skimmed head and cambelt

[mr b]

Heared somewere that if you skim the head on a 1.4spi you have to fit a rover cambelt (one tooth less) to bring the timing back in line any one know

[mowgli]

a standard skim takes about 1/20 of a mm off the head face, you'd need to be skimming massive amounts off to need a shorter belt...

and to debunk it properly, the tooth spacing would need to be the same to fit the pulleys, so considering you can tension the belt up, a belt with one less tooth on it won't actually be any better than the std one, as the increment that the timing can be adjusted is still only one tooth...

so in conclusion, someone's pulling your plonker

[Pistol Pete]

lol thats better than what colour powerbands!

[Andy]

You only fit the shorter belt if you skim the cambox aswell.

[mr b]

i think you had to skim about 0.5mm off the head and the shorter belt was to give better performance the information came from somebody in autograss maybe josh knows

[MK999]

If you skim enough off of the head, then the tensioner won't have enough give in it to tension the belt, hence using a shorter one, and it doesn't alter the timing as that is defined by the size of the pulleys, not the length of the belt.

The performance comes from the high compression found skimming down the head and making the clearance volume smaller.

[mowgli]

the trouble is that a 0.5mm skim won't be enough to need a shorter belt..

the cambox skimming has long been derided on here, including by me, but it dawned on me that it could be used to limit the movement on the hyd. lifters.. to gain some revs

you would most certainly need a vernier pulley

[garyc]

On the junior specials they run a shorter belt. I don't know the exact figure but I suspect the head can be skimmed up to 2mm and also the block decked to ensure the pistons are flush with the top.

[mr b]

I was right the head is skimmed down to 94mm and to get the belt tension you need to fit the belt from a 1.6 rover
so nobody did pull my plonker not this time any way Dave walker builds them for autograss seems they can get 60bhp from a standard 1.2

[mowgli]

are you planning to race one, or do it for the road?

[John]

i'm guessing this is "ineedasmuchbhpoutofmy1.2soicanbeateveryoneelsewho' scheatinganyway" lol

[mowgli]

60hp sounds crap from a 1.2 with 12:1 cr, i thought 60 was easily surpassable with stu's 1.2 tuning guide..and there wasn't much machining in that guide....

[garyc]

Injection not carbed. Basically they are a blueprinted injection engine so everything is pretty much std.

[mowgli]

i saw the spi bit at the start... the 1200st was 55hp std. (and new)

the autograss engine must be a limited class thing, because i'd expect someone with some ability to get a good 100hp from one for racing if it was unlimited.

[Calamity Josh]

on my race car i use the gates 5070 belt (for the rover 100?) which is a tooth shorter. the absolute minimum cylinder head thickness as per the rule book is 94mm

[Calamity Josh]

I was right the head is skimmed down to 94mm and to get the belt tension you need to fit the belt from a 1.6 rover
so nobody did pull my plonker not this time any way Dave walker builds them for autograss seems they can get 60bhp from a standard 1.2

Mines a top speck dave walker junior special engine, and it's 46 bhp at the flywheel! which is all i'd expect as that was around the original bhp figures and what you can do LEGALLY to the engine is very restricted. this was on a old school non electronic rollers with just a gauge! no well you have "60bhp because my rollers run 15bhp higher lol" never believe the figures people tell you, i've been told someones junior special has 80bhp before! BHP doesn't matter anyway at the end of the day as you can have two engines with identical bhp figures and they'll still drive completely differently

[Calamity Josh]

just to clarify all you are allowed is +0.500 overbore, flywheel, rods and crank balanced, NO porting or polishing and the head skimmed to 94mm, everything else standard. so 60 bhp is impossible

[mowgli]

i'm at a loss, the std 1.2 spi gives about 45hp, so surely the hike in compression would surely have an effect.

[Calamity Josh]

i'm at a loss, the std 1.2 spi gives about 45hp, so surely the hike in compression would surely have an effect.
It doesn't really make any noticeable difference as your skimming the head and then the +0.500 overbore so the compression would nearly balance out.

[mowgli]

Not really, an overbore won't lower the cr, it will slightly raise it (by a tiny amount and not worth arguing about) but a head skim will massively raise it.

[Calamity Josh]

i suppose, anyway the main point is 60bhp is pretty much impossible (inside the rules!!) personally i don't see the reason why people would want to cheat as it ruins the fun for everyone

[mowgli]

Its called motorsport josh, and it comes with the territory

[mr b]

I guess you cant always beleave what people tell you about hp and alot of the rolling roads seem to vary. Are you only allowed to vauxhall 1.2 in your class josh and what age can race it

[Calamity Josh]

yes your only allowed vauxhall 1.2 with the modifications i listed. it's 12-16 but i had the choice to go up into adults or stay in juniors for an extra year at 16 because of when my birthday falls, adults can also race it at some meetings

[mr b]

looks awsome fun shame im too old but love to have ago at building one do they cost much to build

[swedge]

Not really, an overbore won't lower the cr, it will slightly raise it (by a tiny amount and not worth arguing about) but a head skim will massively raise it.

i thought an overbore would lower the cr? less space to get "squashed"

[mowgli]

swedge, the piston area is slightly larger, but the dish is the same size, so if you factor in the flat top of the piston getting slightly larger, it will slightly raise the cr by a tiny amount...

re the over bore... i read in a vizard a series book years back that on mini 850 grass trackers, the 40thou made more power than the 60thou overbore, so frictional losses need to be counted in too

[garyc]

vauxhall OS pistons are compression compensated.

[mowgli]

how? the tops are forged the same, they simply run them off a touch bigger on the lathe

[Alex J]

redtop it, no one would notice lol

[mr b]

if you fitted a 1.2 standard head to the 1.4 would that raise compression

[pikey1986]

redtop it, no one would notice lol

i thought them engines came fitted as standard lol

[MK999]

how? the tops are forged the same, they simply run them off a touch bigger on the lathe

Pistons aren't round, so they can't be machined on a lathe to final size. Compression compensation would be fairly simple just by leaving the crown slightly prouder or machining it slightly more.

[mowgli]

mk999, it is a doddle in engineering terms to machine an oval, just like its easy to machine a cam profile, but just how off round do you think a stock piston is? the piston rings manage to fit them pretty easily.....

[MK999]

Less than 2 thou around the skirt the skirt usually, and they're ground to size: http://www.jp.com.au/Made.html

[mowgli]

so spinning them up on a lathe when hot would have the same effect.... its like boring blocks, it should be done with the block at 100oC with a dummy head fitted.... but it isn't.

[MK999]

Well boring is a rough cut, so it's the honing you want to do hot with a deck plate, but yes.

The bigger engine remanufacturers do it hot, but it's less common in smaller companies. I've never seen a round replacement piston though, they've always been properly manufactured.