compression and boost

i have a dx engine and box with a brm head and 270 schrick cam. this was going to go into my mk1 rocco for track/fun days (stripped to the bear neccessaties.
i have since got a turbo kit (garret t3) so decided to go the g60 engine and ecu route.
what i would like to know is, how does the boost and compression affect the character of the engine? is it a case of the more boost=more lag? and does it a case of more boost requires lower compression?
also fuelling. if i were to use a porche 924 turbo metering unit would it run just as well as the g60 ecu route?
i'm quite lazy and would like to just drop the dx engine in and run the turbo with standard compression and a kstar. this is not a good idea, i've been told, if i want reliability. i've heard of people running turbo's with standard compressoin without problems. are these yanks with lower compression u.s. spec blocks?
thanks,
all and any advice/info is welcome

 

I`m aming for 9:1 with the s/c, maybe 8.5:1 would be better? From what I can gather the highter the boost the lower the comression you need for reliablilty. I wouldn`t boost a stock n/a engine.

 

can't answer all these questions. But heres wot i can. In laymans terms, more boost doesnt really mean more lag. The lag will mostly be affected by the turbine wheel size.
Its its ability to not spin up, that gives lag. The boost, generally, is controled by the waste valve. The more boost, the more pressure in the engine, so the compression of the engine needs to be lower, so it dont blow itself to bits!
Obviously with an increase in boost ect, you'll need more fuel too. How your turbo affects your car, is something you could tell us!.....

 

You're talking about the static compression ratio, which is the theoretical max. worked out from piston displacement and combustion chamber size.

At different engine speeds and throttle openings the engine experiences different dynamic compression ratios; as it cannot ever suck in enough air to completely fill each cylinder - meaning the effective compression ratio is lower.

Because a turbo crams in more air than the cylinder could ever suck in on it's own, it raises the dynamic compression ratio. This can lead to pinking if the compression ratio is too high as the mixture gets too hot under compression and ignites before the spark plug fires.
This can be solved to some degree by altering the timing, or lowering the static compression ratio to give a lower dynamic compression ratio.

On lower compression engines running low boost you can get away without altering the static compression ratio - for example the turbo technics ford 2.8i V6 lumps ran standard 9.2:1 comp. ratio, along with about 0.4 bar of boost to give 200bhp. Any more and the comp. ratio had to be lowered.

Ideally you want to retain a static compression ratio as high as possible without any pinking; as the lower the compression ratio the less efficient the engine is.

Basically:

A relatively high compression ratio will give good drivability and fuel economy off boost, but will limit max boost and hence max. power.

A relatively low compression ratio will give poorer fuel economy and power delivery off boost, but will allow higher boost pressure to be used - giving higher max. power figures.

Lag comes from the time the turbo takes to achieve a high enough speed to provide boost. A small turbo will spin up quickly giving a fast response but lower max boost, where as a big fat turbo will take a long time and lots of engine revs to start boosting, but when it does it'll be shifting a lot of air, and giving a lot of power.

So the two ends of the spectrum are basically:

Small turbo, reasonably high comp. ratio, low boost: Lower peak power but a very "driveable" motor,

Big turbo, reasonably low comp. ratio, high boost: High peak power but less low down drivability, lots of lag.

I think the Gti motor runs a comp. ratio somewhere around 10:1 - whihc is really too high for use with any reasonable amount of boost.
The G60 lump is around 8.5:1 I think, so a lot more suited to taking forced induction.

As for ignition and fuelling, you need some means of adding more fuel on boost, as well as altering the ignition timing (think you need to advance it on boost, although I'm not sure).

Poor air fuel ratio / badly set up ignition timing is a big killer of turbo'd lumps.

Hope this helps

 

Assumed you(pswoody) wanted a basic explanation without confusion of different concepts of compression ect.
Not easily to explain in a few words really. Hope youre more wiser not condused m8!

 

double stacked or sized headgaskets are how a lot of people I know who have turbos lower their CR

Or they use a hybrid that has a low CR, 1.8l 16v head on an 2.0l ABA block for example(i think that gives a cr of 8.5:1)
Edited by: Paxle

 

aba block. if that an 8v? same as a 2e? how strong would the engine be in this guise? also how much wider is the kr head compaired to the 8v? (i'm thinking of clearance for the turbo).

thanks all for the info. so the cr could happily run at 10.1 if the airflow is cool enough. i have a dirty great big intercooler but i have been informed that the cooler can be too big.??

thanks again for advice/info.

no such thing as too complicated. just things i don't understand yet.

 

The cooler can be too big, a big intercooler is another thing that will increase lag. But I dont think you can fit one thats gonna make a huge difference in a VW, so obviously a big one (oo err missus) s the way to go! Bigger = cooler = less chance of detonation, which is the reason you would lower the CR. Even with a big intercooler you would still want to though.

