YZF compression increase - cooling problems?

[jescombe]

I took my 453cc YZF Supermoto to the track today - all is not well :(

I've just put it all back together after raising the compression from 11.5:1 to 12.5:1. I had the piston crown machined down to reduce the depth, re-profiled to retain the original squish band width, the valve pockets re-cut, and the barrel decked to retain .040" of squish clearance. I also radiused the edges of the valve pockets and generally cleaned up any sharp edges..

When putting it back together, I degreed the cams to bring them back to spec. So essentially, it looks much like before, just with less depth in the piston crown.

Anyhow, out on the track, it would only manage a couple of fast laps before starting to churn coolant out of the radiator overflow (fortunately into a catch tank)..

I'm thinking that its either running way too hot, or has a blown head gasket..? It doesn't seem to be pressurising the coolant when revved hard on the stand, so I'm leaning towards boiling over under load.

Any suggestions on what I can do to make it run cooler? Would switching from pump (97RON) to race gas help? Atol fuels seem to be the standard choice in the UK, http://www.atolfuel.co.uk/competition_fuels.asp has links to the specs for their products.

Is raising the boiling point with a higher pressure radiator cap sensible - or just masking the problem?

Thanks in advance, Jon E.

 

[Rich Rohrich]

SFO ran his ~14:1 444cc WRF for 1500 mostly WOT through Baja without overheating it, and there are tons more folks running the same CR on WRs and YZFs without issue including me. Running the bore out to 98mm as you did leaves precious little sealing area for the minimal clamping area designed in by Yamaha. If we assume you don't have air in the cooling system (very common cause of this) a mechanical/flow problem with the cooling system or a detonation problem (unlikely and easy enough to spot) the head gasket is a pretty good bet. If Yamaha had used studs to locate the barrel instead of long bolts and simple dowels it would be much easier to get higher more positive clamping at the cylinder/head junction. Thank a pinhead accountant for that decision because no self-respecting engineer would design it without studs if given the option.

Take a look at the CRF and you'll see it done correctly.

Score at Honda :

CRF Engineers 1
Lunkhead bean counters 0 :D

 

[jescombe]

Thanks Rich,

Do you use any additional sealants on your head gaskets?

I'll try to get a thermocouple into the coolant - might give me an insight into whether it's being pressurised instead of boiling... Is there a knack to getting trapped air out?

 

[Rich Rohrich]

Using copper sealer on the head gasket is a pretty good idea on a big bore YZF if you suspect leakage might be a problem.

The bleed screw near the water pump is pretty effective at getting air out of the system.

 

[SFO]

Originally posted by Rich Rohrich
If Yamaha had used studs to locate the barrel instead of long bolts and simple dowels it would be much easier to get higher more positive clamping at the cyclinder/head junction 0 :D

In the early evo days we built a lot of big twins with special iron axtell barrells that stopped the base gasket leakage that was popping up with regularity. They called it a shuvo.
Isolating the clamping forces on base and head gasket surfaces is really requited by intergal cylinder/case castings.
If you have sealing issues I would consider o-ringing the head. A turbo trick, also kinda a hack old school kinda thing.
Sometimes these things will burp an inch out of the radiators, it is just where it wants to be.
I don't run a catch, just a 1.8 cap off of an '01kx65 and laugh.
It will still whistle on knarley uphill slimy rock knarleyness where you are stalled out with the throttle pinned...
But it SHOULD!

 

[jescombe]

Thanks, I'll give the copper sealant a try first.. I'd already torqued the gasket a couple of times during assembly (whilst checking squish & valve clearances) - is this bad practise?

SFO, o-ringing the top of the barrel was the only thing that worked for a friends LC4 after it was taken out to 105mm. I'll bear it in mind if I can't get it to seal with gloop..

At least one good thing happened this weekend - I picked up a gorgeous Akrapovic titanium exhaust system. I'll get some dyno figures for it once I've sorted my top-end. (I looked at some comparison runs from another customers 426, more power everywhere & revved out longer...)

 

[SFO]

How about just putting that 98mm cylinder on the shelf and calling it a learning experience?
Instead of reinventing the wheel just run a 14~1 97mm set-up?

 

[jescombe]

How about just putting that 98mm cylinder on the shelf and calling it a learning experience?

The thought has crossed my mind, not ready to quit on it just yet though! :confused:

Just come back from a quick dyno session, good news is the bikes still putting out decent power (53hp with the standard cams & pipe). I was running a thermocouple in the coolant pipe from the head, and the coolant temp only reached ~80deg C before it started overflowing. So it looks like it's pressurised and not overheating & boiling over??

Also ran a leak-down test, which came out at 12% (has previously varied between 4% at best, and 20% with a stuck compression ring). It's definately leaking past the inlet valves (one in particular), so it looks like I've got a few jobs to do when I pull the head this week..

Cheers, Jon E.

 

[SFO]

If I was in Europe I would go to Friedhelm.

Call him if you have any questions on your future set-ups.
I have had personal dealings with him and he is 100%.


www.gw-racing-parts.de

 

[jescombe]

Thanks for the link SFO, they look like the guys that manufacture the 98mm pistons (from JE blanks)..

Just been checking out my leaky inlet valves - what a mess.. :|

Seriously unhappy with the 'professional' outfit that installed them, the seats are all over the place, and the worst one is cut the deepest into the head (and already on just about the smallest shim I can use) so I don't think it can be re-faced... Is it feasible to put new seats into a YZF head?

