800 ENGINE, WHAT SHOULD THE COMPRESSION BE?

Post by **nickyj** » Mon Mar 08, 2004 6:23 pm

Hello, me again, yet another question if anyone can help. having trouble with the acceleration on my series 2. Binned the idea of changing the engine, too much stress! Can anyone tell me what a good compression reading is in PSI on each cylinder should be?
many thanks
Nicky

PS. can i also be enlightened as to what a 'goldseal' engine is?
(apart from an engine!)

Post by **Matt** » Mon Mar 08, 2004 6:32 pm

a gold seal engine is a factory re conditioned engine. Don't think it will be good just by the name, engines havent been factory reconditioned for a quite a while

Post by **nickyj** » Mon Mar 08, 2004 6:45 pm

thanks very much, I thought it was something special, and costly!

Post by **Matt** » Mon Mar 08, 2004 6:55 pm

it used to be, dont consider it to be any different from any other engine now.....

Post by **rayofleamington** » Mon Mar 08, 2004 7:00 pm

that gold paint seems to have a hallucynagenic property as ppl who see a gold seal engine suddenly get all light headed and full of awe.

Personally, as the goldseal engine were rejects that had to be repaired, and the paint was a marketing gimick, and that there were so many bad ones - only buy one if you can see it running otherwise you are taking sa risk! (same advice is good for for a normal engine)

Post by **Matt** » Mon Mar 08, 2004 7:03 pm

ive got 2x1275 gold seal engines, ones seized and the other one supposidly runs.... wait and see

Post by **nickyj** » Mon Mar 08, 2004 8:24 pm

We definitely need to get our engine reconditioned as its not so good. still need to know the compression readings if anyone knows. cheers Nicky

Post by **rayofleamington** » Mon Mar 08, 2004 8:29 pm

am at home now, so I've got the books out..

for the 803 OHV engine, the compression ration is 7.2:1

Post by **Matt** » Mon Mar 08, 2004 8:43 pm

i was thinking nicky meant pressure, like with a compression tester you screw in to the spark plug socket

Post by **rayofleamington** » Mon Mar 08, 2004 8:59 pm

go on then Matt - you should be able to work it out with all that thermodynamic knowledge...

But hopefully the conversion factor (or a look up table) is specified in the book that comes with the compression tester...

Post by **nickyj** » Mon Mar 08, 2004 9:08 pm

yes Matt. thats what we need to know. Can you tell I haven't got a clue?!!

Post by **Matt** » Mon Mar 08, 2004 9:10 pm

well i could work it out, but its much easier if someone else knows or does it for me ;)

Post by **nickyj** » Mon Mar 08, 2004 9:16 pm

thats what I was hoping!! should it be more than 120 PSI?

Post by **Matt** » Mon Mar 08, 2004 9:29 pm

My quick back of the envelope calculation suggested it should be about 105psi, but thats at 25 degrees centegrade (298K) and assuming the gas doesn't warm up when compressed which it obviously does.....

Post by **rayofleamington** » Mon Mar 08, 2004 9:41 pm

assuming the gas doesn't warm up when compressed

lol - Matt

go read those notes again...

Post by **Matt** » Mon Mar 08, 2004 9:55 pm

I know thats a poor assumption but at pressure below about 50bar we have been told that gasses can be treated as perfect, ie. there is no interaction between them, i could work it out better, and I did say it was a back of the envelope calculation.......

Post by **Matt** » Mon Mar 08, 2004 10:37 pm

Ok, i did it properly, it is slightly rough, I assumed 1 cylinder = 200cc, combustion chamber = 30cc air comping in is at one atm and the engine operates at 100% efficiency, which gave a value of 210PSI. Of course engines don't run at 100% efficiency. If yours is 120PSI it is operating at about 60% efficiency which doesn't sound too bad to me........

(happy now ray?)

Post by **Kevin** » Tue Mar 09, 2004 10:13 am

yes Matt. thats what we need to know. Can you tell I haven't got a clue?!!

Dont worry Nickyi I dont know how these things work either using these calculations, but the later engines should have 160 - 180 PSI, so yours seems a bit low, I am sure Ray will let us know when he has finished pulling Matts Chain, unless someone else has the info to hand

Post by **rayofleamington** » Tue Mar 09, 2004 10:50 am

Had a quick look online, but I couldn't find exact figures for such a low compression.
The closest I found was a 7.8:1 engine where the nominal was 168 psi and 'service limit' (definately needs a rebuild) was 114 psi
Guestimating for your compression ratio, I would say 150 to 154 nominal and lower limit 105.

They should all be pretty close to each other - normally within 10%. If one of the cylinders is low, you should check some things out. Check if the gauge was inserted properly first. Try pouring a little bit of engine oil in the low cylinder's spark plug hole (not a lot of oil - that will give a fals result). Check its compression again. If it goes up, you may have a worn or damaged ring, piston or cylinder wall. If the compression does not rise, you probably have a leaky valve / valve seat (or maybe leaky head gasket).
If 2 adjoining cylinders are low, then headgasket becomes a stronger possibility.

If they are all similar but low - let us know and we'll keep looking for the correct pressure value in case my guestimate was wrong.

Post by **nickyj** » Tue Mar 09, 2004 10:27 pm

really appreciate all your help guys. if i ever bump into you its beers all round!