Compression ratio .

[swifty]

Hi building a 1380 engine . I've a few options I can go down using the parts I have in stock . Could the clever ones figure out what compression I will have with this spec . Longman gt17 offset valve head 16.4cc combustion chambers , omega dished pistons 7cc dish . AF460 head gasket . Think it's 1 thou between piston top to block . .... Also I will be using split 45s , LCB , 649 cam . I'm undecided what would be the best rocker option to use with that spec . I have some Titan roller rockers 1.5 ratio or some forged "S" 1.3 ratio . ..... Ken

[carbon]

Just over 13.5:1, allowing 4cc for head gasket and pistons 1 thou down bore

[ianh1968]

1380cc @ 13.5:1 = 27.6cc Unswept Volume
Carbon's guestimate is quite accurate...

There will be a bit in the "ring-land" as well, but if it all
adds up to about 27.6cc, it will be 13.5:1.

Ian

[swifty]

Thanks for answers fellas , any opinion on what rockers I should use 1.5 or 1.3 ratio .... Ken

[ianh1968]

Rockers:
If you have both, try both... Pick whichever you "like" the best.

If you want numbers, decide using a dyno...

Ian

[CooperTune]

I have vintage raced using the 648/49 cam and used the 1.3 titan as the cam was designed with the 1.3 rocker in mind. Might look at the new 1.4s I think the Harlan Sharp are 1.4. I'm sure the 643 gives up a couple hundred bottom end and picks up nothing on top with 1.5s. Steve (CTR)

[swifty]

I was also told that a 649 cam will work better with 1.3s . Undecided on this point at the moment ... Ken

[Vegard]

What are you using the engine for?

[rich@minispares.com]

What are you using the engine for?

who cares, its going to be a beast!

I bet those webers will sound great when its dropped off cam and its desperately trying to drag its self back up on to the power curve

cough, cough, cough, cough, BARP!

[swifty]

I will be using it as a Sunday morning thrash also undecided on the head as I've also got a Manx 7 port head with twin webers , so may use that . Trying to make me mind up what way to go .... I've just timed the cam in at 100 , hope that's right for a 649 .... Ken

[Vegard]

If using it on the road, fit 1.3 lifters. Every time.

[In the shed]

Awesome. This sounds like ASBO.

13.5:1 is going to need some pretty serious octane. I'd look at Tetraboost and nothing else.

You're going to need quite a lot of it.

I wonder whether making serious octane booster (unlike anything you can get in the shops) might be a side avenue for my skills. I'm going to need some myself and it's quite expensive. When you look at how thirsty a 649ed 1380 will be and then you combine that with the octane you require, you could be looking at running 2% TEL/EDB and that is some volume.

[mk1]

You could always chop a bit off the top of the pistons & drop the CR to a more sensible figure.

[swifty]

Well I've made up my mind today and I'm going to be fitting the Manx head and twin 45s . Pointless having them on the shelf all the time . Not to sure about chamber volume on it . What's the best way to measure it ? .... Ken

[carbon]

There was a thread couple of months back about different ways of measuring combustion chamber volume. The one which looked most practical was using 20cc or 50cc plastic disposable syringe (from vets or chemists) filled with water, together with an electronic balance - you can get 0-500g ones reading to 0.05g for under £20 from places like Maplins.

Use piece of flat perspex or similar with small hole drilled so this is at the edge of the chamber. Seal perspex to head using smear of grease or similar. Weigh the syringe when full of water. Fill the chamber completely until level with perspex, no air bubbles, try not to spill any of the water from syringe. Then reweigh the syringe.

Difference in weight in grams = chamber volume in ccs.

You can then use the balance to check pistons for balance and rods for end-to-end...

[mk1]

I use the traditional method of burette & glass with hole in, using paraffin as my liquid of choice.

Burettes are not expensive nowadays, but I have had mine for probably 20 years now.

http://www.ebay.co.uk/itm/Burette-compr ... 1e912ff042

[Spider]

I use the traditional method of burette & glass with hole in, using paraffin as my liquid of choice.

Burettes are not expensive nowadays, but I have had mine for probably 20 years now.

http://www.ebay.co.uk/itm/Burette-compr ... 1e912ff042

something like,,,,,,,



????

[mk1]

Exactly.

[swifty]

That's great . I will give it a go . Thanks for that link mark , I will be ordering one of those ... Ken

[Vegard]

As Mark points out, much better to use something alcohol based rather than water. The surface tension on water makes it "stick" and it also makes the head rust.