lower the cylinder and ports?

[rd400canuck]

Hi guys,

I'm about to start to disassemble the RD400 for the winter the go through the motor and make sure I did everything right during my rebuild last winter and also to send out cyls and head for squish correction.

From quick older check Im getting 1.3 ~ 1.4 mm of squish. I am just wondering if I use a thinner base gasket to drop the cylinder say .4mm would i notice any difference in low and mid range power? Id have to watch my compression of course.... i think it was 140psi the last time i checked.

Thoughts?

[Yamaha 179]

I sell three thicknesses of base gaskets: .22mm, .35mm, and .56mm.  The .56mm gasket is stock thickness.  The thinnest gasket is made from sturdy material and works well, but it is for adjusting squish clearances much smaller than yours.  You need to send your heads or cylinders to a quality machine shop to alter the sealing surfaces to close up that gap.  If you are going to modify the engine
in other ways in the future you should probably wait and do everything together.  BTW, just closing up the squish clearance won't give
you a noticeable increase in power.  It will however, reduce the chance of detonation and engine destruction. 
Lyn Garland

[rd400canuck]

Thanks Lynn,

Im wondering now... are the aftermarket base gaskets from say those Vesrah kits thicker? I dont have one in my hand but thinking back to my rebuild It seemed like it was 1mm or something like that.

[Yamaha 179]

I've never measured a Vesrah base gasket but I doubt they would be that thick.  That is more in the size of a spacer than a gasket.
Lyn

[pdxjim]

Maybe sticking my neck out and showing more ignorance than knowledge, but ... in the quest for performance, wouldn't one generally want to raise (instead of lower) the cylinders with thicker base gaskets and deck/skim to regain proper compression /squish?

[Yamaha 179]

Maybe sticking my neck out and showing more ignorance than knowledge, but ... in the quest for performance, wouldn't one generally want to raise (instead of lower) the cylinders with thicker base gaskets and deck/skim to regain proper compression /squish?

That was quite a common way to increase power simply, easily, and cheaply years ago and it works really well with stock exhaust systems.  To be rather crude, you don't even have to modify or clean the ports; just raise the cylinder two millimeters with a shim and adjust the top side by milling the top of the cylinder or milling the cylinder and turning a bit off the head to eliminate dead spot at the edge of the combustion chamber where detonation is known to start.  Can be done easily with a Bridgeport and a Logan.  It ain't necessarily pretty, from the inside, but it looks stock and they will run really good for a "stock" bike.  Been there done that.
Lyn Garland

[rd400canuck]

Hi pdxjim...   my quest for more power is basically more grunt. I like a stock bike for this reason but I find my Rd400 could always use more low and mid power. I was hoping that lowering the cylinders by .5mm Id be producing more torque. However... I dont think .5mm will be too noticeable but combined with a .9mm squish it would help out. I ride my bike around town mostly ... I am rarely above 5500 and at cruise im 3000 to 4000 rom max so higher ports and aftermarket pipes arent something that would benefit me.

Plus I LOVE the look and sound of stock pipes.

[Organicjedi]

Hi pdxjim...   my quest for more power is basically more grunt. I like a stock bike for this reason but I find my Rd400 could always use more low and mid power. I was hoping that lowering the cylinders by .5mm Id be producing more torque. However... I dont think .5mm will be too noticeable but combined with a .9mm squish it would help out. I ride my bike around town mostly ... I am rarely above 5500 and at cruise im 3000 to 4000 rom max so higher ports and aftermarket pipes arent something that would benefit me.

Plus I LOVE the look and sound of stock pipes.

You could just lower the final drive gearing and install a lighter 520 chain while you're at it. Probably the best way to go to increase low end power unless you plan to do more extensive engine work.

[m in sc]

get a modern igntion with programmable curve. easier way to get what you want IMHO.

[2T5]

Maybe sticking my neck out and showing more ignorance than knowledge, but ... in the quest for performance, wouldn't one generally want to raise (instead of lower) the cylinders with thicker base gaskets and deck/skim to regain proper compression /squish?

That was quite a common way to increase power simply, easily, and cheaply years ago and it works really well with stock exhaust systems.  To be rather crude, you don't even have to modify or clean the ports; just raise the cylinder two millimeters with a shim and adjust the top side by milling the top of the cylinder or milling the cylinder and turning a bit off the head to eliminate dead spot at the edge of the combustion chamber where detonation is known to start.  Can be done easily with a Bridgeport and a Logan.  It ain't necessarily pretty, from the inside, but it looks stock and they will run really good for a "stock" bike.  Been there done that.
Lyn Garland

Is anyone able to expand on this?

Apart from increasing squish gap, reducing the risk of a gasket sucking in, and reducing compression ratio, what is to be gained from using a thicker base gasket?

[teazer]

Lyn explained that the idea is to raise the ports and machine the same mount off the top of teh cylinder to retain the same static compression and squish depth.

[1976RD400C]

Back in the 80's, when we really didn't know what we were doing, 2 base gaskets, remove head gasket. Just made sure the head was flat and brushed on some copper-coat gasket sealer. 

[2T5]

Maybe sticking my neck out and showing more ignorance than knowledge, but ... in the quest for performance, wouldn't one generally want to raise (instead of lower) the cylinders with thicker base gaskets and deck/skim to regain proper compression /squish?

That was quite a common way to increase power simply, easily, and cheaply years ago and it works really well with stock exhaust systems.  To be rather crude, you don't even have to modify or clean the ports; just raise the cylinder two millimeters with a shim and adjust the top side by milling the top of the cylinder or milling the cylinder and turning a bit off the head to eliminate dead spot at the edge of the combustion chamber where detonation is known to start.  Can be done easily with a Bridgeport and a Logan.  It ain't necessarily pretty, from the inside, but it looks stock and they will run really good for a "stock" bike.  Been there done that.
Lyn Garland

What type of power? At what cost elsewhere? For such an easy, cheap mod, why is it not widely known or used?

I've searched the internet high and low looking for a detailed description of what's to be gained; all people ever mention is it's use for lowering compression or increasing squish gap.
The mention in this thread about a power change is the closest I've seen to an alternative reason. Come on folks, who's gunna talk me through it?   

[m in sc]

I actually did it on a waterbuffalo, used a 2mm plate and I was the one that cut the cyl. (it was for lane on'blue boy')  that thing ran great, would power wheelie in 1st gear under power all day long w jemco chambers and vm34s.

[teazer]

I typically use a 3mm spacer plus extra gasket on a GT750, but they are available in various thicknesses.

The idea is to raise the exhaust port and transfer port roof to increase duration which in turn favors high revs, but it also adds effective time area.

B U T and it's a big But, is that it works on older motors which were made with very little duration and insufficient time-area to make lots of power. But on a modern motor designed for high revs, it is likely to create too much duration.

So here's the magic sauce:  calculate the duration and time-are for your target HP and RPM peak then calculate the stock duration and time-area and from those it becomes clear how much taller and/or wider the ports need to be to achieve the desired outcome.  On something like a GT750, it is necessary to raise the exhaust by 4-6mm and there's not enough metal for that, so we use a lift plate. 

On say an Aprilia RS250, the ports are already as high as you would want to go, so, a lift plate would not be appropriate.

Or say an RD400 looking for TZ port specs, might be better with a lift plate rather than raising the port roofs. I haven't done those calculations to know if that's necessary.  It's just an example.