Replacement pistons

[paul byrne]

G'day all,
I know this subject has been covered before but I would appreciate your views on the best replacement pistons. Here in Aus we have the club supplied JP replicas which I'm sure are fine, but I really would like to take advantage of a few more cc and higher comp. ratio from flat top pistons and slightly bored liners. With modern fuel, 95 or 98 RON, 9to1 CR should be ok and I calculate that a flat top would give a little less than this at standard bore.
People have mentioned Ford Fiesta pistons but I dont think these came to Aus. Checking the catalogue from JP, Triumph 1300 (flat top version) seem suitable and triumph 2000 also seem a possibility although the bore is quite large at 74.7mm compared to the 1300 at 73.65mm.
The main issue sems to be whether the skirts of either can be safely relieved at the sides to clear the opposing conrod big end. Other dimensions seem ok as is, although the compression height is marginally lower than Jup and the distance between gudgeon pin bosses is a bit tight.
Any thoughts or experience welcome!

[Robin Fairservice]

Check with the New Zealand Spares Dept. They sell a Mazda piston, and the sleeve needs a little enlargement which increases the CR a bit. One can also then skim the ehad.

[Keith Andrews]

Compression ratio means squat...it is no more than a marketting ploy used by manufactures...
What is critical is the Dynamic compression ratio...
this is basically calculated the same as the static but takes into a/c the rpm, and valve overlap/duration of the cam profile.

u will find when modern cars are advertised at 10 , 9 what ever :1 the real ratio is around the 8.5:1 for modern fuels.
Even high compression race engines up in the static ratio of 10.5:1 and higher 11:1 have huge valve overlap for the hi rpm ramges, which in effect drop dynamic down in to the mid tohi 9s for octanes 100 to 110.

[pat lockyer]

Well no one else has agreed or disagreed so to keep the thread alive i will put it in my simple way.
Actual c/r is my way of saying what is happening when the engine is running.
The calculated c/r is calculated by taking away the swept volume of the cylinder from the volume of the cylinder,the volume that is left is then divided into the volume of the cylinder and you get a ratio like 8-1, in other words the combustion chamber is 1/8 of the volume of the cylinder.
However the valves are still open when the piston is at BDC so the actual c/r must start when the valves are closed and the piston is already moving up so it is lower than the calculated c/r.
Hot cams usaly have more overlap which actully lowers the actual c/r further.
Hot cams make the engine flow better[at the expence of a broad torque range.
The more air you can stuff in the engine means the actual c/r goes up.
Because of the restriction in the inlet the carbs with extra air flow can only be of real benefit when on full throttle.
The throttle is controlling the actual c/r and when on idle at a guess the engine is running at a c/r of 2/1.

Conclusion more liner protrusion less head torque FT LBS and a 4lb cooling pressure!

[Keith Clements]

Books have been written on getting more performance out of the internal combustion engine; some of which I read years ago. The Jowett engine as originally designed did not utilise much of the technology until the Performance Tuning Notes came out. Much, much more was learnt in the 1960's and 1970's as petrols improved and Universities turned out fluid mechanical engineers that were employed by engine tuners.
The result has been a quadrupling of power for normally aspirated engines. The ability to see flame fronts inside the cylinder greatly improved our understanding of the mixing of charge and the scavenging of spent gasses. I always check the combustion chamber sizes using a burette and make sure they are balanced (within 1 cc). Depending on bore the cc should be about 35cc for high compression. To this will be added the volume of gasket and any depression in the piston crown. Large pistons can give 1600cc or 400cc per cylinder. If you gas flow as per tuning notes, you need to shave the head to regain compression ratio, perhaps at a skew to rebalance chambers.
The most important thing for gas flow is get the inlet valve seat properly profiled with lead in and lead out angles as well as the main closure face angle cut.

[paul byrne]

Surely the static CR is an ok figure to use on a comparative basis.
The original is, I think, 7.8 to 1. From this I calculate the head and piston bowl capacity to be around 54cc total. If the liners are bored to around 75.5 mm the total swept capacity becomes around 1610cc and the CR should rise to about 8.3 to 1 if the same piston bowl capacity was maintained. If they were flat topped the static CR would rise to around 9 to 1. These calcs tend to indicate that it should not be necessary to shave the head to obtain a reasonable CR for modern fuels if a bore around 75mm is used.
I note that Keith quotes around 35cc for the combustion chamber volume in a higher comp engie(of standard bore?), whereas I calculate the standard chamber (without including the piston bowl) to be around 47cc.Is that about right? Of course this is all thoretical because I haven't measured anything!)
Still looking for further leads on suitable pistons, although I have contacted the NZ club, thanks Robin.

