Racing 2021

Joined
Jan 25, 2014
Messages
161
Country flag
When i campaigned a TZ350 engined sidecar back in the late eighties, early nineties i started out using 100LL avgas but very quickly mixed my own concoction by using 4star and acetone in the ratio of 7:1
I got the basic recipe from Alexander Bell's books on two and four stroke tuning.
Never had any engine damage due to detonation and we were running greater compression ratios than 12;1
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
Looking good Chris, my first thought was along the line of Steve's, I have heard you Dunlop guys don't like to give up on them.:)

Steve I hope you are right about the discs.
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
When i campaigned a TZ350 engined sidecar back in the late eighties, early nineties i started out using 100LL avgas but very quickly mixed my own concoction by using 4star and acetone in the ratio of 7:1
I got the basic recipe from Alexander Bell's books on two and four stroke tuning.
Never had any engine damage due to detonation and we were running greater compression ratios than 12;1
12 : 1 on a TZ??, I don’t know a lot about stinkwheels but I would have thought a maximum of around 8:1 would be more in the ball park due to the physical restrictions caused by port height.
I believe Yamaha lowered the static compression in the later TZs, because as they developed more advanced porting the dynamic compression went through the roof and caused detonation.
Must try and find a copy of that book though, sounds interesting.
 
Joined
Jan 25, 2014
Messages
161
Country flag
two ways of measuring the compression ratio of a two stroke. One just the same as a four stroke and the other taking the cylinder volume when the rising piston shuts the exhaust port. That gives the lower reading of the two methods. Same engine, different readings.
When the exhaust valve was introduced that was when things altered.
The ACU regulations back then stipulated that only pump fuel could be used but there were loads of racers using octane booster, indeed it was readily available at the circuits we raced at , Cadwell being one.
I remember seeing a large pallet basket at a local chemical company full of new empty Silkolene octane booster cans ready for filling. I was most surprised as I live over an hour away from silkolenes base near Derby.
 
Last edited:

SteveA

VIP MEMBER
Joined
Dec 20, 2011
Messages
2,304
Country flag
Looking good, though how old is that front tyre it looks like its from another era. We need to get the Norton twins back on the track!
I'm pretty sure I rode one of Chris's bikes with that tyre on in 2011 or was it 2010?, at Pembrey and Anglesey, worked fine ;)
 
Joined
Jan 21, 2008
Messages
1,304
Country flag
Like all of us fine people on here, the tyre is aging well. It's my last Dunlop KR124 IOM soft front. So I will be back on Avons soon.
I'm not sure how much I will get out this season but want some new Avons for next season. Especially a 130/650 rear as I'm running a 110/80 from my Honda at the moment. Got to say handling has been great. Over the winter it's going to get a complete overhaul as I continue to move other bikes & projects on. Only so many years left to ride the track!
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
two ways of measuring the compression ratio of a two stroke. One just the same as a four stroke and the other taking the cylinder volume when the rising piston shuts the exhaust port. That gives the lower reading of the two methods. Same engine, different readings.
Makes sense, like you say it is how we do the 4 strokes, we wouldn’t dream of waiting to start our measurements until the inlet valve has closed.
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
Like all of us fine people on here, the tyre is aging well. It's my last Dunlop KR124 IOM soft front. So I will be back on Avons soon.
I'm not sure how much I will get out this season but want some new Avons for next season. Especially a 130/650 rear as I'm running a 110/80 from my Honda at the moment. Got to say handling has been great. Over the winter it's going to get a complete overhaul as I continue to move other bikes & projects on. Only so many years left to ride the track!
I have heard a rumour that Avon are about to release a new race tyre.
 
Joined
Mar 7, 2020
Messages
1,812
Country flag
Makes sense, like you say it is how we do the 4 strokes, we wouldn’t dream of waiting to start our measurements until the inlet valve has closed.
Sorry, am I reading that wrong... Because in all honesty until the inlet valve closes there can be no compression, yes???
(Not looking to be awkward, just puzzled)
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
On another subject Chris, they are getting really arsey about noise, if your bike is near the limit, it might be prudent to take any add on silencing you may have.
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
Sorry, am I reading that wrong... Because in all honesty until the inlet valve closes there can be no compression, yes???
(Not looking to be awkward, just puzzled)
We like awkward :)
The static compression is a measurement of the difference in the cylinder space between the piston at the bottom of its stroke and the top expressed as a ratio,
The fact that the inlet valve is open for the first part of the piston coming back up is ignored.
 
