Thanks Greg, much appreciated. Here is a story I wrote on the Donohue Camaro: http://www.theroaringseason.com/show...fair-AdvantageOriginally Posted by Greg Stokes
Thanks Greg, much appreciated. Here is a story I wrote on the Donohue Camaro: http://www.theroaringseason.com/show...fair-Advantage
Intake Manifold:
The maximum engine size in the Trans-Am series from 1966 through 1972 was 5000cc. To fit this requirement, Chevrolet created the Camaro Z28, which, using a 3 inch crank with the 4 inch bore provided a near-perfect 302 cu.in engine size for the category. Once specialist engine builders such as Traco, Bartz etc got hold of these engines, they worked their magic, and the Chevy teams were at least on a par with those from Ford and Mercury. By early to mid-1967, teams such as Penske were enjoying around 440hp. The Camaro in the early races may not have stopped or handled too well, but it had plenty of boogie in a straight line.
However, not one to go resting on their laurels, GM engineers were constantly looking for ways to squeeze more power out of these motors within the tight confines of the regulations. One of the rules required by the SCCA was that the cars all have a flat hood, and that the motor and all its parts not protrude through the hood. So too, in 1967, the rules also required that while up to four 2-barrel carburettors could be used, not more than one 4-barrel could be used. But with some pressure from the manufacturers, this rule was changed for the 1968 season, from being not more than one 4-barrel, to not more than two 4-barrels.
So in mid-1967, GM engineers began working on a prototype tunnel ram intake manifold, but in which the tunnels ran horizontally, not vertically, and on top of which would be mounted a pair of 4-barrel carbs. A vertical tunnel ram would produce better results, but wouldn’t fit under the hood. So instead they produced a cross ram, whereby a pair of off-set 4-barrel carbs were mounted in an outboard position to keep the entire system very low. Such systems had been used previously by Chrysler in the early 1960s, although the GM intake was much more compact, as engineers also needed to house it within a special plenum into which cold air could be drawn in through the top of the cowl.
The GM cross ram intake is an aluminium two-piece unit with a main mounting plate featuring equal length long tuned runners that create a ram effect of packing in the fuel mixture when using a long duration camshaft on high revving engines. A lid is bolted to the base onto which the carbs are fitted. The cross ram was designed specifically for this small 5 litre engine, and its ability to rev.
In August 1967, back-to-back tests were undertaken pitting a Chevy Trans-Am engine fitted firstly with a single 4-barrel, then the new cross ram. The single 4-barrel engine used an 800cfm Holley, which was the typical set-up in 1967, and produced 441.5hp @7400rpm. The cross ram intake was then fitted, using a pair of 600cfm Holleys, and it produced 467hp @7400rpm. The cross ram netted an impressive 25hp gain. That was a massive increase for a 5 litre engine, and particularly in a championship where all the various makes produced roughly around the same amount of power. By the end of 1969, the cross ram equipped Traco built Chevys in the Penske Camaros were punching out close to 500hp.
Two 600cfm carbs sound way too much for a small 5 litre engine, but in fact after several tests using different carb sizes and carb types, this was the combination that produced the best result.
This is a cross ram set-up fitted to a Penske Camaro.
Photo courtesy CRG website.
Intake Manifold continued:
But the cross ram can’t function just on its own. It needs large gulps of cold air to help feed the beast. And the best place to source this cold air is the high pressure area at the base of the windscreen. Therefore, GM engineers also produced a special plenum housing which wraps around the cross ram and carbs at one end, and which then follows a wide tunnel which attaches to the firewall on the passenger side at the other end. A hole is cut in the firewall, and cold air is ducted in through the cowl vent at the base of the windscreen.
Clearance between the hood and the cowl plenum created problems early on, as the height of the early air cleaners was only 1 ½ inches tall, which caused air-flow problems to the carbs. This was remedied with a taller air filter. For Chevy teams to be able to use the cross ram set-up on their cars required Chevrolet to manufacturer and sell all the parts involved across their parts counter, for each item to have a part number, and for everything to be homologated. So this was no small job. Most of the really serious teams ditched the factory plastic plenum and made their own taller plenums so they could fit a taller air cleaner. These were usually made from aluminium.
