Compression ratio....amazing what you learn

Right....some of you would have seen my recent posts about deck height, and the issues I was having (mainly due to lack of knowledge as this is my first full rebuild). I was trying to measure the deck height and shim the barrels to get all the compression ratios the same. However...now I been reading more and more about engine building, I've realised that I was going about this all wrong.

I've mainly been using Tom Wilsons' book, and don't think I'm saying it's no good or anything as I've found it brilliant, but like all things, you should gain information from multiple sources to get a fuller understanding.

So, I found that the first thing I should have done was to find out the combustion chamber volume, then figure out the requied deck height for a given compression ratio.

I tested each chamber as they could be different (and were!) by using the method shown below. I made a perspex plate and drilled a small hole in it, then used a large syringe to put coloured water into the hole. I used a little grease to seal the plate down.

You've got to move the head round as it gets full, to get the air bubble at the hole. I filled the 60cc syringe to the 60 line, making sure there were no bubbles trapped in there, then put that into the hole, filled it again and noted what was left once the chamber was full.

Now, the results I got were as follows in order 1-4:
105cc, 105cc, 109cc and 108cc.
Now some information says you should grind out some material to make all the chambers the same, but I wasn't confident to do that, and I was lucky that each head had similar volumes, so the big difference was in different heads, therefore the barrels should be a similar height between 1+2 and 3+4.
Next I measured the step and diameter of the part of the chamber the barrel sits in, so that I could subtract this from the previous figure. Then I did the same procedure but on the pistons to calculate the dish volume. This all gave me total volumes of:
66.4cc, 66.3cc, 70.4cc and 69.3cc

After reading about suggested ratio figures, I decided to go for a ratio of 7:1. This meant I needed deck heights as follows:
1.28mm, 1.28mm, 0.7mm and 0.84mm

I then remeasured the heights with no shims under the barrels to get my starting datum (I took the average of the two measurements at each side of the pistons) and calculated the actual shims needed were as follows:
1.46mm, 1.36mm, 0.29mm and 0.65mm.
Due to the available shim sizes (off the shelf ones) I managed to get the following:
1.52mm, 1.36mm, 0.22mm and 0.58mm.

If you then back calculate using all the figures I measured and got in the end, it gives me the following compression ratios for each cylinder (1-4 again):
6.96:1, 6.98:1, 7.00:1 and 7.02:1

I'm now happy my compression is as even as I could get it, but I was worried that the shim wasn't anywhere near the recommended 1.5mm in cylinders 3+4. So I installed everything and put plasticene on the pistons. I installed solid lifters (as my hyds ones need bleeding and would therefore just compress under load and not move the valves properly) set the valve clearances to 0.006" and rotated the engine by hand. Apart from the resistance of the brand new rings etc, the engine turned fine and I had no clashes at all. In fact only one bit of the plasticene was touched and that gave a thickness of 2.7mm.

So....sorry for waffling on, but after all that, does anyone think that I shouldn't carry on with the build due to the 3+4 shims being really small?

Any advice or comments appreciated as always

 

I haven't read all that but you need a round plate that sits on the sealing surfaces and just measure that.
I think 7:1 is too low, it'll be a bit gutless. I'd aim for at least 8:1

 

What you're measuring in your picture is meaningless, sorry.

 

Can't I just subtract the bit away afterwards? If I get the whole volume as shown, then measure the step you're on about, I should be ok to take that volume away shouldn't I?

 

I'm afraid Zed speaks the truth. You need to measure the volume where the upper cylinder edge meets the head (from your pic, where the "ledge" is), so you'll need to cut a circular piece of perspex to fit the cylinder aperture.

Stock 1600 volume was 50-52cc, as I recall. They vary quite a bit on production heads.

 

What's the difference between putting a plate on the face where rhe heads sit, and putting one on the top and subtracting the volume? I'm not doubting any of you guys, but interested in if I'm missing something?

 

Probably nothing *if* it sits square and you can accurately calculate the "above lip" volume.

