High Compression 3.4 DOHC (Northstar Internals)

[datsun1973]

Okay so I'm trying to figure out how to build a high compression Lq1 without the use of custom pistions. Here is what I have come up with.

1. 3500 forged SUV crankshaft with 2.2500" rod journals

2. Northstar Rods are 5.9449" Long with 2.2500" big end journals (may have to machine big end)

3. Northstar Pistons have a compression height of 1.250" with a 92mm bore (3.622").

This gives a +0.030 above the deck. With a .060 head gasket that should give me about 10.8-12:1 compression.

Any thoughts?

[robertisaar]

wasn't aware that the NS had the same bore as a 3.4... the extra .25" of con rod is what would make me iffy. got a pic or diagram of a NS piston?

[datsun1973]

I don't but the Northstar piston has a reduced compression height of 1.250" vs. the 3.4 DOHC has of 1.476"

Plus the added length of the rod will increase the dwell time at TDC which should result in more complete combustion and more power. This is similar to what the SBC guys do when they install 6.0 rods in a 350 vs. the stock 5.7 rod.

Edit: Just checked and the big end of the Northstar connecting rod is 0.8572 thick. I believe that is thin enough to fit without machine work.

[robertisaar]

.... this all seems to be going together too perfectly, there has to be a catch.

longer rods can also allow for a higher rev limit if the rest of the rotating assembly can handle it.

[datsun1973]

I did more research on the NS rods and they are 2.225 (not 2.250) so the crank will have to be ground slightly to accept the NS rods. I also did the math on the CR (made a spreadsheet). The basic setup above gives about a 10:1 CR. I will most likely deck the heads .005 so that may give me ~ 10.25:1.

NS pistons are forged aluminum and also already have the valve reliefs cut since they are 4V engines.

Since this is all GM stuff I can't imagine there would be a large gotcha.

BTW Eagle makes a set of 4340 H-beam rods for a NS that are 545g each as opposed to the stockers at 594g each. So that would take 250g out of the rotating assembly. They come with 3/8" ARP Bolts and are around $500. The NS pistons from rock auto are $50 each and they are also coated on the skirts for low friction. So thats $300 for pistions. $150 for a good used crankshaft. $200 for the machine shop.

Looks like a grand total of $1150 for the bottom end. Full forged, high compression.

[robertisaar]

Looks like a grand total of $1150 for the bottom end. Full forged, high compression.

that almost comes in line with SBC parts...

and the crank grinding: that's not too bad, it's only .025 needing to be removed. and due to the smaller journals, it will remove friction as well at the cost of some strength.

it's only a .75 increase in compresson for the 91-93 manuals(1.25 for the autos) and all 94-up LQ1s, but the extra strength would be nice, could allow for some spray or a bit of boost.

[datsun1973]

I was actually going for 11:1 or higher to run E85 (100 octane). I'll have to aquire the parts and see how it goes. I'm planning to do the build up this spring. I have a 97 LQ1 that I need to tear apart and make sure its good. It was free and has worked great for the mock up phase. I'm finishing up the chassis now. This is for a full tube frame race car.

[robertisaar]

running E85 full time, i would want a minimum of 12.5:1...

[datsun1973]

True, maybe if I offset grind the crank out to 3.4" stroke that should up the compression ratio.

I should also note that this is not going to be a street motor, its a full on track car so either E85 or 110 Race Fuel.

I'm hoping someone will chime in with some experience on upping the CR on the DOHC.

[RedZMonte]

I have heard this said a few times.... never seen it done... sounds like you have done the reasearch, i would like to see it come to life.

[IsaacHayes]

What RPM are you wanting to turn? It may be cheaper to just get custom high CR pistons and stock bottom end and work just fine...

[Dave96z34]

The money it would take to pull it off you could build a turbo lq1.

[RedZMonte]

i think TimG got a set of custom pistons made for like 5-600 and i wanna say he got custom rods too... for another like $5-600. that might be the ticket to do it that way so you know exactly how long and what compression and everything is to your specs.

S

[timg]

I did get custom forged pistons from CP. I wish they were only $500-600. Double that and I think you're closer to the right ballpark. If you want them, CP should be able to make another set without too much trouble.

Tim

[RedZMonte]

Tim, You ever get that spare motor put together?

S