What are the pro's and con's of destroking an engine?

I'm considering it as an option, but would like the GRM'ers to weigh in on the subject. I'm thinking about bumping the compression back up to the stock, or so with higher compression pistons.

Andrew

Um, destroking (or stroking) an engine involves changing the crank, not the pistons.

Yes, changing the pistons will alter the compression, but not the stroke.

I think I missed something. He only said he would use higher compression pistons to bring the compression back up to stock.

from what I have previously read, you don't always need to change the crank, just the length of the rods. Am I wrong?

Andrew

digdug18 wrote: Am I wrong?

I'm going to say yes. Changing the rods won't change the stroke. Imagine the crankshaft in a position where the piston is at TDC and then turn it 180* so the piston is at its lowest point. The distance between the two piston heights is determined by the crankshaft.

Destroking by itself would lower the compression ratio, so the OP is saying he would counter act that by switching to a higher compression piston to get back to what it was before the destroking.

digdug18 wrote: Am I wrong?

Yes.

Only things I can think of is you'll lose torque but gain a higher redline. With a 4 holer larger than 2 litres, you should lose some of the inherent vibrations that are associated with big 4s. As for things changed, crank has to be swapped if you dont want it reground for a shorter throw, conn rods and/or pistons. Just my uneducated answer.

I would say one negative of destroking is less displacement. Maybe you could operate at higher revs though?

What engine are we talking about anyway?

Destroking will reduce displacement, obviously, and also cost you some power. You can either use a different piston to get back to the same compression ratio, or, even better, use longer connecting rods with the existing piston. Longer rods help a piston dwell near top dead center for a longer period of time than shorter rods, which helps raise cylinder pressures, and thus produce a tad more power, and provides a more complete burn of the air/fuel mixture, which further aids power, reduces emissions, and bumps fuel economy up a little.

Shorter rods with the same pistons on the same crank will only yield a lower compression.

Put a 267 Chevy crank in a 350 (or is it a 400) block and get a 5 liter, 8,000 rpm screamer. That's worth it for the sound alone.

offset grind the rod journals by .020 then get .020 undersize bearings ( yes thicker bearings are called undersized), keep rods, keep crank, keep pistons but mill the head and/or deck surface enough to maintain ratio. cheap way to change stroke

Loss off CC's, higher redline because of decreased piston speed: less stress on bearings and rods. If you can get the engine to spin fast enough to make up for the loss of displacement (think 4000 RPM with a 1 liter block vs 8000 rpm with a 1/2 liter block) you should gain power.

Add a turbo for massive fun and the best gains.

grimmelshanks wrote: offset grind the rod journals by .020 then get .020 undersize bearings ( yes thicker bearings are called undersized), keep rods, keep crank, keep pistons but mill the head and/or deck surface enough to maintain ratio. cheap way to change stroke

uhhh, thats not changing the stroke either.

You simply cannot change stroke with out changing the crank shaft, as stroke is determined from the height from TDC to BDC and that is pretty much set by the offset of the crankshaft.

RedS13Coupe wrote:

grimmelshanks wrote: offset grind the rod journals by .020 then get .020 undersize bearings ( yes thicker bearings are called undersized), keep rods, keep crank, keep pistons but mill the head and/or deck surface enough to maintain ratio. cheap way to change stroke

uhhh, thats not changing the stroke either. You simply cannot change stroke with out changing the crank shaft, as stroke is determined from the height from TDC to BDC and that is pretty much set by the offset of the crankshaft.

I'm pretty sure he was talking about modifying the crankshaft. Rod journal = the part of the crankshaft that the Rod interfaces with.

You can offset grind the crank to gain/loose stroke.

Changing rod length does not change stroke. Ever.

Destroking an engine does not necessarily mean higher redline, higher peak HP RPM, or any of that.

The engine will undoubtedly have less power overall.

Engines are most often RPM limited by their valvetrain, not their connecting rods and crankshaft, seriously. Most destroking was done when engine metallurgy was not what it is today, or to meet rules. An engine with a higher bore/stroke ratio THEORETICALLY has a breathing advantage at very high RPM due to valve (un)shrouding, so rules limiting displacement could still have large bores. Simply reducing the stroke doesn't mean the engine will breathe better, nor does it mean it will have a better high end.

