Reducing Compression

[mahatma]

After half a day at Emerald today, my compression appears to be way too high ... I think the quote of the day was from Dave Walker who said "F*** me, I've never seen the gauge round that far before".

 

So somehow, I have to reduce the compression somewhat to enable the engine to be mapped and the suggestion was to get the pistons dished by a suitable amount (after properly calculating the compression ratio).

 

Can anyone suggest a company who may be able to do this ?

 

Thanks

 

Andy

 

Spec is as follows;

1800 Scholar EVO2 Block

Standard VHPD head (skimmed)

Accralite forged pistons

(285h cams, verniers, throttles, emerald etc)

[Nifty]

"You've got a bl%dy diesel here"

 

Hope you got home okay and get it sorted soon *thumbup*

 

Keep off the straight and narrow 😬

[STEVE GILBERT]

Scholar, Coletec (Holbay), Burtons, Minister, Raceline there must be loads.

 

I bet your really pissed off having to strip a freshly built engine.

It must have been a hell of a head skim, how come it all came out so wrong

I'd be looking for a second opinion before pulling it all apart though. He might have had a faulty tester.

 

Steve

 

See My Caterham Fireblade Here.

 

Edited by - STEVE GILBERT on 22 Mar 2004 19:59:34

[mahatma]

Yep, must the head skim must have been quite a bit, but I think the block had some taken off it at Scholar too (maybe?) and the pistons increase the ratio by one unit too, I think.

 

You're right though, I am annoyed ... going to have to miss the Anglesey track day too that MAV organised.

 

I'll give those companies a try tomorrow ... I was hoping that there might have been someone else who's done this at some point

 

Thanks

 

Andy

 

(Nifty, check your mail.)

[Irrelevant]

Can you not just fit a thicker headgasket?

Didn't a trick used to be to fit 2 headgaskets - they used to do this with turbo/atmo conversions IIRC.

 

J351 TPE - Now a complete car once more (almost)

 

[Luke Beaumont]

I think the K is probably the last engine you want to try crafty head gasket tricks on

 

Run it on av gas (or lpg) 😬

[oilyhands]

It's relatively easy to lower the CR by removing some material from the combustion chamber, if it's removed from the right places the airflow into the cylinder is improved too, you can drop the CR by up to a point by doing this.

 

Oily

[Asklepios]

Andy,

This might seem an insultingly basic observation, but are we talking compression ratio, or compression,as in sealing of the chamber? Most of the replies have assumed CR ,but the gauge too high ,makes me think it was a compression test. Undoubtably the CR will have been raised, but I don`t understand how the CR can have been measured other than with the head off. I am probably mistaken, but hope you can clarify.

 

Luke is correct that detonation can be controlled by Avgas (but lead deposits everywhere), but it is illegal to run Avgas at MSA events I think (not to say not done).

[oilyhands]

With any compression tester which is used regularly it is fairly easy to make a close approximation of the CR. The gauge will reach a zenith point beyond which the needle will no longer move regardless of how many times the engine is cranked, the higher the needle moves the higher the effective CR. The static CR may not be known exactly, but the dynamic or effective CR is known to be too high because of the relative needle position on the guage compared to the position acheived on other engines with a known CR.

 

Oily

[mahatma]

Thanks guys, it was a compression test that was performed, rather than measuring the actual compression but the engine was pinking all over the place. Dave Walker half mapped it so it could be driven back but with had about half the ignition advance that he would normally use.

 

Is the compression ratio that I'm aiming for in the region of 10.5:1 or 11.5:1 ? I was told ... but I can't remember off hand.

 

On my cylinder head, it has 29.5cc stamped on the side and I assume that this is the volume inside the combustion chamber of the head (I'll measure properly a.s.a.p.). Assuming a gasket thickness of 0.5mm and assuming that the piston comes to the top of the liner, I calculated a compression ratio of about 16:1 !!!!

