matching bottom end and engine head

Hiya fellas!

I'm putting an engine together to fit my rabbit mk1, but lately I got a little confused.. About combustion chamber, pistons and compression.

I'm using the golf mkIII/passat bII/bIII bottom end AGG of 1984cc and longer stroke, known as a tall block. Together with the block I planned to use a hydraulic engine head with 40/33mm valves, ported and a 276 degree camshaft. (I'm not sure, but the head is probably late mk1 gti or mk2 gti).

But when I came to think about the compression ratio I got the idea that the combustion chamber will be too large, with low compression as a result.. ? This because of the pistons with a lower part in the centre (see photos) combined with a head with combustion chambers..

Does anyone know for sure what I will get in compression ratio? Or is the an easy method to get the exact numbers? Cause I don't intend to build a supercharged engine, I would rather have compression around 10:1 or so. I've been told that the AGG originally comes with 9,6:1

But skimming the engine head could cause disaster not knowing the present compression ratio.

Pretty please, put your clever heads together

The cylinder with piston in top position


The engine head combustion chambers

You need to know:

The compression ratio and displacement of  both engines to start with.

From that, you can work out the combustion chamber volumes.

For instance, 1781cc total displacement.
CR@10:1
Number of cylinders=4

So, 1781/4= 445.25 cc per cylinder.

CR=VS +VC/VC  (vs is swept volume, Vc is clearance or chamber volume).

So, 445.25 + 49/49 =10.086

Cr = what we know it should be for the example given at 10:1, so the chamber volume must be? 49 cc.
That includes, the head volume, the piston dish volume and the gasket volume.

If you change the VC number ( 49 in this case upwards) the cr goes down in the calc and vice versa.

You can work out total engine volume from this as well.

1781+49 X 4=(196) =1977 total engine volume.


Other way to do it is to physically measure the volumes of the chambers, the voulmes of the bowls in the pistons, the head gasket volumes and do it that way.

Hi, looks very scientific the response, I got lost halfway through, in time I think I would understand it.

My main concern though is the two photographs, maybe they are an optical illusion, but are not the pistons going to hit the cylinder head when it is installed, there does not seem to be any clearance in the head when the pistons are at TDC.

Yep.
I noticed the raised section on the piston also.

Needs to do physical checks on piston to valve and piston to head clearance and make sure the piston has at least 50 thou clearance at tdc, otherwise its likely to say "hiya!" to the head or valves or both.

Thanks Overboost for the explanation, rather scientific hehe, but I'll sort it out somehow.. otherwise I could do the measuring and write the numbers here and you'll do the calculations for me

Malcolm, there's a little optical illusion, the "heel" of the piston is even to the cylinder top, then the multilayer steel gasket is pretty thick. The engine bottom and the pistons are both standard and built together in the original AGG-engine. Nothing is altered so it "has to" work.. hehe..

I'll try to measure the volume of the combustion chambers in the engine head. Can I do it with a piece of glass/lexan or whatever with a hole to pour fluid into the head. I just need to find a vessel with ml or smaller grades.

The clearance from top of the raised section to top of the steel gasket is 1,2mm so there should be no worries about the piston hitting the engine head.

Haven't made any progress in the search for my CR or correct measure on the engine heads..

but at least here's another look at the cylinder, note that the gasket will be compressed by the headbolts.

Looking Good ! Can I ask if you had any problems getting new con rod cap bolts?

My experience none at VW, GSF & Eurocarparts.

Or are you refitting yours?. There seems to be arguments for and against. All the manuals say I should replace them.

Malcolm said

Looking Good ! Can I ask if you had any problems getting new con rod cap bolts?

My experience none at VW, GSF & Eurocarparts.

Or are you refitting yours?. There seems to be arguments for and against. All the manuals say I should replace them.

Thanks!

No problem at all getting new con rod bolts, wouldn't refit those.. especially when everything else is new/balanced/lightened etc. Don't know much of ordering from England cause I live in Sweden..

Got mine from www.autodelar.net, £3,5 each (only in swedish).

Otherwise I would contact ELRING, the make most gaskets and bolts for the vw (and other) engines.

Thanks for your help.

Have you had the bores honed at all?
They look far too shiny to me for a good seal.

Glazed is the word that springs to mind.

Only reason im mentioning this is if the rings are new and bores arent youll be building an oil burner and lose compression and power.

You can do the honing yourself to a better standard than a lot of machine shops for a very reasonable cost using a silicon carbide brush hone from a company called "brush research manufacturing", i get mine from a company in the uk called "Pacehigh".
Very good product they are too.

Heres an example of what it looks like when done, i did this on an audi quattro engine i was refurbing.



Absolutely imperative the bores are scrubbed with hot water and detergent afterwards and then cleaned with auto trans fluid to remove the honing debris.

The bores lookes glazed just because they're oiled up for the pistons to slide in easy, and I don't want to risk any rust growing before I assemble the engine. Before I fit the engine head I'll try to remove any abundant oil. They were honed by the previuos owner and it looked really good.

I tried to measure the total CC today with a pump and a plate of glass.

Theese are the numbers i got

total volume 1984cc
Bore: 82,5
stroke: 92,8

volume of piston dish: 20cc
volume of engine head: 29cc
volume of gasket: 11cc
Total combustion chamber: 60cc

1984/4 = 496
496 + 60 / 60 = 9,2666 = CR = 9,26:1

Which is far to low.. the original CR in AGG engine is 9,6:1

So how much should I skim the engine head to come to somewhere between 10:1 and 11:1 ? Will be run on 98 oktane fuel only.

may i ask why the oil breather resovoir ?  ( i think thats what its called ) is plated up on one of the engine pictures ?

I've running the same conversion but with a mechanical head, no issues with lowered CR. The breather on the 2L blocks is from behind this plate but via the head on 1.8 so is no longer required in this conversion.

looking a little info on here sorry for the high jack

Cero what head are you using as you said your using a mechanical head not a efi head

ive used a 1.8 head with a carb on it but the CR is far to high for some reason turns by hand fine but im having a inkling that it may be the starter aswell as it struggles to turn over off the starter    i have the 2.0gti starter to try also heard that a 1.9tdi one may  be alot powerfull?

Cero said

I've running the same conversion but with a mechanical head, no issues with lowered CR. The breather on the 2L blocks is from behind this plate but via the head on 1.8 so is no longer required in this conversion.

I wondered this too.

Good thing about having a block off plate is it can be easily drilled/tapped for a hose tail for easy catch/breather can fitment  8)

so it would be a bad idea to plate up my 1.8 16v block and get rid of the oil resovoir ? i thought the oil res just returned oil to the crank not sure if im correct

The point is to allows oil/water fumes from hot crankcase to recirc through the intake system and get burnt by the engine, rather than pressurise the crank case and push oil out of seals it shouldn't be coming out of.

Breather tank/can/coke bottle just means that the fumes are captured elsewhere rather than being introduced to intake.

There's probably a better/more accurate description than that but that's the gist of it.

Block it off completely and you'll more than likely just end up with a leaky engine.