JeffHarbert wrote: Destroking will reduce displacement, obviously, and also cost you some power. You can either use a different piston to get back to the same compression ratio, or, even better, use longer connecting rods with the existing piston. Longer rods help a piston dwell near top dead center for a longer period of time than shorter rods, which helps raise cylinder pressures, and thus produce a tad more power, and provides a more complete burn of the air/fuel mixture, which further aids power, reduces emissions, and bumps fuel economy up a little.

What I was going to say.

But as has been pointed out, why? And what engine are you working with?

Usually the only reasons I've ever seen to destroke an engine is to fit into a certain displacement limited racing class.

Can't really imagine any other scenario where I'd want to.

Hell, when I go to Vegas in July it'd be nice to hit the jackpot. I've spent the last week looking at the Crower 4.15L stroker pacakge and Cosworth big valve heads for my 350Z.

Not that I have $14k to build a motor or anything though.

RedS13Coupe wrote:

grimmelshanks wrote: offset grind the rod journals by .020 then get .020 undersize bearings ( yes thicker bearings are called undersized), keep rods, keep crank, keep pistons but mill the head and/or deck surface enough to maintain ratio. cheap way to change stroke

uhhh, thats not changing the stroke either. You simply cannot change stroke with out changing the crank shaft, as stroke is determined from the height from TDC to BDC and that is pretty much set by the offset of the crankshaft.

The key phrase was "offset grind."

currently, I'm on the hunt for a 4g64 out of an evo 8, cheapest I can find for a short block is 1k, 2k for a long block minus the turbo. I'm hoping for a small/quick spooling turbo, but I need to build the block first. Then again, I haven't the car to put it in either, but will most likely grab a 1g awd laser to drop it in.

Andrew

I can think of only one instance where it might make sense. If you're displacement limited and you have a horribly undersquare motor, you can do a big overbore to get displacement where you want it. Possibly decrease valve shrouding issues, etc.

tuna55 wrote: Changing rod length does not change stroke. Ever.

The only way rods could affect stroke is if the crankshaft is not centered on the cylinder bores. Picture the crank being offset a bit to at TDC the rods are not inline with the cylinder bore. The Fit engine is like this, but it to a very small degree...I think on the order of a couple of degrees. If there was an engine with a large enough offset then changing rods would effect stroke a measurable amount.

Changing the rod length does not shorten the stroke. Moving the crankpin closer to the centerline of the crank does. You then have to move the piston pin down the piston to keep the deck height the same. If you stroke the engine, you move the crankpin further from the crank's centerline and you move the piston pin up to keep the deck height the same.

As far as power loss, it's like anything: it all has to work together as a package. Don't have the bore vs stroke specs handy, but the XR400 Honda and the WR400 Yamaha shared a common displacement. The Honda had a smaller bore and a longer stroke and 4 valves and was air cooled. The Yamaha had shorter stroke, larger bore and 5 valves and was liquid cooled. Both had electronic ignition but the Yamaha had a 3D ignition map, meaning it had to have a throttle position sensor. The Honda made 32 HP at around 8K RPM, redline 9600 RPM. The Yamaha made 44 HP at 10,500, redline 11,500. The cool thing: they had similar amounts of bottom end 'grunt' or pulling power. I owned one of each so I know first hand.

BTW, MGB engines had the crank offset from the cylinder bore centerline. I don't really see a way changing the rod length could change the stroke, or maybe I just can't visualize it.

Not rod length, but the offset of the rod journal vs the original rod journal centerline.

I've been trying to figure that out all day and finally got it. I can't "verbalize" it though.

I guess I'm wrong then. Cool beans, could you still give me the pro's and cons of destroking a engine?

Andrew

Destroking without doing anything else will decrease displacement and most likely, power. I can't think of any benefits to just destroking.

If you were to increase the bore sufficiently enough to keep displacement the same as, or at least very close to, the original with the shorter stroke, you can take advantage of being able to fit bigger valves, unshroud the valves, etc, thereby giving the engine the opportunity to pump more air. Pumping more air = moar powah.

destroking gives rpms. it will give you less grunt but it will really come alive on the top end. i think the money and effort is better spent on cams and top end work. everybody is worried about stroke and pistons for compression ratio, google the terms "static compression" and "effective compression" and youll see how much top end work is worth