IDI engine, there IS a future...

[gldgti]

1.5 compression ratio is 23.5:1

1.9 static compression ratio is 22:1 (someone confirm this for me, I am not 100% certain)

volume of 1.9 swept is 474053.67mm3

.: unswept is 21547.89mm3

volume of 1.5 swept is 367707.71mm3

.: compression ratio of a 1.5/6/9 engine would be something like 17.06:1

FWIW the difference in stroke between the 1.6 and 1.5 is actually 8mm, which is more like 5/16" :-)

[carrizog60]

never thought that stroke would afect C.R.
if the piston would project the same as the 1.6 the C.R. would be the same or so i thought...

[libbydiesel]

Changes in stroke affect the swept volume and so directly affect compression ratio.

[libbydiesel]

1.5 compression ratio is 23.5:1

1.9 static compression ratio is 22:1 (someone confirm this for me, I am not 100% certain)

volume of 1.9 swept is 474053.67mm3

.: unswept is 21547.89mm3

volume of 1.5 swept is 367707.71mm3

.: compression ratio of a 1.5/6/9 engine would be something like 17.06:1

FWIW the difference in stroke between the 1.6 and 1.5 is actually 8mm, which is more like 5/16" :-)

I didn't look up the spec for actual displacement, but your math looks right.  It's fun to see someone else crunch numbers.

[RabbitJockey]

I never realized how easy the math actually was. So a 1.6 with 1.9 head would be 18.4:1 not quite the 19:1 that I have normally heard.  And a 1.9 with 1.6 head would be 27.4:1. Wow!!!!! Hillfolk has that combo.

[carrizog60]

what would be the lowest cr for a "normal" start up?here (portugal)is very rare to see 0ēCelcius ...

[RabbitJockey]

From my mad scientist calculations at a cr of 17:1 should put u at around 376psi(25.6bar). Which would normally be an engine that's getting pretty tired, but I'd think it would start.  Something I think can hurt with the lower cr in idi is that to some degree u need the compression to help create the swirl.  The gm idi guys seem to like a cr of 18 or 19:1 I think they probably have it right since the gm idi is probably the swirl chamber engine that had had the most money dumped into it for developing performance parts

[libbydiesel]

When you add a 1.9 head to a 1.6 (or 1.5), the exit of the pre-chamber is in the wrong place which I imagine doesn't help with proper combustion either. 

[Alleslowbuged]

Hi,

may BMW 524 TD (6 cylinder) has now a compression ratio of approximate 18:1 and it is a mess to start it below 5°C . I think part of it is also the hot camshaft i have installed, but mostly due to the lowered compression ratio, stock is also 22:1.

Best Regard
Alleslowbuged   

[carrizog60]

When you add a 1.9 head to a 1.6 (or 1.5), the exit of the pre-chamber is in the wrong place which I imagine doesn't help with proper combustion either. 

but usually 1.6 with 1.9 head is a performance upgrade and show good numbers...

[RabbitJockey]

When you add a 1.9 head to a 1.6 (or 1.5), the exit of the pre-chamber is in the wrong place which I imagine doesn't help with proper combustion either. 

but usually 1.6 with 1.9 head is a performance upgrade and show good numbers...

that is my thought too, i think the real performance advantage of the 1.9 head is the compression drop allowing more advance and a safer increase in boost, the larger itnake valve doesn't hurt i'm sure. but for a performance car u plan to drive in the cold i would think a ported 1.6 head will be very adequate.

[theman53]

I don't have the figures the machinist was telling me but the biggest deal is the bigger valves. 1mm is a huge difference for some reason in performance. He claims the D shaped ports are no better than the O shaped.

I think the precup volume is the best way to drop compression if you wanted to. The space between the piston and head should be almost nothing with the head gasket installed. When thousandths make a difference for the different HG, you know just stuffing the thickest one in isn't going to end up well. It would be interesting to see how much difference one could get with the same everything and just altering precup volume. It isn't worth it for these 30 year old cars, but it would have been nice for someone years ago to have a VW IDI fetish and a dyno with lots of time on their hands.

[RabbitJockey]

I don't have the figures the machinist was telling me but the biggest deal is the bigger valves. 1mm is a huge difference for some reason in performance. He claims the D shaped ports are no better than the O shaped.

I think the precup volume is the best way to drop compression if you wanted to. The space between the piston and head should be almost nothing with the head gasket installed. When thousandths make a difference for the different HG, you know just stuffing the thickest one in isn't going to end up well. It would be interesting to see how much difference one could get with the same everything and just altering precup volume. It isn't worth it for these 30 year old cars, but it would have been nice for someone years ago to have a VW IDI fetish and a dyno with lots of time on their hands.

i think its because the amount of flow area u actually gain

1.9 has 36mm intake valves and 1.6 has 34 mm

so 1.9 has a flow area of 1018mm2  and 1.6 has 907mm2

[Alcaid]

You forgot to take into account the valve stem.

If one has the newer 7mm valves in the 1.9 head you get flow a flow area of 979mm^2 and one a 1.6 with 8mm stems you get 857mm^2

That's 14% increase in flow area right there, also port design is better on the 1.9 heads.

[carrizog60]

i went to the 1y engine because i was told that the valves had 7mm stems...

bad thing that of being hard or expensive to increase compression,i was tempted to do a weird franken engine,just because...