Deck height and compression ratio numbers....

Hi all,

I have measured my deck heights which are 1.37mm, 1.45mm, 1.58mm, and 1.52mm

The CR works out to 8.56:1 on 2 cylinders and 8.34 on the other 2.

Are these numbers good, or is some adjustment required?

Thanks for your experienced opinions.

Ade

 

What engine, cam etc.
You sure the piston wasn't rocking?
Did you measure above the piston pin?

 

If it was a stock cam I'd rather be seeing 8.3:1 than 8.6:1 as this is advice.
If it was mine I'd run the 8.6:1 and be ready to take it to bits again. I think that's on the edge.
If I could avoid such a big spread I would too, the closer you get these the smoother it will run.

 

This is a 1700cc type 4, now with a new 96mm big bore kit and 2000 heads. The engine code is CA and the cam is stock.

Measured in the centre of the piston which didn't rock.

How do I reduce the CR?

 

You have the cylinders tightened down properly?

Your 2 options are shims under the cylinders and/or increasing the head volume.

http://cbperformance.com/v/enginecalc.html

 

I didn't torque them up, just a turn or so more than hand-tight, so they were seated as a guy on YouTube suggested. I was wary of damaging the fins with the copper pipe.

Thanks for the link, what CR should I be aiming for?

 

Yes, be careful if you're temporarily using copper pipe sections to hold the bare barrels down. You will snap fins off if you crank them down too tight.

 

With a standard cam, 8.56:1 CR might be too high, 8.34:1 is okay but I'd probably aim for 8.25:1 and even it up by using thicker cylinder shims on one side (on my 2316cc engine I have 2.26mm shims on one side and 2.31mm on the other).
Does that deck height include the step in the head – if there is one?
Deck heights are a bit scattered and it's possible the crankshaft centre line isn't quite on the crankcase centreline.
What manufacture of heads, pistons and cylinders do you have? Are the pistons flat top with no valve cut-outs? What are the valve sizes?

 

I have a stock cam Type-4, but it's not a stock engine so I hesitate to say that because my engine is happy with 8.7:1 anyone else's would be ok.
The VW 7.whatever is too low IMO, I'd want at least 8:1 and like Geoff I have 8.25 or 8.3:1 as being a good number to head for, but I'd take 8:1 if that's how it worked out.
My approach would be to get the decks tight (1.3mm ish) and as even as you can, then fine tune each one by nibbling at the combustion chambers until you have them all the same at some figure between 8.0 and 8.3:1.
Sounds like a hassle, it is a hassle, but you're in non-standard engine territory now and this is par for the course. It's worth the effort.
You could slap it together how it it, it would run I'm sure, but the 8.6:1 side is a bit ... unnecessary. I probably get away with my 8.7:1 because my big lazy engine is so inefficient it doesn't fill the cylinders as that calc assumes, hence less running CR that yours would make. Hope that makes sense.

 

@Zed Anyone in the US would say the 7.6:1 is too high let alone 8.6:1.
Do you have to use 98 fuel with a standard cam and 8.7:1 CR? Mine will occasionally knock using 95 at low revs and heavy load – a smidge under 9.0:1 CR and a 272 deg duration (234 deg at 0.050") cam.

 

I use the cheapo stuff. I accidentally used the expenive stuff once, it didn't run any differently. Perhaps my ear isn't tuned to hear knock but there was no damage seen when I had the head off after +10,000 miles.
If you read samba performance the eggspurts constantly decry vw low CR as out of date, but 90% of people who tut tut low cr then boast of their 10:1 CR bravery don't mention cam at all and anyone reading their out of context partial advice could get in serious trouble. There should be a law against quoting CR without reference to the cam, it's a major source of confusion to anyone trying to understand.

