Thread: 2.25 Compression ratio

Originally Posted by B.S.F.Nut

Is there a noticeable difference in performance,fuel consumption.,ect. between a 7:1 and a 8:1 head on a 2.25l engine? If I use a 8:1 head can I still use the old Solex carby?
Thank you
W.

Should get a slight improvement in fuel consumption, and power should be slightly up. Carburetter and jets are the same on both, but if the carbie is in need of attention, expect this to mask any improvement.

John

Yes. There is more improvement if you shave the head to even higher comp ratios (e.g. 8.5 ish).

I thought the solex was generally considered not that good, and best swapped for a zenith (or other)

Originally Posted by isuzurover

Yes. There is more improvement if you shave the head to even higher comp ratios (e.g. 8.5 ish).

I thought the solex was generally considered not that good, and best swapped for a zenith (or other)

My view is that the solex is just as good as the Zenith, but is perhaps a little more prone to give problems, with the major factor that all Solexes are older than all Zeniths. Also, I don't think the throttle bushes are as long, which means that not only do they wear more rapidly, but there is more leakage when they do. An additional factor is that the mode of operation is a bit harder to understand than the Zenith.

John

Originally Posted by JDNSW

My view is that the solex is just as good as the Zenith, but is perhaps a little more prone to give problems, with the major factor that all Solexes are older than all Zeniths. Also, I don't think the throttle bushes are as long, which means that not only do they wear more rapidly, but there is more leakage when they do. An additional factor is that the mode of operation is a bit harder to understand than the Zenith.

John

I am not a huge fan of the zenith either. I had plenty of problems with both old and new zeniths.

My father's IIA has a 32/34 DMTL twin barrel webber from a 2.5P - complete with appropriate manifolds (both are larger internally). It goes well, and if I was building another petrol I would either use one of them, or a single stromber CD from a 6-pot.

However, my 109" with rebuilt 2.25P running ~8.7:1 compression and a zenith was quite a bit quicker than my dad's 7:1 2.25P - even though the webber and manifolds can breathe much better.

Originally Posted by isuzurover

I am not a huge fan of the zenith either. I had plenty of problems with both old and new zeniths.

My father's IIA has a 32/34 DMTL twin barrel webber from a 2.5P - complete with appropriate manifolds (both are larger internally). It goes well, and if I was building another petrol I would either use one of them, or a single stromber CD from a 6-pot.

However, my 109" with rebuilt 2.25P running ~8.7:1 compression and a zenith was quite a bit quicker than my dad's 7:1 2.25P - even though the webber and manifolds can breathe much better.

Since I solved the warped top cover problem with the Zenith I have had virtually no problems - I think this is the source of most strife with them. I agree the twin barrel Weber and appropriate manifolds are probably better - but they are pretty scarce in this country. I agree the increased compression ratio should be the best and simplest improvement, although 8.7 may be getting a bit high for standard unleaded.

John

Thank you for all your replies.
Is it possible to crank start a 8:1 or 8.5:1 ?

Originally Posted by B.S.F.Nut

Thank you for all your replies.
Is it possible to crank start a 8:1 or 8.5:1 ?

Yes. No problems. Even a 9.5:1 compression engine (knew someone who had one - but it pinged like hell on unleaded. He ran it on straight LPG).

JD - detonation was usually not an issue on mine. I only had it very occasionally offroad when trying to drive at too low revs in a given gear (i.e. - close to stalling where most engines detonate)).

I lost count of the number of times I refaced the mating surfaces on the Zenith(s) and plugged "the hole" with o-ring.