Gasket match head and intake?

[Bearwen]

Hello All,

  My son and I are fixing up a 79 280zx and we are putting a medium size cam in it and we are putting a new intake on it.

we were looking at the intake gasket and noticed it is about 1-2mm larger than the port all the way around we put it on the the new intake and it's also about 1-2mm bigger than the intake port.

My question is should we gasket match the head and the intake?

Now the head we plan to do a little clean up on and maybe unshroud the valves.

 I know the port goes from big to small at the pinch point from gasket surface to just before the bowl and the bowl widens then from bowl to valve seat small again and then into the chamber and the bowl to cylinder we have a Venturi effect with the big little big through the seat.

I can taper the head runner from the gasket surface if matched so we keep the port taper to the bowl.

 

The BIG issue is the intake side the intake is a straight tube intake and I can grind the flange area and into the intake runner a reasonable distance but at the end of the day the intake runner at the plenum will be smaller in diameter than the gasket surface of the intake runner.

Because of this should I bother to gasket match?

 

I don't want to create weird expansion and contraction areas in the stream. As I understand it the runner should go from big to little from the runner at the plenum all the way to the pinch point of the head just before the bowl where it opens back up.

 

But if I gasket match it will go in the intake from the plenum small to bigger at the head gasket surface and then in the head from gasket surface bigger to smaller at the pinch point and then  bigger again at the bowl.
The head part probably isn't a big issue all though by gasket matching and shaping to keep the taper I am making the port bigger and it may not really need to be.

The part I'm really not sure about is the intake runner being a straight tube and then flairing out bigger to gasket match the head.

Maybe I'm over thinking it I just dont want to create any weird air flows or velocity issues that will rob any gains seen from the cam and intake upgrades.

 

I forgot this is a N47 head.

we are not removing the exhaust liners in fact I'm good with the exhaust side I know what to do there it's the intake side that's really the issue.

Edited September 13, 2021 by Bearwen

[rossman]

Personally, I think you are over thinking it unless you are building a high end race engine but it doesn't sound like that from reading your post.  You didn't talk about the rest of your build. Should we assume that you are optimizing the rest of the system so that the details you are worried about will make a significant difference?  I would assume that you are running full custom EFI.  You should also consider getting a better flowing intake that is actually wider at the plenum - like the one that ProTunerz makes or you could design and build your own.  As an alternative you could cut off the stock manifold air plenum and open up the other end and weld it back on if you want to keep the stocker for some reason. Anyway, it's just my $0.02.

[Bearwen]

Rossman,

you are correct we are going to be running a custom intake and a full megasquirt with modern injectors and LS coils and a 3-2-1 header going MS3X and will probably run the jeep cam sensor mod so we can run full sequential ( not needed but since it's capable why not) we plane to add boost and launch control etc etc so we need the MS3x so figured we would go sequential.

The bottom end is all stock with the dished pistons as we plan to boost it later.

we are going to be running the comp cams 280/280 .460 lift cam for a nice little lop ( son likes the sound ) and to get a little more out of her but without having to replace springs.

Now the intake we were looking at making our own that is basically straight tubes to a plenum roughly like the intake you mentioned. We were planning to add air horns to the ends of the intake runners inside the intake plenum.

You mentioned an intake that is wider at the plenum?

are you referring to the shape of the plenum being wider on the throttle body side and narrowing towards the rear of the intake or are you referring to the intake runners from the plenum down to the head tapering from larger ID at the plenum down to smaller ID at the head?

the current thought was to use a straight tube same inner diameter from the plenum down to the head and match the head port inner diameter at the gasket surface. We are planing to do some port work but mostly deshroud the valves and smooth the rough casting, radius sharp edges, streamline the valve guide bosses and smooth up the valve throat just below the seats.

Am I wrong in my thinking of using a straight pipe from the plenum to the head? And if not shouldnt it be the same size as the head port size at the gasket surface?

Thats what I have in materials at the moment and because of the tubes and the heads ID being the same at the head gasket surface well the head gasket is a little bigger and that's what got me thinking that gasket matching will change the velocity (but leaving it unmatched leaves a gap from the gasket which might create turbulence) and what not as the air enters the head because we are adding an expansion at the gasket surface and then as it travels down the port to the bowl and short side it will contract and then after the short side into the bowl will expand again and then contract again as it goes through the valve throat and again expand as it goes into the combustion chamber.

I am not sure if the extra expansions and contractions will mess with the velocity and the general flow and we're not even thinking about what will happen to the wave when the valve shuts.

This is not a race car it is a nice daily that will be given a little something here and there ( my 17 year olds car ) he wanted a cool old school car not a Honda ( and he's a bit of a purest didn't want to do a motor swap) but we want to be able to hold our own with the Honda's ( not the crazy Honda's but what most of the 16-20 year olds daily drive ).

Thanks for any help you guys can give.

[rossman]

Honestly I'm not the right guy to answered your questions regarding porting an enngine. There are far more experienced people on HZ, or at least there used to be. I was referring to the runners being tapered. Wider at the plenum than the head. I port matched my intake and head but I'm running a custom tapered runner intake. I also turbo'd my car for the most bang for buck while keeping the l6. I will also be turboing my son's ZX. He has the later p79 round port head with exhaust liners. We plan to remove the liners.

[rturbo 930]

2 hours ago, rossman said:

He has the later p79 round port head with exhaust liners. We plan to remove the liners.

Based on what I've read, removing the liners ruins the port flow because the port is too big. 

[rossman]

 

10 hours ago, rturbo 930 said:

Based on what I've read, removing the liners ruins the port flow because the port is too big. 

That's interesting. Thanks for the heads up. It's certainly worth more reading as it's a PITA to get the liners out. Most of the info I've found on the heads with liners is dated, and written when better heads were still plentiful.

Edited September 14, 2021 by rossman