Curious about performance upgrades

[powerlifter]

I have a 1991 ZR1. Originally Dyno at 303HP. Rippie chip, k&n, corsa catback and replaced bad injectors. Dyno now at 343HP. What might I expect to gain adding the Ported plenum and injector housings / 63 mm throttle body and retune?

Any feedback appreciated!

I'm hesitant to make any real mods to the LT5

[91King]

Long tube headers will get you an additional 20hp and a topend port job will get you another 30. These are not RWHP numbers.

With what you have done so far, plus the mods I mentioned above, you are getting the best bang for your buck on LT5 mods. After doing these mods the price for horsepower gets a bit pricey.

[Hib Halverson]

The 63mm TB will gain northing and may even cost torque at the low-end.

Porting the plenum and the housings might get you 10-15hp. It kinda depends on how the port work is done.

Now, if you port the heads as well as the housing and plenum, reset the cams and add the right cal., you're lookin' at 35 or so hp. If you add an aftermarket exhaust (that works) you'll see another 8-15hp. Remove the cats and put headers on and you'll see another 25hp.

[locobob]

Intake porting and headers are the two best bang for the buck mods you can do at this point. Here are a few links to check out.

http://www.zr1netregistry.com/forum/...ead.php?t=1128

http://www.zr1netregistry.com/forum/...ead.php?t=1224

http://www.zr1specialist.com/HAT%20W...%20porting.htm

[Aurora40]

Originally Posted by Hib Halverson

The 63mm TB will gain northing and may even cost torque at the low-end.

What's the information you're basing that on? Haibeck, DRM, and the Vette Doctors all include this in their plenum porting packages.

[Hib Halverson]

They may include it but I think it's a waste of money.

Automasters (Jim Van Dorn) built a several of his "Street Skinner" engines (350s with top end jobs and headers) with TBs having 63mm bores. During testing as time went on, he learned the bigger TB on 350s didn't make any more power but cost a lot of money. So, he stopped offering 63mm bores on stock displacement engines with "top-end" jobs.

Where a 63mm throttle bores become useful is with engines of larger displacement. It might be a good value on a 368 and once you get to 385, you probably need it.

I think I'd ask Marc Haibeck, Doug Rippie and Carmen/Dennis for their data showing that a 63mm bores on engines with only plenum porting increases power and torque.

And really, if "bang for the buck" is the major issue as far as performance goes, nitrous oxide is a far better value than plenum porting work and headers. I'm not necessarily advocating that in lieu of the engine work, because you have the complexity of the nitrous injection hardware, the hassle of refiling the bottle and lack the coolness of a modified engine *but* if initial-cost-per-performance-gained is the benchmark: nitrous is a better choice. A properly installed and calibrated nitrous injection on a 350 inch LT5 can get you to 550hp with ease whereas you'd never see that from a 350 with engine mods that also had reasonable driveability.

[Aurora40]

It's certainly pricey. I'll have my car at Marc's later this week, I'll ask him about the TB. It was more the part where you said it may cost low-end power that I was wondering if you had any data to that effect. My car definitely had more pull in 6th gear after the Marc H top end package than it did before it. I generally can keep it in 6th at 50mph and above and not have to downshift to speed up or when the road inclines, etc.

[Hib Halverson]

While the LT5 is electronically fuel injected, it's still a normally aspirated, gasoline burning, spark ignition engine.

Remember in ancient times what happened when you bolted an 850 holley on a stock 350, well...the same problem can exist with a stock or near stock LT5 and a TB with big bores.

To date, my information has been that the 63mm conversion is of little or no performance value on stock or near stock 350s which see the typical street high-performance duty cycle. Additionally, on a stock or mild 350, performance and throttle response from idle up to the mid range might be slightly degraded with the net sum of slight losses at low-speed with little or no improvement up high being a "wash". I'd talk the 63mm bores over with Marc and go with his recomendations.

On the other hand, if you've got a 350 with a lot of mods and which runs safely to 7200 or so and is going to be race more than driven on the street, then, well....the 63 mm secondary bores might be worth the money.

