IT465 vs YZ465

[uts]

Hi there,

My friend and I both have IT465's. I'm leaving mine as is but am doing some research for him. He has the DG YZ465 exhaust system on his and is wanting to experiment with a YZ465 jug as well.
I've heard Eric has done much work in the past with these Dinosaurs and I am wanting to know the porting differences and power delivery differences between the IT and YZ. All I know is that the IT is tuned more for bottom end.
Also I suppose I should ask if you can just bolt the YZ465 jug onto the IT465 engine.
Any info on this topic would be sweet.

UTS

 

[cujet]

I knew about the 490's which are a big version of the 465. The following may apply?

The YZ had a larger reed assy with 8 petals vs. 6 for the IT. The YZ had a higher exhaust port, but still just as narrow.

Today, we understand that the blow down area of each (YZ and IT) is insufficient for the transfer port timing, which is quite similar between the two.

Both the IT and the YZ needed a wider and taller exhaust port. The IT just needed about 1.5mm more material removed. The trick used to be making the exhaust port look like a bread loaf. The top is wider than the bottom. This provided good ring support and good blow down area.

We used to get about 65RWHP, no problem. Porting, good reeds, good pipe. The trouble came when it got hot out. It would ping like crazy unless race fuel was used. Even then, the engine would get too hot during an extended race. That is where the CR500 would excell, at the end of a race.

Chris

 

[Joe in Florida]

Hi Chris,
I have an 87' 490 and I noticed in your reply that you mentioned the blow down time. What effects does this have in the power range ( bottom end vs. top end )
Thanks
Joe

 

[2 strokes for life]

uts, the flywheels are also different on both modles and the Jug will fit right in place of the IT one.

cujet, i have a 82 IT465 that was bored to a 510 with a Pro-Lite piston and DG pipe and must have made the same power you are talking about or mabe even more, but try to start it with that much compression form the milled head. You had to jump off the side of a house to start it but at top dead center it would start hot or cold. Now i am returning it to its old standard bore and compression. Fun bike.

 

[cujet]

Joe,

While this discussion is not "technical" in nature, blow down time/area affects how the engine runs in it's proper state of tune for each setup.

So, in our example, Yamaha opted to go with very small exhaust ports as compared to the transfer port time/area. The result was better piston ring support, good mid range torque and a sharp reduction in high RPM torque. In other words, OK performance for someone used to 20+HP dirt bikes of the time.

However, Yamaha did understand the exhaust ports well, even back then. They published data on the modifications necessary to make the bike competitive with the Honda's.

Simply taking an IT cylinder and modifying it to YZ port specs was not enough. Sure, it helped, but the exhaust port was still too small. Following the published Yamaha YZ port modifications resulted in the best of all worlds. Good power, clean running, and much more fun.

Chris

 

[Joe in Florida]

Chris,
Thanks for the reply.
So if I understand it correctly, if the exhaust port opens @ 89 deg after TDC and the tranfer ports open @ 121 deg after TDC, that would give a blow down time of 32 deg? And to optain more bottom end torque, the exhaust port needs to be able to flow more volume within that 32 deg?

Thanks again,
Joe

 

[Joe in Florida]

What I have is an 87' YZ 490 that I race on a gokart and am trying to understand the effects of the port timings to power ranges. I would like to be able to get more bottom end torque.
If anyone can help, I would appreciate it very much.

 

[cujet]

Joe, in particular, the IT cylinder was tuned for low end torque. I am not certain that much can be gained with regard to more low end torque on that engine. It seem to me that the YZ cylinder had a bit less low end.

Are you running higher compression?

 

[jayg]

How did this project turn out? Did you replace the IT cylinder with one from a YZ?