Narrow engine, short chains.

[davidppp]

hello all.

Has anyone tried to build a radically narrower engine?

ie flycut heads a lot, shorter pistons, and all?

The chain pitch would permit one fulllink taken out ..

Obviously

Kind regards
David

[racing97]

Yes you can use a 1 and 1/2 link extension and remove two links to shorten the chain. The chain I believe is 3/8 pitch double row. This was done a long time ago for compression on complicated displaced (bore and stroke) motors for racing when aftermarket pistons where not developed such as they are today.

regards

[Henry Schmidt]

Head to cam tower spacers are an option that can accommodate almost any dimension.

[davidppp]

Very Clever, Henry!

Is the valve/rocker relationship a limiting factr?
What would the maximum thickness of that spacer be, please?

Many thanks
David

[Henry Schmidt]

I would guess the .080" might be the limit. We make .060"
If the goal is to increase compression from a existing piston, perhaps the simplest, most effective way might be to lengthen the connecting rod, there by forcing the piston further into the combustion chamber.
This keeps the overall dimension for the engine correct and improves the rod length to stroke ratio.
So what is the importance of rod ratio anyway?
rod ratio describes piston geometry (3 things):
1. piston speed away from TDC and BDC
2. piston dwell time at TDC
3. the amount piston sideloading against the cylinder wall as the piston travels up and down the swept volume.

So rod ratio can affect how high you can rev, since a low rod ratio places more side loads on the wall.

The rod ratio in a naturally aspirated engine affects how well the cylinder is filled and dictates cylinder head port size. The faster the piston pulls away from TDC on the intake stroke means you can get more suck to fill the cylinder. How fast the piston transitions or flip-flops from squeezing the exhaust gas out at TDC for the exhaust stroke to dropping down and begin filling on the intake stroke (i.e. TDC dwell time) affects your cam overlap and cylinder filling.

low rod ratio engines have short piston dwell times at TDC and fast piston speeds away from TDC (or faster piston speed dropping down on the intake stroke compared to a long rod ratio). So a low rod ratio motor generates high flow velocities for filling through the intake port at low-mid rpm. These engines like bigger cylinder head intake port sizes compared to a long rod ratio motor like the Civic Si's b16a.

Low or short rod ratio ALL MOTOR engines like a cam with more lobe separation angle, more duration, and more cam overlap, since it has short piston dwell time at TDC and needs help scavenging in fresh air/fuel.

This all has to do with revving ability, proper intake port sizing, and cylinder filling IN AN ALL MOTOR SETUP which depends on passive filling using lower pressure in the cylinder (called pressure gradient) compared to the atmospheric pressure (14.7 psi). In turbos you push in the air to fill the cylinder and so rod ratio plays a very MINOR role in cylinder filling.

The importance of rod ratio in a boosted engine relates to how efficiently the inert burnt exhaust gases are removed from the cylinder after combustion. The piston speed away from BDC to push the exhaust gas out is important. Remember exhaust gases aren't burnt twice and cannot make power and so if they aren't removed, they just take up space in the cylinder ... preventing room for fresh air/fuel from coming in to do their job. Burnt exhaust gases are like unwanted tenants not paying rent: you want to evict them from the cylinder."


Rod length can be changes by either offset boring the wrist pin bushing (simple and cheap) or just buying a custom set of rods (expensive but stronger & lighter)

Combine longer rod length and chamber volume reduction (mill head and install spacer) and compression goes up.

Some minor piston machining is requires when rods are lengthened.

[PFM]

Henry,

Do you make the tower spacers in .050? Trying foolishly to make 3.2 pistons work in a 3.0.

Thanks,

PFM

[YTNUKLR]

PFM ,

offset bore the small end bushing 0.8mm to extend the 3.0 rod from 127.8mm to 128.6mm. Also bore the wristpin bushing 23mm instead of 22mm.

Thanks Henry

[PFM]

YTNUKLR,

The rods are already at 23 mm pins and you can get a bit more rod length out of the re-bush but not much. The compression height difference is 1.2 mm so no way to pick up that much in a stock rod re-bush. I try to save offset bushings for final adjustment of deck height. This is a bunch to make up for, the cam bos spacers Henry shows look like the fix but still checking everything out before trimming a nice set of cylinders and all this work. A fresh set of pistons may be a better idea. The sad part is these pistons are ceramic coated, skirts slipper coated, balanced and had a cryogenic treatment done.

Thanks for the input.

PFM

[Henry Schmidt]

Scott is correct.
The rods can be rebushed to achieve the proper pin location.
You just need a machinist that has experience with custom bushing work.
We have been doing this modification for years.
The cam tower spacers can be make in almost any dimension needed but I would think the rod length modification is the better option.
Not only is it cheaper but you get the many benefits offered by a longer rod.

[PFM]

Scott, Henry,

I may be getting older but... How will a .8 mm pin offset resolve a piston compression height difference of 1.2 mm? The bushing is 1.04 mm thin on the end of the rod. Are you saying to bore the steel rod oversize to stretch it? The stock rods are strong but... The bushing would be near zero at the thinnest spot. I guess it would work but????

I guess a little rod offset and a little cylinder trim could get the job done or sell these nice 3.2 pistons and buy the correct ones or maybe have new ones built to better suit my combination.

Thanks,

PFM

[Wayne 962]

You can also pull a link from the chain and use smaller tensioner gears. I think there's a pic of that in Bruce's book...

-Wayne

[Henry Schmidt]

Originally Posted by PFM

Scott, Henry,

I may be getting older but... How will a .8 mm pin offset resolve a piston compression height difference of 1.2 mm? The bushing is 1.04 mm thin on the end of the rod. Are you saying to bore the steel rod oversize to stretch it? The stock rods are strong but... The bushing would be near zero at the thinnest spot. I guess it would work but????

I guess a little rod offset and a little cylinder trim could get the job done or sell these nice 3.2 pistons and buy the correct ones or maybe have new ones built to better suit my combination.

Thanks,

PFM

Interesting, it's advise based on years of experience. Take it or leave it, but question it ? Sometimes this seems like a waist of time.
Good luck