The GM cross ram and its various associated parts was homologated in early 1968. Interestingly, while a well-connected team such as Penske were using the piece from Round 1, these were the prototype models, as GM hadn’t actually started producing them yet. They became available to the regular privateer teams part-way through the year.
As a side note, for the 1969 Trans-Am season, the SCCA relaxed their rules slightly and allowed the use of small hood scoops. But these scoops weren’t allowed to interfere in any way with the drivers vision. To that end, Chevrolet produced its beautiful and now very familiar RPO ZL2 steel hood, with its rear-facing scoop that forced cool air to the carb/s from the base on of the windscreen, just as the 1968 set-up did, but now the air was fed via the rear facing scoop allowing for a much taller cowl plenum and air cleaner, thus creating even more horsepower. In order for the ZL2 hood to be accepted in the Trans-Am series, GM had to sell 1000 units, which they did by making it a regular production option.
In addition, GM also homologated a fibreglass version of the ZL2 hood, which was only available over the parts counter and which they labelled a ‘heavy duty option’, which was in-house code for ‘racing option’. The fibreglass hood was designed specifically for Camaro racers, and was never actually available on road cars, not even as an option. Exactly how many fibreglass ZL2 hoods GM made is not clear, but its fair to say if you sent down to your local Chevy dealer in 1969 and tried to order one, they wouldn’t have any in stock.
Intake Manifold continued:
Now, both the Craig Fisher and Jerry Titus Pontiac Firebirds used the GM cross ram set-up on their Chevy engines in 1968, and I’d love to do the same on my Firebird. Not only is this authentic for my car, it also just looks so damn cool. But for 1970, the SCCA changed its rules once more and stipulated only a single 4-barrel carb would be allowed. So GM had no need to keep producing the cross ram.
So what this means is that, given the cross ram and its associated parts was only produced in small numbers to begin with, and given not all of them would have survived, originals intake manifolds are extremely rare, and also extremely expensive. I’ve seen bare intakes being offered for sale upwards of US$4000, and Heartbeat City Camaro currently have a 1968 cross ram with date matching carbs for sale at US$27500: http://www.heartbeatcitycamaro.com/s...-4-10-68-1968/
As much as I’d love to fit my car with an original GM cross ram intake, these prices are way out of my league. However, I want to build an engine that looks just like the one in the Fisher Firebird. Fortunately, both Edelbrock and Offenhauser also produced their own cross ram intakes in period. From what I can tell, neither are still in production, or have been for a long time. However, when they do come on the market, they’re a lot more affordable than a GM unit.
This was where Jon Mello’s excellent research shared on the CRG website came in handy. The following photos compare the three intakes. The Edelbrock looks quite different to the original GM part, and doesn’t have the ridge travelling around the top outer perimeter. However, the Offy does, and in fact, other than the Offenhauser lettering, could easily be mistaken for the GM item, unless sat right next to it.
GM cross ram
Offenhauser cross ram
Edelbrock cross ram
These photos are courtesy of the CRG website.
To that end, I set my sights on finding an Offenhauser cross ram, and my search soon led me to an Ebay auction where a seller had a beautiful NOS item for sale. I bid, and ended up owning the intake for just under US$1000. I was well pleased with that. In fact, when it arrived in New Zealand, I thought it was such a beautiful looking piece, I have since displayed it on my office desk. Whether I grind off the Offenhauser lettering or not I’ve not fully decided.
I’ll purchase a pair of Holley 600 carbs, model number 4150, to get the right period look.
As interest Steve I sold the cross ram from the Tulloch Camaro in 2006 back to a guy in Texas. I had advertised it on Trade Me with bidding starting at $6500. No one wanted it but the guy in Texas had been chasing on for years and typed in cross ram on his computer and my add came up. In all complete with the carbs I got $10K for it.
I had asked Inky did he want it with the car but they had only run the car with it in 3 meetings and said it was difficult to tune so removed it and replaced it with the LT1 Z/28 intake with single carb.
I sold the car with a Smoke Ram on it.
Bet I could of sold it here today now that historic racing is on the up.
