LT1 Aluminum Rocker Arms And 58mm Throttle Body Upgrade - Top End
Part III: A Little Elbow Grease And A Few More Bolt-Ons Help Free Up More Power On Our 355 LT1
From the February, 2009 issue of GM High-Tech Performance
By Scott Parker
Photography by Scott Parker
In our last installment, you may recall RaceKrafters swapped out the refurbished stock LT1 heads and small Comp Cams bumpstick for the Edelbrock Performer LT1 heads and cam package. At which point, we retested the Edelbrock Air-Gap intake manifold and 52mm throttle body, which got us to a grand total of 393 horsepower and 393 lb-ft of torque. But like eating at a French restaurant, this tasty meal didn't come close to satiating our appetites. So for this go-round, we figured we would stop off for a snack before grilling up a big juicy steak in our final installment. Before moving on, we were hell-bent on wringing every ounce out of the current Edelbrock combo, and even comparing a ported Air-Gap intake to a ported stocker.
Before I arrived at the Lancaster, Pennsylvania shop, RaceKrafters went ahead and ported the openings of the intake runners on the Edelbrock heads to better match the gaskets and intake manifold runners. The smoother transition was carried over to the Edelbrock intake as well, and taken all the way up the runners to the plenum-and any excess casting flash was removed. At the same time, RaceKrafters also used its own proprietary methods of prepping a stock LT1 intake, which included porting the runners and smoothing the throttle body opening using a special military-spec filler that is resistant to gasoline and virtually any type of corrosive agent likely to be found in an engine-including methanol. In the interest of eliminating any weak links or cause for doubt, we decided to throw an Edelbrock 58mm throttle body (PN 9310) and Summit Racing 1.5-ratio aluminum roller rockers (PN SUM-G6935-16) into the mix. With an increase in airflow and decrease in valvetrain friction and weight, we were sure to drain the last bit of power left in this combo. Enjoy this snack in anticipation of a big meal to follow.
While in Part II we simply...
While in Part II we simply bolted up Edelbrock's Performer LT1 heads to the Air-Gap intake, this time RaceKrafters took the liberty of hand porting the intake runners one inch from the surface of the heads (matching them to the intake gasket). If you recall, these heads came from Edelbrock with some very minimal CNC work that protruded only a half-inch from the opening. But our hypothesis was that an uneven transition from the intake to the gasket to the heads caused a restriction in flow.
Additionally, RaceKrafters port-matched the runners on the intake manifold and removed the rough cast finish, as well as excess casting flash, all the way into the plenum.
Since we had also hypothesized...
Since we had also hypothesized that perhaps the intake manifold was better suited or designed with the intent of using a 58mm throttle body, we called Edelbrock to replace the 52mm with its larger cousin (PN 9310, $399). Without measuring or looking at the engraving on the throttle blade, you couldn't tell the difference between them. But we wanted to see if the dyno could.
The bottom plate for the IAC...
The bottom plate for the IAC and coolant is transferred over from the 52mm throttle body, which was originally from the stocker.
After baselining the Edelbrock...
After baselining the Edelbrock ported intake and matched heads, the 58mm throttle body is affixed using its supplied Allen head bolts and gasket.
These beefy, full-roller-tip...
These beefy, full-roller-tip aluminum rocker arms are from Summit Racing (PN SUM-G6935-16). They are made from high-strength extruded aluminum and are heat-treated for reliability. Though they are still the factory 1.5 ratio, we expected to see a few ponies due to the reduction of valvetrain weight, deflection, and friction. With a higher-revving cam, these gains would be increased exponentially. And at $250, they are quite affordable.
The taller rockers required...
The taller rockers required shorter pushrods. We went with Comp Cams 7.200-inch pushrods, and a different set of valve covers.
At first Bob and Craig thought...
At first Bob and Craig thought they might have to switch to ARP rocker studs due to the taller rocker arms, which have been known to bend factory studs, but found the Edelbrock studs up to the task. Modified Isky Racing pushrod guide plates were necessary, though.
An Allen head and a 5/8-inch...
An Allen head and a 5/8-inch open-end wrench are needed to tighten down the rockers as the motor is turned over.
In just doing the gasket matching...
In just doing the gasket matching on the heads and porting the Edelbrock intake, we had already upped output from 393.3hp to 406.3hp, and torque even climbed to 402 lb-ft from 393.8. Without any changes to the tune, the 58mm throttle body alone added another 2hp, and in combination with the Summit roller rockers, the 355 LT1 was now making 416.1hp and 409.4 lb-ft.
After many years of building...
After many years of building LT1s and quite a bit of experimenting, RaceKrafters has found the best method for porting the intake manifold is to cut off the lid and hand port the runners...
...The throttle body opening...
...The throttle body opening is bored to match a 58mm throttle body. A special military-spec epoxy is used as filler just behind the throttle body opening as well, which helps direct air particularly to the starved #1 cylinder...
...With sequential fuel injection,...
...With sequential fuel injection, Bob says he would typically wind up cutting fuel enrichment by 2% as the result (of the difference in air flow), which is more pronounced in 383ci and forced induction builds.
With the 58mm throttle body-as...
With the 58mm throttle body-as well as the injectors and fuel rails-bolted up, Bob dropped the RaceKrafters-modified factory LT1 intake back on top of the Edelbrock heads and sealed it with silicone.
The ported factory intake...
The ported factory intake made nearly the same average power with the same tune, but when Bob began tweaking the fuel with the FAST XFI software he found this intake performed much better in the upper rpm with a leaner mixture (closer to 13.1:1 AFR).
The gains from each round...
The gains from each round of modification appear linear in terms of horsepower. However, the average numbers are decidedly in favor of the ported Edelbrock intake with the 52mm throttle body and stock rockers at 332.33hp. Peak, though, was at its highest with the 58mm throttle body, roller rockers, and ported factory intake at 422.7hp (at 5900rpm).
Though there was an initial...
Though there was an initial dip, the 58mm throttle body didn't hurt torque. Average numbers went from 392 lb-ft to 399.24. Once again, peak was highest with the 58mm throttle body, roller rockers, and ported factory intake at 414.3 lb-ft (at 4900rpm).
| ||Ported Edelbrock Heads/Intake, 52mm T-body ||Ported Edelbrock Heads/Intake, 58mm T-body, roller rockers ||Ported Edelbrock Heads, ported LT1 intake, 58mm T- body, roller rockers |
|Peak HP ||406.5@5700rpm ||416.1@5800rpm ||422.7@5900rpm |
|Peak TQ ||402.0@4900rpm ||409.4@4900rpm ||414.3@4900rpm |
|Avg HP ||332.33 ||323.81 ||327.18 |
|Avg TQ ||392.36 ||399.24 ||403.39 |
First, I'd say we can discount the idea that a 58mm throttle body is too large for a stock cube motor. Though its average horsepower suffered a little, its peak was higher and it did not manage to kill the torque like many have suggested. Since these gains would have only increased with a larger cube motor or forced induction, it also gives the motor room to grow. Second, despite the fact that the roller rockers were the stock 1.5 ratio, they still managed to pick up power just through valvetrain dynamics. Never underestimate how much power friction and weight can rob from your motor. Lastly, Edelbrock put in a valiant effort with the LT1 Air-Gap intake manifold, but for a 350 to 355-cube motor it is hard to beat a stock or modified stock intake.
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