Rb26 Compression With Poncams Type-B

[XKLABA]

How much variation between cylinders is there. Are they all close cause if they are it is most likely a machanical problem

[Ben C34]

http://www.skylinesaustralia.com/forums/topic/378661-valve-overlap-rb26-tomei-260-pon-cam/

[portaz]

It worries me a little that you have honed bores and reused old pistons that's in essence increasing the piston to bore clearance which would also increase your ring gap. Problem is unless you measured the Bore with telescopic gauge and micrometer you couldn't be sure how much taper the bore has, I've made a few assumptions here but it's just from experience also where did u set your ring gaps at what point in the cylinder ? How far from top?

Scott

[GOD12A]

Time to take it for a blat as per the earlier advice given to create pressure to push the rings on to the bore and bed them in

good luck

Post back results

Agree, run it in hard.

My fresh engine did 30x >300kw power runs after I drove it from first start to the dyno ~15min away.

Give the rings a chance to bed in and see where you're at after some (loaded) driving.

Mark

[andre_gtr]

It worries me a little that you have honed bores and reused old pistons that's in essence increasing the piston to bore clearance which would also increase your ring gap. Problem is unless you measured the Bore with telescopic gauge and micrometer you couldn't be sure how much taper the bore has, I've made a few assumptions here but it's just from experience also where did u set your ring gaps at what point in the cylinder ? How far from top?

Scott

Was about 20-30mm from the top of the cylinder

[portaz]

Was the bore measured at different spots?

[Ben C34]

It worries me a little that you have honed bores and reused old pistons that's in essence increasing the piston to bore clearance which would also increase your ring gap. Problem is unless you measured the Bore with telescopic gauge and micrometer you couldn't be sure how much taper the bore has, I've made a few assumptions here but it's just from experience also where did u set your ring gaps at what point in the cylinder ? How far from top?

Scott

Not to completely disagree, but just chuck some more Info in the mix.

The larger bore will reduce the amount of force a ring will exert on the bore (by a tiny bit)

Which will make the ring gap bigger, but in the scheme of things a larger ring gap gap by itself really has a negligible impact on the engine

http://www.diagnosticengineers.org/journal_%20articles/Ring%20Gaps%20vs%20Knowledge%20Gaps.php

[XKLABA]

If he has fitted new rings then they really should have been gapped to the bore after honing it, and those honing tools take 2/5ths of F all anyway, this is how I did my 32 last time and it has held together for a few good hard years of abuse and ran some really good numbers given its set up

To give a consistent 100psi then I would say something is a miss machanically, most likely cam timing

[andre_gtr]

Thanks for all the info guys been giving it some stick over the weekend boosting from second gear upto 7000rpm 3rd about 700rpm and then just a little in 4th upto about 200km/h then down to about 80km from 2nd again and again

now I havnt checked the compression as yet I will try get a diffrent gauge aswell and lets see what the outcome is will keep you posted

[GTSBoy]

It's actually funny how the old and sensible instruction to avoid idling a newly built engine coupled with the old and sensible advice to run the engine through a variety of loads and speeds (but not high load or high speed) has translated through the last 20 years into thrash the living f**k out of it from cold startup.

You don't need to arse rape an engine to get the rings to bed in. You just need to drive it sensibly.

[andre_gtr]

I think ass rape is the order of the day in my situation

[portaz]

True if numbers are even that should be an indicator of another issue , I would personally never just hone. A 26 as it's such a big job to get to that point but each to their own I guess really in worst case could be a bit more blow by or just a more noisy engine but as was states earlier honing doesn't take much off but my point was more that an engine that has run for some time may have a taper in the bore, on some of the lesser engines I've built eg Honda lol if just being honed only I would check

The ring gaps at 3 different spots in the bore to make sure I wasn't going to have any dramas . Hopefully it's just a gauge issue in this case as sometimes it's the simple things that cause the most drama . Best of luck to the op ,

Scott

[Ben C34]

Did anyone read the thread I linked to at the top of the page?

A guy put type b cams in his 26 and ended up with 100 psi comp , put stock cams back in went to 150.

[portaz]

Wow big change

[andre_gtr]

I have seen that before hence I tried this thread to see if there was any others with the same issue

[andre_gtr]

when I get to checking the compression again I will add some oil to it to see if the compression increases if not then the guys that did my porting and shimming of the head made a f**kup

[portaz]

If your worried about the head work do a leak down test that will tell you straight away if any valve issues etc

[andre_gtr]

Ok so tested compression again

And all seem to be the same 100psi when i put s9me oil in the chamber the increased to about 116psi

This was done with all 6 plugs out and the apexi read about 180 rpm should it not be 300rpm?

[Shoota_77]

Wouldn't imagine it's rings then. You would expect a lot greater increase in comp with oil added (provided you added enough).

[Ben C34]

Retard the exhaust and advance the intake and see what happens.