 

So, to bring it back down to 11.6:1 ... I would need to increase the combustion area in each chamber by 13cc. Could anyone help with the following ?

 

What is the volume in the combustion area in a standard VHPD k series head ?

 

Is it worth carrying on and getting the pistons machined ... or would I be better with another head ?

 

Thanks

 

Andy

 

[scooby dooby doo]

as discussed on se7ens.net (ever at the forefront of seven's technology ) the accralite pistons seem to be too tall - a few others have experienced this as well.

 

taking some material off them does the trick. I think Mick reckoned 10-12 thou did it for him.

 

HOOPY R706KGU Hoopylight R

[Rob Walker]

A company called Gosnays 01708 740668 sell compression adjustment shims. Basically a stainless steel shim which takes the shape of a head gasket, this is sealed to the head with a special sealant and you fit the head in the normal way.

[mahatma]

Thanks Rob, I'm not sure that I'm keen to introduce another layer in to the 'k sandwich' though. The head gasket area seems to have a lot of problems historically and adding steel between the layers of aluminium *might* be asking for trouble.

 

I'll put up some more details on this for interest over the weekend when I've had a proper chance to do some accurate measurements. I think that I've overestimated several areas when calculating the CR and the current CR is probably more like 13 or 14:1 which is still too high, but a damn site easier to bring it back down from this sort of level.

 

A bit more research has suggested that the combustion chamber volume for a VHPD head is 29.5cc and for a standard k is 27cc. This would be un-ported and not skimmed. I'll be measuring mine a.s.a.p.

 

Andy

[oilyhands]

The gasket thickness is 60 thou (1.5mm) and the gasket bore is 82mm. The pistons dont come to the top of the liner, if they are Omegas they will have a 3cc dish and there is the volume in the valve cut-outs too. Do not use a shim *EVER*. If you have only skimmed by 20 thou then it should be relatively easy to recover the lost volume by reworking the combustion chambers. FWIW I would estimate your CR at around 12.75/13:1 .

 

Oily

[Peter Carmichael]

Longer duration cams allow more tolerance of high compression so ditch the 285h and fit some 320s.

[murph7355]

Many moons ago when I first had my K done, I had a similar problem with compression ratio.

 

Accralite pistons are not ideal in these circumstances from what I gleaned at the time, Omegas being a better bet. I ended up having the Accralites skimmed, but all this adds to the cost.

 

That said, by careful work with the head it was possible to sort the CR out. I think it started at 12.5 or 13 and went down to just over 11. But don't hold me to those numbers! Note that taking too much out of the head can cause other hassles later...

 

However, I'd get a bit more info together before proceeding:

 

1) Ask whether the block has been skimmed or not, and by how much.

 

2) Ask if the head has been skimmed or not and by how much.

 

If too much has been taken off either, you're going to have more difficulty getting the CR sorted, and should you need to have any work done in the future (if a head gasket goes and the head is affected etc) you have little or no room for manoeuvre.

 

I wouldn't mess with the head gasket at all (use one of the beefed up versions that are available regardless).

 

oily and PC are much more knowledgable than I, but depending on what info you get from above, you *may* be better off replacing some of the bits to give you a better long term bet.

[Richard_E]

Sorry about your engine but Daves quote is 😬

"F*** me, I've never seen the gauge round that far before".

[Peter Carmichael]

Seriously...

 

The CR is something you can calculate very accurately. The only thing you need to measure is the combustion chamber volume. I did the maths to integrate the volume of the valve cutouts in my Omega pistons when my big engine was first built and came up with exactly the same answer as Roger King did through measurement...

 

...the point being that you can model a target CR very accurately although it does help to start with an absolute set of know conditions, including the height of the block...

 

The accralite pistons that QED supply have always set the CR too high. I think at first build my engine was around 11.6:1. After its rebuild following my forgetfulness with the oil filler cap it went up to 11.9:1. When I had it skimmed (unnecessarily as it turned out) before Curborough it went up to 12.3:1. It was still running fine with no pinking on Piper 1227 inlet cam and 300 degree duration exhaust, although I did get more than a little paranoid about fuel quality.