 

Agreed, and some don't understand what the cam does to torque and you get people building engines with 285+ deg duration cams (Scat C35 for instance) and saying it is built for low down torque.
Have you read what CR John at Aircooled.net recommends? 7.5-8:1 for a 272 deg cam, far too low. But then he also told me not to increase the capacity or use Biral cylinders.
Anyway, back to deck height, I doubt if there really is so much difference between cylinders and for a road engine I wouldn't bother to tickle the combustion chambers for the sake of a point of CR. We don't know the combustion chamber volume and we don't know if the deck includes any step in the head. It looks like the combustion chamber volume is about 52 or 53cc so I'd average the deck to 1.41 and 1.55 (assuming the two lowest numbers are on one side of the engine and the two highest are on the other) and I'd use a 0.10mm shim on the side with the highest deck and a 0.20mm shim on the lowest deck and call it done. The deck would be close to 1.6mm and the CR would be about 8.45:1. But if the deck doesn't include the step the squish could be in the zone to be avoided. There is too much assuming and guessing for me to be comfortable with, but you get the idea.

 

I too chuckle at people defending (because that's what they're doing) their bad bus cam choice like that, I want to ask what gear that's in when they say it pulls from tickover. In first maybe, with some throttle.

Mine goes up this hill in 2nd with my foot off the pedal. It's a lot steeper than it looks.


A C35 has no hope of that, it would stall in 1st - been there as you know. Cracking way to get power above 3,500rpm though.... if you build the rest of the engine to rev to 6,000rpm so you can actually take advantage of the power band.


I don't think 1700 heads had a step, they had the discontinued head gaskets?

 

VW 1800 heads don't have a step either, both originally had head gaskets. The OP has after-market 2.0l heads bought from Stateside but I don't know who manufactured them.
It's not easy to give advice when you only know part of a story, each new thread gives a bit of information but you have to remember what has gone on before to make any sense of it. And I have trouble remembering what happened yesterday.
EDIT: I'm hoping the new heads don't have steps or he's including it in the deck, if not it will be big compromise between deck and CR.

 

Bottom end still won`t like it ;-(

 

Right then gents. Thanks for all your input so far (and your patience), it is greatly appreciated.

I have remeasured everything, with deck height in the middle of the piston (with new callipers rather than a feeler gauge). The pistons are flat, there is no step in the head (55cc), and the details of what I have are in the photos (valve sizes etc).

A stock 1700 engine but now with a big bore kit and new heads from Stateside Tuning.

The numbers are:
No. 1. Deck height - 1.36mm, CR - 8.4:1
No. 2. Deck height - 1.24mm, CR - 8.5:1
No. 3 Deck height - 1.6mm, CR - 8.2:1
No. 4 Deck height - 1.45mm, CR - 8.3:1

Is the solution to shim 1 and 2 to bring the CR to 8.25:1 (i.e. 0.14 to no. 1 and 0.26 to no 2)? Where am I likely to be able to get the shims I need from?

I welcome your thoughts and feedback.

Ade

 

There should not be a difference in deck between #1 and #2 or between #3 and #4 - are the cylinders the same length, at the same height and flat across the tops?
You shouldn't use different size shims on the same side of the engine, whatever you add to #1 should add the same to #2. The thinnest shims I have seen available are 0.25mm but you could cut some from shim stock. https://www.ahnendorp.com/VW-Type-4...and-machining/Cylinder-Shims-276/?language=en
You must check the ring end gaps on those pistons, AA will tell you they are correct and ready to fit but they may not be.

 

If difficult to measure with a caliper a depth mic is preferred . If your barrels are different lengths you heads won’t seal plus your deck heights differ worryingly. Everything needs to be measured super accurately. Usually you’d start by decking case then accurately measuring barrel lengths, measure piston weights and from gudgen pin to piston top etc . You have some big ass differences in your measurements.

 

OK, I have had a second opinion on my deck height measurements. It seems a more consistent approach such as ensuring the mating surface is clean, and tightening each barrel the same amount and in the same manner, yields more consistent results. The deck height is 1.4mm across all 4 cylinders, giving a CR of 8.3:1.

The new barrels are all the same length, and everything is flat and true.

Previous comments indicate that this CR would be acceptable. Adding a 0.25 mm shim would reduce the CR to 8.1.

 

That's more like it.
Either would be fine, you wouldn't notice the difference. That's just my opinion, other opinions are available.