'lo all,

I'm v.close to having all the bits for my engine rebuilt (only took me 9months so far ) and I've got a couple of final questions:

A. what compression ratio to go for? Forged internals with 8.0:1 wossner pistons, target pressure will be 1.3-1.4 constant. I hear from a friend talking with various Italian builders that nowadays all engine builders target for 9.0:1 comp on integrale engines (which I find a bit too much, judging from what I've read and what G. Croft argues) So I'm thinking of sticking to 8.5:1 unless otherwise instructed

B. assuming so, what thickness gasket do I go for? I'm planning to buy one of these pure copper ones (which apparently are reuseable). Going from the stock 1.9mm to a 1.3 or so, how much do I "gain" in comp.ratio? (head will most likely be skimmed slightly so some thou are going to be missed from there as well... )

cheers

V.

Up to you at the end of the day, although i do think 9.0:1 maybe a little too much.

Having said that, here is a list of my head of std compression ratios :

Coupe 20vt - 8.5:1
16vt - 8.0:1
Evo VI - 8.8:1 dropped to 8.5:1 for 2 bar boost
Nissan SX - 9.0:1
Porsche 911 turbo - 9.0:1
Porsche GT2 - 9.4:1

So make of that what you will

The days of low compression + high boost are/should be long gone. We have much better fuels to help prevent detonation, improved charge cooling and then you have water/methanol injection if you so require. I would suggest that 8.5:1 is the minimum so maybe try 8.8:1 VAG engines are running 9.5:1 out of the factory so if you get it mapped well you'll be getting a much more driveable car

the optimum compression ratio depends on the quality of petrol that your running on. High boost heros would like to run low comp and lots of boost. That makes a lazy engine.

If you can get 99/100RON shell or other good quality equivalent fuel and be sure you're always going to use it then I would go for at least 8.5:1. I personally am in favour of 9:1 or higher but that is because I know I will make the same power as 8.5:1 but the car will be transformed off boost/part boost and in terms of responsiveness. No prizes for guessing that I run well above the stock 20VT compression ratio of 8.5:1.
In terms of gasket thickness you should use the thinnest one that is effective as it will improve the squish properties and enable more advance.

If going forged I would suggest reconfiguring the pistons and rods. The 16VT would benefit from a longer rod and shorter piston.
I am also a big believer that you do NOT want forged replicas of the std cast pistons. This is because they are designed to be optimised for the piston manufacturing process (ie. larger than necessary ring lands and in the case of the 16VT a WAY too long skirt). My pistons bear no resemblance to OE pistons, and I spent hours of consulting time with the manufacturer to get the design right.
John

Begbie: thanks for the sum-up

Flea: didn't realise VAG engines run 9.5:1!
Driveabily as in wait wait wait and then whooosh bang to the limiter, right 16VTs are much worse to 20VTs on this respect:(

John: 99RON is widely available down here (my unofficial testing shows that BP and a local firm EKO are much better than the Shell stuff but I've yet to see any official testings).
Regarding reconfiguring rods and pistons that's probably a step too far for me. I've already ordered pauter rods which are standard length.
I agree with what you say regarding forged pistons being exact copies of forged ones. We have one such example with JE doing so for a 20VT. I hope that the Wossners I'll be getting are improved compared to the stock ones with proper sized ringlands and nice short skirts (not coated though and no gapless rings either )

Regarding question B. anyone knows what I'll end up with a 1.3mm gasket compared to the 1.9 stock one?
It seems rather complicated to calculate it on paper as Rsquared*pi*0.05cm is the volume of the extra compression available (diff to the stock 8:1) where stock volume is Rsquared*pi*stroke and I now need to somehow relate these two .

thanks for all your replies!

in regard to question B id say a bit more of a smidge increase in compression

Also engine managment has come a long way able to run closer to the danger zone in terms of detonation. Remeber that the emission targets are getting tougher to meet so the quest for a clean burn is required, which a higher comp allows. But on an IAW ecu I would er on the side of safety, some of it depends on whay you are doing and what boost, racing high boost on pump fuel would indicate a different comp to the situation of a road car spending most of its life off boost on light throttle cruise. Me I'd go the safer route, but may just depend on how deep your pockets are.

rich

Looks like the new S3 2 littre turbo runs 9.8:1 and 1.1 bar boost

kp: I was hoping for .2-.3 increase tbh will try and do my maths (unless of course someone else has done it already )
Rich: true, car will be weekend play thing, plus a bit/a lot of motorway traveling at 4-5K revs which means 0, 0.2bar . Pockets are slowly emptying, so with the stock ECU I'd probably go to 8.5:1 from what I've read from you guys!

