compression psi

Rufustbear

Rufustbear

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Location
Manchester
Reading back through postings i see psi readings for Jotas at 155 to 160 as i understand Jotas uk should read 190,any tell me what a 1200 with Mondial high com pistons should be i thought they would read the same as a Jota at 190 psi,or does the larger surface area of 1200 bores mean less,we are reading 170 psi at the moment,many thanks Rufus
 
I think it depends on whose comp tester you use Ray. I have a Sykes Pickavant and I've got 190 psi at last check.. my Jota is at best circa 155-160

I think you might get more compression on a bigger laverda bore because the extra portion of bore compresses an area (squish) that doesnt comprise a hemi chamber, dont make  me do a PLayskool picture !

Rob
 
Didn't know you got hi comp mondials for the 1200,thought that was why mirages were low comp.


                                                    Dave
 
Rufustbear said:
what are u getting 190 psi on ie setup,cheers rufus
Click to expand...

I think there's too many variables to make a comparism Ray ?  I use, Ross pistons, you say 1200,  its bollocks for the most part, you mean 1117, 1130 or 1144, 1172  :o , the head, and barrel has been skimmed, and the other usual generic stuff, Holley down draught carb etc

Rob

Readings would have been taken with a hot engine at WOT.. Cam's point is valid. If your readings are all consistent, +/- 5 psi  then theres a good chance all is well..Where is the problem, is the head standard?
 
Yeah, I always thought my guage read high- they're only useful for comparison purposes.
 
Dave is right the "1200" or should I say 1116cc Mondials are 7.8:1 the high comp pistons were Asso and they (might) be 9.5:1

These days there are a lot of Accrolite (both Phils version,good and another bad) J&E, Ross, Cosworth,and probably even Halfords, my own experiance is that there is nothing like Asso, but these have changed over the years as well, I have a brand new set of three Asso high comp 80mm bore pistons, which I cant use because one of them is 5 thou out of round, and may have been dropped, I have had a new Asso from Wolfgang (old stock) which did not match and a new Asso (recent manufacture) from Andy wagner and that also didnt match, so thats three different types of Asso high comp pistons known to me, maybe there are more.

Ray, are you telling us that on your race engine you  not have degree adjustable timimg chain wheels?
that might be costing you a bit of top end, and is cheap horspower gain.
CLEM
 
Clem

I wasn't aware pistons were round ? Appreciate for comparism purpose you would like 3 that match but it's common for them to be oval, particularly below the neck .
 
There are a lot of variables when testing compression. Absolute values (as long as they are somewhere near reasonable) are not as important as the variation between cylinders.

If you want to compare two different engines, you need them at the same ambient temperature, same engine temperature, use the same procedure, crank at the same speed, and preferably use the same gauge, etc.

Air compression is simple in theory. There are only 3 variables - pressure, volume and temperature. A compression gauge only tells you what one of these variables is doing. You can work out the volume ratio by looking at the valve timing (it'll be something lower than the compression ratio), but it's difficult to get an idea of temperature. When air is compressed, it gets hot (that's how Diesel engines ignite the mixture). The heat effectively raises the pressure. It would be expected that the incoming air will be heated more in a hot engine than it would in a cold engine, so a hot engine should give a higher compression gauge reading.

As an example, I just put a refurbished cylinder head on a Kawasaki Z1300 and cranked it over to confirm compression before fitting the fuel, exhaust, ignition and cooling systems. The compression readings (between 110 and 120 psi on all 6 cylinders) were lower than the numbers in the workshop manual. But I wasn't bothered by that because the manual figures are for a hot engine, whereas I had no option but to test it cold.

A compression test is a crude diagnostic tool. A leakdown test will tell you more about the condition of the engine.

Cheers,
Cam
 
Crispin and I did a compression test on the Oz Laverda Black Mamba race engine, was above 250 psi somewhere
it blew my compression gauge up, that I have had for around 30 years

bitch  >:(



 
Mar,
No Dyno yet.
I have $19,000 worth of Dyno parts at home but I need to assemble everything first
End of this year hopefully.

Red
 
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