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HarryB

Too much compression?

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HarryB

Hi guys, I love this forum. I've been using this nearly 10 years to learn more about my 205 gti. 

 

The problem is that I have made a mistake. I'm assembling an engine using a 1.9 DKZ block, liners and crank with 1.6 B6D ported and skimmed head. Along with that goes Kent cams PT36 sports injection cam (270degrees, 11.63mm lift), double valve springs and competition followers. The mistake happened with the pistons. I assembled and installed them using the pistons from the 1.6 and rods from the DKZ. I thought that I don't want to lose compression with the deep dishes of the DKZ. What I didn't know is that the 1.6 pistons come 2mm higher than the 1.9 pistons normally would (true?). 

 

So the head being skimmed, I measured the chamber volume using a syringe and plexiglass to approximately 29-30cc.
With the following numbers:
Bore 83mm
Stroke 88mm
Gasket diameter 84mm
Gasket thickness 1.5mm
Piston dish volume 9cc
Pistons below deck at TDC 0.2mm (I this real? Or does the head squeeze the liners so that this disappears?)

...I get the CR of 11.05:1 (correct?)

 

What should I do? Is that too much compression? The car is for daily driving and occasional fun. Thicker head gasket? Change the pistons back? Put the head on and start driving?
 

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petert

Sounds ok to me, if you have mappable ignition and 98RON fuel.

Edited by petert

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DamirGTI

Kinda same as mine except the cam ... i'm on a 26cc combustion chamber volume , and CR over 11.3 .

 

100 RON fuel , but kinda struggling with ignition ... as Peter says , highly recommended go mappable ignition it'll be best + you'll pull more power/torque from such spec. engine .

 

D

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HarryB

How much do gaskets compress? The gasket that came with my kit is 1.65-1.75 thick, but I can't find any info whats the official thickness. If I play with the calculator the final CR seems very sensitive to differences there.

I measured the dish volume on the piston and liner and that was a total of 8cc so I should count the CR with the piston deck clearance at 0.

11.51...

 

Feels like I should retreat to a 1.8mm gasket

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DamirGTI

I'm crap with math/numbers , Peter will know !

 

Compressed thickness goes around steady 1mm +/- 0.50mm .

 

Thicker "repair" gaskets are usually 1.4/1.5mm , normal/std. are 1.2/1.3mm .. i run thinner 1.2mm on mine (to retain lower squish clearance) .

Not much point nor help tearing it apart just to fit "thicker" gasket ...

 

With the properly adjusted ignition , mappable ignition , on higher octane fuel (98 is enough) CR in excess of 11.1 wont be a problem .. only what i have found with mine , oil and coolant temp. tends to get a bit high when you press on a bit faster or on long trips/drive at autobahn (not so much the coolant , but the oil certainly does heat up considerably).. during the summer hotter ambient temps. especially .. thus fitting proper oil cooler (with thermostat) would be good idea , and cooler grade water/coolant thermostat and fan thermoswitch combination .

 

D

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HarryB

Thank you, really appreciate your replies.

The head is still on the table so fitting the gasket is the next step. I have two gaskets. One that was the original plan, says 1.3mm on the sticker and measures 1.4mm with the calipers. Other one came with the gasket/seal -kit, ~1.65mm with calipers. So the gasket must squeeze about 0.1mm when installed. I think.

 

With your reassuring advice I'll put on the thicker one and not order any more gaskets. And I'll stop worrying also.

Ignition mapping solution is still on the drawing board. I have a dyno-owner contact who said he should be able to help with that, possibly even using the original ECU, but first I'll get the engine in the car. I have no idea whether one can really remap a 30+ year old ECU.

 

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welshpug

if its a motronic ecu then yes, the jetronic ecu no!  they dont have programmable chips in them!

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petert

Jetronic just needs stiffer springs in dizzy. Quite simple really.

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HarryB

Ok so no remap for me then. I'm pretty sure I have jetronic. There are no sensors in the exhaust. The car was originally 1.6 Gti. Year -88 no cat.

 

Adjusting the ignition mechanically sounds awesome. Just need to understand better how it would work. Do you guys have any links to a guide or a thread?

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HarryB

Getting slightly offtopic, but the distributor is 0237009063 Bosch and when I started opening it out came this guy in the photo. Copper and seems very worn. Pencil for size comparison. Does this mean that I need a new distributor?

20211105_151227.jpg

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SRDT

You just need a new rotor, and most likely a new cap too.

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HarryB

Ah yes, that is a contact end of a rotor arm. Funny thing is that I had new cap and rotor already some 20k ago installed. And even the previous one was just worn, not broken. So this must be a piece of some earlier rotor that somehow had flown inside the distributor when it broke :D.

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DamirGTI
13 hours ago, HarryB said:

Adjusting the ignition mechanically sounds awesome. Just need to understand better how it would work. Do you guys have any links to a guide or a thread?

 

Some info here :

 

 

 

And here how to adjust the springs (scroll down a bit you'll see pictures) :

 

 

 

Or , send the distributor to the : https://www.h-h-ignitionsolutions.co.uk/

Write all the engine mods/specs on a piece of paper , and pack it up with the distributor , they'll then re-adjust the advance curve to suit such modified engine . 

 

But really , if money isn't the limiting factor , and especially if you have there someone who knows how to properly adjust/remap ignition i'd highly recommend digital mappable ignition over mechanical/modified distributor . 

 

D

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petert

DamirGTI is right, in that mapped aftermarket ignition is the best solution. However, if you’re handy enough mechanically, it’s easy to swap the springs and use a timing light to see what you’ve done.  Google distributor advance springs. 

 

How will you adjust the fueling?

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HarryB

Thanks Damir! Great stuff. I'll have a look at those.

The plan was to do the injection adjustments on the dyno. Nut now, if I'll adjust the ignition timing myself I'm considering just taking it to a workshop so that they can adjust and do the official emission measurement also. My understanding is that beyond idle the air flow sensor is linear so that if its adjusted properly at low loads it should be fine through the whole range.
 

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DamirGTI

Can fit adjustable fuel pressure regulator , and up the pressure + 0.5 Bar .

 

Or adjust the mixture via AFM , but you'll need someone with either WB (wideband) sensor and gauge or exhaust gas analyzer .

 

D

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Richard309Sri XU5JA 205GTi

 

this also seems a good posting on the general subject, if not found already 

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HarryB

Thanks everyone, I'll look into these links.

 

And I forgot to mention that I have the yellow injectors that have part number ending 762. Those should be bigger flow than the 1.6 originals.

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welshpug

best bet is to bin the whole factory system and get a proper mappable ecu on it.

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HarryB
4 hours ago, welshpug said:

best bet is to bin the whole factory system and get a proper mappable ecu on it.

Yeah, this is starting to look tempting also. That would just ruin my budget.

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welshpug

i doubt you will be that short once you faff with getting a new afm, get the ignition curve adjusted, rolling road time to tune it..

 

flapper type afm also reall dont get along with longer overlap cams, been there done that :lol:

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