Joined
Mar 7, 2020
Messages
1,812
Country flag
Thank you, this I know.... But if the valve is not fully closed until, lets say after the piston has travelled upwards one quarter of the stroke, then actual volume fuel/air compressed will be only three quarters of that number, no??
The first figure just compares 'swept volumes' no?
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
Thank you, this I know.... But if the valve is not fully closed until, lets say after the piston has travelled upwards one quarter of the stroke, then actual volume fuel/air compressed will be only three quarters of that number, no??
The first figure just compares 'swept volumes' no?
True, but those volumes change with different cams and timing so as a base I suppose it makes sense to use the differential ratio.
If you measured cylinder pressure with a gage at kickstart speed, a motor with a hot cam could produce less pressure than a cooking cam motor, that is why a motor can loose torque when a hot cam is fitted without bumping up the compression, I think!!

A two stroke would give miserable pressure readings.
 
Last edited:
Joined
Mar 7, 2020
Messages
1,812
Country flag
True, but those volumes change with different cams and timing so as a base I suppose it makes sense to use the differential ratio.
If you measured cylinder pressure with a gage at kickstart speed, a motor with a hot cam could produce less pressure than a cooking cam motor, that is why a motor can loose torque when a hot cam is fitted without bumping up the. Compression. I think.
Two stroke would give miserable pressure readings.
Thank you. Yes... I honestly wasn't trying to change the automotive industry's accepted 'norm'.. Just promoting healthy discussion... :)
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
I was thinking the same though with the two stroke, Yamaha stated that the TZ had an 8:1 compression that was dropped to 7:1 in the later motors, when dobba99 said he had 12:1, I had never considered that the 4 stroke method of stating compression was valid, then I realised the inlet valve was still open whilst the piston is on its way back up and all of a sudden dobba99’s 12:1 is valid.
Never liked stinkwheels how can something so simple be so hard to understand
 
Joined
Mar 7, 2020
Messages
1,812
Country flag
Apologies if it's a granny/eggs scenario, but without over simplifying, with a four stroke it all happens above the piston, two strokes use the area underneath too, very canny!!
 

SteveA

VIP MEMBER
Joined
Dec 20, 2011
Messages
2,304
Country flag
Thank you. Yes... I honestly wasn't trying to change the automotive industry's accepted 'norm'.. Just promoting healthy discussion... :)
I think...really I mean that rather than 'I know'....

I think you are forgetting the pressure in the ports! Both of them. And that it varies during the cycle and can be positive, or negative.

At the start of the compression stroke, the inlet port has fresh charge coming in that will initially continue to flow, increasing pressure in the cylinder, and then stop the charge that is still flowing into the cylinder going back out the way it came in, even before the inlet valve has closed.

In the same dynamic way that exhaust pressures can either prevent, or assist, exhaust gases in their escape down the pipe dependent on design.

Good designs exploit these phenomena at the rpm your engine is designed to work at.

Meaning, at least partially, a good road cam, inlet tract/carb and exhaust works at road rpm and a good race cam, inlet tract/carb and exhaust works at race rpm!

The challenge of the two stroke has always been developing sufficient power out of the power band to make it initially move and then to get into the power band, without being spat off by an instant hit of power!
 
Last edited:
Joined
Jan 21, 2008
Messages
1,304
Country flag
Hi Ralph
Thanks, I will get my bikes tested at Donington. It's an unsilenced weekend! So I will have a reading to work from.
 

storm42

VIP MEMBER
Joined
Jul 20, 2011
Messages
1,045
Country flag
Apologies if it's a granny/eggs scenario, but without over simplifying, with a four stroke it all happens above the piston, two strokes use the area underneath too, very canny!!
Yes, a form of forced induction as far as I am concerned. Two strokes should have been banned from racing in 1946 along with all the other supercharged bikes, they stopped DKW with their spare piston thing and should have taken it to logical conclusion and banned them all. Horrible noisy smelly things. :D Mind you, I was at the dyno with an RGB 500 the other day and it is quite impressive stood at the side of one of those things at full chat, only made about 122 bhp but sounded and felt a lot more.
 
Top