LSx engines position level or little tilted ?

[Bowtieracing]

Should i try to make the engine sit as level as possible or could it be few degrees tilted ?

[chicane]

Originally Posted by Bowtieracing

Should i try to make the engine sit as level as possible or could it be few degrees tilted ?

The answer to this lies within the necessity of working angles of the universal joints in the driveshaft.

The 67-69 F-body specifically (as well as most other GM vehicles produced), as a standard, were produced with the engine and transmission at a 4 degree down angle (tailshaft -4*)... and the differential housing with a 4 degree up angle (pinion +4*). This sets up the proper geometrical relationship for the universal-joint working angles.

This 'number' can be skewed some what... BUT... no matter what, the angle between the engine/transmission and differential/pinion needs to be equal and opposite. For instance, if the transmission is -3.5*... the pinion needs to be at +3.5*.

With the availibility of newer materials and the product of building a more solid platform (chassis)... the numbers can be reduced to, in my opinion, no less than 2*. Utilizing both poly engine and transmission mounts with a manual transmission... you can get away with 2, but... I wouldnt run less. There needs to be some measureable working angle for the universal joints to operate properly without inducing problems... like vibration and jump roping the shaft at higher RPM's. if you run less... the probability of u-joint problems increases greatly.

In other instances... engine horsepower/torque, suspension travel, spring rates and driveshaft material... as well as its length... are other variables that come into play when configuring drivetrain angles. Some are more influential than others but thay all have effect in the equation.

For most... "the equal and opposite angle" between 2.5 and 4.5 degrees will cover just about every application dealing with a 2 joint shaft and the drivetrain speeds that are delt with on this board.

[Bowtieracing]

Thanks Chicane once again for professional answer

[Vegas69]

While I agree with the equal and opposite you will find that the cars that are lowered similar to mine can't run equal and opposite. It would actually give you a positive pitch on the driveshaft and increase the working angle of the rear u joint upon suspension articulation.

[chicane]

Huh... that is peculiar. My 67 is lower than your 69 and I surely don't have that issue... nor have I had any issues with client cars. In fact, one client at Bonneville last week, that has a 69 and I know that its lower than anyone would ever consider here (its about an inch to the frame rail) doesn't have that issue either.

How are you measuring your shaft angles.. ? How much initial angle do you have in the shaft ends at static ride height ? Do you still utilize the rear spring rubbers... ? What is your rear spring rate... ? What busings and/or rear suspension are you using... ?

What you describe maybe more of a spring control issue... because I do know for a fact that the majority of the off the shelf - after market springs that I have seen in the past twenty years or so... cannot locate the IC well enough to negate the issue that you described.

Not to mention, in relation to your specific application... that with a big block, and the power/torque it can produce... if you are at more than about 2 to 2.5*... that is most likely going to be the larger problem, especially with the flexibility in soft rates springs.

[Vegas69]

I measured them with a socket and digital gauge. I removed the c clips and leveled the rockers before hand. Have 4.5 degree driveline angle. I do have a triangulated 4 link so pinion wrap won't be an issue. Talking to DSE and the instructions with the g bar instruct a negative pinion angle. 2-3 degrees.

[chicane]

Ah yes... the one suspension we have yet to delve into... a short armed link suspension.

For the most part, 'pinion wrap' shouldn't be a real issue with a solid link suspension... but I am sure with the power of your big block... it does have some influence in pinion angle change under load, not very much... but its there.

The thing about link suspensions... is where "designed ride height" actually is. In particular to the suspension you chose, once the system is installed you may not have adjust-ability with-in an appropriate kinematic arch. So... if one were to go lower than the "designed ride height"... it would put the suspension at a different place initially, in the arch at static ride height. Since link type suspensions are actually limited by there respective arm lengths in a specific range of motion... and with the suspension being at or closer to the top of the geometric arch... the pinion angle change becomes more exponential as the suspension compresses. So if you were to start out with 4 to 4.5 initially... and compressed the suspension... it could possibly open that up beyond the working angle range of the universal, or enough to induce related problems.

So yeah... big block power, unknown designed ride height and possibly lowered below the previously mentioned unknown value... and I would expect issues.

The instructions for pinion angle with the G-Bar state, "As an example, a two-degree downhill drivetrain angle would require the third member to be set at two degrees with the snout slightly raised." And the note at the bottom of page 12... drives a point to tighten the number by 1 or 2 degrees down from what you have at static ride height (your 4.5 number) to combat system flex from high power drivetrains and high traction... but not actually to set it negative at static ride.

Good thing you brought this up... it was a good refresher for me to re-hash this in my head... and it passes more tech onto the masses.

[Vegas69]

Good call Chicane. I'll keep that in mind while I fine tune the bugs out of this baby. Frank send me those adjustable uppers.