NefMoto

Hi guys, I'm finally going to be starting work on my BT build and I was hoping to get some advice on the matter of CR.
I will try and keep this short and to the point, if any more information is required please let me know.

AUQ Engine - 9.5:1 CR
GTX3071R Gen 2 DBB 0.63
Twin Scroll Manifold
Bosch EV14 1000cc Injectors
U.K Super Unleaded, which I believe is 95-99 RON Depending on where I go.
WMI Eventually

Basically I was under the impression that higher compression was better for off boost response and would give me more low down torque at the cost of some up top timing and boost.
I spent hours reading and I keep coming across posts that say these engines are seriously knock limited and it happens as soon as you go to WOT regardless of whether your spooled up or not.
Is this true? I keep reading things like "lower compression is better everywhere, just add more timing low down"

What compression ratio would you guys recommend for my setup? And should I pick up a set of BAM pistons now before I get my rods and get 144x20's instead or go with a 2.5mm head gasket, or should I get both and get closer to 8.0:1 which seems to be the "sweet spot"?

95RON is pretty much piss water. I'd aim for 8.5 or so.

8.0 might be overkill, though for small turbos you won't lose too much spool or off boost torque.

YMMV, its a debatable topic.

I'm struggling to find a set of pistons on ebay atm, there's a set of BAM's for £80/$112 but there looks to be a few scratches on the skirts looking at the pics so not too happy getting them. Are there any issues with sealing when it comes to thicker head gaskets?

https://www.ebay.co.uk/itm/254961368718?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3D47c4ece03035470aadc33cc3efedb53f%26pid%3D100675%26rk%3D5%26rkt%3D15%26mehot%3Dnone%26sd%3D233994998731%26itm%3D254961368718%26pmt%3D0%26noa%3D1%26pg%3D2380057%26brand%3DAudi&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3A3e5e2a15-b356-11eb-b4bb-3ece3227de6f%7Cparentrq%3A620047021790a9bdee3119b0ffd5a0ff%7Ciid%3A1

I've been looking at a 2.5mm Decompression plate style mls head gasket which is around $80. I know it's not ideal and more costly when it comes to replacing it but as long as it seals it sounds like a good option. It's supposed to drop compression to 8.55:1

https://spaturbousa.com/products/copy-of-vw-vr6-r32-3-2l-24v-3-65mm-decompression-plate-mls-head-gasket

I've had good luck with gaskets on the 2.7t (throttlebody placement movement notwithstanding) but no experience with 1.8t motors.

What about squish effect when you put 2.5 mm head gaket/decompresion plate. Is that too much?

Do you mean it wont squish properly around the water jackets etc because it's too thick? I think I'm going to go for that set of BAM Pistons, I was just a bit worried because the 3rd one down looked a little shabby with a couple of light scratches here and there, I'm hoping they will clean up and be good to go. Worst case scenario I just buy a single 1 for a little extra. I'd much rather go 9.0:1 with a proper set of the BAM pistons rather than experiment with a thicker head gasket. I just figured here is a good place to ask before I buy anything, as I know this forum is full of BT builders that daily these cars and I value your opinions.

I'd also have the option to try the HG or an inbetween OEM and the 2.5mm thickness in the future if I wanted more like 8.5:1 etc

With that turbo 7.5 on 95 RON. But that will start feeling shitty off-boost. You don't want to go lower than 8.0 on stuff driven every day, because below that you start to actually feel the effects of low CR.
8.0 on 99 only.

Any higher won't make power and will run into severe EGT issues.
That turbo will run 2.3+ bar boost on this engine, and since it's port injected you need a very low CR to be able to run that boost on pump fuel. The dynamic compression is just too high.

Also IIRC AUQ is a small port head, you need a big port head from AMK/BAM.

I actually run 10.3 on my converted engine still, but its nothing crazy like you are planning to do, only 0.5bar on t04e. on 95 sometimes 98 ron(european/russian gas). Still got aight timing , ofcourse lambda is decently rich. AEB head with VVT too. Daily for 2 years now.. but changes are coming.

Obviously if you could you'd wanna keep the highest CR you can but if you dont have access to good gas(with wallet ofcourse too) or toulene maybe, then yeah I'd forget and just go for boost/timing with lower CR. oh and get VVT if you dont have it already.

Once I'll start pushing higher levels I am contemplating on what do too, because I might wanna bore the engine to 82.5 because I have a set of pistons made for 20v with 82.5mm with high CR. But I have access to toulene too thats that. Future plans anyways we'll see.

