there are some pikey ways of increasing CR I have seen people using weld to make the combustion chamber smaller, filling out the bowl in pistons etc. but realistically maximillion its piston design that can be most effective domed pistons with valve cutouts are a good way or custom designs for bowl in piston jobbies.

Who the hell did that?

Yeah its the pistons that need modifying, but that needs to be in conjunction with block height modification, or decreasing the HG thickness etc.

can i just say i will be boosting my afh next year and tbh i am looking for 200bhp+ its given me alot of shit so far but that was the old owner abusing the engine and me being lazy not checking the engine thoroughly and they have all been standard problems: headgasket failure and piston rings gone i have spent alot on my afh and will spend alot more and i didnt drop it in just for the easy 100bhp from the start it was nice but i wanted to boost one from the start and prove it can be done and so far all i need to boost it now would be a decomp plate and that is to drop the CR to a suitable level which i dont see as a bodge tbf as it will be a legitimate laser cut plate of aluminium. There isnt many ways of dropping the CR enough for boost on the afh due to how the piston are you cant skim enough off them without making them too weak on top, so the only was i properly see of dropping the CR from the standard 10.5:1 to what I'm aiming at which is 9:1 for driveability on and off boost is to use a decomp plate. i am also looking at forged pistons and vernier pulleys to correct timing but i already have forged rods and this was because i got a good deal and could machine them myself to fit the engine. I have nothing against 8v's and if i hadnt of gone down the 16v route i would have already boosted an 8v easily from the money I've spent on the afh already but thats my choice and I'm aiming at something different with my car to most other people on here.

I say go for it Max but people do need to stop slagging off the afh's thats for sure

Nice input Pete thanks.

It's good to hear your going all out on yours, when I said a de-compression plate being a bodge , I meant it in the context of using one with standard internals to get round pre-det and boost issues.

oh right i will need to work out a proper thickness for mine to be but I'm guessing maybe around 1mm thick as i have looked at other ways of dropping the CR just none seem enough and this is the simplest and most straight forward way to me of doing it.

and i wasnt aiming the "people need to stop slagging afh's" off at anyone it was just to put it out there for the general people lol

oh and i need a strong and reliable engine for the power i want as the power will be needed for a reason.

and just to add I'm sure with a decent standard afh thats been looked after i could achieve a reasonable amount of boost on standard internals cant prove this as like hell am i building up another engine lol would rather just put all the effort into my one polo but thats just my opinion

Edited 13 December, 201114 yr by StinkyPete

So, you could skim the head right down so the valves stand proud and deck the block.

Also add a protruding spark plug (not sure about that, have you seen the plug placement? Its crap).

Whether the pistons sit above the deck or not is up to you and the gasket you fit.

1mm (0.040") is usually stated as minimum head to piston clearance and should be easy to calculate if you can find out the installed thickness of the gasket.

Skimming the pistons will reduce the bowl volume, and also reduce crevice volume and weight which has added benefits.

Of course you will need to measure your valve to piston clearance. Easily done with the head on the block, valves installed with weak testing springs and a degree wheel. Clearance is usually closest just ATDC, but you'd have to test to be sure.

2mm piston to valve clearance is a safe figure to work to. Maybe too safe, but that's up to you.

Pete did you look at early head gaskets? Back when we had a few different engines stripped down here we were comparing head gaskets and the afh was the same kind of layout as the mh gasket bit that was thicker which should drop the cr some? Not sure if any other of the older lumps use even thicker gaskets? I doubt it would be enough of a drop to run boost but maybe if coupled with the avy pistons it might just do it?