The problem now is that I'll probably have to buy another set of pistons (no, haven't bought the Wossners yet, but was about to!) as they are ok designwise (from what I can see, short skirts, coated, ringland looks smaller as well) but they only do them in 8.0:1. Just asked for custom ones, but not sure if the price will be right (wossners were 625euro delivered!)
Any other options in similar prices?
plz don't mention CPS (too expensive, last time I checked!)

cheers

skim the block then the pistons can be any comp ratio you like

is that true? wont I get pistons too high up and in close contact to the valves John?
I thought that comp ratio changes are accomodated on different design (variations in the hollow of the piston) pistons...

Wouldn't happen to have any idea how much we are talking about skimming would you

V.

give me all the dimensions as OE and I will tell you what you need to do. Also re the std gasket size and the alternatives you have as you may be able to do it all with a tighter head gasket.

You will only have to change the piston if there is a risk of valve-piston clash and that is very unlikely at this sort of compression ratio unless you are also using rally-spec cams or something like that. usually to change the piston design they reduce the bowl on the crown but that also affects the squish properties.

Stuff I need to calculate the comp ratios

std bore
std stroke
std volume of cylinder head
std crushed head gasket height
std bore of head gasket (optional)
std comp ratio
std stroked volume (CCs)

target bore
Target crushed head gasket height (or the options you have)
target comp ratio

I can do it without the volume in the head as well as long as you have the crushed head gasket height

Cheers

IIRC i think you need to skim 1mm off the top of the block to raise the CR height to 8.5:1

It would be approx 1mm too

well you could probably do that with a head gasket rather than a skim which is also better from a tuning perspective.

John, thanks very much, but you really want to know too much (don't know half of them )

Cams are going to be C&B maximum road (not race) which according to their literature doesn't need any extra pocketing on the pistons. However I don't know if that is calculated with the stock 1.9mm gasket or a 1mm or less gasket.

However, if indeed a 1mm drop in gasket is moreorless okay, then I can get the pure copper 0.9mm gasket and be done with it! (stock gasket is 1.9mm)

Obviously I haven't got a clue of what thickness the standard gasket is once fitted and squashed in place, compared to the full copper job.

But I can also ask for 0.7mm copper gasket (again not sure when copper gasket thicknesses go silly and there's nothing left in them to do the sealing! (I mean would a 0.5mm gasket be considered safe or stupid?)

God there are so many variables to choose from...

Again any ideas welcomed

This is what I'm guessing - tell me if anything is incorrect

Std bore = 84mm
Std stroke = 90mm
Std gasket height (crushed) = 1.9mm
Std compression ration = 8.0:1

If you overbore to 84.4mm that will bring the compression ratio up to 8.06:1. If you then take 1mm off the head gasket that will give you 8.68:1. If you do not overbore but do use the gasket that will give 8.61:1.
If you use a 0.7mm gasket that would put it up to 8.74:1 without any overbore or 8.81:1 with an overbore of 0.4mm

Hope that gives you a good idea. These are only approx though as to do it properly you need more info...

i vote for low CR and high boost

if i remember correct, the stock head gasket is about 1,7mm

Remember to put copper coat on it or big change it'll leak when cold.


vas, take a look at this, it appears that they have both the 10mm and GpA 12mm uprated head bolts and gaskets at a reasonable price. http://www.lancia.org.uk

Spot on John

pistons are 84 (Stock) so I assume cyl dia is a bit more, no
My forged pistons are going to be 84.4 (so again cyl dia will be a bit more - I remember 7thou clearance mentioned somewhere on Wossner site...)

