Is the 3900 a true 60*?

[ForcedFirebird]

Making some deck surfaces out of aluminum, I have discovered that the bores were moved towards the outside of the block by .09571". So isn't that closer to 73 degrees?

[geoffinbc]

I though I recalled reading they were staggered in order to keep the same bore centers.

Edit: Just read this

The new 3.9L V-6 differs from previous GM 60-degree designs in its offset cylinder bores. The centerlines through the bores on each bank do not intersect at the crank axis; rather, they intersect 3 mm below the crank axis. The offset bores present a number of advantages, including room for larger cam journals and flexibility to stroke the engine for more displacement.

[ForcedFirebird]

The bores don't appear to be staggered. I'm measuring a 3900 head gasket now and they are in-line. Even placing it on a 3500 block proves that they are linear.

The intersection of the piston centers being below the crank makes sense though (Ben and I thought maybe the crank was lowered), but what does that do for wristpins? Are the 3900 pins offset to compensate? Wouldn't make sense to have the rod driving up that far off center, but maybe it does work?

[geoffinbc]

No they would leave the rods alone. It just means the rods work on a bit of an angle. Maybe the pistons have an offset pin location. More so than a 3400 would anyway.

[ForcedFirebird]

No they would leave the rods alone. It just means the rods work on a bit of an angle. Maybe the pistons have an offset pin location. More so than a 3400 would anyway.

Yeah, I was talking bout wristpins, not crank pins, sorry.

That's the only way I can think that could take up the slack, but several degrees seems like a lot. Even taking the angle into consideration as the wristpin is closer to the crank, there's still several thousands offset for the angle to be corrected. (probably closer to .050" or .070") This just seems like an awful lot of wristpin offset.

[Joseph Upson]

The pin end may be offset in the piston, I recall reading that in some of the LS1 piston descriptions and since the engines share the same bore they may share that aspect as well.

I recall reading a thread here somewhere regarding some customized chevy rods having an offset that wasn't realized before they were modified for a V6. It didn't cross my mind at the time but now that I think about it, if the big end is offset slightly that equates to a stroke increase depending on how they are installed which I don't recall being mentioned in the thread.

[montess408]

well on the wikipidia page(not sure how accurate it is) but it they state that the 3900 and 3500 vvt is the "high value" V6 and is based on the 60*V6
so yeah if that is correct then the 3900 is not a true 60*

[ForcedFirebird]

How can an offse on a rod cause stroke increase? If a rod is bent even, then the center to center would not change by reversing it, both ends will still travel the entire sroke of the crank. Installing the pistons in reverse perhaps was the conversation? Then I could see a rise in compression.

I like this conversation

[Joseph Upson]

How can an offse on a rod cause stroke increase? If a rod is bent even, then the center to center would not change by reversing it, both ends will still travel the entire sroke of the crank. Installing the pistons in reverse perhaps was the conversation? Then I could see a rise in compression.

I like this conversation

A bent rod would not increase stroke because the small end and the big end center would still be in the same location.

You're right it wouldn't on second look because although the mass of the connecting rod would be off center relative to the big end circle the hinge point would be in the same place giving the appearance of applying force at a point further out. A lapse of imagining judgement on my part for a moment. Trigonometry works best for me when I apply it to paper first and then visuallize.

The conversation was about the rod being off center at the time because he was trying to sell them as a result from what I recall.

[geoffinbc]

That rod in a previous thread was offset for and aft in the engine because it was a rod from a 4.3L which uses a split pin crank which sets each rod journal off center from the bore of the piston. A different situation.

In the case of the 3900 they have widened the block a bit but the included angle between cylinders is the same. So if you create a line going straight down the center of the bore on each side the 2 lines would intersect 3mm below the crankshaft bearing centers.

[ForcedFirebird]

The block was widened? The head gaskets are exactly the same on all the holes, except the cylinder bores were moved to the outside.

This makes me want to get a 3900 even if it's just to reverse-engineer it.

...off to Joseph's thread to look at it some more.

Maybe a 3900 could make it's way into the Firebird instead of a 3500

[geoffinbc]

That's what I meant the bores were moved out. Sorry bad choice of words.