Engine mounting plates are underway

Mounting the Chevrolet LS street engine in the car is going to be a challenge because the original race engine was stress mounted, that’s what formula car designers say when they mean that there isn’t any frame between the front of the car and the rear suspension.  The engine block itself acts as the structural member and carries all of the static and dynamic loads.  The front of the engine bolts to the back of the tub and then comes the bellhousing and the transaxle.  The rear suspension then bolts to the sides and top of the bellhousing and transaxle.  It’s all very compact and elegant, but not really re-producible with the street car components that I’m trying to use.

I’m not even sure if an LS block is strong enough for stress mounting, but I know it’s been done in other Indy cars while using a lower supporting brace so that’s what I’m going to do.  So at this point, I am fabricating drag race style front and mid plates to 1) bolt the front of the block to the back of the tub and 2) bolt the rear of the engine to the bellhousing.  I’ll figure out something for the rear suspension subframe after I figure out what transaxle I’m going to use.

I started with the engine-to-bellhousing mid plate because it was easier.  It was easier because there are several good dimensional drawings of the back of an LS engine block out there on the Internet and these drawings gave me the positions of the six mounting bolt holes.

Figure 1: I found this on the Internet a couple of months ago, but I forgot where, so I'm sorry that I can't acknowledge the original author. 

Then it was a matter of determining the positions of the six mounting holes for the bellhousing.

Figure 2:

This is when I learned about transfer punches. 

Figure 3: I bought a set of these from Harbor Freight.

I figured out pretty quickly that I was never going to get good measurements of the bellhousing bolt locations by trying to measure with a tape measure from the center of one hole to the center of another hole.  Luckily a little bit of web surfing turned up information on how to use transfer punches on drilled holes and I was able to make a template of the bellhousing on a piece of scrap plywood.  Then I combined the six hole locations for the back of the block with the six holes for the bellhousing on my mid plate prototype (a ¾” piece of plywood sandwiched together with a piece of ½” MDF because I couldn’t find any material that was the needed 1 ¼" thick).  With that done, I was able to marry the engine to the bellhousing.

Figure 4: Mid plate installed.

At this point I should say something about the thickness of the two plates.  From the best that I can tell with a hand tape measure, I have approximately 22 ½" from the back of the tub to the front of the bellhousing.  I measured my LS block at 20 ½" from the front to back mounting faces so that means that I have two inches to work with (That’s what she said!).  I could have just made each plate one inch thick, but I decided to make the front plate ¾ of an inch thick in order to give myself the chance to use a stock mechanical water pump.  This is an LS engine specific topic that will come up in the future.

The front plate to join the engine to the tub was a littler more work.  Unbelievable to me, I couldn’t find a dimensional drawing of the front of an LS block anywhere on the Internet.  But once again a little Internet searching led to the discovery of threaded hole punches.  Like the transfer punches, they help you locate the center of a hole, but these are little inserts with spikes right at the center of the hole that screw into threaded holes.

Figure 5:

I screwed the punches into the six holes around the water inlets and outlets, the eight holes for the timing cover and the one bracket mounting hole that there is on the lower driver’s side of my Gen III LS truck block.  Then I took a hammer and transferred the hole locations to a piece of scrap Melamine by whacking my template with a hammer at each hole locations.

Figure 6: LS engine block front mounting hole locations.

Figure 7: Close-up of the dimples made by the threaded hole punches.

The six mounting holes for the back of the tub were relatively easy because I had the old bulkhead panel to use as a template.  Those are drilled holes so I used the transfer punches again.

Figure 8: Original bulkhead panel that separated the engine from the tub and the fuel cell.

You’ll notice that I left a lot of extra expanse on the sides and that is on purpose to help me figure out where I can attach a future rear subframe and the middle supporting brace.

Figure 9: Front plate prototype with six mounting holes for the tub and nine for the engine: six around the water inlets/outlets, the two lowest timing cover bolts and the lower accessory bracket mounting hole.

These prototypes are made of wood, but eventually I’ll put everything into a CAD drawing and have the plates made out of metal.  For right now though they are going to allow me to marry the engine to the car and start measuring for some transaxle options.  Oh, but first I have to test fit the body work…