Also I do not think you'd have any issues with squish with lowering plate, the 1.8T headbolts are pretty much gold until like big power territory, so they must have some decent clamping force. VR5T/VR6T run decompression plates without issues AFAIK.


Also a warning SPA shipping is a lot to europe afaik. I believe on ebay you can find thicker decompression HG's for 1.8T's when I checked few years back. Also BAM is a small port head.

I'll be running a maximum of 22 psi until I get everything dialled in anyways. I don't want to start attempting 5120 hacks until I get everything else right, engine bedded in and running stable at a reasonable boost level. I remember reading somewhere that the small port head is good up to 500 bhp and actually helps with bottom end torque. AUQ is also a small port VVT afaik. Bam is small port non-vvt.

The plan is to do injectors, pump, maf housing, exhaust valves and rods. Once I start pushing for big boost and power I'll add things like WMI and valve springs, clutch etc as the money comes in.

My focus was on squish band
https://upload.wikimedia.org/wikipedia/commons/thumb/8/8f/Squish_band.svg/1200px-Squish_band.svg.png

But on 5V engine there is not much squish area, sii I think it wont effect too much on that.

BAM have same CR as any other 1.8T acording to ETKA, so piston crown schould be same? Or I am wrong

Yeah, I forgot, only the AEB and AUQ were big port.
Forget stock clutch, waste of time. And CR needs to be 8.0, so you need to change the pistons as well.

The big port head is simply better for your turbo, if you want one get from ADR(early version, will require 1 cam change atleast), AEB, AGU or AJL(AUQ is small port too yes). Prj said in the past that the big port head is a better design which definitely goes against the grain, tho havent seen concrete comparsion between small and big  I tend to agree, the runners look better into the combustion chamber., also the latest ever 20V head had the big port design too(i think even enlarged, havent measured) wasnt on a 1.8 though :) I think the velocity loss talk came from the fact that most people who put the big port heads havent added vvt lol, since all of them came without VVT, so the loss would be visible vs a small port head with vvt on the cars that have it enabled, or something along those lines I guess.


You are wrong yes, the BAM runs 8.9(~9) CR, the pistons have bigger combustion chamber diameter and lower depth too.

Found a German supplier for decompression plates. They offer a lot of different thicknesses, from 1.5mm-3mm so from 9.27:1 - 8.23:1 Assuming AUQ 9.5:1 starting point.

The effects with the 9.0:1 BAM Pistons would net anywhere from 8.80:1 - 7.87:1. I was thinking of picking up the BAM pistons anyway so I don't have to go too thick on the gasket. Plus if I ever decide to get a set of aftermarket forged pistons, the 20mm wrist pins would probably be better supported.

A good combination looks like BAM Pistons with the 2.5mm gasket, which should net 8.15:1, My head has already had a light skim, so I'm assuming it'll end up slightly higher, maybe 8.2:1 or 8.25:1 etc. Seems like a good compromise

I chose a similar solution in my 2.7 AGB with K04 RS6 Hybrids. It has Mahle pistons with 9.0:1 compression(stock was 9.3), +0.5mm in diameter + 034 Motorsport headgaskets with 0.5 CR drop. Heads(2.8l big port) were already shaved 3 times, so i assume im somewhere around 8.6-8,7CR. I already put 40k km on this setup and it works great.
Moreover, BAM is small port, but it has VVT.

And how about spooling time. I have 2 cars with same turbos, same exhaust manifolds, 1,8t engines. One car is AEB stock bottom and head, compression ratio 9,5:1. Another car 06A block, AEB(big ported head), S3 pistons and stronger rods - compression ratio 9,0:1. With 9,0:1 ratio,  turbo spools about 500rpm later than with 9,5:1. It;s normal, or maybe problem somewhere else?

Do they both have the same tune? I'm no expert, but my understanding is that lower compression allows for more boost and timing due to lower cylinder pressure and therefore would be less susceptible to knock. So would the lower compression engine not benefit from more ignition timing? Maybe you need some more timing low down at WOT where it's trying to spool up?

Lower CR = lower N/A efficiency, so yes, it will spool a little bit worse. Nevertheless, I don't think 0.5CR should make a difference of 500rpm. I suppose that there's something different affecting your spooling time.

