Dished Piston vs rod lenghts

[NextGen]

I have a customer that took a 1.7 case installed 2.0 dished pistons using the stock 1.7 heads. He also used a stock 2.0 crank 71mm he said he checked and has 8.5 compression

Was wondering what the compression would be if he kept the 1.7 dished pistons and the stock 66mm crank.

I know there are different dishes.

[Piledriver]

All very approximate of course w/o deck measurements, head CC, piston dish/dome cc etc...

if he used the 1.7l domes, ~8.4:1, with no head gaskets/min deck.
With the bus dished 1.7 pistons about 7.5:1

the former will make 15-20 HP more, all else being equal.

With the stock cam he may need to run premium, and stock timing may be a little too advanced to be optimal.

[NextGen]

OK how about the 2.0 DISHED Pistons, 1.7 head and 71mm rods would you say he was right on with the 8.5

sorry I said crank, meant to say rods, stoked.

[Piledriver]

The 1.7/1.8/2L bus chambers are different shapes, but all about the same 55ish cc chamber size, so it probably won't matter much.

The 1.7L/1.8L dishes are ~5-6cc, the 1.7L domes +5-6. I have never see a set of the euro domed 1.8 pistons.

1.7 heads flow a bit better than the bus 2L heads with the same valves.
They also tend to be in better shape, but that's a very generalized statement, the history of the heads is the driver.

The 2L BUS pistons have ~15cc-18cc dishes, so the CR will be much, much lower.
Keep the deck screwed down tight and it can still be tuned ~OK, but that limits you to a pretty mild cam if you want efficiency.

If you are not building this for a bus and maybe even then...I strongly recommend flat tops,
The bus pistons can be useful building a mild stroker as they can take quite a shave off the top.

The 2L bus piston compression ratio is too low for much other than a bus with a web73, will kill power and not help much else.

[NextGen]

I totally got it, thanks and I know there are a heck of a lot of guys running the 1.7 heads with the 2.0 dished pistons and the 71.mm rods. I was just talking to one of them after reading your numbers and he said as stocker with the stock cam, he would have never known it could be that much better, seeing the engine is still so much better then any stock t-1 he owned.

One other thing he said he read at a 914 site that combo is a long living engine as a daily driver with reg gas???

[Piledriver]

Long life? Well, if you aren't making any power I suppose you can't hurt it, but the stock cam is a turd and will.

The 2L (bus) was rated by the factory at ~70ish HP, the 914 1.7 was closer to 85, with just the compression bump.
(the domed pistons being the only difference)

In a 914 I put close to 400K miles on one, that was literally all used parts to begin with save for having the heads refreshed...the only thing that eventually caused a tear down was the cam thrust bearing wearing away, so it made "funny" noises at idle but ran fine. I reused the main bearings with a fresher crank on the rebuild, 80K miles so far.

With a cam even as mild as a Web73 or the Raby version with a little more ex duration would benefit from flattops and really wake up, probably pushing >100HP with either carbs or well tuned EFI, even the stock EFI with some massaging. (Although you can do MUCH better cheaply these days)

I would suggest flattops and a better than stock cam, perhaps especially for a Bus, as the factory cam ~melts the heads by design for emissions purposes. Jake used to rave how bad it was, and in anything other than a 1700 it totally sucks, and even then ~anything else is a huge improvement.

If you run the jugs with no head or base gaskets and have stock length jugs, the deck will be <1mm, the 12cc bus pistons will give you about 8.2:1, which can work ok with a web73ish cam. if the dishes are larger less cr.

[NextGen]

Perfect that was exactly the answer he was looking for, Cam and flat tops are the winner. Funny seeing the T4 engine is in a light bug and never gives any problems, fun to drive and not really a race engine just a daily driver. I know the guy until he has a reason he is not going to split the case.

Actually other then potential power he is missing, there is no real down side. Will till the cam, says Hello!!

Thanks