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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
I've got an L98 which was harvested from a totaled 1988 Trans Am. The engine has been carb-converted (Eddy 1406 & Performer man.) Everything else remains stock, but freshly rebuilt. I'd like to keep my current gas mileage, but improve power.
Comparing specs from the L98 to the L31 5.7 Vortec, its remarkable how similar they are. Seems the only major differences are; TPI vs. TBI, and L98 heads vs. the infamous Vortec heads.
L98 vs. L31 Vortec
Dipslacement 5.7L / 5.7L
Compression 9.3 / 9.4
Comb. Chamber 64cc / 64cc
In valve 1.94 / 1.94
Ex valve 1.50 / 1.50
Cam in lift .415 / .414
Cam ex lift .430 / .428
In dur 207 / 191
Ex dur 213 / 196
LSA 117 / 111
Yet, the L98 is rated at only 225HP vs. the Vortec's 255HP. So, my question, is it probable that my L98 would get that 30HP gain by installing even better heads than the Vortec (Dart, AFR, etc.) with no other changes? I've heard NO WAY without changing the stock ex. mani's to headers, yet the Vortec manifolds look far more restrictive than the L98's...
Vortec on the left, L98 on the right. The larger "looking" area on the Vortec's are actually sheet metal heat deflectors, mounted on the outside of the casting.
2-1/8" I.D. Dual exhaust through non-restrictive Flowmasters, 700R4 with 3.73's. I love the look and fit of my stock manifolds, and gawd I don't want to mess with headers. And, I'd rather not risk losing mileage by bumping up the cam. So what do you say? Do you s'pose top-notch heads would get me 30 hp more?
Last edited by Maker-Wright; 01-11-2024 at 04:19 PM.
The TPI manifolds are terrible, the L31s are much better flowing by comparison. Headers even shorties are a massive improvement on a SBC. The inner passageways are larger on vortec manifolds, my L31 had 2 1/4" OD outlets and immediately stepped up to dual 3" to the muffler.
If you're concerned about fuel mileage, do not modify your engine and consider buying a modem vehicle.
That being said, yeah 20-30hp is a reasonable guess if you're moving to a nice set of heads. A larger cam doesn't necessarily mean less fuel mileage, but a little extra duration and lift could really help you hit your power goals. It really depends on how you drive it. Factory manifolds are a pretty bad choke point, I don't think I'd invest in nice heads just to plop those back on. There exists performance aftermarket cast iron manifolds if headers aren't for you.
Your biggest cork currently is the manifolds, especially if those are the 2' outlet version. Swapping the heads alone will gain very little. If you cannot get the exhaust out, you cannot get the air/fuel mixture in. I just built a healthy L31 and decided to try the Hooker 2.5" manifolds on it. I know the manifolds will cost me some HP and a lot of torque compared to headers but this is a cruiser and a L03 305 TBI moved it a long well enough. The manifolds were just over $200 when I bought them. I put LT1 flat tops and the GM 94666492 camshaft with 1.6 rockers into it. Used the ProFlow4 because I got it new in the box for $1200.
Your biggest cork currently is the manifolds, especially if those are the 2' outlet version. Swapping the heads alone will gain very little. If you cannot get the exhaust out, you cannot get the air/fuel mixture in. I just built a healthy L31 and decided to try the Hooker 2.5" manifolds on it. I know the manifolds will cost me some HP and a lot of torque compared to headers but this is a cruiser and a L03 305 TBI moved it a long well enough. The manifolds were just over $200 when I bought them. I put LT1 flat tops and the GM 94666492 camshaft with 1.6 rockers into it. Used the ProFlow4 because I got it new in the box for $1200.
Okay... I ordered a set of Hedman 68478 mid/shortie headers, and I'm going to have to change my long straight-boot plug wires to 90-degree angle boot wires. What brand/part number of under-manifold wires do you guys suggest for an L98 with headers? I'd prefer gray or black wires.
I am talking about the 90* aluminum stamped heat shields that went on a few cylinders of the LT1. Actually I recall these in use even before that on TBIs and maybe TPIs on #5 & #7. I know the 93 Fleetwood I had used them there. The heatshields clip to the spark plug hex and the boot, preventing the wires from flopping around as well.
Taylor had them for the longest as part number 2570, although they are currently not showing in stock at places. Rock Auto had them for $3-4 ea in the Standard Ignition line. I bought a few sets of 8 from Oreilly as well for about the same. Before that I bought them from GM. I say they are alumimum because I dropped one under the motor mount on my Express and it would not stick to a magnet. Ended up having to pull out the remote operated retrieval claw to get it out.
01-11-2024, 04:04 PM