 

Big thumbs up from me for reading up and trying to get your head round it yourself
A second thumbs up for giving what you read ago with this attitude you will really get to understand your engine and it will love you back for it

Do yourself a favour though and cut that perspex down to fit in the lip the barrel sits on
Maths is all fine and well and is very much needed but tolerances build up quicker in real time than on paper

Its the hands on time measuring and adjustments and more measuring and adjustments that performance cylinder head workers can get an extra 10 bhp on a stock engine

Just to chuck it out there if you just want it to run and keep you running then a few cc here and there makes no difference
Ive seen on the forum aircooled engines with different valve sizes on each side where the owner had no complaints untill it broke for a different reason

 

Not sure if I read it correct but you can't shim the barrels independantly ,they have to end up perfectly parallel to each other to allow the head to sit square

 

Unless the case aint square

 

Thanks for that. I can understand what you mean about tolerance build up, so I'll recheck tomorrow once I've cut the plate down to fit the bore

 

@lhu1281 's right. All barrel tops must sit at the same height, otherwise the head will be cocked and possibly not seal correctly. Understand what you're trying to do, but it's likely to be a bit self-defeating.

Also re compression ratios: as Zed said, 7:1 isn't too exciting. A boggo 1600 is 7.5:1. Up to 8 will be fine, possibly more. When I fitted my stock Autolinea heads yonks ago, the CR went above 8:1, with no ill effects.

 

Lower compression is probably more ok with large capacity. Our engines were designed for poorer fuel than we run in them, hence the low design CR.
My 2020 has about 9:1 CR.
Agree with the shimming bit - using different shims to correct the case machining is a +ve move but the tops of the barrels must end up as square as possible for the seal. You can't just heave on the head bolts if it leaks. If your pots have very slightly different CR you won't notice, but go too low on the CR overall and it will not have any grunt.
If you have a standard engine with standard heads I really would go for the standard deck height and build it like that.

All very interesting in theory, but I can vouch that a 1600 with about 6.5:1 CR will not accelerate in 4th AT ALL unless it's down hill. 7:1 doesn't sound much more to me. Does anyone know what the std head capacity is for a T4 engine your size?

Sorry for appearing rude last night, I was very tired. You have been over complicating the measuring in a way that's not neccessary, measuring to a surface that is basically a tidied up casting with no engineering use - that surface floats in mid-air when it's all assembled.

 

Thanks @zed and don't worry about last night....you grumpy git

Just to clarify, the engine is an 1800 AP with 93mm std barrels and Pistons (brand new AA ones) and the heads are AMC ones off my old 2ltr CU.

I'm going to redo the cc measuring, even if it's just for interest, today hopefully. So, are you saying that to a certain extent, the barrels being level is more important than a uniform balance of compression ratio? So get the barrels level with a deck height about the correct amount for say 8:1 but just accept the difference in ratios? I suppose then that doing it that way would mean if you really wanted to get them all the same, you'd then machine material off the heads to compensate, thereby leaving the barrel heights all the same?

 

Even when I built my stroker I didn't go as far as you are, though maybe I should have. Yes to what you are suggesting - get them level and adjust head capacities would be the way to even CR's across the engine. 1cc of head material will make quite a hole in your head so if you remove a fair bit from the worst offender and leave the best alone you'll be altering the burn to a degree and swapping one "difference" for another?
Though my engine is T1 based I have 92mm barrels - pretty well same as yours - and 60.5mm cc heads. The stroke is different though so the calcs will diverge a bit. I ended up with about 1.15mm deck height after building with 0.4mm initially, then loosing my nerve after a few miles. That was about 9.5:1 CR but if I'd got it good and hot I think even hypereutectic pistons would have kissed the heads. I wasn't building for longevity though, I was after prooving that a plonker can make an engine with almost the same bits but double the power the so-called professional manged. I did achieve that goal, so now looking to tone it down a bit and make it more of a cruising bus engine than a thrash off the lights jobby (which it does very nicely).

 

Usually when you cc heads you just dremmel or die grind material out of the chamber , measure the barrel lengths and machine if different ...you could take material off piston tops .but where do you stop as there are so many variables like carbs ,

 

Or you could just fit a 123 distributor, which will compensate for any small differences in chamber volume ...

 

Really ?how does it know .?

 

Ok, I've re measured using a round plate in the head this time and I was shocked to see the inaccuracy of my first attempt! Each chamber is 56cc's. The problem I have now is that if 7:1 is too low, 8:1 gives me a negative shim!
7:1 would be a deck height of 1.28mm but 8:1 works out at -0.29 mm!
The new pistons are adding 10cc due to being dished (I presume I'm right to include this in the calc) so I've got 66cc chamber, 93mm bore and 66mm stroke.
According to my calcs, With zero shim that would give me 7.8:1.

Am I getting something completely wrong here? (Oh yes......that is definitely possible )

 

Pistons - if they were flat top you'd be at 7.8:1 with 1.5mm deck height.
But in reality it's probably the 2L heads which would have needed comparatively bigger chambers to reduce the CR.