6 replies to this topic

[datsun100a]

Just curious to know what some of the more experienced engine builders on here think about back pressure requirements on the a series engine. I would have though that you have a hot expanding gas that you want to get out of the chamber completely so you can suck in a full chamber of nice new stuff? It's just that the exhaust ports on the 12g940 head look very restrictive to me and I am thinking I should spend more effort on them than the inlet side?

Why do we need back pressure on a 4 stroke? I understand why on a 2 stroke but some comments on the web suggest a 4 stroke needs it too.

[Paul Wiginton]

Ive recently done a lot of research in this area. You want as little as possible.

Google anti reversion step and have a good read

Paul

[liirge]

I was lead to believe that on some Cams where the overlap is quite long, it is a necessary as to stop some of the inlet charge going out of the Exhaust valve.

Edited by liirge, 14 October 2009 - 09:26 AM.

[Paul Wiginton]

There are 2 ways of looking at this and it depends on how well your induction and exhaust systems work.

You can get exhaust reversion where spent gases get drawn back into the cylinder and then back through the inlet valve into the intake manifold.
If there is minimal back pressure, yes some inlet charge will go out of the exhaust valve but its not a bad thing as that little bit is that spent exhaust gas that has returned.

The other thing is while the valves are both open the piston is moving downwards so drawing air inward. In which case charge is being pulled in the exhaust valve rather than inlet charge being pushed out of it. If you use an anti-reversion baffle of some sort you will minimise this effect and you will get mostly combustible inlet charge in the cylinder.

As I said it all depends on how efficient the induction and exhaust systems are as to which scenario will apply.

Paul

[Cooperman]

As Paul says, this is a very complicated area and there are so many considerations. Yes, the exhaust ports are very restricted and will take some decent opening up. If you have a high-overlap cam, like a Kent 286 and upwards, at low revs there will be an element of draw-back from the exhaust as well as new mixture being thrown back out of the carb mouth. I once fitted a 649 race cam to a 1071 cc 'S' and at tickover of 1400 rpm, you could see the petrol droplets coming back out of the carb mouths when the air filter was off and running down over the exhaust manifold!
However, once the inlet and exhaust are flowing properly at higher revs, this goes away and the engine will pull smoothly through its upper rev range. In fact, at high flow rates, in and out, the moilecules of exhaust gas going down the pipe will be travelling fast enough to effectively 'pull' the exhaust along with it (I think that's the best way to describe it). When the exhaust pipe is too large in diameter, then the exhaust gas moves more slowly and, contrary to what some think, the back pressure increases. There is an optimum size depending on cam duration and revs. For example, on a 1275/1380 pulling up to 7000 rpom, it's generally accepted that a 1.75" internal diameter exhaust is the very best size and that diameter should go from the end of the exhaust manifold right to the end of the tail pipe. For a 998 the ID is 1.625". If a larger final tail pipe is fitted after the exhaust box, the same applies and back pressure will increase as gas velocity drops.
With a shorter duration cam, there is less of an issue.
With the A-series head, the basic design is so badly in need of gas-flowing, that opening up the inlet and exhaust ports and throats is almost always a good way to get some more power, especially when combined with a better cam like the MG Metro or Kent 266 (for a road car that is).
I've over-simplified this a lot, but it may give you a feel for what's involved.

[datsun100a]

Thanks for the replies.

Havn't got the spare cash at the moment for a pro head, due to new classic, so I am going to mod my head very slightly in accordance with mr vizards guides. Just looking at the head, now that it is all stripped, it is really restricted in the exhaust side. I am thinking of just cleaning up the inlets a little and opening the exhaust exits and throats as much as I feel confident to do. I hope this will give enough to complement the sw10 cam I am going to fit in the new rebuild.

[Paul Wiginton]

Following Vizaeds guide is a very good start and should give good results. Please do wear a breathing mask and goggles though.

Make sure you pay attention to the manifolds too

Paul