Combustion chamber squish and design

[Alakso]

For you gear-heads out there does anyone know what the stock FJR combustion chamber squish is?

What kind of combustion chamber design does the FJR use, Hemi, D-shaped, Bathtub, etc.?

What is the thickness of the stock head gasket?

 

[03HiYoSilver]

Alakso,

Don't know what design the head is but here are some pictures for you to look at:

https://www.fjrforum.com/forum//index.php?s...st&p=271113

 

[johnny80s]

Looks like a hemi.

Edit - Link for info on combustion chamber design.

 

[Alakso]

Alakso,
Don't know what design the head is but here are some pictures for you to look at:

https://www.fjrforum.com/forum//index.php?s...st&p=271113

Thanks for the link 03HiYoSilver, I appreciate it very much especially seeing how I have dial-up and it takes me forever to do forum searches. I did go through my FSM today and there was only one drawing I could find of the CC. From the drawing it looks like a bathtub or modified bathtub CC design? I'm waiting for the link you gave me to load so I can get a look at the '03 CC design. I wonder if the CC design changed with the second Gen bikes?

 

[johnny80s]

From the drawing it looks like a bathtub or modified bathtub CC design?

Why would you say that? I am not an engineer but it looks like a hemi to me.

 

[Alakso]

Looks like a hemi.
Edit - Link for info on combustion chamber design.

Thanks for the link johnny80s, as above I really appreciate it! From the pics in the link 03HiYoSilver sent me it looks like a modified bathtub CC. A Hemi or hemispherical CC design is what HD used for years until the EVO motor. And of course the Hemi CC design was made famous by the 426 Hemi Chrysler engines that ruled the roads back in the 60's and 70's. Plus I believe the Hemi CC design is still the CC design of choice for Drag racing especially in the Top Fuel class.

 

[Geezer]

The FJR combustion chamber is definitely a hemi design. In fact it is much closer to a true hemispherical shape than the new hemi's being used in Dodge cars and trucks.

 

[Alakso]

As an add on to my original post does anyone know if the FJR CC uses a double squish?

Setting the squish to between .025"-.030" makes the CC very efficent and will help promote good turbulence which in turn creates better thermal effiency which produces better power along with better gas milage. But one has to be careful with warm-ups and not blip the throttle until the engine is at operating temp.

And finally from what I saw in my FSM drawings it looks like the pistons are flat tops.

 

[Alakso]

The FJR combustion chamber is definitely a hemi design. In fact it is much closer to a true hemispherical shape than the new hemi's being used in Dodge cars and trucks.

Are we looking at the same pic? Take another look at the '03 head in the link 03HiYosilver posted. If it were a Hemi then the whole CC would be round as in Hemispherical. I see small shelfs in between the exhaust valves and also between the intake valves. Looks to me like a bathtub shape with flattened ends and pinched in sides.

 

[03HiYoSilver]

By the way, the Piston Tops are Flat in structure and those were not shown in the pictures.

The area by the Spark Plug is about 1/4 deeper from the flush area of the head, in case you wanted to know and it does look like a dome configuration.

 

[johnny80s]

I am more confused then when we started. Maybe Fred can come in and confuse me more. He is good at that.

 

[Geezer]

The FJR combustion chamber is definitely a hemi design. In fact it is much closer to a true hemispherical shape than the new hemi's being used in Dodge cars and trucks.

Are we looking at the same pic? Take another look at the '03 head in the link 03HiYosilver posted. If it were a Hemi then the whole CC would be round as in Hemispherical. I see small shelfs in between the exhaust valves and also between the intake valves. Looks to me like a bathtub shape with flattened ends and pinched in sides.

And the one in the diagram linked to earlier actually looks like a pyramid.

The main thing in a hemi is that the spark plug is in the center, the exhaust valves are on one side and the intake valves are on the other side. With four big holes for valves, and the valves being flat, you will never have a perfectly round chamber.

I guess when you think about it, it doesn't matter what you call it as long as it works as well as it does.

 

[radman]

Commonly known as a "Pent Roof" chamber. Central plug, nice even flame front, squish fore and aft. MUCH better flame front characteristics than a true Hemi, but slightly longer and less free flowing ports due to the right angle turn at the valve. Hemis make power, but need boost to work well and have a big 'ol knob of a piston crown in the way of the FF-bad for emissions. They were great due to largest possible valves before multi-valve heads became common.

 

[johnny80s]

Hemi is also know as the Pent-Roof chamber.

 

[FJRBluesman]

Hemi is also know as the Pent-Roof chamber.

+1
Without getting into absolute technical design chamber verbiage, it's basically a hemi.

 

[radman]

No, a Hemi is a Hemi. Pent Roof a Pent Roof-if the terms didn't have different meanings, then why do both exist?

Hemispherical Chamber
The hemispherical combustion chamber is a very popular design. This chamber is a symmetrical design. It is also an open chamber due to the concavity of the cylinder head. The valves are placed on an arc-shaped head. The hemi (abridgement of hemispherical) chamber is very popular in high performance automobiles. A high performance engine known as the “426 Hemi” used this type of combustion chamber. This engine dominated the high performance automobile scene during its era. Also, the hemi chamber was very successful in drag racing. Figure 2 shows a picture of a hemispherical chamber.

Crescent (Pent-Roof) Chamber

The crescent combustion chamber is very similar to the hemi chamber. The main difference between the two chambers is that in the crescent chamber the valves are placed on a trapezoidal-shaped head instead of an arc-shaped head. The valves of a crescent shaped chamber are placed at an angle on flat surfaces (rather than curved) on the head. The crescent chamber is also called the “pent-head” or “pent-roof” combustion chamber. The “pent-head” chamber is used the Jaguar XK engine.

Again, a Pent Roof-

A Hemi-

In addition-Hemi has no squish-the piston crown fills the chamber to provide enough compression to run. Huge valves come at a cost-flow is fantastic with boost, that's why they do so well in supercharged applications.

 

[Constant Mesh]

Question for Radman --

Looking at the photo you provided of the head and cylinders how does the coolant get to the backside (intake side) of the cylinders?

You can see where the coolant enters the block on the lower front middle area. So I can see how it flows to the exhaust side of the cylinders.

Does all the coolant for the intake side of the cylinders have to flow around the outside of cylinders 1 & 4?

 

[Alakso]

Commonly known as a "Pent Roof" chamber. Central plug, nice even flame front, squish fore and aft. MUCH better flame front characteristics than a true Hemi, but slightly longer and less free flowing ports due to the right angle turn at the valve. Hemis make power, but need boost to work well and have a big 'ol knob of a piston crown in the way of the FF-bad for emissions. They were great due to largest possible valves before multi-valve heads became common.

Thanks Radman for the info!!! Never heard of the "Pent Roof" CC so it's cool to learn something new!! Great pics also and the one of the Hemi CC is what I'm used to seeing on all the older HD's and 426 Hemi's from back in the day.

On another note, if I remember correctly Jerry Branch did a bunch of development on the Bathtub CC design for the XR1000 which was a two front head engine with a Delorto for each head. But the throttle pull was a beast with these slide carbs.

Radman, am I correct to assume that the FJR pistons for the Pent Roof CC are flat tops and as such only have 1 squish band? Or are the FJR pistons slightly domed or slightly dished?

How does the FJR head respond to some porting and who if anybody can do some professional porting on the FJR head?

Again thanks!!

 

[radman]

Flat-tops, and squish fore and aft.