Intake port design questions

[amichie]

Hi Guy

I hope the following questions make some sense.

1. If there is a large angle between the intake and exhaust valves does this reduce the importance of having a downdraft design in the intake port?
Modern 4v designs seem to be using relatively small angles such as 20- 25 degrees whereas in the past the inatke and exhaust valves were set at large angles presumably to fit larger valves.

2. If your engine is benefitting from delayed ( or retarded) intake cam timing is this indicating intake design problems such as inadequate port flow? By benefiiting I mean greatly improved performance from about 4000 to 6500rpm.

Thanks Andrew.

[miro-1980]

I want to see Guy's response to these questions, as well.

By the way : what do you mean by :" downdraft design in the intake port"?

Miro

[Guy Croft]

"If there is a large angle between the intake and exhaust valves does this reduce the importance of having a downdraft design in the intake port? Modern multivalve designs seem to be using relatively small angles such as 20- 25 degrees whereas in the past the intake and exhaust valves were set at large angles presumably to fit larger valves"

Take a look at the sketch (from the GC Head Prep DVD). Yes, the greater the angle of the port to horiz the better (because gravity assists the flow of air and fuel) and the more acute the angle betw valve stem and port axis the better too, which is more or less what I think you meant.

Of course whilst this can (I stress 'can') be a help on a conventional engine for fitting bigger valves (although it's not always a prerequisite) this makes the engine bigger and can mean a bigger combustion chamber (and thus a bigger intruder on the piston - bad for emissions etc etc..!)

If your engine genuinely gives more power with late ignition timing it is more a sign of good burn than not.

All engines have to start the igntion even early in the compression stroke otherwise you don't get optimum pressure on the piston at the right time, ie: you shorten the effective power stroke. The amount of advance varies with load (throttle) and speed and also it varies from one engine to another. Normaly aspirated (n/a) SOHC Fiats, for example need less that TCs and n/a multivalve (eg 16v, 20v) units of almost any kind need less than 8v ones. The big danger besides power loss from running the timing retarded (ie: starting later in the compression stroke than standard) is that the burn continues too late in the power stroke and overheats everything. High velocity with turbulence (either or both or even velocity induced turbulence itself) in the inlet tract can enhance burn. However, excessive ex back pressure due to tortuous arrangements of pipes and silencer baffes can require the ignition timing to be retarded and that could show as a more powerful engine (than it was before) though it's hardly what you'd call an optimum setup and both can be damaging to seats and rings..

You better tell me more about the engine before I remark further. Pictures too..

GC

[amichie]

Thanks Guy

The first part of your answer regarding downdraft port design was spot on.

My second question was about cam timing and not ignition timing.

The engine is a toyota 7mge non turbo engine. It is a 3 litre straight 6 with a 84mm bore and 91 mm stroke.

It uses a 4V head and has relatively small valves for the cylinder displacement. (32mm Inlet and 27.5mm Exhaust).

It has large and very horizontal inlet ports and the valves are set at a wide angle (about 50 degrees).

I and others have found that this engine performs strongly upto the redline with the intake cam retarded. Late closing of the intake valve is the most likely contributor.

Back to the question. Does this indicate inadequate flow in the intake port. Are the valves too small perhaps?

Thanks again Andrew.

[Guy Croft]

Doh! Well done me, cam timing of course now that I read it again, sorry!


32mm inlet is certainy not huge for a 500cc cylinder, no. Retarding the inlet cam reduces the overlap and ordinarily cuts off some of the top end performance, but in this case, judging by what you say, the extension of the intake stroke resulting from late closure may well be developing better filling. Remembering of course that the filling continues after BDC as a result of the charge momentum. It can also be the case that holding the inlet valve closed for a longer period (relatively speaking) does allow a higher depression to build up in the cylinder, giving potentially a higher pressure ratio across the valve throat. Could give better (quicker) filling, sure..

So much of cam timing is dyno trial and error really and if you have quantifiable results as described there is no doubt something along the lines I've described is at work.

GC

[superbike]

Hi Guys

I haven't spent much time on the internet for a fair few months hench my low postings.

Cam timing is something that interests me and is something I get to play around with daily on the 4v V twin engines. From what I can gather so far a port with a low velocity or too big benefits from inlet cam advance . One head I work with has such a low airspeed intake cams can be advanced up to 15 degrees with no loss in top end power.

Other engines with smaller ports don't like as much inlet cam advance and the ports I make smaller by filling with weld make best power with standard or retarded intake valve timing. I think a port with a higher airspeed can take advantage of closing the intake valve a little later than usual. Obviously there are many other factors that come into play and my theory of how it works may be wrong . All my findings are based on the same engine but just different variations of it.