Hi Steve. These are nice manifolds, I picked up 2 NOS Offy manifolds about 4-5 years ago for $600 ea from a hotrod shop in the U.S via ebay. The carbies I am planning on getting are 600 double pump model #4295 and have the progressive secondary linkage like the ones from the original setup.
https://www.camarocentral.com/1968_1..._p/enc-843.htm
Last edited by Mark Fiddyment; 02-25-2017 at 05:01 AM.
Reckon leave the casting as is Steve, later on who knows what they worth. (original)
Hi Rod, yeah I'll probably have to. It'd be almost impossible to grind it off and not make it noticeable.
Thanks Mark, yeah I've looked at those carbs before. What will you be fitting yours to?
Steve, I have 2 HK Monaro's a genuine 327 that I want to restore as close to original as I can money allowing, plus a base model that will get one of the following that I already have, cross ram, moon with 4 DCOE Webbers, Formula 5000 mechanical injection or 80's Camaro Tune Port Injection.
moon with 4 DCOE Webbers----excellent taste, Mark. Very period for a HK
Last edited by John McKechnie; 02-26-2017 at 07:58 PM.
Mark, those sound like cool projects. The mechanical fuel-injection would really be cool.
Intake Manifold continued:
As mentioned earlier, the carb induction system used on the Camaros and Firebirds in 1968 was fed via the cowl vent, from the base of the windscreen. GM produced a plastic cowl plenum which attached to the passenger side of the firewall, and wrapped around the carbs. A hole was then cut into the firewall, from which cool air was drawn in through the cowl vent. If you look at period photos of the Camaros and Pontiacs that raced the Trans-Am in 1968, you’ll see they had flat hoods. It was only in 1969 that Chevrolet homologated the ZL2 cowl scoop hood for the Camaro.
Another person who is helping me a great deal with information, particularly with regards the cross ram, is Ron Atchley. Ron owns the beautiful British Green 1968 Z28 that is featured elsewhere on this site. The car was originally raced by Johnny Ware, and competed in the Trans-Am series from 1970 onwards. From the time it was bought new until it was converted to a road racer, the Ware family drag raced it.
Ron has been messing about with cross ram systems since the 1970s, and has produced a number of plenums for them, plus various other parts. The original units were made from plastic. Ron’s are fibreglass. Many of the plenums he produced are now being sold through Heartbeat City Camaro.
One thing I wasn’t aware of, until Ron told me, was that the Offy intake is slightly taller than the GM, most notably the carb plates. Ron is currently building an Offy cross ram to fit to his Trans-Am Camaro, and I’ll be looking forward to seeing this come together.
Ron said he plans to produce his own fibreglass plenum for the Offy intake he will be running on his car this year, and I’m hoping to purchase one of these off him. If not, I might try and make my own.
Hey Steve,
I found another tow vehicle for you as I drove around at the weekend. Just 22 K (US) and you could mix with both the Ford and GM boys when you arrived with your 1970 Boss Mustang. Put on some silver pin striping and a silver fern etc. and you would be a big hit.
You could even sell John McKechnie the rear spoiler as he can't find a suitable one for his Monaro. (Don't tell him I suggested this and I have also supplied some instructions ! BTW this site has some good Mustang Tech Tips.http://www.cjponyparts.com/mustang-r...uctions/a/354/ )
I had to put the Mini Cooper in to show that I actually did stop by to check it out. p.s. It was not there this morning.)
(It seemed like a good car for the price. Do not get many like this now around here for sale.)
Ken Hyndman.
Last edited by khyndart in CA; 03-01-2017 at 08:57 AM.
Thanks Ken, thats a good looking car. I don't think its a Boss, they had the brake duct inlet thingies on the rear fenders removed. But still a neat looking car and the price seems good for a '70 fastback.
You sure see some cool cars over there! I love your Mini Cooper.
Thanks Steve regarding the Cooper.
Do you note the high quality third brake light so as to try to warn others that I am slowing down.
It works almost as well as when I use my oil can to put some drops on the exhaust when I am driving and someone is tail-gating right on my butt. They will endanger everyone with their tail gating but they hate to inhale any blue smoke as it might affect their health and they drop way back. ( So a small hole for the oiler nozzle and 2 squirts, works every time ! )
Ken H
Ha ha, thats brilliant!
Posting Permissions
Reply With Quote