marlinspike

Should the size of the throttle body match the size of the opening for it on the intake manifold lower? I got a large factory manifold and throttle body that will fit my car, but I noticed the opening on the intake manifold is 2mm bigger than the throttle body. Would it be worth finding a machine shop to bore out my throttle body and machine a large plate so that it's flush, or is there some reason behind this?

Fabrik8

Not that big of a deal unless you're chasing every elusive horsepower. It's much worse to have a throttle body that's bigger than the manifold port, which makes an obvious flow restriction. In your case, you don't really have a flow restriction, you have sort of a minor flow disruption (the transition from a smaller to a larger diameter), which isn't as bad. Ideally that transition would be tapered, and that might be a nice easy thing to do if your throttle body is long enough so it can be tapered without affecting the travel area of the throttle plate. I'm not sure I'd be overly concerned with that though.

Anyway, changing the throttle body size it's something trivial to do, because you'll decrease flow velocity for any given flow volume (because you're making the cross sectional area larger). So there may be tuning involved or else you may lose throttle response at part throttle or have idling problems. You can also gain power this way obviously, IF you need the extra flow because the throttle body is too restrictive at wide open throttle.

I'm guessing the reason that the manifold port is larger is because that manifold casting is used on multiple engine sizes (and therefore different throttle bodies), or they made the casting patterns before they had fine tuned the throttle body size for production.

marlinspike

It's actually the reverse, they used the throttle body on multiple engine sizes but the manifold on only one engine.

How every is the every elusive hp? If I can expect to get 8hp at the wheels, it'd be worth it to me, but if we're talking 2hp, forget it. Right now, the engine is starved for air. This setup as it stands will give me a 4mm bigger throttle body and an overall bigger intake manifold lower with each port on it also being 1mm bigger (8 cylinder).

Any thoughts on who does the best heat barrier coating for intake manifolds? I'm thinking Swaintech.

Fabrik8

I've got nothing then. LOL. I'd be really surprised if there was any actual reason behind it other than something having to do with production though. I can't imagine they'd be trying to induce turbulence by having that interface at different diameters, because there are much better ways to do that. Usually they'd make sure that the casting was big enough to allow for casting tolerances without worrying about having to post-machine the manifold casting to get it as big as the throttle body. You don't want the casting right on the edge where it would be fine if the tolerance was toward the large side, and then have to bore the manifold if the tolerance was toward the small side. It's just easier to make it too big over the whole +/- range. That's what I suspect is happening at least, even though my first guess was already wrong.

There are TONS of things on every production engine that can be optimized with enough thought and testing, because it's very difficult to make a good engine and make it cheap too, and take into account the tolerance ranges of all the parts over the length of the production life. There are reasons I never want to get into production automotive engineering, and that is just one of them. I'll take expensive low volume racecars instead, thanks.


You could also make a throttle body spacer and do the tapering there instead of modifying the throttle body. The added distance won't do anything except add a tiny amount of intake plenum volume. If you're worried about that, you could make a thin spacer with a sleeve section that slid into the plenum, so you'd have the tapered length without lots of spacer thickness.

I'm so burned out from school that I'd much rather discuss your throttle body than finish studying for my two tests tomorrow.

marlinspike

I don't think there's much I can do in the way of spacers. It's a pretty confused setup, but you only have so much room to work with things. These screen caps of the parts catalog should give you some idea. The throttle body attaches to the big opening at the front of the lower manifold, then peeks up through the gap in the upper manifold to attach to the plenum, which has the fuel injection metering plate (think mechanical MAF) sitting on top of it, and then the air filter housing, not pictured, sits on top of that. The fuel distributor sits on the back of the same piece of metal that has the metering plate built into it. If I push up the throttle body, I'll be pushing up the metering plate housing, which means the fuel distributor will be higher, which means my fuel lines will no longer be the right dimensions. I also reckon the upper manifold's ports will no longer line up with the engine.

I hear you on the studying btw - somehow I'm no further along in studying for my test upcoming on Monday than I was this Tuesday. 8 + 4 + 4 + 3 is too many years in school.

Fabrik8

Ah, that's a bit different than I had imagined.

Looking at the flow path into the manifold, I don't think the size of the opening matters at all. You're just dumping air into the flat side of a shoebox (the plenum) so the size of the opening is a really minor consideration. If it was a setup where the throttle body opening had a smooth transition into something (for example if the throttle body opening was radiused into the plenum like the 8 individual exits are) then it would be more of an issue. The flow is abruptly changing from a tube to a shallow chamber (and then turns some corners) so it's already screwed up, and just changing the opening size isn't going to make a difference because the flow is already pretty f'ing turbulent through the rest of that manifold. The much more important part is what happens at those 8 exits and through the upper manifold. That part looks decently designed for the most part, at least from what I can tell from a line-drawing parts blowup.

So the short answer is that I wouldn't worry about it.

marlinspike

Fair point, but there's got to be some reason that they went to the bigger throttle body on the 1986+ cars, right?

Given all the screwy stuff going on, is there any point in having that shoebox polished?

b18b

Fabrik8 is correct, it's the other way around that becomes issue when the TB is larger than the manifold opening.

If you know for a fact that the engine is starving for air(still pulling vacuum at WOT). I would just get a bigger TB instead of having a machine shop bore it out and replacing the plate due to costs being similar. Your engine should respond very well to this bolt on and depending on your current modifications.

Coating your manifold will probably do next to nothing as long as you have it bolted to the heads. Your engine will just transfer the heat right to it. So your best bet as far as isolating the heat from it, I would suggest a thermal barrier intake manifold gasket such as those made by hondata but since I'm not familiar with v8s I do not know which aftermarket companies to refer you to.

marlinspike

Here are some of the parts involved btw

Fabrik8

The bigger throttle body would only be for greater flow.

If you want to polish something, polish the inside of the 8 outlet ports on the shoebox. If it's a multi-piece shoebox, you could internally polish the whole thing, but the gains will depend on what the surface finish already is, etc..

Arguably the best possible thing you could probably do is to make the shoebox deeper, so there is more space between the 8 outlets and the floor of the shoebox. Then you lose a little velocity (because of the greater volume) but gain a straighter flow path into the 8 outlets. Those outlets, and the runners they attach to, are really what sets most of the velocity into the head. The shoebox should affect the velocity a little, but mostly it's the geometry of the damn thing that will have the most effect on how well it works. It's the same reason that you see velocity stacks on race engines. A plenum box will be around them, but the best thing a plenum box can do is to not interfere with the velocity stacks. There are the obvious tradeoffs with plenum volume and throttle response too, but poor geometry is one of the worst things you can possibly have, because then you can't make the best of the other parts you're working with.

I'm starting to ramble, sorry.

Remember though, the biggest flow restriction is always a throttle plate that isn't open all the way.