[Jason Plato]

The accralites havent "always set the compression ratio too high "

 

People dont account for the increase made before they skim heads too far - its as simple as that .

 

As you point out yourself Peter , you had 3 skims and the mapping , performance and fuel was still OK .

 

Dave

 

 

 

 

[Peter Carmichael]

Sorry. My two statements were unconnected...

 

The accralites set a high compression ratio out of the box.

 

My engine was built with omegas and even they were dished to hit the design CR of 11.6:1.

 

My point was rather that after three skims with omegas, my engine was approaching the sort of CR that you'd be getting straight off with accralites.

[murph7355]

I think something along the lines of "Accralites have a different profile to Omegas" is probably what PC meant Dave.

 

They are very different and, unless you have the info up front, this can catch you out. It doesn't make them bad pistons - they're not - but it can lead to hassles if all elements aren't checked properly.

 

Fuel quality is a definite thing to watch with high compression. Especially in these days of pinko environmentalist vegetarian types who want us all to run our steeds on Palmolive

[mahatma]

Hmm ... hindsight is a wonderful thing, it's in short supply round these parts though. From the research that I'd done, I understood that forged pistons generally raise the compression ratio by 1 unit. It appears that the accralites may be more than this.

 

I assume that Peter's head was well and truly ported when it was first installed - within an inch of it's life if its the one that I saw at Oily's ... and as Oily mentioned, a full unit can be reclaimed from porting the combustion chamber in the head. This suggests that Peter's CR would have been greater than 12.6:1 (or so) with my head, which has not been ported but has been skimmed.

 

So combining this with the accralites, my CR may well be over 13:1 (as per Oily's suggestion). I've got the next couple of days off work, so I'll do some measuring.

 

Andy

[mahatma]

Well, to complete the story a little I've managed to do some measuring and thought I'd post up my findings.

 

The pistons come to the top of the bores, so no volume in that area.

 

Displaced volume = 448.9cc

Gasket thickness = 1.5mm

Gasket Volume = 7.92cc (82mm bore)

Head volume = 25.5cc (measured with paraffin, using a burette)

Piston pocket vol = 2.8cc

 

 

Volume at BDC = 448.9 + 7.92 + 25.5 + 2.8

Volume at TDC = 7.92 + 25.5 + 2.8

 

 

Therefore CR = 485.12 / 36.22

= 13.39 : 1

 

Doesn't look particularly good.

 

I have a couple of pictures of the head one large and one small of each picture;

 

Small picture of whole head

Large picture of whole head

Small picture of combustion chamber

Large picture of combustion chamber

 

I spoke to Oily on the phone and described my findings and he obviously couldn't comment exactly on the situation but I described the marks on the head where the gasket had been sitting which are clearly visible on the pics above and this is after only a few hundred miles. It was suggested that this *may* be associated with a soft head, especially after such a short term - this situation was also suggested by Scholar when I spoke to them about machining the pistons. I don't have any hardness testing kit so I can't be certain.

 

The head has clearly had a massive skim though as the 'D' part of the casting in the combustion chamber has been completely removed.

 

As a keen novice, I was unaware of this when I bought the head ... a very expensive lesson indeed, not to mention how frustrating it is. So it looks like I'll be needing a new head and probably still a little work on the pistons.

 

Once again, thanks for everyones help with this.

 

Andy

 

Edited by - mahatma on 27 Mar 2004 14:08:50

[Nifty]

Good to hear you've got to the bottom of your problems Andy.

 

Give me a bell when you're ready to return to Carrotland, save on the B&B.

 

Keep off the straight and narrow 😬

[mahatma]

Thanks Nifty, much appreciated. That would also give us the opportunity for more steak and a few more pints of IPA in that rather nice pub near your place.

 

Andy