33 replies to this topic

[lsto]

Hi all, just before everyone starts telling me that decompression plates are evil and I should never mention the word again I would just like a little information on these. By information I mean I would like educated facts rather than just opinions.

(My apoligies if that sounds rude, I'm not trying to offend!!!)

Basically what spurs this is that I have been building a 998cc turbocharged engine, and during my research a while ago when I was working out compression ratios and deciding what items to buy, I couldn't help but notice every time someone mentioned using a decompression plate they were always told its a bodge and it shouldn't be considered.

I know that it has pros and cons but I cant really understand why it is such a terrible way to lower compression, I am not an expert on the A+ engine so please correct me if there is a fundamental flaw which makes it so unreliable.

As I have said I am not an expert on the a series, but I work as a heavy diesel fitter and some of our engines run decompression plates as standard, and I know it creates a weak point in the engine, but in other respects that is an advantage because when something does go wrong it tends to blow the plate rather than cause more expensive internal damage, and I would have thought if your trying to run a high boost engine it would be better to blow that rather than burn out a piston?

As I said I am not an expert and I'm not trying to prove anything, I would just like some more information on why its a bad idea.

(And by the way my engine is not running a de-com plate, has a nice 12G295 head )

[Globule]

You may find the pushrods are too short afterwards which would require spacers (or there may be enough adjustment, depends on the thickness I guess)

[BritishRacingGreen]

I believe (I haven't used one, but know people who have, and only a few have ever had any issues) that most people's aversion to them stems from that you have to use 2 head gaskets with them, which makes it twice as likely to go wrong (especially with a forced induction engine).

[Alex_B]

It's not so much the act of using one that's a bodge its why you are using one that is the bodge, if you are engineering it properly then that's fine as you are engineering in a weak spot as you have mentioned, if you are using a decamp plate because you don't want to spend the money on milling out the compression chamber to get the right c.r then it's a bodge

[bully]

my brother runs a decompression plate with headgasket on his short stroke race engine as the head thats fitted (slark race head) has been skimmed many times so c/r was too high, he ran it all last season flat out in 2 nd screaminig at 8500 revs with no problems at all and yes he won ther championship and i came second! (git) im not saying "its the way to do it" but buying another £700 head instead of a £90 decompresion plate was not something he wanted to do.
as for turbo wise im not sure if it will cope, something you would have to ask manufactuer

[lsto]

Its just curiousity more than anything, I just wondered because some of the turbo diesel power units I work on have run them standard from day one. It just struck me as strange that if a motor designed to work can run on them quite happily then why do people seem to shun them for performance work?

I totally agree that if your building an engine you may as well do it properly and get the head or pistons worked, but if you are on a budget then personally I cant see any reason why an engine with a de-com plate would be any more prone to break down then one without. (Providing it is done properly!!)

Just a thought...

[ANON]

you've answered your own question when you said about commercial diesels using them as standard.

i know loads of people who've used them without issue, as long as it's going onto a fresh block then there is no reason why you should have a problem with one. the naysayers will tell you how it's a bodge and your engine will blow up etc.

speak to mike tansky at ferriday if you're unsure about them.

[ANON]

thinking out loud...if a head gasket thickness has to be taken into account when calculating a cr then surely a gasket is also a decompression plate??

[JustSteve]

thinking out loud...if a head gasket thickness has to be taken into account when calculating a cr then surely a gasket is also a decompression plate??

Exactly!

People on this forum always go on about how they're a bodge, but they're not.

For the last 25 years my dads 1380 has ran with two head gaskets which he says was normal when he was growing up and as the OP states, it is recognised as the 'done thing' in other areas.

Maybe we shouldn't be using steering drop brackets, adjustable suspension and 'spaced drums'. These are all as much a bodge as fitting a decompression plate

[Alex_B]

Well like I said they are a bodge if you are using them because you arnt doing it properly and getting the head worked to lower the compression...



and not really compareable to a steering drop bracket as that is specifically allowing the steering column to be mounted at a lower angle, adjustable suspension well thats just common sense and spacered drums came about because of the cooper s getting discs at the front which gave a wider track

[JustSteve]

Well like I said they are a bodge if you are using them because you arnt doing it properly and getting the head worked to lower the compression...



and not really compareable to a steering drop bracket as that is specifically allowing the steering column to be mounted at a lower angle, adjustable suspension well thats just common sense and spacered drums came about because of the cooper s getting discs at the front which gave a wider track

In what way is raising the cylinder head any more of a bodge?? It has EXACTLY the same affect as having head work done

They are all 'afterthoughts' and there to allow changes from the way the car was designed.

[Alex_B]

they are a weak spot, when raising the head slightly needing two headgaskets thats doubling the chance of headgasket failure, and unless you are specifically designing in a weakspot (which is a very tricky thing to do correctly, just look at the tech and design in ring pulls on cans as they are essentially a engineered weak spot ) then its just a quick "this will do the job"


If you had the choice to either get the C.R measured and adjusted by changing size of the combustion chamber or using a plate which will have more chance of failure, I know which I would rather do.

and modifications to original design are fine provided they are done correctly and not done instead of something correctly, like using cable ties to hang the steering column instead of a specially made bracket

[jd24-7]

I can't see any real problem with compression plates, i intend to try it out soon on my otherwise standard 1275 turbo. Past of the reason is certainly cost, but also it's easily reversible, even to the point of taking off the turbo.
As regards a bodge, it's just engineering. No worse than machining pistons , or grinding out heads, both of which cause their own issues.

Jd

Edited by jd24-7, 26 December 2012 - 11:08 PM.

[ANON]

thinking out loud...again...a head gasket is just a bodge really, if you have two 'perfect' surfaces then you don't really need a gasket between them.


and before anyone asks yes i've done it with a few two stroke engines years ago when we were playing around with squish on them. takes a while but it can be done.

also managed to get a sprite running ok years ago without any inlet manifold or exhaust manifold gaskets.

was a bloody pain to do though

[Alex_B]

you wont be getting a head and block sealed without a gasket though as it was designed with one in mind, everything nowadays is designed and manufactured with a certain tolerance, yes theoretically if it was made with zero tolerance and error then yes no gasket is needed but its just not practical in the real world.

and a gasket isnt considered a bodge as it was designed with it in mind so it would technically be a bodge not using a gasket