 

[SFO]

You could do seats or have custom valves made to fit what you have that will put the stem protrusion back into the middle of the shim range.
Freidhelm can point you in the right direction, or try...

http://www.blackdiamondvalves.com/

 

[SFO]

one more thing

Jescombe,
Try talking to the person who built your head.
They need to be advised that they set it up at the end of the shim range.
I swear I get more education from my customers than I give.
Did he tell you were it was?
If the stem protrusion is at the end of the shim range was the spring installed height altered?
If the installed height wasn’t corrected you could also be looking at too light a seat pressure.
You can also butt grind the valves to correct your stem protrusion but then you will have to be very careful about the bucket/retainer clearance, I have seen people set up heads were the bucket pushes on the retainer instead of the shim. CATASTROPHIC!

Keep us posted,
Bill


These guys also appear to be squared away but I have no experience with them...

http://www.cylinderheadshop.co.uk/services.html

 

[jescombe]

Thanks SFO,

I hadn't considered the spring height, and I don't think they did. The shop re-cut the seats for Kibblewhite oversize valves, and ground the original shims to regain the correct clearance.. (Well, a vaguely correct clearance, I still had to change shims when I got it home). I think it was just a sloppy job - but you live & learn!

Funnily enough, it was the Cylinder Head Shop that I had in mind for new seats. They fixed up a friends VOR head after another shop installed a valve guide off-centre. I've heard they're very busy though, so I may have a long wait..

I've just found a stock YZ400 head for £200, I'm hoping it'll fit the 426, so I may well be back in business for this weekends racing after all..

Cheers, Jon E.

 

[SFO]

Originally posted by jescombe
Thanks SFO,

I hadn't considered the spring height, and I don't think they did. The shop re-cut the seats for Kibblewhite oversize valves, and ground the original shims to regain the correct clearance.. (Well, a vaguely correct clearance, I still had to change shims when I got it home). I think it was just a sloppy job - but you live & learn!

Jon E.

Jon,
when you are shopping for more cylinder head work the shop that does it for you should be able to provide...
#1, Knowledge of the shim range and were the middle of it is for stem protrusion.
#2, Seat pressure at closed and at full lift for your cam.
#3, Assured concentricity/perpendicularity of seats to guides and how do they gauge it?
#4, Free travel of valves, if the cam is not supplied the head shop needs to know how much cam you are installing, or tell you how much cam you can install.
#5,What kind of machine do they use to cut/grind seats and why?
Hope this helps...
Bill

 

[jescombe]

Hope this helps...

Certainly does, thanks Bill...

 

[SFO]

I must confess...

At first I was going to reference you to Phillip Allen at R&D performance.
I went to find him and was informed that he had just passed away upon his shop floor of a heart attack.
RIP, Phil.
It was my pleasure to have known you.
Sincerely, your comrade, William Jeffery Berndt.
Godspeed Phillip.

 

[SFO]

If Yamaha had used studs to locate the barrel instead of long bolts and simple dowels it would be much easier to get higher more positive clamping at the cylinder/head junction. Thank a pinhead accountant for that decision because no self-respecting engineer would design it without studs if given the option.
[/B]

I think you might have to pull the motor to do a top end if you used studs...
We shall see soon enough.

 

[jescombe]

Ok, I've got my hands on a stock YZ400 head that I'm going to run until I get my valve seats fixed. I've measured the head volume and it's a couple of cc's less than my flowed 426 head.

Using this head will push compression from 12.5:1 to ~13:1 unless I run a looser squish clearance, and I'm getting nervous about staying with (97 RON) pump fuel. It seems UK race fuel is more expensive and lower octane than you get in the US. However, the consistency can only be a good thing, so I'm hoping for some advice on specs..

Atol (http://www.atolfuel.co.uk/) seem to have the market pretty much cornered, and have two fuels aimed at 4stroke motorcycle engines. The first fuel is the cheaper @ £50/25L vs £70/25L, but the high end of the distillation curve seems very high compared to (for instance) the Phillips fuels that have been discussed here?

ATOL ULR

Density Kg/l at 15 C
0.745

Reid Vapor Pressure bar at 37,8 C
670

Research Octane Number
99.0

Motor Octane Number
88.0

Distillation (c)
30% 66
50% 100
90% 170
FBP 195

Oxygen % % Mass
1.3%

Benzene % % Vol
0.60

Lead Content g/litre
<0.005


ELF MOTO 4T

Density Kg/l at 15 C
0.766

Reid Vapour Pressure bar at 37,8 C
0.502

Research Octane Number
101.5

Motor Octane Number
88

Distillation (c)
30% 70
50% 82
90% 119
FBP 139

Oxygen % % Mass
2.68

Benzene % % Vol
0.1

Lead Content g/litre
<0.005

Any pointers before I rush out and buy the wrong one would be gratefully received.. :eek:

 

[SFO]

http://www.four-stroke.com/Dyno_runs/SFO_index.htm

These are my results w/pump vs. race fuel...

 

[jescombe]

Wow, there's not as much difference between the fuels as I'd have expected... I keep meaning to web up my dyno runs, they are pretty similar numbers to yours (with a slightly wider spread of torque).

Do you know what was pulling your torque curve down around 5k? I've seen it throughout most of my engine development with the stock pipe, but it had completely gone during the last dyno session. (Only thing I can think of was that I repacked the silencer before the runs).

 

[SFO]

Jon there is another thread going about this now.
Check it out and post there.
http://www.dirtrider.net/forums3/showthread.php?s=&threadid=46643

 

[jescombe]

Good news, I'm up & running with the stock head. I lost 2-3hp over my gas-flowed/big-valve head, but still managed 55hp on the dyno today with new cams and the Akrapovic pipe. (And no coolant leak!)

I will try to web up a selection of dyno runs this weekend...