 

16v/aba block is a rather popular hybrid, Yes its an 8v block. Check out the hybrid forum on vwvortex.com for more info

 

Yeah, when looking into this a short time ago I understood that what the US guys refer to as ABA is our 2E over here.

And I just had a look at Vortex as Paxle suggested and came across this thread that looks like it could be useful.

Like KeithMac, I'm doing a 16V with S/C conversion and need to get a similar CR too. I'm not sure if I'd have the talent to start putting together a hybrid KR/2E engine though. I might just fork out for custom pistons for my KR block if that's going to be more straightforward.
Edited by: Trev16v

 

Must confess at this stage I don't fully understand the reasoning behind using the 16V / 2E hybrid, unless it's simply down to low cost. Like the fact that you can gain low CR without custom pistons?

 

The ABA is the US mk3 Golf 2.0 8V which has the crossflow head. As far
as I know the block's exactly the same as our 2E (TSC will confirm this).
The reasoning is that it lowers the compression to a suitable level for
boost cheaply and effectively. Not much good if you're trying to build a
turbo 8V though...

 

For a turbo 8v engine, I've heard mixed results with the double stacked Headgaskets, IMO I think they are ok for low boost only.
I think there is also one company out there making double thickness copper head gaskets, these would be a lot better for force induction. I wish I could remember the name of the company....

And yeah the 1.8 16v/ABA is done for cost reasons (you can pick up a low mileage 2.0 block for less than $100 here, the pistons would be a lot more. You can up the compression ratio if you use ABF engine Pistons.

BTW what octane gas is sold at the pumps in england anyways? I have 87, 89, and 93 around here, a few places also sell 94
Edited by: Paxle

 

In the UK, the common research octane number, or RON, for Gas (Petrol) is 95 (Regular), 97 (Super) and 98 (Super Plus). I believe that you can get 100 or 101 octane fuel in Japan... that's like aviation fuel grade. 94 sounds pants, surely US octane numbers differ to UK grades?

Edit: Spellings!
Edited by: Edition96

 

Ah ha!

Must be the RON, MON, PON debate

Clicky

 

Ohh... trust me our 93 is good.... I have to advance my timing on my bronco if i run it, otherwise it smokes and floods when i try and start it.

Looks like english pumps might list the Ron, instead of the average between Mon and Ron.

So your regluar gas would a bit worse than our regular your mid grade is about our regular or a little above, and your premium is a little above our premium.

You guys have better diesel Fuel than us I hear, Diesel in the US is still primarly used for trucks, very few cars use it. So its made to be cheap as possible for the trucking companies to use.

I got pulled over by a cop in DC for having an "Loud aftermarket exhaust" on my Golf diesel. Argued for a half hour trying to tell him that it was a diesel and had the stock exhaust on it. Finally had to pop the hood, and break out the bentley manual to prove it.
His response was to scratch his head and say, I didnt know they made diesel engines that small.

 

That sounds fair enough to me. I use 98 RON (Shell Optimax) in my Valver. It's expensive but worth the extra over 95 RON. UK fuel is a rip off due to the silly tax that is put on it. My last fill up cost 85.9p/litre (391p/gall) so that would be $7.49 per gallon... well per UK gallon... i think US gall are different.

Welcome to European motoring. We've had small diesels for years. Typically the 1.7-2.0 litre range was common but with Commonrail and Pump duse injection, smaller engines with comparable power outputs are now being used, and this in turn gives more economy. A few years back now, the UK diesel fuel was changed from regular diesel to the lower carbon output 'city' diesel. This resulted in a drop in economy for most diesel owners. Today diesel fuel prices and regular 95 RON fuel prices are comparable... so diesel generally benefit:

a) Tight fisted, high milage owners
b) Companies (due to the tax band thingy)
c) Those who want lots of torque (towing?)

IMHO diesels have a market in the US but that is another topic altogether. However the VW diesels are great. Take the smallest and largest models, the 1.4l 3 cylinder pump duse 75bhp model in the Lupo and the 5.0l V10 twin turbo in the Toureg and Phaeton... awesome

 

even with stacked gaskets to bring the CR to an acceptable level you can still run into problems such as your ringlands collapsing, most FI motors have the piston rings further down the piston, and the tops of the pistons are much thicker and stronger to cope with the extra temperatures, stresses produced, for reliabilty your best off forking out and buying the proper low compression pistons, or use pistons from a FI motor, I.e if you are building a turbo 16v use pistons from an Audi S2 engine if you are lucky enough to find some,,,
Edited by: rradogolfman

 

S2 pistons are relatively cheap to buy used from Germany, and brand new ones are something like 80 quid each apparently... which still isn't that bad. But, I get the impression that there has to be a downside of simply using S2 pistons in a KR block. It kind of sounds too good to be true. There was quite a lot of discussion about this on the Yahoo 16V G60 list.

 

well an audi 20v is basically a 16v with another cyl on the end so it all sounds pretty good, havent actually heard from anyone that uses them though, wonder if anyone on here has first hand experience with this conversion?
Edited by: rradogolfman