Maximum bore size and compression

[fe350chev]

Hi

A simple question and I don't have time to reasearch but want opinions.

I have an old EJ motor here that needs a big rebore anyway and I was wondering if I bored it to fit standard 149 pistons, what would be the final compression ratio with a normal head? Also, what would a mild cammed engine run best at? Is anything over 10:1 too much running premium, or I was thinking of a high lift cam running straight gas setup.

But would prefer to stick with petrol.

It is a 138 grey not 132. What the maximum anyone has rebored, I think 149 would be sage though from what ive heard and read.

[Tortoise]

Hi fewithrb30,

I assume you are talking about 149 pistons from a red motor.  Pistons from a 149 red motor are 3.25" diameter and (because of the 3.125" stroke of the grey motor crank as opposed to the 3.000" stroke of the red) will give you 155 cubic inches in a grey motor.  The red motor pistons may have a different compression height (distance from the centre of the gudgeon pin hole to the top of the piston) and therefore MAY give you a different compression ratio.  You will have to measure them - or someone on this site may know the difference.

The gudgeon pin is also larger in the red motor pistons, so you may have to have the little ends of your grey conrods honed out to match for an interference fit (grey motors were a small diameter fully floating gudgeon whereas reds were a larger diameter and interference fit) - although this has been done before.  For all the stuffing around, and cost, you may do just as well using oversized grey motor pistons and your grey motor rods.  Grey motor pistons are commonly available in 3 & 3/16" (3.187") bore (for a displacement of about 149 or 150ci), and less commonly available in other sizes.  I think JP Engineering in SA still make varying grey motor pistons, and can even make dome-topped 12.5 to 1 compression ratio pistons for race applications.

Old FX & FJ hotrodders used to bore their grey motors to 3.25" but because of poor quality manufacturing in the early Holden blocks, they would sometimes risk boring into the water jackets, and have to scrap their block.  The later model grey motor blocks were slightly improved in this regard.  If you have a 'J' prefix engine number on your block you are in the best shape.  Generally 'No prefix letter' = FX/FJ; 'L' prefix = FE/FC; 'B' prefix = FB/EK; and 'J' = EJ.  The more common 3 & 3/16" bore grey motor pistons would probably be your best bet, would still give you about 150 cu in (not much difference to 155), be more reliable in regards to the block, and even give your block another life if you ever wanted to rebuild it again.

The largest bore size that I have heard of in a grey motor without special sleeves being pressed-in and welded (a big job), was 3 & 1/4 + .030" (3.280") giving a total displacement of 158 cubic inches.  This was done a couple of times by racers in the early 60s, and the engines generally held together, however it must be pointed out that in the 60s, grey motors were relatively new and wouldn't have suffered much water jacket corrosion at that time.  The grey motor blocks now are around 50 to 60 years old.  Racers more commonly bored their grey motors to a maximum of 3 & 1/4" (155ci) or even 3 & 3/16" (149 to 150ci).  These sizes were more reliable.  More exotic experiments were done by re-sleeving with bigger sleeves but these had to be welded into the block top and bottom to give some rigidity back to the block.

Lots of people overbore their engines to the limit, but the extra few cubic inches do not usually make that much of a difference.  If you are using the same head, cam, carbs and exhaust, the percentage difference in the power between a slightly overbored engine and a hugely overbored engine, is nowhere near the percentage increase in the cubic capacity.  For example, I did up a 3 & 3/16" (149 or 150 cu in) block to replace the old stock (3" bore) 132 in my FE.  The stock block was still in good nick and was holding compression well, but I wanted what I thought would be a reliable increase in horsepower.  I used the same head, cam, carb & exhaust from my stock-bore FC engine.  The difference was hardly noticeable.  Now I only bore my engines to the next available oversize - and give my engines many more lives at the same time.

I don't know what the difference in compression ratio will be, but the standard ratio for an FC was about 7:1 and an EJ about 7.5:1.  I don't imagine it will increase the ratio much above 8.5:1.  Red motors with a standard ignition timing curve will handle a maximum of about 10:1 on premium.  I imagine that the grey motor head being less advanced would probably handle something less than that - maybe 9.5:1 (but I am only guessing).  And I also imagine that this would be a good compression ration for a mild cam. 

If you are going to go straight gas, you can increase the compression ratio heaps.  You could probably even use those special dome-topped 12.5:1 compression pistons.

Hope this helps.  Tortoise.

[fe350chev]

yeah i agree with 9.5 for good unleaded and higher for gas. that was a big response. i think i would rather go the oversize with the grey pistons as you suggest but i have read that the red ones are much cheaper, thats only reason. ill have to check it out. i also agree that extra horsepower is not achieved by the maximum bore. but this engine i stripped had a broken ring and heaps bad bores anyway. i have 2 grey motors and i can see what you mean about the corrosion.

thanks for the reply, its good to exchange ideas.

[Tortoise]

I know what you mean about the red motor pistons being cheaper.  You can pick up fairly cheap red motor piston sets on eBay (although 149s might be a little rarer).  Most grey motor pistons will be more expensive, but if you go with the red motor pistons you will have to add the cost of having the little ends of your grey motor rods honed out to fit the red motor gudgeon.

Tortoise.

[fe350chev]

yep. cool.