Almost the same tune, tried rewrite tune to each other, 9,0:1 spools later always. There is some diferences in intake (9,5 longitudal big ported manifold, 9,0 transverse big ported manifold ant bigger IC). Still searching why, i also dont believe, that 0,5 compression ratio can make 500rpm difference.

CR does not affect spool in any noticeable way.

The bigger intercooler on the other hand would make a difference to spool, as you are losing charge air velocity whilst filling the extra volume of piping and core. I'm assuming the larger cooler setup is on the car with the extra lag?

There's something else completely screwed, the bigger intercooler is not going to give you 500rpm spool difference.

Most likely causes:
1) Boost leaks, probably not pressure tested.
2) Incorrectly set turbo actuator
3) Damaged turbo
4) Wrong cam timing

Either way, CR makes no difference on this.

Think of compression ratio as a brick wall. If you are not running special fuel, and just stick a big turbo on, there will come a moment where you can not run any more boost without melting the engine, because the timing efficiency will be so low and the EGT so high.
It's not a "more power, different off-boost response" kind of thing. It's more a "if you want to pass this amount of HP/TQ on pump fuel, your CR is going to have to be this or lower".

Yes, if you are already maxing your turbo on the higher CR and EGT is not getting too high, then dropping the CR will do very little in power terms.
But running a GTX3071 you're gonna need 2.3+ bar boost on the engine, and anything over 1.5 will give you problems on 9.5 CR and normal pump fuel from an EGT standpoint.
So you will be stuck making max 350-400hp on the turbo (with all the associated turbo lag), when you could be making 500 with the correct setup.

Another thing with a GTX3071 is you need to upgrade the valvetrain, because 7200 rpm will not be enough to do a 1->2 and 2->3 shift properly, and anything higher has a risk of floating valves on the 1.8T.
At a bare minimum you need solid exhaust valves and uprated springs to rev to 8000 rpm or so.

As for the performance - if you did a blind test on correctly tuned CR8.0 and CR9.5 1.8T on pump fuel even on the stock turbo you wouldn't notice anything, apart from the marginally higher fuel consumption on the CR8.0 car. And the CR8.0 would still make a little more power when both are tuned to the maximum.
The only reason the CR is pushed up so high on these cars is for fuel efficiency.
Unless you are planning to run the car primarily on ethanol or VP MS109, you absolutely need to significantly drop the static compression ratio if you want to make any power.

I personally think OP has not done his homework. For the type of mods he wants to do, a GTX2860 would be a much better turbo.
With the GTX3071 the car won't make any more power, and will have a ton of turbolag.

IMO this part is critical

So i get main answer - this is not CR problem.
IC bigger about 50percent and pipes lenght about 20 perc in slow-spool car. Intake checked with pressure (all intake system new, also ckeched after welding). DV doesnt leak(tested and tried to replace it). Turbo actuators set identical in both cars. Cam timing good(i have had set timing in 1,8/1,8t engines many times - always double check before adding valve cover). New chain, new OEM VVT. That part about  damaged turbo sounds bad, because there is almost nothing left to check

I've been researching this build for a couple of years now. I plan to have the entire engine built eventually but it's obviously a cost issue. I will be fitting some Inconel exhaust valves from the get go, and aiming for around 8.5 cr. I've decided on a dual ball bearing turbo, as well as a twin scroll manifold and twin scroll exhaust housing to try and keep the lag to a minimum on the 3071. Yes I'd like to put some valve springs in and a water methanol kit on there, sachs 4 puck clutch etc.

The thing is, I know it's going to take me hundreds of hours of logs and tuning to get anywhere near maxing out the turbo. I need to dial in my fuel injectors, re-scale and dial in the maf housing, get the torque model and boost pid somewhere near sane. I'm not even thinking about 2.3 bar of boost and 8k+ rpm right now. I'm just thinking about getting the majority of the parts fitted and getting a stable, low boost base map to work on. 5120 Hack etc will come much, much later. I just wanted to make sure I started the build correctly and wanted some simple advice on best compression ratios and people's experiences with thicker head gaskets. If a thicker gasket is fine I could get 8-8.5:1 with my AUQ pistons. The only reason I want BAM pistons, or any forged 9.0:1 piston with a 20mm wrist pin is because aftermarket pistons seem to be better supported in 20mm pins and it would give me some level of future proofing.

I appreciate your input prj, but just because I'm not taking the most optimum path. It doesn't mean I haven't been doing my research. Otherwise everyone would just have a stroked 1.8t running a k04-064 :P

There is absolutely nothing wrong with that approach, imo.