[Keith Andrews]

u are looking around the 8:2 to 8:7 close to dynamic CR for modern pump gas

[Keith Clements]

http://jowett.org/jowettnet/dt/tech/JAVELIN-TUNING.pdf
are the tuning notes with compression and combustion space figures for Jowett pistons. Anyone know what the difference was between those three piston types?

Bear in mind that that was for Javelin gasket; not the thinner Jupiter one.

I will have to dig out what I set the CC space for the Fiesta pistons; from memory I aimed for 9:1.

[Leo Bolter]

Hello all,

I have used Mitsubishi Lancer pistons in my Jupiter, and my son’s Javelin with good success.



The pistons are the ones used in Mitsubishi engine type: 4G-36 (1238 cc)
Part Number: MD 000796 (with pin, no rings)
Dimensions: Nominal Diameter 73mm
Gudgeon Diameter 19mm
Compression Height 39.5mm
“Domeâ€

[Drummond Black]

Leo Your info is good. Are rings sold separately. ? What mileage have you done so far ?. Thanks for giving us another approach.

Drummond

[Leo Bolter]

Hi Drummond,

My notes tell me that at the time (March, 1996) a set of genuine Lancer piston rings were quoted at about $200, while locally made aftermarket ones were more like $80 . . . . I went for the aftermarket set!

Incidentally the pistons were $283.84 for a set, back then. And the liner boring was $112 per set. I'd be rather interested to know what the present day costs would be!

The ring manufacturers' technician, Mr. Morgan, took the trouble to give me advice as to the required bore finish for their product, as follows:

"The rings will be fine in superior material, cast iron liners such as used in Jowetts (harder). Tell the reconditioner that the bore finish is really important and should be "500 finish" (whatever that is!) using 400 grit hone, with 45/60 degree crosshatch." - My comments in brackets!

As far as your request for the mileage covered is concerned, I can't tell you that right now as the logbook with the information in it is in the car which is stored off site. I may get there sometime in the next couple of days . . . . Watch this space!

Cheers, Leo

[Mike Allfrey]

G'dday From The Wintry But Sunny South,

I have made a concsious decision to "stay out of this one", believing that the JP pistons available from our club are entirely suitable.

We need to consider some points:

High compression ratios and fifty-plus year old components do not mix well.

Jowett Cars Limited experienced problems with the 'thin walled' cylinder liner type. Why re-introduce such problems by over boring thick wall liners?

For most of the car's use, it will be on heavily policed roads at speeds up to 100 kph. Much as we would like to, the nanny society does not let us thrash our cars any more.

Regards,

Mike Allfrey.

[Leo Bolter]

I said previously . .

As far as your request for the mileage covered is concerned, I can't tell you that right now as the logbook with the information in it is in the car which is stored off site. I may get there sometime in the next couple of days . . . . Watch this space!

The pistons in my Jupiter have done a mere 12,420 miles since the date of fitting them . . . . I obviously "don't get out enough!" ':)'
. . . thats the problem with so many choices of vehicles! (2 x Princesses, a Jupiter and a LE Velocette motorcycle).

The Javelin/Mitzi pistons would have done considerably more miles as it has traveled almost the length of the North Island of NZ several times and had also been used as regular transport around Auckland and Wellington . . . but I can't quote an exact figure for them at this stage, except to say that it appears to be about 39,000 miles. Incidently, the total miles amassed by the Javelin, up to the present, is approximately 212,000.

It still runs beautifully . . . it was FULLY balanced when overhauled in March 1996

Mike, I agree entirely with your comments about "wasting" the valuable liners that don't need boring out so as to accommodate larger (and heavier?) pistons.
Balancing AND driving at a sensible rate (revs and speed) is healthier for "man and beast" . . . . I haven't had my Jupiter over an indicated 80mph (and that was at Manfield and Ohakea race tracks). But, I must confess, the revs were still rising although the speedo was "stuck" at 80!!! He heee ':o'