QED intruder pistons, head gasket, CR question

[holywood3645]

Hi all, I need a litte help and confirmation On CR and head gasket thickness
I’m rebuilding my QED 420s spec 1700cc engine. Previously I used flat top pistons and std head gasket. It’s a 711M block that was machined to suit the previous flat top pistons that were flush with the deck when assembled

For this build I purchased a set of Omega 83.65mm with a 4.4cc intruder.
However when fitting the pistons the top shoulder of pistons sit 35 thou proud of the deck..
Head cc is 37.75.
Stroke 77.62 and std 1600cc rods.
420S cams (not that aggressive)

This is for a road going car on 91 RON octane gas.
I’d appreciate it if I could get recommendations on CR and head gasket thickness to achieve that CR.
Plan to use multilayer 85mm head gasket.

Appreciate the input
Thanks
James

[Foxie]

Hi James,

My last engine before the present was a 711 block, 82mm crank, QED 420 cams and Omega intruder pistons. I ordered a set of rods which would give me pistons level with the deck, and machined off the intruders to give me a reasonable (10.5:1 ) compression. It was a great engine.

Talk to QED, Burtons or Dave Bean to see your options.

[rgh0]

Lots of variables here to consider in setting up the engine details

1. I would aim for 10.3 to 10.5 compression ratio
2. Whats needed in terms of piston mods or gasket thickness depends on current head chamber size which needs to be checked
3. Is sounds like the deck height has been machined more than normal if the pistons with standard deck height protrude by 35 thou. I personally would be worried about block stability but as its a used block its probably OK for road use.
4.I personally would machine the intruders of the pistons and potentially have the top machined down also depending on whether their is enough metal in the top or new pistons with a custom deck height sourced.
5. You could compensate for the piston being above deck height and use a thicker head gasket but I personally would stay away from the MLS gaskets as they are to stiff for the twin cam head and sealing is not reliable with them. Having said that a thicker one with more layers will be more flexible and may work better ? You can get composite fibre head gaskets made to 2.2mm uncompressed thickness I think which will compress to around 1.5 mm which may be a better option.

cheers
Rohan

[holywood3645]

Cylinder.head volume is 37.75.
Was planning on a 5 layer MLS gasket.
No issues with the block, it had about 18000 miles since last rebuild. Water pump failure, and took opportunity to get valve stem seals installed, and a fresh rebore/pistons.
Preferr not to redeck block or remove intruder. And adjust CR with thicker gasket.
Thanks

[seniorchristo]

QED should be able to tell you the volume of the intruder which you will need compute compression ratio.

[2cams70]

10.5:1 compression sounds a little too much for my liking for an engine only running 91 RON fuel. I’d be running it on 98 RON at that CR.

Why are you changing from flat top pistons for this build? Flat top pistons are the best for a road build. All those lumps and bumps on race spec. pistons to allow for large valves and high lift cams are unnecessary. They increase the surface area and reduce thermal efficiency.

[rgh0]

While the original poster quoted a 91 "RON". i think he meant the normal USA 91 Octane rating which is RON +MON/2 and equivalent to Australian 98 Octane RON rating ... roughly ... depending on who the refiner or blender is as much more variation in the US.

cheers
Rohan

[rgh0]

With the intruder machined off ( its about 5-6 cc in volume at 4mm high depending on valve cut outs and about -1 cc with it machined off due to valve pockets remaining) and 37.75 cc combustion chambers and 1.7 mm head gasket thickness ( which is a 5 layer MLS I think) I get a compression ratio of around 11:1, see below.

The piston coming 35 thou / 0.89 mm above the deck is the major contributor to this. With piston at 0.5 mm below deck height which is my preferred design and standard gasket compressed thickness of 0.7mm the compression ratio reduces to 10.6:1 which is close to where you want to be.

Compression ratio :
swept volume 426.57 cc
4 cyl capacity 1706.30 cc
head chamber volume 37.75 cc
gasket space volume 9.65 cc
deck clearance volume -4.89 cc
piston head land volume 0.91 cc
piston intrusion volume -1.00 cc

Clearance volume 42.42
CR 11.06

cheers
Rohan

[holywood3645]

Rohan, thanks for the reply.
Cometic make 85mm bore gaskets in several thicknesses up to .120 MLS thick. The Higher range is (.080, .086, .092, .095, .098, .12)
The bottom end has been totally assembled, and I’d preferr not to disassemble and machine (if possible)
Would one of those thick multilayer gasket provide enough to get my CR into range?
Thanks
James

[rgh0]

The 0.120 inch MLS gets you 10.5 comp ratio with the current bottom end if my assumptions are correct.

You can check the intruder and piston head land volume versus my assumptions by moving a piston a known distance down the bore and filling the bore with a measured volume of liquid to the deck and then calculating the volume lost due to the intruder and gained due to the valve cut outs and piston head land versus the ideal cylinder from piston deck to block deck.

I have attached the spreadsheet calculation with a PDF extension as the site does not allow spreadsheet attachment. Rename the file as .xlsx extension to open.

James comp ratio.pdf

(13.01 KiB) Downloaded 202 times

cheers
Rohan

[HCA]

QED should be able to tell you the volume of the intruder which you will need compute compression ratio.

Emphasis being on the word 'should'.

In reality, I fear they cannot. I am waiting months for answers to simpler questions - questions that accompany an order worth several thousand to them. QED have become just another sales organisation without any technical knowledge.

[SimonH]

QED quite some time ago told me that they were stopping doing in house engine work and giving free advise and just becoming somewhere to buy parts. Too many people were taking said advise and going elsewhere rather than coming to them to have the work done. Sounds like that is possibly still true.

[rgh0]

Yes many years ago they stopped doing engine work. The problem is they struggle to give any detailed information on the parts they supply such as the intrusion volume of the Omega pistons it appears.

cheers
Rohan

[SimonH]

That was what I meant. I was told it was going to be up to the customer to work it out, or the person the customer was using.
That might have just been a flippant comment by whoever it was though

[RichardHawkins]

I have been similarly disappointed with QED. I wanted to buy a sensor bracket for a trigger wheel flywheel, and needed to know where it fits. I phoned QED, who didn’t know! They offered to sell me their version of the flywheel to go with the bracket but still could not tell me where the bracket fits. QED then said they would find out, I said I would phone back in a few days. After about a month of this farce QED asked me to stop bothering them, as they would pass the information when they had it. More than a year later, I have given up!

Richard Hawkins