Compression

Post by **David53** » Wed Oct 14, 2009 5:46 am

Just tested my compression on my totally original 803cc.

120
115
120
120

I think this is good for an 803 yes?

Post by **ssnjimb** » Wed Oct 14, 2009 8:46 am

Your compression ratio is 7.5:1 so i think that is still a little low Put some oil in the bores just a drop and see if it changes.

How many miles has the engine done and have you ever had it rebuilt or the headgasket changed.

Jamie

Post by **mike.perry** » Wed Oct 14, 2009 9:51 am

CR on an 803 is 7.2:1 to be precise. As a guide the book gives the pressures an a low comp. (7.5:1) 1098 as 130psi so I wouldn't start stripping the engine to look for the odd 10psi. The engine might benefit from a valve regrind some time.

Post by **David53** » Wed Oct 14, 2009 1:34 pm

The engine was rebuilt during the restoration, as was the head. If 130 is the low level for a 1098 I would have thought 120 was about right for an 803? Remember it is original with the original head etc.

Post by **bmcecosse** » Wed Oct 14, 2009 4:00 pm

I would say it is desperately low - for any hope of efficient running. But maybe it's all you can hope for from the pathetic 803! At least they are all much the same!!

Post by **David53** » Wed Oct 14, 2009 10:53 pm

Where could I find out the factory spec for the correct compression on the 803?

Post by **David53** » Wed Oct 14, 2009 10:56 pm

And stop calling my nice little engine pathetic!

It tries hard

Post by **bmcecosse** » Wed Oct 14, 2009 10:56 pm

Not normally quoted anywhere. The same-ness suggests there is nothing seriously wrong - it's just a low power motor.
Only jangling your chain with the 'pathetic'! It is what it is! Crank in these engines has been described as 'bent piece of wire' !

Post by **David53** » Wed Oct 14, 2009 11:05 pm

Yes they were very close. The 115 on No 2 was prob more like 116-117 but my gauge isn't that accurate, only goes up in 5's.

So if the CR is 7.2:1 how do you convert that into what the gauge should read? Is there a formula?

She doesn't blow any blue smoke and doesn't eat too much oil so I'm thinking it's basically sound (as it should be after a rebuild!)

Post by **ssnjimb** » Wed Oct 14, 2009 11:19 pm

is 7 reffering to 7lbs of pressure in the cylinder at tdc ie when on the compression stroke

Jamie

Post by **ssnjimb** » Wed Oct 14, 2009 11:21 pm

If the car goes ok and runs fine then leave it alone its what it was intended for. yes the 803 is a weak engine but if its looked after it will run for many years more. it has lasted at least 40 years

Jamie

Post by **bmcecosse** » Wed Oct 14, 2009 11:22 pm

It's 120 psi in the cylinder - just before the spark plug fires! Good competition engine will be more like 220 psi. Good running 1098 or 1275 for road use would be 160/180 psi range.

Post by **David53** » Wed Oct 14, 2009 11:32 pm

Yes she runs fine and I should leave it alone but after 10 years of restoring I have itchy hands and can't help tinkering!!

If only I had space for a new project.........

Post by **ssnjimb** » Wed Oct 14, 2009 11:44 pm

keeping up with maintenance is not a bad thing david. how often do you service her. Just drive her as she was intended. what condition is the body in etc.

Jamie

Post by **David53** » Wed Oct 14, 2009 11:53 pm

Full nut and bolt restoration over 10 years, body is in excellent condition. Only thing I didn't overhaul is the gearbox.

Judge for yourself

Post by **ssnjimb** » Wed Oct 14, 2009 11:57 pm

very very mice david. keep it original and standard. the engine was not meant to be powerful. keep its oil clean and service it more often.

Jamie

Post by **mike.perry** » Thu Oct 15, 2009 12:26 am

Look after that front bumper blade. They are different to the M1000 and replacements as rare as rocking horse poo.

Post by **David53** » Thu Oct 15, 2009 3:46 am

Just back on topic for a moment, found the following info on a very informative website:

"The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units (psi) as between 15 and 20 times the compression ratio"

Thus if we take the 803 with it's CR of 7.2:1 we get a low of 108 and a high of 144, which makes my 120 in range, but a little on the low side.

Based on this formula the 948 range would be 112 to 150.

Post by **bmcecosse** » Thu Oct 15, 2009 10:17 am

The 15 to 20 is simply based on what happens. It's only a rough guide. Air temperature/engine temperature/moisture content/fuel content/heat transfer rate etc all come into the equation - as well as the ease with which air can flow into the cylinder during the intake stroke. Don't worry about it - as long as it runs - that's the main thing!

Post by **ssnjimb** » Thu Oct 15, 2009 1:04 pm

what sort of psi would i be looking at for a healthy 1098 i will test mine tomorrow with my tester

Jamie