Fabrik8

That probably means that the stock runner design is perfectly adequate, or that his manifold isn't very good, I don't know which.


I'm going to go do something more productive but less fun. I'll be back tomorrow afternoon.

marlinspike

The problem is I'm running high compression, cams, and a custom exhaust, so whatever was perfectly adequate for a stock engine is not for mine. The estonian guy did say the biggest problem is the manifold gets super hot, so maybe I'll see gains from a thermal coating...

And yeah, I'm going to bed myself. Good luck with your tests.

marlinspike

In case you were curious to see what it all looks like, I realized these were still on my memory card

The whole shebang, in the car


The cavity everything goes into


Intake port on head


What the head looks like without a gasket


Top view of the intake stuff, out of the car, with the injection stuff off


Rear view


Side view


More side view


Other side view


More side view


Close up of port from upper manifold to head

Fabrik8

So how much hood clearance do you have again? And what's with the strange asymmetrical port spacing on the head?

marlinspike

No idea why they made it asymetrical. I could come up with a theory if they were wider spaced at the back on both sides, but as you can see on the passenger side the back ones are close together and on the driver's side it's the opposite. Maybe it's related to the firing order? 1-5-4-8-6-3-7-2 where 1 is the front passenger side and 5 is the front driver's side

I'll go and measure it later if it's important, but to give you an idea of hood clearance - the rubber strip on the false-firewall seals against the hood.

Fabrik8

It would be hard to keep the mechanical injection if you wanted to make a different intake manifold, mostly because you'd have to move it somewhere else. I guess the big problems I see are keeping the intake runners long (if that's what you want for low-end torque) and then packaging under the hood. The stock manifold isn't horrible, it's just not a very direct intake path in that airbox. You have a single throttle body, so one of the big problems is how to keep the intake path as direct as possible without making the plenum box (after the throttle body) too large and therefore affecting throttle response.

You could do a dual plenum setup where the runners crossed over each other like a 300ZX, which would probably be possible if you could use some lower profile valve covers. It doesn't look like you have the hood clearance to do a high rise manifold, so you'd be stuck making the runners shorter like a traditional domestic V8 manifold. That would work great if you wanted to slide the powerband up in the RPM range, which is the anti-Mercedes train of thought.

It's pretty obvious from looking at the stock manifold that the easiest thing (to keep the runner length long) was to go down instead of going up.

Fabrik8

Oh, so to continue from the other thread, if you expand the upper manifold ports, it will respond like any other manifold porting job. You'll gain flow at high RPM, and lose a little at low-end because of the change in runner velocity. This all assumes that you have enough flow through the throttle body and that end of the plenum box, because you're not going to gain any power if the ports are already being choked upstream. That's a really hard thing to predict without getting the whole assembly put on a flow bench.

I'm going to go read this thread again because I'm probably repeating myself from a few months ago.

marlinspike

So let me ask these questions.

When you say high in the RPM range, how high? Redline is 6000rpm (though, whenever I finally switch to an MSD I'm going to move it up to about 6200rpm)

I'd be all for more power in the 3500rpm to 6000rpm range at the expense of power before there. Here's a dyno chart (no correctoin factor, ambient temp 59F) http://davidson.smugmug.com/photos/7...2_uEtn2-XL.jpg - as you can see, I don't really have a down low power problem, it's an up high problem.

One of the forum members looked at my throttle body/lower plenum and said he could bore the throttle body out another 6mm (I forget the exact number, but it was around 6mm). You think that's a good idea? IIRC it's like 72mm now and he said he could go to almost 78mm.

FWIW, the intake ports on the head are 41.3mm, the intake port cutouts in the head gasket at 42.3mm, and the exit intake ports on the upper manifold are 39.4mm.

Fabrik8

You'll pick up a tiny amount of power by port matching the runners and the ports in the head, just by reducing turbulence at the change in area where the two parts meet. Ultimately you'll still be flow limited by the rest of the intake runner though, unless you get that part Extrude Honed or something like that. The whole point here is that you need to find out where the flow restriction actually is, whether it is the throttle body or the plenum box or the intake runners. If you think it's the throttle body, try a larger throttle body and see what happens. If you think it's the plenum box I'm not sure what you can do because that part isn't an easy one to change because of the packaging. If the problem is the throttle body, you can make it bigger until the gain drops off, and then the problem will shift to the next problem in the flow chain. Unless you have access to a flow bench and some $$$ software, the only way to do this is to keep tweaking things until it doesn't make an improvement or the power starts dropping off (or you don't like the shape of the power band). So you could start stockpiling a couple of manifolds and throttle bodies to experiment with....

If you just slap on a huge throttle body, the worst thing that can happen is a loss in throttle response. If you use a throttle body that's 25% more flow than you need (for example), you'll reach max power at less than full throttle, so you won't use all the available throttle travel. You'll lose velocity too, but I don't think it will be as bad as something like a Honda manifold because your flow path isn't nearly as direct to start with.

marlinspike

Yeah, right now I've got one spare throttle body and one spare lower plenum, but upper manifolds are haaaard to come by.

Would a place like Abacus have said flow bench and software?