CORVETTE RESTORATION UPDATES

MARCH 26, 2006 - OIL PUMP PRESSURE BALANCING

 

 

 

 

   

No major news on the engine side of things quite yet. I dropped off the block about a week ago and it's just now moved towards the top of the shop's to-do list. Thus far they have done some of the basics - hot tank, magnaflux & take preliminary measurements - and said the block is in great condition for its age. I'm hoping to have more news in the next update. In the meantime I decided to do a little work with the stock oil pump.

While reading several books on building performance engines I found one recurring message - stick with the stock big block oil pump! Apparently these pumps have a solid reputation for being bulletproof.

Here's what the stock oil pump looked like after a thorough cleaning.

Copyright - Tim Cote 2007

Oftentimes people replace the stock oil pump with a high flow or high pressure aftermarket pump as a matter of course when completing an engine build. However, much of my research seriously questioned this practice and made a compelling case in the process. First, for street and street performance use the stock oil pump is perfectly capable of pushing more than enough of oil (provided the correct main bearings are used and that bearing clearances are not ridiculously large). Secondly, high pressure and high volume pumps certainly do push a lot of oil, but much of that oil is simply forced out of the pressure release valve and never makes it through the engine. Also, don't forget that pushing all that extra oil takes more work and more horsepower. Lastly, although a high volume pump could easily have been provided by GM engineers, they didn't feel it was necessary - even on the high performance L-88 and ZL-1 models.

With that in mind I started checking my stock pump to see how it looked, overall it was in very good condition. Below is a photo of the pump with the cover removed.

Copyright - Tim Cote 2007

Disassembly of the pump is quite simple, the parts are all slip fit so they're easily removed. Prior to removing the drive and idler gears I marked their relative position so that they could be reinstalled in the same orientation.

Copyright - Tim Cote 2007

Despite the bullet-proof reputation of the stock big block oil pump several of my references recommended a few minor modifications when using them in a performance application. After giving this some consideration I opted to go ahead and make the modifications as they were explained and outlined in the book "How to Hotrod Big-Block Chevys". The book is a good reference, although it's a bit dated, but it provides some good info nonetheless. Considering used copies can be had for less than $10 it's certainly a worthwhile investment.

First on the list of modifications was to lightly chamfer the edges of the gear teeth with a file.

Copyright - Tim Cote 2007

Next, and perhaps the scariest modification, was pressure balancing the oil pump. By creating small grooves in the pump housing the hydrostatic forces on the idler and drive gears can be more evenly distributed. This reduces the amount of pulsating forces that are transmitted up the oil pump drive shaft and ultimately into the distributor. Although not a major problem, this pulsating has been known to contribute to spark scatter (changes in timing) at high RPM. The pressure balancing also helps prevent cavitation and allows the pump to operate more efficiently and on less horsepower. GM engineers provided some of these pressure balancing grooves on their high performance pumps (used on L-88 Corvettes), but these modifications go a step further.

Having said that, below I've laid out the locations of the pressure balancing grooves on the pump housing.

Copyright - Tim Cote 2007

To make the grooves I used a Dremel tool with a bit intended for chain saw blade sharpening. The size of the bit (5/32" diameter) was just about the perfect size and it worked very well.

Copyright - Tim Cote 2007

After a few hours of careful work the grooves were complete. Below is a picture of the modified housing. Note that care needs to be taken to assure the grooves in the cover (#1 & #2) need to line up with the vertical grooves on the pump housing (#1 & #2). The other two red arrows are meant to point out vertical grooves in the pump housing (a bit hard to see in the photo).

Copyright - Tim Cote 2007

Lastly, and probably most importantly, the proper end clearance between the gears and pump housing cover needs to be established. Excessive clearance results in poor priming ability and inefficient pump operation. Ideally end clearance should be about 0.0025 inches. If excessive clearance is found the housing can be sanded down by using a piece of fine to medium grit sandpaper placed on a flat, hard surface. Be sure to apply even pressure and rotate the pump often during this process to ensure material is removed evenly. Conversely, if the clearance is too small the gear height can be reduced by using the same sanding procedure above.

Copyright - Tim Cote 2007

After establishing the proper clearance and a very, very thorough cleaning I reinstalled the pump cover and screws using Loctite on the screw threads.

That's all for this update!

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Total time spent on restoration to date:

283 HOURS

JUMP TO THE NEXT UPDATE -  ENGINE BLOCK PAINTING

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