78vette78

Im about to get my H pipe with 2 cats and flowmaster one chamber exhaust on my 78 vette. Do I gain some horsepower and if I do.... How much??????????

mrvette

Hard to say, depends on the condition of the present, presumably OEM Cat.....them things were restrictive as hell, and so worth maybe 10-15 hp right there....modern cats are much less problem, but you maybe want to check with the local smog ***** to see what they require.....I suspect your total gain to be maybe 20 horse....

A header company owner told me, once long time ago, that any time you put a pipe and ANY muffler after a header, you loose ALL of the effect of the header.....no benefit what so ever.....

hey, the man could have lost a sale by that statement, he didn't, but could have....so I believe him, and it furthermore makes sense.....

jb78L-82

On my 78 L-82 4 speed, over the past 25 years I incrementally changed the totally stock OEM Y pipe cat exhaust to currently Mcjack's shorty headers, 2.5 in true duals (no H pipe), and monza Turbo mufflers (pretty much straight through) with dual outlets (3 in total area each muffler) with no emissions at all ( no cat, AIR pump, EGR, running 180 degree thermostat and K&N filter). The difference in power between the original exhaust and the current exhaust borders on the unbelieveable! It definitely is not 10-15 Hp but more in the range of 30-40 HP. The Ecklers and Mid America vendors claim 30+ HP gain just going to true duals on an 80 corvette and 30+ as well on the C4's 84-90, again just going to true duals and no cat. Another good reference is to compare a 1974 L-82 rated at 250 HP versus the 1978 L-82 rated at 220 HP with the only real difference in the engines is the exhaust on the 74 being true duals (not even 2.5 in pipes), no cat and the mufflers (which are more restrictive than any number of aftermarket mufflers available for your 78).

TERRY CLARK

Not sure if the H pipe will give you any more horses....It balances the exhaust and gives it a better note.

jb78L-82

Correct on balancing the exhaust as well as a better sound. The primary advantage of an H pipe is enhanced low end torque.

Reggie Dunlop

The H-pipe is a proven performance enhancer. Many of the "original" muscle cars had them right from the factory - specifically Hemi cars. In recent years the X-pipe has become more prevalant, but for a street car there's really little to no difference between the two. The H-pipe will increase low and mid-range power and depeding on the placement, reduce resonance.

mrvette

I not disputing what you say above, but he is going for the dual cats.....and they are very restrictive compared to what you are saying above....amounts to yet another muffler set, at BEST.....

so I guess at 20 hp increase....I'll spot you 25 for his case, but no more.....

TERRY CLARK

I have an H pipe on '75 Blazer ( 40 over 350 ) with Headers & Glasspacks......sounds a little like a Big Block !

jb78L-82

From what I have read, the newer high flow cats (especially the smaller bullet types on modern cars which is what I would fit to duals) have almost no restriction versus an open pipe. Now, running OEM type cats with duals which I absolutely (I would not even bother since the restriction would be worse than a stock system) would not do, is a totally different story. If the latter is the case, I agree with you, correcto mundo!

Reggie Dunlop

I can say for a fact that the modern cats have very little to no impact on power or E.T. I have a '77 Pontiac Firebird powered by a 400 Pontiac, with a 214/224/112 cam, 8.5:1 compression, q-jet and headers with duals, a TH350 and 3.23 posi. 3 years ago I moved to an area that requires emission testing and had to put cats on it. I chose the Dynomax 13" cats. This car ran consistant 13.75s @ 101 mph day in and day out. The trans is set up to shift by itself at 5,000 RPM - so I just leave it in drive and step down on the long skinny pedal & the car does the rest. With the cats it runs 13.80s @ 101. Holding your hand over each tail pipe at an idle you'd never think it had cats on it. It is just a tad quieter.

rayluka

Being you are going from a fully restrictive exhaust to duals and hi flow cats should make a very reasonable difference. 20 horses sounds about right. What part of Cal are you in? I ask because of your smog issue.

jb78L-82

Thanx for confirming that info on the modern high flow cats. If I ever had an issue, I would look for the smallest high flowing cat that I could get on my 2.5 in pipes as close to the head pipe as possible. Many of the high flow cats I have seen are as small as 6-8 inches in the bullet shape.

v2racing

This is not the case at all. I do a fair amount of exhaust design and testing and you would be surprised at what can be done with secondary tuning (beyond the headers).

This article is the best I have read on the subject.

http://www.superchevy.com/technical/...exh/index.html

78vette78

Im in san fancisco CA.

jb78L-82

Revised estimate on the HP with High flow cats, 2.5 in true duals, and any unrestricted muffler such as Magnaflows, based on my own personal experience and Reggie's comment above, 30 HP. The OEM y pipe exhaust and the boat anchor original cat is VERY restrictive! Add headers for 10 HP more! Remember that the original Y pipe exhaust is essentially acting like a single 2.5 in exhaust (the single 2.5 in pipe running down the passenger side) which for all you physics majors out there represents the system restriction. Add the stock cat acting like a tennis ball in the single 2.5 in pipe and you have a fubar! Going to dual 2.5 in pipes with performance mufflers and high flow cats will increase the exhaust flow by 4X, not twice, as one would think. That is an awful lot more exhaust flow! Lots more exhaust flow equals lots more horsepower. Still think 20 hp is way conservative. I refer back to my 74 L-82 250 HP analogy with smaller pipes (no cats, though) and relatively restrictive mufflers to the 78 L-82 with the Y pipe OEM exhaust, very restrictive mufflers and the boat anchor cat. Dual high flow cats, 2.5 in dual pipes, efficient performance mufflers, at least the horsepower of the 74 L-82, 250 HP or 250Hp- 220HP>=30HP difference.

Roughrider

To determine the proper place for the crossover pipe, simply draw a line with a wax crayon from the header collector to the muffler. Run the engine to normal temperature. Follow where the crayon has been burned off, and where you see the crayon mark again is where you locate the crossover.

eta_78vette

the burned place is due to a crest(peak) wave(much hotter).

i also have a 78 stock but i bought it with true dual already in place from the previous owner.

i have a question if duals give you ~20hp over stock.

how much would hooker sidepipe header with 4" collector and 4" straight sidepipe give you?

jetnoise

I would assume he is out of business by now because he had no clue

toddalin

This is proper and great in theory, but seldom achievable in a Corvette due to stuff in the way at that spot (like the crossmember).

This is about the easiest place to insert it..., right before the rear end.

DIP51

Doesn't Magnaflow make an "x" pipe system for C-3 Corvettes?? I read the x is better at scavenging gasses and equalizing pressure. True???