Compression Test

[Henrik P 0]

Hey. My plan is to start port and give the head an overhaul this weekend. But before i do that i have done a compression test on my XU9JAZ engine and it says:

1: 10 bar

2: 10 bar

3: 11 bar

4: 9 Bar

 

On the 205GTIdrivers technical specs site it says that if the compression is under 10 bar it needs to be checked. But is that on the XU9JA engine ore the

XU9JAZ? Because i now that the XU9JA has a little bit highere compression ratio than the XU9JAZ.

What do you say guys? Is it ok?

[RINRIN 1]

I checked mine lastweek, and it was around 7.5 bars

[Henrik P 0]

I checked mine lastweek, and it was around 7.5 bars

 

7.5 bar? That was not much. Is that also on a XU9JAZ engine?

[dch1950 37 1 Cars]

Hi,

a general rule of thumb is that measured cranking pressure (in psi) should be between 17 and 20 times the compression ratio for the engine. This allows for engineering tolerance differences over the 4 cylinders.

Pug cites a minimum of 10 bar (145 psi) with 5% variation overall.

Thus for my 1.6 XU5JA (9.8:1) we get between 166 psi and 196 psi or 11.4 to 13.5 bar.

or if we say as a median value 18.5 x 9.8 would gives 181.3 psi +/- 9 psi (approx) for an engine in sound order and normally aspirated.

This converts to 12.5 bar +/- 0.62

Hope this helps

Dave.

[Henrik P 0]

Hi,

a general rule of thumb is that measured cranking pressure (in psi) should be between 17 and 20 times the compression ratio for the engine. This allows for engineering tolerance differences over the 4 cylinders.

Pug cites a minimum of 10 bar (145 psi) with 5% variation overall.

Thus for my 1.6 XU5JA (9.8:1) we get between 166 psi and 196 psi or 11.4 to 13.5 bar.

or if we say as a median value 18.5 x 9.8 would gives 181.3 psi +/- 9 psi (approx) for an engine in sound order and normally aspirated.

This converts to 12.5 bar +/- 0.62

Hope this helps

Dave.

 

Ok. Then for my engine it's 156 PSI-184 PSI. (10,7 bar-12.6 bar). So that means my engine has lost some compression over the years (190000 km). I gues it's ok then. But don't you think the biggest chance for a leak is in the valve seats? Ore can it be the piston rings that are starting to get a little weak?

[Cameron 16]

There's an easy way to check. Do another compression test but this time squirt a few drops of oil down the spark plug hole beforehand and then post your results on here. If the rings are getting on a bit the compression should have increased.

[Henrik P 0]

There's an easy way to check. Do another compression test but this time squirt a few drops of oil down the spark plug hole beforehand and then post your results on here. If the rings are getting on a bit the compression should have increased.

 

Ok. Thanks guys. I will try that

[Simsy 0]

Mine was around 12 bar for all

 

XU9JA though

Edited November 5, 2008 by Simsy

[macaroni 17 1 Cars]

Just done mine after a head rebuild and skim; 200+ psi for all 4 (13.7 bar).

[Henrik P 0]

Just done mine after a head rebuild and skim; 200+ psi for all 4 (13.7 bar).

 

Thats a lot. Is that with the std cam and how much did you skim off? What's your compression ratio?

 

By the way. I have another question. This winter my plan is to get the head portet with 3 anglet valve seat, higher compression ratio, another cam, KN airfilter, get the manifolds blueprintet and then get it on a dyno. My gold is 150-160 hp. My questions are:

 

1: I have had a look an these 2 cams from CATCAMS. 4900365 and 4900366. Wich ones do you think is the best for my setup? I don't care if the idel becomes a little rough but offcourse not to rough.

 

2: How high do you think my compression ratio shall be? My plan is to use V-Power fuel ore somthing like that

Edited November 5, 2008 by Henrik P

[dch1950 37 1 Cars]

Hi,

wet and dry tests will indicate compression ring wear. However if the wet reading peaks quickly then goes down after 4 or 5 cranks then secondary indication is valve stem seals, or the valve stems themselves are leaking. Similar readings for both wet and dry show valve seat and or exhaust valve burnt.

When actually driving the car overrun exhaust smoke will point to oil stem seals - usually!

I had my lump stripped and rebuilt last year and have been really kind to it for the first 5K miles. I did 500,1500, and 3000 mile oil change and new oil filter. I think that it is revving quite nicely now and the 4-6k power is quite sweet.

Now the run in is out of the way I am looking to get some head and cam work done. Lovely!

I always thought the cam profile was used to alter the duration of the suck and blow parts of the cycle and thus shouldn't affect compression.

good luck Hendrik

regards

Dave

[Simsy 0]

Thats a lot. Is that with the std cam and how much did you skim off? What's your compression ratio?

