41 replies to this topic

[alpder]

Also worth noting that a decompression plate needs a head gasket on either side, so you need to factor in 2 x head gaskets + the decompression plate in your calculations

Agree with what you say, but just a note that, according to http://www.ferriday.co.uk/ not only don't decompression plates need two gaskets, it's actually a bad idea to use two. Especially in my case where the decompression plate would end up being almost zero thickness to get the CR required.

 

I suppose the main trouble with two gaskets (either fitted together or sandwiching a decompression plate) is heat extraction from the fire rings: when two gaskets are together each fire ring is only against one large and [relatively] cold piece of metal: the head or the block, instead of the head and the block. So heat is being conducted away from each fire ring at half the designed rate. Hence parts of the fire ring get hotter than designed, and fail quicker. We can get away with fitting two gaskets only because they're well over-designed, but we may've gone from being at (say) 45% of the design temperature to 90%... a massive decrease in the reserve. Similarly, sandwiching a thin decompression plate between two gaskets means the plate is thermally insulated from both the head and the block leaving it at risk of burning, whereas fitting it tight to the block pretty much thermally connects it to the block, cooling both itself and the side of the gasket it's against.

 

A secondary problem may be that the gaskets are designed to crush to different clamp pressures in different locations for the same crush distance. I can't get my head around whether or not doubling-up would result in less pressure than designed in the "squishier" parts of the gasket. (Logic says not - each gasket gets a 50% share of twice the crush distance... but intuition says that's wrong.) In any case I suspect this is less important than the heat-flow issues.

[alpder]

I don't know if Boosters are legal or even which ones are snakeskin oil and which are as claimed. Some may also achieve the octane boost in a way our old engines don't like ( Oxygen enriched???)

 

I had considered this, and scanned the Demon Tweeks octane-booster page. Most that have a serious effect (a handful of octane numbers) aren't street-legal. While most that are street-legal don't even say how much they boost the numbers by. And those few that are both street-legal and do quote figures, tend to give those figures with "up to" before the number and "points" after it. A "point" being 0.1 of an octane number. So, yeah, a lot of snake oil out there.

 

I notice ethanol has a very high octane rating. Maybe I should just pour a gallon in the tank before each fill-up. It's not outrageously expensive. Oh, hang on... would have to declare and pay the road fuel tax. Ahem.

[KTS]

if i understand correctly; retarding the cam timing should reduce the dynamic compression ratio which would help 

 

downside is that it's likely to effect the engine characteristics far more than just doubling up the head gaskets

[timmy850]

Ferriday might make specific material decomp plates, but the usual mini ones are steel and need one gasket each side - eg
https://vmaxscart.co...sion-plate-1275

[alpder]

Ferriday might make specific material decomp plates, but the usual mini ones are steel and need one gasket each side

Ferriday make copper plates. Soft enough to seal, I guess. They recommend a smear of Wellseal on the non-gasket side. Turnaround is days - and they're not far from me - so I'm tempted to whip the head off, measure the CR, and get a copper one made. £125 plus the gasket and the only faff being adjusting the rocker height.

 

The response to my email query was to tell me that it'll work fine but shouldn't I consider having an engineer machine the chamber volumes out instead. It's always encouraging when you don't get the hard sell treatment.

[ACDodd]

The problem here is by increasing the gasket volume is you adding quench volume. This reducing a the engines ability to surpress pinging/pre ignition and detonation. There is a reason why I suggested the piston method, and that is because it maintains head casting thickness, and maintains minimal quench volume while reducing the compression ratio.

[alpder]

Originally, to up the CR the engineer decked the block (rather than skimming the head) so I guess adding a decompression plate is only undoing some of what he already did... hopefully, therefore, the results would be acceptable even though not optimal.

 

If the engine was out, I would absolutely get those pistons machined right now. But I would also like to spend a lot of this summer driving the thing... not crawling around under it... so it's a compromise between what's best and what's quick. I just don't have kind of energy - or practice - required to extract, strip, rebuild, and refit an engine in a short period. And engineers have weeks of lead-time around here. So I reckon it's a plate this month, and pistons in the winter when salt keeps me off the road anyway.

[Shooter63]

If you want to go down the octane booster route, this stuff is about as good as it gets and is available fairly easily. There are other methods of boosting octane at a decent cost but slightly more dubious.

Shooter

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[sonscar]

Did you ever measure your total timing?Steve..

[alpder]

Currently playing with ign timing, to see if I can get a summer's motoring without doing any real work

 

The engineer who did the original machining agrees the CR is "probably a bit high then", says he suggested 10.5 because his dad (who used to build A-series race engines) claims to be running 11:1 in his moggy. Tho' having chatted to his dad it seems he has a mad cam in it giving much lower dynamic CR, uses only E5, and has re-curved his distributor to have very delayed mechanical advance. So his engine doesn't really compare to my daliy/towing engine. Anyway, he offers to do the piston work if not free at least at a heavy discount. Happy enough with that: his local rep is good and at the end of the day the original CR decision was mine to call and it's not his fault if I didn't know my sump from my wrist-pin.

 

As for timing: according to spec my own dizzy has pretty flat mechanical advance: it starts at 2.5 degrees at 1800 engine-rpm and tops-out at 7 degrees at 5300 engine-rpm. The only time my engine's ever going quicker than that is during a power run on the rolling road. I've tried retarding to just 4 degrees at idle and - unladen/no-trailer - I now can't provoke pinking. I'll dilute the current half-tank of E5 with some E10, head out to our biggest hills with a trailer attached later, and update.

[alpder]

Did you ever measure your total timing?Steve..

I'm intending to buy a 'proper' timing light to replace my 1980's Gunson cheapie. Any advice on what to choose - at the budget/mid-range end of the spectrum - would be very welcome.

[timmy850]

Mine still ran 30 degrees total with 11.4:1 compression. If yours is pinging at under 10 degrees it sounds like there's something else going on?

[alpder]

Mine still ran 30 degrees total with 11.4:1 compression. If yours is pinging at under 10 degrees it sounds like there's something else going on?

The something else may be my cam? It's fairly short duration and I'm seeing compressions ~215psi - tho' admittedly these numbers come from a compression gauge that cost a tenner.

[alpder]

Distributor mech advance specifications are generally stated in degrees at (distributor) rpm. We need to double the dizzy rpms to get the engine rpms. Do we need to do the same with the degrees? So my "7 degrees" is actually 14?

[timmy850]

Very little timing can also lead to overheated valves and hotspots on the head. They could be causing pre-ignition?

Mine had +220psi on the gauge too