The Jensen Owners' Club

I am planning my engine build around a 4.150” stroke Molnar crank with 2.200” rod journals and Molnar steel 6.760” rods.

I already have a pair of Trickflow 240 alloy heads with 78cc chambers, so with -6cc SRP pistons and a 0.039” gasket I will, according to the Eagle engine calculator, be at 0.025” deck clearance and 11.15:1. Less one full point of compression due to alloy heads, is that right? So actual 10.15:1.

If I plan on using premium unleaded at euro octane of 99 (Shell), or 100 (Tesco Super) plus, if necessary, an octane booster like Torco or Tetraboost will this compression be ok? Guess I could go to a thicker gasket if needs be but I assume that will have a negative effect on quench which I understand will itself be likely to cause detonation.

An alternative piston would appear to be a Mahle -8cc piston that comes out at 10.83:1 or 9.83:1 with the alloy heads and the same gasket.

God, this is so complicated.....!

My new engine calculated to be 10.9:1 CR. This is STATIC compression. Dynamic compression depends on where the inlet and exhaust valves close / opens (I.e. cam timing).

Mine seems to be OK on our 95 octane fuel (if one can believe that its really 95 octane).

Thank you Philip,
I have my cam specs so how do I work out dynamic? Does it tend to be lower than static as a rule?

Sorry, I have no idea how dynamic relates to static. Things most likely change when intake air momentum and exhaust extraction effects come into play.

I found a couple of calculators on the web, but helpfully got two different answers in spite of entering the same data! Both came out quite a bit lower than the static number, one being 9.63:1 and the other 10.73:1

For comparison, I'm running 10.6:1 without incident on 93 octane. It's important to do your homework, however you never really know until you have it all put together.

Hi all,
as we are a quite global community here the following info might be relevant (borrowed out of www )
rgds
Thomas

American fuel vs European fuel – Fuel quality on both sides of the Atlantic explained
“Fuel in the US is really bad compared to Europe and the rest of the world” – is a very untrue statement. Fuel in the United States is as good as anywhere else in the world. What changes is how Americans describe their fuels in terms of quality – octane number. They use a different system so their numbers do not translate well into the octane numbers that the rest of the world uses.

How is fuel quality measured and rated

Gasoline quality is measured experimentally but using different tests. Depending on the test that is done to control fuel quality, we get

Research Octane Number (RON) , used in Europe and most of the world
Motor Octane Number (MON)

Because two different tests define RON and MON, with MON testing done under more difficult conditions, MON number is always lower than RON.

Europe vs the States

In Europe, gas stations describe different types of gasoline based on their RON rating. It is typical to have 100, 99, 98, 97 or 95 RON with prices varying accordingly. In the States the descriptions look a lot like RON, but they are not.

Pump gas

In the United States gas stations describe the types of gasoline based on the Pump Octane Number (PON). That number is the average between RON and MON. This causes many problems.

If a fuel is 98 RON then it will be 93 PON -> 93 PUMP
If a fuel is 95 RON and 87 MON then it will be 91 PON -> 91 PUMP

Is American fuel quality bad after all?

No, not at all. Contrary to popular belief, fuel quality in the States is as good as anywhere else in Europe. Most Americans think that because they are running 91 octane fuel, their engine will blow up. It is the same as if they were using European 95oct fuel.

In our website you will find RON ratings required for each remap stage.
98/100 RON means you will need to use 93 PON in the States.

95 RON means you can use 91 PON in the States.

Everywhere else in the world, 98 RON and 95 RON gasoline usually are fit.Anything lower then that and your modern engine will have a lot of trouble adjusting, even on the stock calibration.

An excellent description Thomas. I never knew all of that before today. What it is possible to learn from the JOC forum!