Now, once you start increasing displacement...368, 385 and so forth, then the bigger bores become increasingly more valuable.

With a stock or near stock 350 for street high-performance the big pay-off from "top-end" jobs come in the head porting, (if it's a 2-bolt motor) the housing work, freeing up the exhaust (if you live in a state with no exhaust emissions test, include cat removal) and changing the calibration. From their the gains get progressively smaller with reductions in restriction upstream of the injector housings (plenum, air intake system, fitler, etc) being worthwhile but not getting you as much power. In addition, optimizing cam timing can add a little performance but, the idea that there's a "silver bullet" cam timing configuration on a 350 with stock cam profiles that's going to make a ton more power is urban myth.

My guess is that if you port the heads, housings and plenum, free-up the exhaust, free-up the air filtering, set the O.E. cams accurately and go with one of Marc Haibeck's cals you'll be at 400-410 SAE-corrected at the wheels. Add a set of headers and remove the cats and you'll be at 425-435. If you need cats, the right aftermarket units seem to cause little loss.

Good luck.

[Aurora40]

Well I talked with Marc, and he feels the 63mm TB is worth it with a ported top end. He said he's seen gains from adding the ported TB.

[90 Corvette ZR-1]

after talking with Kurt White and Jim Vandorn, I'm fairly comfortable saying that a 63MM tb is a waste of money

[BOB SWEETING]

DID CHEVROLET OR MERCURY MARINE EVER MAKE HIGH LIFT CAMS FOR THE LT-5? IT WOULD SEEM TO ME THAT .390" LIFT IS PRETTY LOW IN THAT DEPARTMENT. THE DURATION IS NOT THAT CRITICAL DUE TO THE DUAL PATTERN LOBES, BUT THE LIFT IS WEAK. I KNOW THAT THE CAMS ARE RETARDED TO DROP COMPRESSION AND MEET SMOG. BUT I BELIEVE THE HORSEPOWER IS HIDDEN IN THE LIFT AND CAM PHASING. THE INTAKE HAS PLENTY OF VOLUME AVAILABLE. I WAS DISAPPOINTED THAT THE BORE IS SO SMALL. THIS ENGINE IS SIMILIAR IN DESIGN TO THE MAGNIFICENT "ALLISON" V12 1710 ENGINE. WHICH IS A BEAUTY. MY TURQUOISE CAR NOW HAS 235,000 MILES ON IT, AND STILL HAULS. MY COMPANY IS "POWER BRAKE SERVICE" AND I AM THE HYDROBOOST GUY.

[90 Corvette ZR-1]

Bob, Great post and I'll be happy to answer it but for future reference all caps makes it very hard to read. Yes there are a few ZR-1s out there with high lift cams and the general conscience is that they are not needed on anything smaller than a 397 and even then they aren't really necessary. you can make big power on stock cams.

[Aurora40]

As it's a multi-valve motor, I'm not sure if lift is comparable to valve lift for SBC's and the like. I'm also not sure what kind of multiplier was used for OHC motors though there may not be one at all.

[jonszr1]

i have headers flowmaster ex no cats and partial siamesed ported inj and plenum along with a custom prom all work done by henderson performance tech(corey) results 409.9rwhp 373.3tq . doing the heads would be cool just to much cash outlay for me as everyone that does the porting on these wants the car. NO ONE will port the heads if i want to send them and install myself. believe me I have asked and conrary to what others have said they want the car or no deal. so in the end 6-7k to do the heads is just too much for an average working stiff..

[powerlifter]

Originally Posted by powerlifter

I have a 1991 ZR1. Originally Dyno at 303HP. Rippie chip, k&n, corsa catback and replaced bad injectors. Dyno now at 343HP. What might I expect to gain adding the Ported plenum and injector housings / 63 mm throttle body and retune?

Any feedback appreciated!

I'm hesitant to make any real mods to the LT5

Now at 364 RWHP on Doug's dyno.

Hmmmm so anyone have first hand experience with a DRM500 upgrade?