Rd400 barrels on gt550

[Vintageman]

Arneg,

What are you doing in that picture? Porting …almost looks like your welding

Did you attempt to restore compression?

What or you having done on crank?

[arneg]

masking out how much material to remove to end up with (hopefully) the same port times on every cylinder. That's just a lamp shining in from behind to see better. The crank needs new seals.

As for the GT600 project, I just scored 4 RD400 cyls for super cheap. Can't wait to do a first check. I'll post pics.

[Vintageman]

Wow

As for the GT600 project, I just scored 4 RD400 cyls for super cheap. Can't wait to do a first check. I'll post pics.

Back to Gt550 port mod

Will you try and restore compression after raising GT550 exhaust? Maybe you have not made decision yet.

Thanks

[arneg]

why would that change compression? we ported the port, didn't raise the cyl.

[tz375]

Arne, people get fixated on so called corrected compression which suggests that raising an exhaust port will decrease the compression ratio. Of course that ignores wave strength from a higher port at reasonable revs. It's true that at low speed, compression will drop slightly if the port is closed later.

[ja-moo]

I personally always compensate for a raised EX port. Wave strength isn't doing a lot until in the powerband when everything get into sync. Low end power suffers.

[Vintageman]

If you raise the port you lose compression. This can be seen using compression gage before and after. Since this is a domed head compression all ready low (although 6.8: 1 not too bad you may be 6.6 :1 now)

Every time I have not restored the compression I lost "everything" but top end (only then if I had a good exhaut). This is noticeable more on smaller engine too with little to spare. The GT550 may have room to spare. But, ugly street /touring mod per my experience if greedy or not careful.

Note, you will lose some regardless for you have shortened your gas expansion degrees You gain RPM and thus HP for based on the rate you do work. Faster you go pumping loss (heat) becomes a factor and why high compression is undesired at high rpm. This is how I am understanding it.

The four stroke anaolgy is if you have high degree camshaft you are then required to run higher static compression pistons else you loose or won't get as much power due to dynamic compression loss

The trouble (why I have been asking) is if you want to restore compression (I see now not everyone does) not easy when you take a lot off for example you may need to reshape head. If you lift barrel then you can just deck top same amount. I think I will lift barrel and deck top,. Only because tranfers are real low too.

[tz375]

And there's the rub. It's not possible to restore all that lost compression if you raise the exhaust port by 5-7mm without having the compression too high when it's on the pipe.

As Ja Moo explained, raising the exhaust costs compression and power at the bottom end, but raises power at the top. Bringing static compression back up after porting to the same level as stock may create a situation where the effective compression becomes too high at high revs when the pipe is created a considerable stuffing wave.

So, like everything on old bikes, it's a compromise. I have a motor with a 3 into 1 pipe that doesn't generate much of a stuffing wave, so on that motor the compression has been raised much higher than stock after it was ported (slightly).

So it all comes down to how much the ports are raised and how comfortable the engine builder is with high compression. With a decent squish head (Yamaha RD and Kawasaki triples), the motor can stand a lot more compression than one with an open quiescent chamber (ours). Engines with programmable ignition (MSD, Ignitech, Zeeltronic etc) can take advantage of high compression at high revs by progressively retarding the ignition as revs and combustion speed rise.

It's all a balance. Think of it as an optimization exercise. For the street I want lots of low end and a reasonable rush to the top. If I want a more sporty ride the objective might be less at the bottom end and more mid to top end. That creates a faster bike if I can ride it, and because the bottom end is weaker, it feels more powerful as it comes on the pipe. Someone else may want more top end and his optimum will be a different combination of ports and compression and pipes. For a track day bike or drag bike, the optimum is way more top end oriented.

The key is to take lots of time to measure your motor and then measure it again before starting to grind metal out of ports.

[arneg]

Thank you guys!! I learned something new. Wow. Yes I had always noticed that a taller exhaust port made a bike easier to kick start.

[tz375]

The reason I belabor the point about measuring twice is that I have seen several sets of numbers for GT550 ports that are not consistent and they are not the same as I measured on a motor that was here a few months ago.

I use a degree disk AND a dial gauge to determine opening and closing points and I then translate one to the other and often the two dimensions are not the same, so I have to go back and measure again and calibrate the results. For example if we simply measure down from teh top surface, we not only have a margin of error in measuring, but we also have to adjust those numbers for deck height. If the piston at TDC is say 0.5mm below the top of the cylinder, we have to subtract that from our measured numbers to get the real opening and closing relative to TDC and not just relative to the cylinder top face.

The data I have seen for port dimensions appears to be most incorrect on transfers which are also hardest to measure.

[arneg]

Maybe you can correct my approach once more, but I don't even measure TDC, just use the degree disc to measure the opening time of a port. That is independent of TDC. I use a lamp to determine whether a port is open or closed. There's a lot of inaccuracy in that part.

[tz375]

Most people use a measuring stick to measure the distance the port roof/floor is below the top of the block. So we tend to talk about the port being 40.5mm down and so on. The problem with that approach is that it does not take into account how far down the piston is at TDC and therefore at every other point compared to the measurement taken above.

So when we look at mm data we have to adjust for deck height.

Using a degree wheel sounds like it's more accurate but on ports part way down the barrel it's almost impossible to see the actual point of opening. If the ports have a decent chamfer we might mistake light through the chamfer, for normal port opening and that leads to incorrect interpretation because of that parallax effect.

That's why I use a very long dial gauge and a large degree disk. I use a piston (positive) stop to set TDC accurately and zero the gauge. Then read the dial for actual distance before/after TDC that port events happen.

[Vintageman]

arneg (et al),

You motivated me.

I just took nice spare set of 73 GT550 jugs and raised exhaust 2mm ( maybe too much if I don't like I'll go back). I really did not widen other than a good casting cleanup and kept the same general shape. I have not done an exhuast rasiing in quite a while, hope I get lucky on this one

I plan to recover at least 1mm of this compression loss by shaving head and or thinner gasket. Will check clearance and reshape head if needed

If I recall the piston comes pretty close to top of jug , so little deck height, I will measure.

I will check compression before and after.

Unfortunately, I have too much snow to get access to this bike and 10" - 20" snow predicated on the way.

Third engine project now awaiting access to bikes to finish

The GT750 is in my heated workspace awaiting crank rebuilt from B.Bune

I may start my fifth engine mod project if spring won't come here until May. Nothing wrong with snowmobiling (2 stroke of course)

Let me know how your GT550 turns out and what your final changes are either a post or (PM me)

[dyrberg123]

I am sucking information in here. Thanks for all your inspiring replys. I have absolutely know idea what effect raising exhaust roof 2 mm will give. But widening it 1mm to each side is not alot according to Bell's book. Cheers

[tz375]

It's not. You can open it up much further without causing rings to crack up or bulge into the exhaust port. You can raise the roof a lot further than 2mm. check that timing in degrees and compare it to a warm RD400 and you'll see what I mean.

Thin in terms of a 5-6mm rise for exhaust and transfers with a spacer