ericjon262

has anyone on here taken a crossfire intake and made a carb top plate for it?

Apeiron

You're trying to make a dual carb manifold?

ericjon262

either dual or single I havn't decided.

Apeiron

I wouldn't bother with either. The crossfire manifold suffers from a horridly small plenum volume.

Stekman

The crossfire intake base is one of the most restrictive pieces of metal GM bolted onto cylinder heads. Extensive port work is required just to make them meet flow specs of the TPI intakes.

ericjon262

I definately planned on porting it, and possibly a spacer for the top plate to increas plenum volume.

Stekman

Even with porting, it still will choke an engine.

ericjon262

worse than a torker II?

Stekman

Yes.

ericjon262

do you have flow #'s and is that with a factory tbi top plate? I would be running at least one 750 cfm 4 bbl, instead of the 2 275cfm tbis

Stekman

In stock form, the baseplate only flows out at about 175cfm peak. There is not enough metal present on the intake to remove anything substantial enough to get enough air to supply any healthy build.

ericjon262

I found this artical on porting them, you can remove quite a bit of material. I Imagine it would perform way better. than stock.

http://www.technovelocity.com/chevyh...rt_polish.html

the third pic down on the left shows the head mating furface on the intake.

Apeiron

It might perform better than it did stock, but it'll still get beat all over by a stock 4bbl carb manifold, let alone by something aftermarket.

Stekman

I've held a crossram intake. I know exactly how much meat is on there. I also know people who have gone with that option. Just because there's "enough" metal on the intake, once ported, to feed a stocker CFI engine, does not mean there's enough air to supply an engine that's been redone with heads and cam.

ericjon262

I was hoping that wouldn't be the case, how well does a good cross ram perform? does anyone make an aftermarket cross ram?

Apeiron

A good cross ram performs slightly worse than a bad modern dual plane.

ericjon262

do you know of any particular reason why, one that couldn't be fixed?

Apeiron

Long runners, low plenum volume, an emulsion path that includes a sharp 90 degree bend, and the headaches of tuning dual carbs are some of the reasons that people gave up on them 30 years ago.

I almost forgot poor fuel distribution, too.

ericjon262

so, in other words, they look cool, but are a lot of trouble for less than great performance.

Stekman

Pretty much.

Apeiron

Pretty much. It took Chevrolet a lot of R&D effort to come up with an entire package that worked. The matching cam and headers were designed specifically to complement the narrow powerband of the tuned runners. The carburetors had to have mileage-unfriendly large dual accelerator pumps and a custom secondary opening rate to keep the mixture from going lean on throttle opening, and dozens of iterations were necessary on the dyno to come up with a working baffle arrangement inside the plenum that would keep the cylinders equally supplied with fuel across the RPM range.

Even after all that, the end product was something that was only suitable for a narrowly-defined category of road racing with a skilled driver who knew how to use a close ratio transmission.

Vader

GM Powetrain probably viewed the whole exercise as a good basis for designing the TPI system, which was a good exercise for building the LT1, which was a good exercise for building the induction for the LSx engines. Even TPI, with its 19" runners was more forgiving. It's easy enough to get them to run, but they're not quite an optimal system.

Apeiron

If you want a not-unreasonable comparison, take a look at the kinds of problems the long runner TPI guys have when they try to make power, and then consider that on a cross ram you don't have the option of just bolting on different runners, you don't have an injector per cylinder to help fuel distribution, you have wet-flow manifold that's just waiting for an excuse to drop liquid fuel out of suspension and puddle it, and instead of a computer controlling the mixture you've got a pair of cantankerous mechanical devices arguing back and forth.

Vader

Sounds like my front seat on a long trip - Don't tell Mrs. Vader I said that...

ericjon262

I had heard that crossrams made good power somewhere along the line, but it was probably a quote from 40 years ago,

something I should have thought about it though, if a crossram was any good, aftermarket companys would still make them.

Apeiron

As long as things aren't mechanical and cantankerous in the back seat, I think you'll be ok.

Air_Adam

A GOOD Crossram intake, like the one GM used 40 years ago, is essentially a tunnel ram laid down sideways, so it behaves similarly. Typically having a very strong, but very narrow, powerband, usually at very high RPM. Outside that powerband, the power just kinda falls on its face. Thats why GM put runners with such a small cross-section (roughly 2/3 the size of the ports on the heads) on the CrossFire manifold - to try to bring air velocity up to create some low speed torque and drivebility, and prevent fuel coming out of suspension.

Back in the '60s, the Z28 was a monster with the Crossram and dual 650 Holleys... as long as you kept the revs around 5500 or higher. Below that, it was pretty hopeless. Guys that used those crossram Z28s at the dragstrip usually needed a 4.88 or shorter gear just to keep pace with the single 750 4bbl Holley Z28s that used the factory 3.73 gears.