Clearing up some of the myths regarding engine compression

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Re: Clearing up some of the myths regarding engine compressi

Post by DanBass » Mon Mar 04, 2013 6:37 pm

To be honest I fitted it for both of those reasons. I like the idea that the oil I'm burning in the chambers is clean 2T designed for the job, and the fully synthetic is offering better bearing protection for longer, plus I'm using 5w40 grade, which offers (in theory) the best of both frequently recommended grades. I will still perform regular oil changes, as I've always done, but it was the combined advantages for me, not one in particular.
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Re: Clearing up some of the myths regarding engine compressi

Post by Phil Bate » Mon Mar 04, 2013 11:08 pm

From what I have learnt (and I am in no way accomplished in oil knowledge) proper synthetics are able to have a higher shear resistance and can stay in grade at higher temperatures for longer. It makes sense to me that this may be an advantage in our engines once you don't have to worry about burning it.

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Re: Clearing up some of the myths regarding engine compressi

Post by Dadiodude » Mon Mar 04, 2013 11:33 pm

Phil, the advantage of higher shear resistance is that you can run a lighter grade. With something like Royal Purple (arguably one of the highest) you should be able to confidently return to 5W30 and get reduced resistance and friction therefore a performance gain over a mineral/semi 10W40 offering similar characteristics. It's a molecular shear not a mechanical shear that's referred to and it means the oil will not increase in viscosity beyond its stated spec when hot, whilst a mineral oil would. Rather than cold as on a piston engine its quite possible that much of the bearing wear on a rotary is when the engine gets very hot and the bearing oil film breaks down. Cold the oil is thick and at quite high pressure.
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Re: Clearing up some of the myths regarding engine compressi

Post by Phil Bate » Mon Mar 04, 2013 11:56 pm

Do you mean will not decrease in viscosity beyond it's spec when hot?
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Re: Clearing up some of the myths regarding engine compressi

Post by Dadiodude » Tue Mar 05, 2013 7:49 am

No mate. If an oil gets too hot it can start to break down and become too thin. The perfect oil would be a single grade that wasn't affected by temp but they all become more viscous as temp falls and less when it rises so every engin oil is a compromise. A fully synth is more consistent at high temp and is less prone to weakening and vaporising.

Its similar really to a standard grease. It works fine until it gets warm then it starts to liquefy and becomes useless. If you synthesise it by adding something like PTFE it'll continue to perform at temps where the non- synth grease has dribbled all over the floor.

Edit: Sorry Phil, just realised why you'd asked if I meant "decreases" and yes I did :).I should really plug the laptop in and not rely on the micro mobile screen. Anyway, your Sohn let's you use better sump and OMP oil and I can't see any downsides to them.
Last edited by Dadiodude on Tue Mar 05, 2013 6:43 pm, edited 1 time in total.
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Re: Clearing up some of the myths regarding engine compressi

Post by Phil Bate » Tue Mar 05, 2013 9:24 am

Are you sure about which way around you have viscous, or am I having an arse-backwards moment here? I understood more viscous = more resistance to flow (thicker) - oils lose viscosity as they warm up (the 10w and 40 not being ratings of the same scale).
Wikipedia wrote:The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal notion of "thickness". For example, honey has a higher viscosity than water.
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Re: Clearing up some of the myths regarding engine compressi

Post by fernandofan08 » Wed Apr 03, 2013 5:09 pm

im fixated as mines 5.4/5.2 basically. but i think when shes running again 2 stroke oil and my usual checks should keep her going
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Re: Clearing up some of the myths regarding engine compressi

Post by super » Wed Aug 21, 2013 9:50 am

Good thread this. I took a similar opinion to the OP.

I paid 4200 quid for a mint car, even if I have to pay 2.5 - 3k for a rebuild, the car is still good value compared to other similar cars on the market.
It felt good on the test drive, and had no visible issues.
Guess I'm a glass half full type of guy.... ;)

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Re: Clearing up some of the myths regarding engine compressi

Post by Soul » Wed Aug 21, 2013 10:25 am

Let's face it, even if you had to rebuild every 4 years, that's still a lot cheaper than shelling out for a new car :D
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Re: Clearing up some of the myths regarding engine compressi

Post by ChrisHolmes » Sun Apr 05, 2015 11:52 pm

Just read all this and thought it worth bumping to make it visible to more people again.
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Re: Clearing up some of the myths regarding engine compressi

Post by Shagrington » Mon Apr 06, 2015 12:01 am

It is a sticky, so stays at the top of the page ... but yes, I guess it wouldn't be on the 'recent/new posts' or the 'active topics' lists for people who view the forum that way.
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Re: Clearing up some of the myths regarding engine compression

Post by suzzu » Mon Oct 19, 2020 1:34 pm

Just did compression test today 4.8 4.8 4.8 front, 6.4 6.3 6.2 back. The car has not hot or cold problem start, drive really smooth and not noise at all. I know is matter of time a rebuild, but can I postpone till spring? I’m new with the car and I will rather get easy with a bit low power engine

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Re: Clearing up some of the myths regarding engine compression

Post by kopite72 » Mon Oct 19, 2020 2:10 pm

You can't postpone it at all when you think about it. The engine will decide when you rebuild it.

