27 replies to this topic

[Turbo Phil]

As a rule of thumb the more boost you run, the more you must limit the timing when under boost to avoid detonation. With something like Megajolt this is easy as you can allow plenty of advance at low revs/load, improving economy and response, but retard the ignition more, the higher the boost goes.
With a dizzy this situation is less than ideal. You have to limit the advance by either locking the dizzy and running fixed timing, or putting a blob of weld on the mechanical advance stop, thus limiting the total timing. A dizzy is very crude in this situation and a programmable ECU wins hands down here.

Phil.

Edited by Turbo Phil, 07 June 2015 - 06:35 PM.

[connorwmc]

[quote name="Turbo Phil" post="3272538" timestamp="1433682402"]The Omegas sit about 0.040" down the bore at TDC on an unskimmed block. This gives another 4cc plus the 10cc dish. Looking at the picture of the head he posted I'd guess the chambers were at least about 27cc to.
Phil.[/quote Aahhh I see now there a completely new shape to mine.. I'll probaly get this when I get the engine bored!!! And go for the Omega pistions... Sounds like the mega jolt is a bit of a no brainer.. I'll have to read up on these as I m not to familer... Thanks Phil for all your help... I've got enough info to keep my busy for a few months haha

[connorwmc]

As a rule of thumb the more boost you run, the more you must limit the timing when under boost to avoid detonation. With something like Megajolt this is easy as you can allow plenty of advance at low revs/load, improving economy and response, but retard the ignition more, the higher the boost goes.
With a dizzy this situation is less than ideal. You have to limit the advance by either locking the dizzy and running fixed timing, or putting a blob of weld on the mechanical advance stop, thus limiting the total timing. A dizzy is very crude in this situation and a programmable ECU wins hands down here.
Phil.

I've just picked up my turbo today, it might be a stupid question but could you identify these? I found them at the bottom of the box I'm not sure what they are where they go or what they do

[connorwmc]

oops forgot to upload them 

Attached Files

•  IMAG1719.jpg   43.18K   23 downloads

[Jon937]

I think the site is jonspeed which sells various thickness compression plates which may save you replacing the head. Although you would need to use two head gaskets. You could also try water injection to cool the air/fuel mix which allows you to still run a high CR despite turbocharged. Hope this helps

[connorwmc]

I think the site is jonspeed which sells various thickness compression plates which may save you replacing the head. Although you would need to use two head gaskets. You could also try water injection to cool the air/fuel mix which allows you to still run a high CR despite turbocharged. Hope this helps

ahhhh cool.. i didn't know such thing existed, just checked them out.. 

[Turbo Phil]

The items in your picture above are the anti run on valve and the electronic boost solenoid. The boost solenoid is controlled by the standard ECU to limit boost to 4psi below 4,500rpm and the increase it to 7 psi thereafter. Most folk don't use either.
As for decompression plates I would avoid fitting one of these and only consider it as an absolute last resort. Far better to get the chambers enlarged or fit some bigger dish pistons to drop the compression.

Phil.

[connorwmc]

The items in your picture above are the anti run on valve and the electronic boost solenoid. The boost solenoid is controlled by the standard ECU to limit boost to 4psi below 4,500rpm and the increase it to 7 psi thereafter. Most folk don't use either.
As for decompression plates I would avoid fitting one of these and only consider it as an absolute last resort. Far better to get the chambers enlarged or fit some bigger dish pistons to drop the compression.

Phil.

 

I've seen on your website you enlarge chambers, what sort of gains could i see from this cc wise? would £70 be the total cost? and could i fit the standard valves back in, i tried to measure my chambers today but the water kept leaking through the values haha 

[connorwmc]

Okay so I'm at the Point now I need to some work done on the engine and cylinder head..
But from what I've read I cant get my engine bored out with out the pistons already and I cant get my cylinder head opened up untill I've had work done to the engine?

[69k1100]

If you're clever then you can use the stroke, con rod length and the wrist pin to crown height to work out where the piston will sit in relation to your proposed bore, then you can work out the compression ratio from your existing head and gasket.

From there you can talk with your engine builder on whether to possibly machine the Pistons or enlarge the head chambers.

[connorwmc]

would it be wise to get the block and head skimmed as the engine I'm using for my turbo project had previously overheated and melted one piston... would this have warped anything?

 

also it looks like theres bits of the piston fused to the bore wall, will this still be okay to overbore?

[Jon937]

I would say if the head is ally definitely get it skimmed, if cast iron I would check with a straight edge.
I would see no problem with boring the bore out as that material would be removed in the process anyway but I'd ask the machine shop before paying foebthe work to be carried out.
I hope this is of some use

[Ethel]

Turbo's generally, but especially on old engines like the A, make their power by filling the cylinders when a normally aspirated engine would start to struggle in the reduced time available, each stroke, as the revs increase.

 

For the normally aspirated engine, the lowering of the pressure in the combustion chamber, just before the ignition fires, means combustion happens more slowly: hence the need for even more advance to get the most oomph applied to the piston once it passes top dead centre and can push the the same direction as the crank is rotating.

 

Since forced induction can keep the cylinders fuller, the burn rate stays more constant and the advance is more just to compensate for the faster rotating crank getting the piston past TDC sooner. Too much advance can mean the piston gets forced against the rotation of the crank, which allows the pressure to get much higher, before it's relieved by the piston sweeping down the bore, high pressure translates to high temperature and melted pistons.

 

Boost pressure and engine speed having opposite and independent effects on the ignition timing requirements is why it makes a lot of sense to fit programmable ignition, that can measure and compensate for both independently, to turbo engines.