Timing chain length on a tall block

[oldchieft]

Hi All
Can anyone tell me how well the timing chain fits on a tall block?

The chains are 3/8" pitch, the tall block has two extra links in it, that will add one link to each side of the chain run.

The information I have is the Xflow block is 209mm tall and the Lotus block is 198mm.

The 11mm difference in the is not quite matched by the extra links in the timing chain.

By my reckoning 3/8 is 0.375" or 9.525mm, 11.0mm - 9.525mm = 1.475mm tighter chain.

If say I'm building up a tall block, with new sprockets, new chain and a thick head gasket for correct compression will I find getting the sprockets on a struggle?

Jon the Chief

[promotor]

You can buy a longer chain for tall block conversions - it has 122 links as opposed to the normal one having 120 links.

You may also need a slightly longer timing chain tensioner plunger - all available from QED. Usually you need the longer plunger when using a 1600 block that has had approx 5.75mm taken from the top of the block (as you are likely to be using lotus pistons which have a compression height 5.75mm lower than the crossflow pistons that were originally in a 1600 block). If you use a standard height 1600 block then I think you only use the longer chain with a standard plunger.

The longer plunger takes up any slack in the chain you may encounter that can't be taken up with the normal plunger.

EDIT - when using the 1600 block you would want to use the added stroke of the 1600 crank, and therefore are likely to need the 1600 con rods.

[types26/36]

I've done a couple of tall block conversions and never had a problem with using a 122 link chain, as stated the 711 block is normally decked anyway and I believe there is a mod to the brass tensioner although I have never done this mod.
Have a read over this thread which has some details of the mod.
lotus-twincam-f39/timing-chain-for-tall-block-t21006-15.html

[oldchieft]

Hi Brian and Promotor thanks for your input.

My situation is complex as I have an 84mm stroke crank, 84/2 = 42, for the crank height.
At present I am running 122mm rods
Accralite do a 44.75 compression height piston in my size (83.5mm)

42, for the crank height,
44.75 compression height
122 C to C rod length
208.75 total height, this is close to the required 209 Kent block height.

Accralite also do a piston in my size (83.5mm) that has a un-finished crown, but their web site is down at present so I can't check the compression height.

Edit Thanks for the pointing to the correct post, I forget you guys have been round a while and seen it all before.

Jon the Chief

[elanman999]

Jon,
Obviously it's your engine but aren't 122mm rods a little short for 84mm stroke? With longer rods you might need bespoke pistons but that's not difficult these days.
Also from running a tall block conversion myself (in a +2) I can see that you could well have a problem with the cam box touching the bonnet if you are using a standard height 1600 block.
I like your style though - the more cc's the better.
Cheers
John

[promotor]

.

My situation is complex as I have an 84mm stroke crank,

Hi Jon,

Sorry I forgot about your crank!

I'm pretty sure your engine would be standard 1600 height then if you've got the 84mm crank as the engine builder would need all the space they could to get everything in there.

If you put in lower comp height pistons you could put in a slightly longer rod ie a crossflow at 4.93"/125.22mm.

[oldchieft]

Hi John Elanman
You are right about short rods.
The reason is I am running 1841cc in a Kent cross flow with a small chamber of 10.5 cc.

I binned the heavy bowel in crown pistons, and ran short rods and twin cam pistons to get more space under the head. even then I had to take some material off the pistons to get 6mm short of the deck height, this gave me 35cc in the cylinder.

The gasket I estimated at about 7 cc.

This gave me Vc of about 52.5 cc and a CR of about 10:1 or a bit less.

It sounds like a horrid bodge, but it ran very well with bags of low down pull and is nice to drive.

I now have a brand new SAS head chambered at 40.6 cc.

(Promotor I will be starting a thread about this as you asked with lots of pictures)

This is were I am now, the rods are old but good Cosworths, so if i can just buy suitable pistons that will keep some nice old historic parts in the game.

The other bits to build the front end is what I am planning now, hence questions about chains.

A bit off topic is there any opinion about timing cases? i.e. original water pump or cassette style? And who makes the best ones?

Jon the Chief

[avro]

Jon

I would have to agree with John and Al regarding conrod length. Using a crank with such a long stroke the conrod will be at too acute an angle when the journal is in the 90 degree position. I also have a vague recollection, that you may find when the journal rotates toward BDC that the larger diameter counterbalance weights of an 84mm stroke crank may come into contact with the bottom skirt of the piston with such a short rod.
Go for a longer rod and a shorter compression height piston.

Nick

[elanman999]

Jon,
OK I see where you're coming from.
In my comment on the rods I was thinking the same as Nick, that the angle at 90 BTDC is too acute. It's far from ideal but if it runs OK that's up to you.
If the Accralite 83.5 pistons you mention are the Burton FB120/83.5 then they have a crown height of 37.2mm. I have used those pistons and they are OK but heavy as supplied. So if you are running them at their supplied height then you will need to machine the underside of the crown to get the weight down. I got the weight down to that of a standard T/C piston. If you use these pistons you will then need to deck the block and it will be around the usual "tall block" height so should be able to use the 122 link chain.
Provided you have a good distributor or better still mapped ignition you will be able to use more compression.
Sounds an interesting project.
Cheers
John

[promotor]

Pictures are always good!!

So on your original crossflow engine with the 84mm stroke crank were the pistons down the bore by 6mm or am I reading that wrong? Did the crossflow head have chambers in it or was it a flat head?

[oldchieft]

Hi Promotor
Yes you got it right, a small chamber in the head, and most of the combustion in the space above the piston.
Jon

[oldchieft]

Hi Nick
Your vague recollection is spot on, the counterbalance needed turning down. I don't have it in my notes but it was not by much.

Rod to Stroke ratio affects a lot of things (low can be 1.45 to 1.75), 122/84 is 1.45, so this is the low end of low.

This is the ratio for the Small Block Chevy 400 with std. rod, so it is not so extreme as to be beyond main stream manufacturing.

Low ratios are good at low down power due to the faster acceleration of the piston and the better gas velocity at the start of the induction stroke also better squish turbulence on compression, with my bodge set up both these work well for me!

Also earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation.

On the down side at high revs the piston can out pace the flame front.

A higher level of vibration, my balance man back in the day was one of best in the game. (I hope I can still find him)

High piston side load giving rise to wear and higher heat load, so short piston skirts and high revs are not good.

I did not know all this when I built the engine, so dumb luck played a big part in building an engine that suited what I wanted. I love low pulling power for relaxed cruising.

For what it is worth that is my take on short rods, if anyone wants to put the other side I would be glad to hear it.

Jon the Chief