The 18R-GTE reference manual ...

[Steve M]

The compression ratio of an 18RG with 18R-C flat top pistons of 88.5mm bore and no other modifications to the engine (based on 72cc combustion chamber and ACL STD gasket) works out to be:
7.011:1 before the gasket in torqued up and about
7.07:1 after it has been torqued.

So basically, pretty crap!

A 1mm head gasket would achieve 7.263:1 and most likely some dead valves at some stage.

Note that the pistons should also be valve reliefed which would further drop the compression significantly.

[JustenGT8]

That's what i recalled Steve. Comp ratio doesn't worry me too much as even that low the RG is surpringly torquey...the valve relief is an issue though. If they are simple flattop, which i have run in the past i thought it might sneak through ?? You don't think so?

[ra_28]

If you compare an 18rg piston to an 18rc you will see that the only difference is that the 18rg has a dome on top. There is no actual valve relife below block height so why should the valves hit?

[timbosaurus]

That's what I thought about the RG pistons, no recess below deck height. but I might be mistaken. haven seen an RG piston for 3 years

The 18R is definately flat top, so I'm wondering why a thinner head gasket would result in dead valves? Shave a fair bit of 18R blocks and running hi lift cams is possible, and I thought this would have brought the valves closer to pistons than a 1mm HG.

Maybe the RG cams have far more lift than 18R items? Then againg, I would've thought their advantage was in flow and duration.... not necessarily lift?

Anyways, good to find out more about it

[JustenGT8]

If flat top will clear then no probs...i can't recall whether my old forged flatops had valve reliefs in them or not? i vaguely recall not but this isn't something i want to take a chance on.

RA_28 PM with a price for your short block

[timbosaurus]

Justen... just tell Zane to sell me his car. I'll get it going

[JustenGT8]

LOL we can build it bigger, better, faster than before....... now where did i leave that 6 mil??

[timbosaurus]

I reckon some forgies, ARP bolts and a decent HG would get that baby capable of pulling some big numbers!!!

Shame it's not mine

[DQIKST]

how about some extra dowels, PTS seals large angle in the housing and a big mutha fuker of a hair dryer,










yeah i thought so

[Steve M]

Comp ratio doesn't worry me too much as even that low the RG is surpringly torquey...the valve relief is an issue though. If they are simple flattop, which i have run in the past i thought it might sneak through ?? You don't think so?

If the turbo is well suited to the engine, it should be fine. Usable boost from 2000RPM would negate the need for high compression. That is my plan as the engine will redline about 7200. As I plan to go N/A for a bit, I went for high comp pistons anyway. I just have trouble committing to anything with this engine and like room for improvement.

Also note: 92mm bore x 22r crank and flat tops all adds upto 8.2:1 compression.

If you compare an 18rg piston to an 18rc you will see that the only difference is that the 18rg has a dome on top. There is no actual valve relife below block height so why should the valves hit?

Sorry mate, not correct. The valve reliefs do extend below deck height.

88270 (8.3) piston: ~ 1.1mm below deck height on inlet, zero on exhaust side.
88233 (9.7) piston: 2.5mm below deck on inlet, 0.5mm below deck on exhaust side.

[JustenGT8]

ah thanks Steve, i couldn't recall what my old forged flat tops looked like but 1.1mm is pretty serious and i wouldn't like to try and get that with just a HG.

I amd a avid hi comp fan no doubt but if budget dictates then i'll live with low comp. Looks like we will be trying a forged 9.5:1 2 litre bottom end in the short term anyway while we rebuild the 2.4 big block.

[timbosaurus]

88270 (8.3) piston: ~ 1.1mm below deck height on inlet, zero on exhaust side.
88233 (9.7) piston: 2.5mm below deck on inlet, 0.5mm below deck on exhaust side.

Why does it need a bigger recess (compared to deck) on the 9.7's?

Assuming the cams are the same, the depth below deck height could remain the same. Or did the cams on these engines have more lift/duration to necessitate the bigger recesses?

[Steve M]

The early cams had more duration and bigger lift, also no ability to fix timing as chains stretch.
In my opinion, the later model piston reliefs are too small, I've seen ones where the normal carbon build up has actually been compressed by the valve they come so close. These were 18RGU 88251 pistons. 88270 pistons were about the same if not slightly worse.
88252 pistons were basically a re-release of the 88233 9.7 piston and are good.

[SillyCarS]

That is my plan as the engine will redline about 7200.

may not want to build based on redlines that are above 6000/6500 rpm (although i realise with the rg you dont have to much choice) the reasons rg's are so prone to spinning bearings is because the oil pump starts to push air at about 6000/6500 rpm. Some mods to the block are needed if you expect your gunna be running with lag and a rev range of say 3500 to 7500 with a turbo on full boost

craig


ps. im half tempted at the moment to grab a 1j and be done with it all

[Steve M]

I'll have to watch the oil pressure gauge next time the old RG sees 8000. I don't reckon it's ever started to drop before.
I reckon one issue the 18RGs spinning bearings is that too many people are using 18R-C oil pumps too much and they are using old motors that are tired.
I've checked out a few pumps and some have been really unhealthy with lots of scoring and bits of stuff stuck into the rotors. I wouldn't be using these in a hurry.
Another issue that may cause spun bearings is crap oil.
Using average 20w50 oil like that used in low revving engines is no good for high revving old engines.
Oil for high revving engines, two valve or otherwise should be chosen for the purpose, multivalve engine oil like one of the Shell high performance/turbo synthetic jobs has done very well in my brother's engine, as has the 10mm overfill strategy. Never get oil pressure drop around any hard corners with that arrangement, but it did start to drop with STD level and GTX2.

That said, my 18RGTE will have a redline of 7200 as it is being stroked. This figure was matched to the piston f/sec. of the standard 18RG 8000RPM baseline.