Standard Pistons @ 11.5-1 Ratio?

[edbar 6]

Money is getting tight on the rebuild and need to know or at least have an idea of if anyone has done it and if they have had trouble? Have a newman 300 degree cam/ported head. Wanted to get rods and pistons at next years freshen up so could do without the expense even willing to sacrifice a few revs. Cheers, Edd.

[petert 588]

I've raced Mi16 pistons at 11.5:1. No problems at all.

[Andy F 0]

hmm interesting how were you planning to this??

 

Thinner head gasket or skimming the head more or decking the block?

[petert 588]

It was a 16V block, decked and 8V head. Possibly a thinner gasket. I can't recall.

[edbar 6]

Most probably head skim and decking.Leave that to the machine shop to decide best route.

[welshpug 1,661]

if its a 1.9 you could fit 1.6 pistons, I've not yet found exactly what compression that would give, but it saves skimming the head and makes it far easier to set the cam timing as it wont be far off the pinned position unlike when you skim the head a lot, its nowhere near!

[Saveit 0]

Will the timing be wrong if you skim a head? I am going to put an XU10 head on my 1.9 GTI, but what about the timing then? I have to skim the head to keep the compression up with the new head..

[philfingers 5]

you'd probably need to run offset keys or a vernier pulley to get it right.

[Saveit 0]

you'd probably need to run offset keys or a vernier pulley to get it right.

 

Oh thats bad news.

[philfingers 5]

offset keys shouldn't be much. Maybe someone else can comment, depends on how much time you've invested in this and how much you want out of it. Also how much you've taken off the head! Effectively you've shortened the distance between the pulley, so instead of needing maybe 50 teeth on the belt you may need 49.5 which obviously you can't move the belt 1/2 a tooth so you may need a method of setting the timing. If you know how much has been taken off then maybe some one will say yep you need a *****

[petert 588]

8V

1mm off the head needs approx. 4 deg. of advance (crank). If you also install a bigger cam at the same you need to advance it more. This could be 6-7 degrees depending on the cam grind.

 

I assemble the head/cam/belt first, then measure how much valve lift there is at TDC. I then work out how much the cam needs to be advanced to get the desired lift @TDC, in crankshaft degrees. Halve this figure for camshaft degrees.

 

The location pin is then removed from the pulley and the hole drilled oversize to accept an offset pin, turned to the required offset from above in camshaft degrees.

 

It's a little more tricky than a 16V offset woodruff key, but a fraction of the price of a vernier pulley.

 

Many folks skim a head, then loose what ever performance they've gained by the camshaft being retarded.

 

I can do exchange 8V pulleys if you tell me how much advance you want.

 

16V

Theoretically on a 16V, any deck or head skimming should be equalized because there's two pulleys. In reality, for a big skim (eg 1+mm off the block), this is not the case as it's not possible to pull enough tension on the inlet side. Thus most of the tension is taken up on the exhaust side, retarding the cam timing.

Edited January 16, 2009 by petert

[Saveit 0]

Hi Peter

 

I am going to put an 8v head on from a XU10J2 engine on my 1.9 GTI. And i am going with a really aggressive cam (Kent PT27) of 304* and high lift. I am going to skim the head 1.7mm to raise the CR sine the CR will decrease if you just put a standard XU10J2 head on without skimming it.

 

Now i dont quite understand what you mean? Do you perhaps have some pictures of this location pin and so on?

[ifcho 0]

How do you measure the valve lift at TDC with assembled engine

[welshpug 1,661]

16V

Theoretically on a 16V, any deck or head skimming should be equalized because there's two pulleys. In reality, for a big skim (eg 1+mm off the block), this is not the case as it's not possible to pull enough tension on the inlet side. Thus most of the tension is taken up on the exhaust side, retarding the cam timing.

 

unless you're talking R or RS heads, which use the adjustable pulleys which would negate that problem (if they have enough adjustment that is)

[28CRAIG 2]

Will the timing be wrong if you skim a head? I am going to put an XU10 head on my 1.9 GTI, but what about the timing then? I have to skim the head to keep the compression up with the new head..

 

I have put a XU10 head on mine with a 1.6mm skim and the cam timing was out and the cam belt was working lose but been fine since fitting a vernier pulley. The other problem with high cr is the ignition curve is all wrong on the standard dizzy and i lost 8bhp and a load of torque by fitting the XU10 head but is has run fine for over 25k.

 

Not sure on my exact compression ratio but is between 240-250 psi across all four on std pistons.

[petert 588]

How do you measure the valve lift at TDC with assembled engine

 

With a dial indicator guage & magnetic base.

 

Here's the pin on an early 8V pulley. It can be removed. On later versions the pin is part of the casting but can still be removed by drilling from the back.

[edbar 6]

I have a catcam pulley and my engine builder will be using a dial gauge so i shouldnt have any problems as he will use his own timing figure anyway. I was more concerned with the pistons breaking up at revs and making a mess?

[tony perks 1]

I use std (perfect circle) pistons in my rally engine 1.9 Mi 16 pistons, using oops up to 8500rpm, now need to fit new springs as theyve gone a bit soft , no problems to date the only thing I find is the rings we get at work not the OE ones do tend to hammer the lands a bit more than the goertze type rings.

[BackStreetRacecars 3]

With a dial indicator guage & magnetic base.

 

Here's the pin on an early 8V pulley. It can be removed. On later versions the pin is part of the casting but can still be removed by drilling from the back.

 

 

When you remover the pin do you then set up the timing and find the new home for the pin,drill and replace? or do you rely on the bolt being tight and trusting threadlok?

 

maybe i'm asking a very stupid question!!

[petert 588]

A new stepped pin goes into an enlarged hole. Because the washer covers the pin it can't fall out.