" skim may be 0.005" to file the slot to re-time....
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Post by wannabe on Jun 21, 2019 20:21:20 GMT
re: skimming, I think measuring the head before taking anything off will be a good idea, lol. 35thou is a 'safe' skim with benefits, AIUI, so if it's already had a skim, you might not need to take much off! IIRC 40thou is still 'safe' but could be borderline in terms of non-interference. Mine appears to be 50thou+ so I'll let you know how mine goes. LoL
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Post by Zed. on Jun 21, 2019 20:56:14 GMT
when you skim the head, the camshafts are moved closer to the cylinderblock & the timing belt length between the EXHAUST CAMSHAFT & crankshaft (THE TENSION SIDE) is shortened which retards the cam/crank timing. I don't understand this and it makes my brain hurt lol In my head, if the cams and pistons are correctly timed (e.g. no.1 piston at TDC and both inlet and exhaust valves closed on that cylinder) what does it matter how short the belt is between crank pulley and cam pulley? Surely they are now fixed/timed to each other and will just both rotate together, regardless of distance between them?? Or is it that a toothed belt means the cam pulley only has so many positions it can be in in relation to the crank pulley, and if half a tooth is needed to get the cams correct, but it's (obviously) not possible, the cam pulley/cam will be up to half a tooth out? At which point adjustable cam pulleys will enable that half a tooth to be recovered? Which is effectively the expensive version of your DIY adjustment, as below? you can modify the standard camshaft pulleys to 'adjust' for the skim with a file (file the 'slot' that the camshaft's alignment 'pin' locates to allow it to index an ammount clockwise. - important note being that the skim thickness does not relate to the ammount of cam-retard / advance correction so 0.020" skim may be 0.005" to file the slot to re-time.... Sorry, I'd written it slightly incorrectly Edited now SORRY as to the rest, yup also, due to manufacturing tolerences, engines can be dimensionally perfect or on the outer edges of the tolerences with most in between (some cars are faster, some better on fuel etc.) so theres no garantee that a new genuine factory mass-produced engine is perfectly timed etc. Rich. (dislexic)
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Post by scottydugg on Jun 21, 2019 21:54:12 GMT
Sorry for the lack of help but how much is the skim costing? For future reference It's 60 quid I've been quote, with me stripping the head down. You will gain performance by having higher compression, and lose performance by altering the timing. You really need adjustable timing gears if you skim over the Mazda specifications. I understand that it is possible to re drill the timing wheels or move the pegs in the camshaft.( not an easy job to do correctly ) Offset dowels have been used for some engines. Is your machinist a tuning shop ? if so he should be able to advise you for the best. It is easy to gain 10.5 -1 compression with a Mk 2.5 Block Swap and use your existing head and sensors. Its a case of how far do you want to go on what budget. He hasn't done an mx5 head before m, but I'll go with whatever he recommends. How easy are adjustable gears to dial in? I've tried searching and can't find a clear guide. What do you want from the car? What do you use the car for? What supporting mods have you done? What do you expect from said mods? (ie, will a skim become a mod or as part of a whole?) if you want a noticable performance increase a skim can work towards this BUT also re-seating the valves will help increase performance by restoring the cylinder heads capabilities to keep the combustion in the cylinder when you skim the head, the camshafts are moved closer to the cylinderblock & the timing belt length between the crankshaft & camshafts is shortened which retards the cam/crank timing. it's debatable if you will notice this retarding with small 'clean-up' skims especially as you can set the 'CAS' to the Mazda setitngs for the ecu to run as designed you can modify the standard camshaft pulleys to 'adjust' for the skim with a file (file the 'slot' that the camshaft's alignment 'pin' locates to allow it to index an ammount clockwise. - important note being that the skim thickness does not relate to the ammount of cam-retard / advance correction so 0.020" skim may be 0.005" to file the slot to re-time.... FOR EXAMPLE!) The above means nothing if the pistons / bores are worn & the cylinder / combustion pressures vent into the sump Rich. Thanks Rich, car has randal intake, arc chamber, ful exhaust. Swapping the head and inlet chamber are next. Then eventually it's forced induction or itb. If I could find out what to do with adjustable gears and it's not a fortune to tune them in then maybe, but I'm already further into this than I wanted. Cars a daily/weekend car that I want to autocross/track eventually. Skims part of development of the car fitting the updated head. My two penneth... Do you care about originality? If I was refreshing an early mark1 engine and didn't care about keeping it that original I'd find a mk2 head and a mk2.5 (square top) intake manifold. Will flow better than an early head even with a lot of head work done on it, can rev higher, bolts straight on and is cheap and available. If I had an aftermarket ECU I'd prob go the whole hog and fit the mk2.5 VVT head, but I don't think you list one in your spec... Originality is already long gone, 2.5 inlet mani is on the cards to avoid VICS. Aftermarket ECU is a long way off for now.
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jon
Chats A Lot
Posts: 270
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Post by jon on Jun 21, 2019 22:08:55 GMT
Actually, just realised the BP4W is a mk2 head, doh..
VICS is supposed to be better for midrange, square top for high end at the loss of a little bit of mid.
I've just swapped from VICS to ST and can't say I notice much difference midrange, but over about 6000rpm I had to add quite a bit of extra fuel...
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Post by scottydugg on Jun 21, 2019 22:18:46 GMT
re: skimming, I think measuring the head before taking anything off will be a good idea, lol. 35thou is a 'safe' skim with benefits, AIUI, so if it's already had a skim, you might not need to take much off! IIRC 40thou is still 'safe' but could be borderline in terms of non-interference. Mine appears to be 50thou+ so I'll let you know how mine goes. LoL My limited knowledge, it looks like it hasn't been skimmed already. Do we know what the original dimension should be?
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Post by dadbif on Jun 22, 2019 9:38:19 GMT
If you are intending to go for turbo or supercharger in the future the last thing you need is a higher compression engine..
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Post by wannabe on Jun 22, 2019 11:29:04 GMT
This is a good point ^^^ re: skimming, I think measuring the head before taking anything off will be a good idea, lol. 35thou is a 'safe' skim with benefits, AIUI, so if it's already had a skim, you might not need to take much off! IIRC 40thou is still 'safe' but could be borderline in terms of non-interference. Mine appears to be 50thou+ so I'll let you know how mine goes. LoL My limited knowledge, it looks like it hasn't been skimmed already. Do we know what the original dimension should be? This is my thread with measurements: mighty5s.com/post/40525/threadTo copy and paste for ease, "The workshop manual I have says (page B-48) that head height/depth should be 133.8 to 134.0mm (5.268-5.276in), with max grind being 0.20mm (0.008in).", although that's an early 1.6 so I'm not sure if the 1.8 is different.
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Post by wannabe on Jun 22, 2019 11:45:21 GMT
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Post by wannabe on Jun 22, 2019 11:47:13 GMT
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