After getting some details Dave passed on Graham has got into measuring up both blocks and just sent me some info regarding the differences and the potential of the 3.5 and 3.8 engine. He hasn't sent pics yet to show the differences but I'll get them up when he does.
3.5/6G74 block and crank
- Weight = 48kg
- Main bearing saddle weight = 5kg
- Crank weight = 20 kg (crank has cross drilled pins and mains)
- Cylinder wall thickness at bottom of No 1 and No 6 = 7 - 8mm (rough measure, not ultra sonic)
- Cylinder wall thickness through No.1 water inlet = 7.5mm
- Thickness at bottom of all cylinders above main bearing webs = 7 - 9 mm
- Depth from main bearing tunnel to top of crankcase "vee" = 65mm
3.8/6G75 block and crank
- Weight = 46kg
- Main bearing saddle weight = 6kg
- Crank weight = 20 kg (crank has cross drilled mains, through drilled pins)
- Cylinder wall thickness at bottom of No 1 and No 6 = 5.5 - 5.7mm (rough measure, not ultra sonic)
- Cylinder wall thickness through No.1 water inlet = 6.4mm
- Thickness at bottom of all cylinders above main bearing webs = 6.2 mm
- Depth from main bearing tunnel to top of crankcase "vee" = 67mm
So differences are:
3.5 block is heavier and has thicker walls so same maximum bore potential unless better grade iron is used in the 3.8 block. Apparently the 3.8 has a big external rib along both sides of the block, and the main bearing webs are cast full depth so there is more strength/rigidity there.
3.8 bearing saddle is 20% heavier due to substantially thicker/deeper side beams.
2mm more space in the top of the 6G75 crankcase so less relieving required for long stroke crank.
Cross drilling of 3.5 crank is as per that in most high performance and race engines. It gets good flows to both the pins and mains. The through drilling of the 3.8 may reduce main lubrication and help the pins, but it may just have been to save cost and there may have been no benefit to pin flows. Have there been any 3.8 bearing issues reported?
Cylinder bore potential
Unless the block is subjected to unusual harmonics or has especially poor grade iron it should be OK up to a 99mm bore! That's close to 4.2 ltr with the 90mm crank or 4 ltr with 85.8mm crank. Rubbish American iron regularly handles 110 HP per cylinder and 10,000 rpm on 3.5" stroke for 5,000 miles with good ring seal on 0.180" wall so a Mitsubishi block should be good for 70 HP per cylinder and 7,000 rpm without block fatigue with similar wall thickness. (Early Chev blocks regularly run down to 0.120" wall but racing block life was short and ring seal not perfect).
Rings would not be a worry due to Chev LS1 Gen III bore size but pistons would have to be made, also head gaskets.
A cheap option would be 97mm bore, 4 ltr with 90mm crank and 3.8 ltr with 85.8mm crank. Graham thinks the stock 6G75 head gasket would be OK enlarged to suit that bore size and 20 thou oversize low block series III Buick/Holden Ecotec V6 rings would fit. The forged 3.8 Supercharged piston could possibly be used (ACL #6MKRY9705).
In a 6G75 that piston is about 2mm too high but Graham thinks it could be machined. He's been told the top ring is 6mm down and the crown is thick enough for blower use and a 3.55mm deep bowl so it should be more than strong enough NA. Machined 2mm it would end up with about a 6cc bowl. It has a 23mm pin so the rod would need to be honed/rebushed.
In a 6G74 these pistons would be the correct height but the stock 14cc bowl would have to be machined a bit wider to get the compression ratio down, or the combustion chambers enlarged.
Already a long stroke engine so why bother? Graham says limit would be about 96mm. Going down to a 50mm Nissan VG30 pin and rod the 3.8 crank could be offset ground for about 95mm stroke and 3.5 crank for 91mm stroke.
Graham would be interested in any info you come across on the strength and bore potential of these blocks.