9 replies to this topic

[dennismini93]

as above, long story but this would solve many problems

[Magic jason]

i believe you can put any box on any block, my 1098 has the orignal magic wand box!

Im not a head man myself thou

[Cooperman]

You could do this, but the compression ratio would need to be measured, the calculations done and the volume of the combustion chambers enlarged as necessary to suit the larger compressed volume per cylinder.
Gearbox would fit OK.
Engine would not breathe very well as the 850 valves are very small.

[dennismini93]

the volume of the combustion chambers enlarged as necessary to suit the larger compressed volume per cylinder.

thank you :) what do you mean above? do you (for want of a better phase) 'bore out' the chambers in the head. also i gess you could port and polish it to improve the breathing?

[mini-luke]

Not really worth the hassle when a 998 head is much more suitable and they're not exactly rare.

[dennismini93]

my mate has a spare 850 and cant buy any more bits. would the 998 require mods or could you bolt it straight on?

[Cooperman]

It is VERY important to get the compression ratio correct. If you think about it like this you'll better understand: when the piston moves from bottom to top, the mixture in the cylinder and head is being squeezed into the head chamber. So an 850 will be squeezing a swept volume of only 212.5 cc into the head. With a 1098 engine, the swept volume being squeezed will be 274.5 cc. So the head chamber volume on a 1098 head will be much bigger. To fit an 850 head would require grinding out several cc's from each chamber. The time taken to do this will be several hours and each chamber must be exactly the same volume. If you can do this yourself it's fine, but to pay someone would cost several hundred pounds. If you just fit an 850 head onto a 1098 the CR will be so high that you'll probably get 'detonation' and break the pistons! Again, you can't build engines using guessing. You need engineering measuring and machining.

Edited by Cooperman, 13 April 2012 - 09:01 PM.

[dennismini93]

It is VERY important to get the compression ratio correct. If you think about it like this you'll better understand: when the piston moves from bottom to top, the mixture in the cylinder and head is being squeezed into the head chamber. So an 850 will be squeezing a swept volume of only 212.5 cc into the head. With a 1098 engine, the swept volume being squeezed will be 274.5 cc. So the head chamber volume on a 1098 head will be much bigger. To fit an 850 head would require grinding out several cc's from each chamber. The time taken to do this will be several hours and each chamber must be exactly the same volume. If you can do this yourself it's fine, but to pay someone would cost several hundred pounds. If you just fit an 850 head onto a 1098 the CR will be so high that you'll probably get 'detonation' and break the pistons! Again, you can't build engines using guessing. You need engineering measuring and machining.

ok thank you. same problemm for a 998 head as well i gess? would low comp pistons help?

Edited by dennismini93, 13 April 2012 - 09:04 PM.

[Cooperman]

Yes, but if you're going to strip, re-bore & fit lower comp pistons, you might as well get the correct head for the car. The 850 has very small valves which will 'strangle' the performance. It seems a very expensive thing to do in order to lose a lot of power.
Whatever you do when building an engine with non-original parts you should always measure & check the CR.

[Ethel]

I don't think it'll be that bad, both the 850 & 1098 had fairly low compression as standard, 8.5 to or there abouts, and you'd be swapping a 26cc head chamber to 24.5cc. that's only about 6% and should give around 9-9.5 to 1, actually a useful improvement. The head doesn't flow as well of course and I am assuming everything is standard factory spec.