So can go ahead with the 8.0:1 wossner pistons and get myself a thin 0.7-0.8mm copper gasket and I'm fine

I owe you (and the other guys) many...

V.

Piston sizes are based on the bore they require not the piston size. So an 84.4mm piston is the bore size( 84.4mm) MINUS the clearance required for expansion. I wouldn't mix mm with inches either - you should get a spec sheet from the piston manufacturer which you pass over to the machine shop to bore your block out to. Don't bore before you see that sheet! As if they are using inches the bore might not be quite what you expect...

You must use vernie timing wheels with that cam, esp if you play around with the deck height as you alter the number of teth i.e. the length between the crank pulley and the cam wheels. If you use the cb marks the timing will be out. Dont worry cam timing isnt that difficult to do properly, but you will need a degree wheel and a DTI to do it properly, id also use tripple race springs, but hey thats me I like to do things right (ie not allowed to do it wrong).

Rich

Rich,
not talking about the racing cams, just the maximum road. Pretty sure there wasn't anything mentioned on vernier timing wheels for the maximum road cams by C&B
will have to double check.

Triple springs is something I did consider (briefly).
Does it mean changing anything in the valve setup, do I need 3 new set of springs, or simply add a third in the setup? Any pointers would be really helpful!

cheers

V.

cheers dink!

already called them for the 12mm GrA bolts and trimetalic bearings. Waiting for their reply.
What gasket thickness do you use btw?

V.

does the stock 16vt not have vernier timing anyway?

It truly is a poor cousin to the 20VT

Actually with 1mm difference the timing is probably within manufacturers tolerances anyway. The tolerances are there because you may potentially have to skim up to 2mm off a head over the course of the engines life (and 2-3mm is normally the stated limit).

Forget triple valve springs - get hydraluic lifters like the big boys

Vas

12mm bolts requires you to drill the head out and the block for them to fit

I have the 1,9mm spesso gasket with uprated bolts. No problems with 1,3 bar of boost.



i.c.w. a copper gasket they usually do a wire ring conversion but don't think that can be done on an overbore engine as it's tight already.


Unless you're going to revv the crap out of your engine you won't need triple valve springs.

Vernier pulleys are always usefull

Hydraulic lifters have no place in a high performance engine hence solid lifter conversion - soft 20v

Trust me the right way to time up is with verniers you wont get spot on without them, wrong way and the right way, I dont care what CandB say either. Anal perhaps; I dont care. Triple springs are cheap anyway.

I missed a gear the tell tale went off the scale over 8500rpm!!??

Should be good either way.

Rich

some guys here in the forum compare direct injeted cars/engines with indirect injected cars/engines this is not comparable because with direct injected engine you can run higher comp ratio this is well known.

What you need to know is the Dynamic comp. Ratio!! to fix the stat comp. Ratio!!

With "high" compression ratio 9,5:1 you wont never get +500HP on 100 Oktan fuel but you get broken Ringlands melted pistons on coupe engines.

If we look at the hall off fame we see that the limit is about 400-450hp on a 8,8:1 Engine with standard fuel and maybe waterinjection.

John: relax, we all know you have an inferior engine (who the hell thought that an odd # of pots is a good idea?) and you resort to overstroking them to get some power out of them
Begbie: I know, and I hope it's not a big trouble for my mechanic to do so...
dink: I'm a bit skeptical on the copper gasket, may go for a thin spresso after all!
rich: I'll have a look for vernier pulleys. Assuming I get them, how good my mechanic must be to get the timing right, that's what I wonder! Is there some theory behind the cam timing that is a matter of careful application of certain rules or is it an "ear" thing???

God, I sorted all the "large" and "expensive" pieces (namely pistons, rods, cams) and turns out that the small unimportant ones may end up costing as much as the expensive ones

again thanks for the input.

If your engine was built to rev to 9k then I would agree. The 20VT is short-stroke and the 16VT is long stroke so ideally the 20VT should have the solid lifters and the 16VT should have the hydraulic ones