 

By the way. I have another question. This winter my plan is to get the head portet with 3 anglet valve seat, higher compression ratio, another cam, KN airfilter, get the manifolds blueprintet and then get it on a dyno. My gold is 150-160 hp. My questions are:

 

1: I have had a look an these 2 cams from CATCAMS. 4900365 and 4900366. Wich ones do you think is the best for my setup? I don't care if the idel becomes a little rough but offcourse not to rough.

 

2: How high do you think my compression ratio shall be? My plan is to use V-Power fuel ore somthing like that

 

You want 160 from a 1.9 8v ?

 

Wishful thinking i reckon

 

Maybe a solid 140+

 

As for your static CR, that's up to how much you skim, but i wouldn't go too mad.

 

Hi,

wet and dry tests will indicate compression ring wear. However if the wet reading peaks quickly then goes down after 4 or 5 cranks then secondary indication is valve stem seals, or the valve stems themselves are leaking. Similar readings for both wet and dry show valve seat and or exhaust valve burnt.

When actually driving the car overrun exhaust smoke will point to oil stem seals - usually!

I had my lump stripped and rebuilt last year and have been really kind to it for the first 5K miles. I did 500,1500, and 3000 mile oil change and new oil filter. I think that it is revving quite nicely now and the 4-6k power is quite sweet.

Now the run in is out of the way I am looking to get some head and cam work done. Lovely!

I always thought the cam profile was used to alter the duration of the suck and blow parts of the cycle and thus shouldn't affect compression.

good luck Hendrik

regards

Dave

 

You can increase the dynamic compression ratio by using different cam timing, difficult to go too mad with a longer duration cam though.

 

If i were tuning a 1.9, based on what i've read, i'd prob go for a light skim, bigger valves, a different cam and advance the ign to run on the highest octane fuel i could get

Edited November 5, 2008 by Simsy

[dch1950 37 1 Cars]

Hi Hendrik,

This guy is always worth a read ( although I think he's had fall outs with people on this forum)

http://www.pumaracing.co.uk/PUG2058V.htm

Dave

[projectpug 0]

160 isn't wishfull thinking on an 1.9 8v its achievable with some choice mods

[Cameron 16]

I win, my Mi / 6 hybrid was 16 bar the last time I checked. Shows the benefits of running in hard.

[dch1950 37 1 Cars]

160 isn't wishfull thinking on an 1.9 8v its achievable with some choice mods

agreed but doesn't it point towards TB's and modded heads?

Dave

[projectpug 0]

Lairy cams ,twin carbs/bodies and modded head are what you want for going to and above 160 hp on an 8v.

[macaroni 17 1 Cars]

Thats a lot. Is that with the std cam and how much did you skim off? What's your compression ratio?

 

Not sure about the cam, I should have measured while it was out, but I couldn't be bothered.

Quite a bit was taken off the head, as parts of it were corroded, but the guy said it wasn't as much as for the race heads he does.

No idea what the CR is!

[Henrik P 0]

You want 160 from a 1.9 8v ?

 

Wishful thinking i reckon

 

Maybe a solid 140+

 

Not what i have heard from others who has the same kind off tuning i am about to do. It's 150-160 on the engine my gold is.

 

About the compression ratio my plan is to calculate how much needs to bee skimed off. Don't you think 10.5-1 is ok?

 

What do you say to the two CATCAMS i suggestet?

Edited November 6, 2008 by Henrik P

[Henrik P 0]

I win, my Mi / 6 hybrid was 16 bar the last time I checked. Shows the benefits of running in hard.

 

damm. How high is your compression ratio?

[Jon_Bmw 13]

Standard 1.9 mi16 checked last night.

 

All around 12 bar, although cylinder 3 took slightly more cranking to reach it. What does that mean?

[pee vee 1]

I win, my Mi / 6 hybrid was 16 bar the last time I checked. Shows the benefits of running in hard.

 

 

 

just got a brand new compression tester, warmed the engine up and tested mine

 

and got very similar results. standard mi16 engine that i rebuilt about 5000 miles ago. with PeterT cams..

 

just about 15.75 bar on all cylinders.

Edited November 7, 2008 by pee vee

[Cameron 16]

Nice . To be honest, I don't know my compression ratio. I didn't have much time to get the car built so I just kinda whacked it all together and hoped it worked. I imagine it's fairly high, 11:1 or something, maybe more. Maybe I'll actually check it sometime.

[Henrik P 0]

Nice . To be honest, I don't know my compression ratio. I didn't have much time to get the car built so I just kinda whacked it all together and hoped it worked. I imagine it's fairly high, 11:1 or something, maybe more. Maybe I'll actually check it sometime.

[/quote

 

Is it nessesary for the engine to be warm? Because mine was'nt.

what kind off petrol do ypu run on?

[pee vee 1]

yea the engine is supposed to be at operating temp when you do the test, so that all the ring gaps etc, are what

they would be under normal operating conditions,.