This is just some thing that's been rolling around in my head and I haven't looked any more into it than comparing the hg's so I have no idea if it would even work, just thinking out loud

also just to say I'm on golf coilies all around on lowest setting on front so a supposed 120mm down running a stage 2 subby and the mk3 sump and pickup on my afh and i have no clearance issues or oil problems so far and i have a lot of speed bumps to get to my house and have done 600 odd miles problem free other than blowing the diff bearings and going through a set of gsf top mounts and this in in a mk2 bready

Pete, you don't necessarily have to drop the cr to run boost. Just use a massive intercooler to cool your charge. Have you read up on turbo charging?

simon i haven't read up alot on turbo charging no but that doesnt mean i dont know anything i know enough for what I'm doing atm and anything i dont know i will research thoroughly or ask for help i do know a higher cr creates more heat and tbh i have no room for a massive intercooler and its easier to drop cr than to cool the air coming in, also i would rather not stack headgaskets or anything like that and i know thats not what you meant guy but i have heard of people doing it i would prefer to run a metal hg but i doubt a single one is thick enough to drop the cr to the level i want and so coupled with an alloy decomp plate is what i plan on doing.

As for spark plugs, you could use indexing washers , or maybe surface discharge plugs if clearance is a problem.

You can't skim a heron design cylinder head to increase C/R

The abd head is Hydro

How manufacturing used to be?

No one ever asked how to raise an engines C/R...

Pete, I didn't mean to suggest you didn't know anything about turbo charging. I just wondered if you'd done much research. We could share info. Looking forward to seeing your progress on charged Afh.

Harris, wouldnt deepening the valve seats cause problems with the valve stems when they close i.e they'll be too 'long'?

i take it the heron design is with the CC in the block and hydro is in the head??

and max i beleive the topic is the best way to raise the cr for an abd?? lol

Pete, you don't necessarily have to drop the cr to run boost. Just use a massive intercooler to cool your charge. Have you read up on turbo charging?

I am interested to know the theory behind this? Cooling the charge increases the density, and therefore increasing the quantity of charge you can cram into the cylinder, increasing cylinder pressure further under compression, creating more heat and more force on the pistons when it is detonated- a nightmare for a motor on stock CR!

Max - I understand more why you're doing it now. I still think that it would be a lot of work for not a lot of gain. Thought about bigger valves at all? Build a manifold for a bit of resonance tuning?

Edited 14 December, 201114 yr by Iscariot

Apologies not familiar with the head design.

Well about the valve stems, mines fine with .8mm off the head and valves sank into head. All about measuring and checking clearance.

Decking the blocks still gonna give valve clearance issues as the heads still going to be closer to the pistons (or the CR wouldn't change)

What cam you planning on using?

Nick

Pete, you don't necessarily have to drop the cr to run boost. Just use a massive intercooler to cool your charge. Have you read up on turbo charging?

Cylinder pressures?

Det limit vs MBT?

It's not just heat that's an issue.

Just going on what I've read. I'll be honest and say I can't explain it ;-?

Andy, what's det limit and mbt?

Colder air isn't gonna cure a high CR

i take it the heron design is with the CC in the block and hydro is in the head??

and max i beleive the topic is the best way to raise the cr for an abd?? lol

Yeah, the BEST way, not how to do it?

Can we actually look what people are writing here, not just scanning over it and then answering something incorrectly.

Edited 14 December, 201114 yr by MaxMax

surely the best way is some custom designed pistons?

thread tidied... as requested by OP.

Indeed custom pistons are the way to go, due to the, Ahem, unique pistons and combustion chamber on an ABD the ideal scenario would be similar but with a smaller bowl.

Can we keep this thread on topic now and what the thread was intended for, not "Buy a 16v instead", everything has been said and i'm sure we can carry on the discussion at a the RR day.

@Peteg40

Yeah that's the best way of doing it, but how much would a set of custom high comp pistons cost?

I'd say you would need to design some unless there is someone out who is willing to either design it for you or give you an existing design. it could be as simple as a smaller bowl or it could be more elaborately design to get the maximum efficiency out of the engine for exampe a protrusion in the combustion chamber can reduce CR. too small and it'll become a hot spot causing pre-ignition and too big it'll hit the valves so a happy medium needs to be found.

prob about £500ish for the pistons i'd say.

And you did say best! not best under £200 or best under £400! just best!