Just drive it and enjoy it and in the meantime save money until it decides to give up the ghost
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Re: Clearing up some of the myths regarding engine compression

Post by warpc0il » Mon Oct 19, 2020 2:13 pm

Just keep one of these in your back pocket 8-[ :paranoid: [-X :lol:
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Re: Clearing up some of the myths regarding engine compression

Post by v-rex » Mon Oct 19, 2020 3:57 pm

suzzu wrote:
Mon Oct 19, 2020 1:34 pm
Just did compression test today 4.8 4.8 4.8 front, 6.4 6.3 6.2 back. The car has not hot or cold problem start, drive really smooth and not noise at all. I know is matter of time a rebuild, but can I postpone till spring? I’m new with the car and I will rather get easy with a bit low power engine
I had about ~6.5's at the rear and low 4's at the front for a few years and all was fine. No problems with cold/hot starts and it still pulled ok. Was told the rear carries the front. I've rebuilt now but more just cos I wanted to rather than any problems rearing their head.

Stay ready, but you'll prob be ok till spring if not more.
Last edited by v-rex on Mon Oct 19, 2020 4:17 pm, edited 2 times in total.

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Re: Clearing up some of the myths regarding engine compression

Post by New Duke » Mon Oct 19, 2020 4:03 pm

warpc0il wrote:
Mon Oct 19, 2020 2:13 pm
Just keep one of these in your back pocket 8-[ :paranoid: [-X :lol:
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Re: Clearing up some of the myths regarding engine compression

Post by kopite72 » Mon Oct 19, 2020 4:05 pm

:giggle:
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Re: Clearing up some of the myths regarding engine compression

Post by Koren » Tue Oct 20, 2020 12:45 am

Hmm, the OP in this thread was interesting to read. Mine is now relatively ok other than hot starting. So maybe I can just run it until it doesn't run any more, and try to plan around things to avoid stops of less than 20 minutes. Getting petrol seems to be the biggest headache, but if i do it before it's warmed up much at the most local one...
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Re: Clearing up some of the myths regarding engine compression

Post by warpc0il » Tue Oct 20, 2020 1:02 am

Just fit an R3 starter, to spin it over faster...
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Re: Clearing up some of the myths regarding engine compression

Post by Proactive » Tue Oct 20, 2020 8:59 am

My son builds track cars for a hobby, he is paranoid about every squeak rattle and rumble, he recently passed on an engine he built that developed a noise at 3000 rpm he could not diagnose or resolve to another enthusiast, who then strapped a turbo on it and pushed it to over 300 BP and tracks it monthly without any issues.
My point is there is a lot of paranoia out there, for my own car I have upgraded where required and adopted all the recommended servicing and maintenance options aimed at keeping the car going.
I haven't and wont have a compression test done as I dont need to know, all I need to know is that the car gives me pleasure every time I get in.
I will deal with any issues if and when they appear, until then I will just enjoy, I have no need to feed a paranoia...
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Re: Clearing up some of the myths regarding engine compression

Post by V8 Power » Tue Oct 20, 2020 9:06 am

I have never had a compression test done on any of my RX8's as I knew that unless the results were very high it would feed paranoia.
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Re: Clearing up some of the myths regarding engine compression

Post by Koren » Tue Oct 20, 2020 11:10 am

warpc0il wrote:
Tue Oct 20, 2020 1:02 am
Just fit an R3 starter, to spin it over faster...
Mine does spin at about 250 rpm, which from what I understand isn't bad, is an r3 item better? I don't know what starter is on there actually...
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Re: Clearing up some of the myths regarding engine compression

Post by warpc0il » Tue Oct 20, 2020 11:36 am

250 rpm is reasonable and is what all compression test readings are adjusted too.

My R3 starter spins the engine at 320 rpm when the battery is fully charged and the engine warm..

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Re: Clearing up some of the myths regarding engine compression

Post by Freddie » Tue Apr 06, 2021 3:17 pm

Just a couple of points.....

1) Thanks for seconding my regime of 6 week oil changes.....have been ridiculed on here for doing it but good oil means good engine....2 stroke up th spout too. Unfortunately i haven't found a valid replacement to the r3 oil pumps for independent oiling of rotors.

) have fitted a new AGM battery that was very "tight" in the modified compartment. Just that S1 or S2, if your batteries on the way out, you wont get optimum revs on the starter.

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