Jeep Strokers

For the same build recipe, how much extra power would .030 and .060 overbore create compared to the stock bore size?

I'm planning a stroker build and I was curious to know if overboring the cylinders on my existing 4.0 is worth it. The HP and torque numbers, that I've seen for 4.5 and 4.6 recipes, have been similar.

In my opinion there's no reason to run a standard bore block.
I've sonic tested a dozen or so 4.0 cylinder blocks from Renix era to late HO's.
My tests have shown that the blocks easily have enough material to support a 3.935 .060 bore.
80% of the Strokers that I build go to a 3.935 bore. My race Strokers are bored to 3.970 and finish honed with a torque.

wldc10 wrote:For the same build recipe, how much extra power would .030 and .060 overbore create compared to the stock bore size?

I'm planning a stroker build and I was curious to know if overboring the cylinders on my existing 4.0 is worth it. The HP and torque numbers, that I've seen for 4.5 and 4.6 recipes, have been similar.

The difference between a stock 4.0 bore and a stock 4.0 bore at .030 oversize is 1.55% of the total displacement. Stock 4.0 is 241.5 cu in With a .030 over bore you get 245.26 cubic inches. That is 1.55% more displacement and 1.55% more power. Not sure dyno runs are accurate enough to actually show that much of a difference. You are talking about 2 or 3 horsepower on a stock 190 hp engine.

On the other hand, it cost nothing extra to bore the block out as far as you can. Any power gain is just free power, so why not go to .060 or 080 over. Or in the case of Russ's race motor to .095 over size bore? That is 253.49 cu in with the stock 4.0 stroke. and a 4.9648% increase in both displacement and horsepower.

I'm glad this question was asked...and answered. I was wondering the same thing. Have read differing opinions about whether going .060 over as opposed to .030 was worth it...assuming .030 would be sufficient to attain a clean workable cylinder. Sounds like .060 is the way to go...

ronazon wrote:Sounds like .060 is the way to go...

The only disadvantage is that if the cylinders are already +0.060", the block will basically become scrap if they should ever need to be rebored again. A +0.030" overbore at least gives you a chance for a second rebore.

Cheromaniac wrote:
The only disadvantage is that if the cylinders are already +0.060", the block will basically become scrap if they should ever need to be rebored again. A +0.030" overbore at least gives you a chance for a second rebore.

I use that same approach on my race motors. .030 first then .060

SIXPAK wrote:

Cheromaniac wrote:
The only disadvantage is that if the cylinders are already +0.060", the block will basically become scrap if they should ever need to be rebored again. A +0.030" overbore at least gives you a chance for a second rebore.

I use that same approach on my race motors. .030 first then .060

All good points and I certainly can't disagree with them.

But maybe we could look at it in a different way.
When we're building a stroker, probably the least expensive component in the build is the cylinder block core.
You could buy three blocks for the cost of a set of forged pistons. Even if you're at 3.935, a Hastings Molly ring is available in .080 overbore.

Just something to think about.

Russ Pottenger wrote:You could buy three blocks for the cost of a set of forged pistons.

Depends on which part of the World you live in. Outside of the US we're not that fortunate.

I read somewhere that increasing bore size will also increase the chances for detonation. Is that true?

optmaxx wrote:I read somewhere that increasing bore size will also increase the chances for detonation. Is that true?

As long as the static compression ratio remained the same I don't see how that could be true