Best OEM pistons for pikey small block 16v turbo

leave the pistons stock and work out what two metal headgaskets (from a an afh) would give you compression wise on the polo gt (3f) engine. a single metal headgasket gives valve to piston contact but two might just be the ticket.

For reference, all the rods from the 1.3 mk2/3 polos are the same (possibly the one litre as well, can't remember) be it 55bhp spi or 112bhp G40 and are very robust, i certainly know of nobody bending/breaking any

i think that turboing a 3f at stock compresion would work well for a responsive engine due to high'ish compression, couple it to a ko3 on relatively low boost and it would be a very good package in my opinion...

 

Pretty sure I have a few old pistons and rods from a mk3 Polo SPI 1043 (AAU) engine lying around Mike. They are not fit for use but fine for measuring up. The comp height of the pistons is huge due the the shorter stroke of the 1043 compared to the 1272, and the fact they use the same length rod.

 

I mapped an AAV (1272cc, 9.5:1 CR) engined Mk3 Polo running G40 management and a K03 turbo. The owner runs it on 10psi or so daily, but we mapped it up to 15psi and it made 175bhp on the dyno. It's been going for 5 months so far without issue.

My motor's 9.3:1 CR, but it does have a G40 head (sodium filled exhaust valves) and forged pistons, so it doesn't seem to complain about the 22psi being shoved up its @rse by an overworked K03.

 

What head gaskets are used with these conversions? Is it the G40 ones?
The standard MH to AAU types seem to give up fairly regularly around the waterways and the front oil feed.
Often wondered if the G40s used an uprated type.

 

danster....the headgasket of choice is a metal one from the mk4 polo 1.4 16v (afh)

 

The car I was talking about runs a standard AAV gasket.

The metal one Gregggggg's thinking about is from an 8v Polo 6N (e.g. AEX motor) - mainly used because it's thinner; to up the CR slightly.

 

I sit corrected, cheers Andy

 

Cheers - do you happen to know what the thicknesses off the various head gaskets are? According to ETKA all the ones for the 75mm bore 1043/1272cc engines are the same, with the exception of the G40 and Diesel varients, the latter of which has three possible thicknesses listed (1.53, 1.57 and 1.61mm) presumably to account for head skimming and probably not enough to make much of a difference on a lower comp. petrol engine.

Yes, the 1043cc engines use the same 122mm rods as the 1272cc lumps.. I've also heard they're tough little blighters.

What's the crack with turboing an engine with a >9:1 CR? I'd like to keep the CR as high as possible in the interests of component strength and off boost performance as you say - if retaining the stock CR do you lose power / efficiency on boost through excessively retarded timing?

Thanks
I've already pillaged a fair amount of info from the Mahle website; however I'd be interested to know the top ring land height and the respective masses of the piston, gudgeon pin and rod though, if you have a spare minute

According to Mahle the 1043cc pistons have a compression height of 47mm That said I suppose they shouldn't weigh that much since they house the combustion chamber, and I suppose the large amounts of meat on the top of such pistons is probably a good thing for heat dissipation in force fed applications.

Crikey Any idea how retarded the ignition needs to be under boost compared with a standard G40 lump running similar boost pressure? I'm not overly familiar with the G40 setup, but believe it uses a knock sensor? Has to be a good thing when looking to do cheeky things with standard parts

As mentioned above, according the ETKA the G40 does have a different gasket. I assume there's nowt wrong with using one that's slightly too large then, since the AEX has a 76.5mm bore if I'm not mistaken.

On a different note, does anyone know of any projects where a later AHW type 16v head (only one cam driven from the crank) has been successfully fitted to an earlier 8v block? It appears that there might be a bit of a problem with oil and water ways.. wondering if there are any solutions that don't involve major surgery to the head or block.

Thanks for your thoughts chaps - it's great to be able to discuss such stuff

 

Lots of stock bore G40 boys run the AEX gasket. I run it because I've got a 2mm overbore too, so it fits a bit nicer than a stock gasket.

Comparing an ~8.3:1 CR G40 turbo (stock pistons, AEX gasket, BV head, K03 turbo, Aquamist, 'turbo' cam) with a 9:1 CR G40 turbo (Accralite pistons, stock gasket, stock head, K03 turbo, Aquamist, 'turbo' cam) the lower compression car has about 4-7 more of advance. They both ran the same boost pressure and made within 1bhp of each other on the same dyno on the same day!

Based on this and other experiences, I'd argue that on the Polo engine at least, going up to 9:1CR doesn't have any power penalty if you're going to calibrate the ECU to suit. The higher compression car made about 10lb/ft more torque lower down the rev range too.

Bare in mind that these are all on K03 turbos being worked very hard, so the results may be different if you use a more efficient larger turbo - but I have no data to prove that either way!

Also of interest is that on my car (~9.3:1 CR, standard head) I hit det limit before MBT, but then mapping the ~8.3:1 CR turbo'd G40 I found the same - though det limit and MBT were much closer on the lower CR car.

 

Cool, didn't think about the overbore situation.

Thanks for the info, it's good to have a reasonably direct comparison. You mention that the K03 is overworked (it appears that it would be operating well outside it's peak efficiency zone, looking at a compressor map) I assume this is the reason for the water injection; to remove some of the excess heat generated by the inefficient compressor. I trust you're also running a decent sized intercooler on both motors too..?

From this I suspect that using a "correctly specced" turbo could only cool things down more.

MBT and det limit; am understanding this correctly that MBT (max brake torque?) is the point at which advancing the igntion further yields no extra torque? But this point is not reached because det / knock / preignition sets in first?

Thanks for your insight - it's good to talk to someone with some experience

 

I don't actually run water injection on mine, mainly through laziness - but there doesn't seem to be a power penalty. In fact my car made more power/torque than the 9:1CR turbo'd G40 I mentioned, but it was a different dyno on a different day - and was within 5% so in reality probably not too much different.

I do run a sizeable intercooler, but inlet temps aren't too crazy - only 20-25C above ambient if my Emerald ECU is to be believed.

I'm fairly sure the K03 is overworked to make ~200bhp on a 1.3 8v motor, but people push them much harder on a 1.8 20v lump without them failing every 2 miles - so for a cheap, easily available turbo they work well. I'm sure something like a GT17 or even GT2554R with a milder cam might be a more efficient way of doing things, but it'd cost more.

You're right about MBT. The likes of Dave Walker will tell you that's the sign of a badly designed engine setup - but lots of OEM boosted gasoline motors will hit det limit before MBT, certainly both my Saabs do.

I suspect if I whacked on a more suitable GT2554R and ran it at similar boost levels I'm sure I'd make more power with a lower CR bottom end than I currently have - but it'd have slower response due to the larger turbo and lower CR. So swings and roundabouts. All I can safely say, is I have some pseudo-scientific evidence to suggest that with K03 and 'turbo' spec cam 9:1 CR only seems to give benefits.