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post #1 of 10 (permalink) Old 01-16-2016, 09:14 AM Thread Starter
 
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Advice on 74 GTO Exhaust

I'm the original owner of a 74 GTO with 350 engine. Working on restoring for past year or so after decades of sitting and only occasional starts, always garage kept. Has less than 28,000 miles on it. 40 years ago I put on Holley carb, Edelbrock intake, 1.65 rockers and pushrods, headers and dual exhaust (original exhaust fed into single muffler behind rear axle), TransGo shift kit in TH350, aluminum 4 core radiator, lighter springs in distributor to give full advance by about 2,800 rpm. Recently put original stock Pontiac manifold back on, along with original Q-jet (which I rebuilt). Still has 1.65 rockers and headers. Still working to shaker hood scoop back into operation.

Car starts and runs fine. Acceleration is OK, but low end torque could be improved, really starts to move at about 2,000 rpm, 0-2,000 rpm OK, but could be better. Exhaust needs to be replaced and would like to remove the old headers. The headers keep the engine compartment extremely hot in the summer and I am thinking about going with the RARE ceramic coated LB-1 factory headers to replace the headers, get more room back in engine compartment, and cool the engine compartment down. I will need to replace the entire exhaust back to the exhaust tips. Stock 2" exhaust pipes still in place. May do a few mild mods to the engine over time, such as mild cam and thinner head gaskets to slightly raise CR (stock is 7.6:1). I have heard that too much diameter in exhaust is counter productive, so wondering if going with 2.25" or 2.5" diameter exhaust would be the best option.

Any thoughts on the best diameter exhaust for now and some mild future engine mods?
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post #2 of 10 (permalink) Old 01-17-2016, 02:23 PM
 
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They say you need a little back pressure, but then as you build the engine, you want to minimize the back pressure by going bigger pipes.

I would think the 2.25" would work with the 2" dual pipes. If you go 2.5", then you will have to go with 2.5" pipes and I think that might be too big for your application.

I posted a build for a Pontiac 350 that took the stock engine and improved its performance with basic add-ons and adjustments. Search for "350CI Dyno-Tune" and it will list the parts used.

Gearing has a lot to do with it. If you have a set of 2 series gears that were installed at the factory to obtain better gas mileage, that could be a lot of the problem on the lower end. A gear swap would make a big difference in bottom end performance.

The 1.65 rockers may be hurting the bottom end feel on the stock engine. They may be opening the valves too fast and even too high for the stock cam so port velocity is not building up at the lower RPM's, but does just fine as you said from 2,000 RPM's and up. You could play around and perhaps go back to the 1.5's on the intakes only and see what happens and then put the 1.65's back on the intakes and swap the 1.5's to the exhaust side. A better cam of course would improve things all around.

Also, tune the Q-jet secondary flap. It may be closed up to 2,000 RPM's and then pops open when it is possible for it to open sooner without causing a bog/dead spot.

Just some thoughts.
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post #3 of 10 (permalink) Old 01-17-2016, 09:33 PM Thread Starter
 
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Jim,
Thanks for the suggestions with your 350 Dyno Tune changes, I just looked it up, never came across your list before.

I'll work through your list and see what happens. Once done with the Dyno Tune changes, if I'm still struggling with the very bottom end torque, I'll look at playing with the 1.65 rockers and with the stock 1.5 rockers on the intake and exhaust side. I'll have to wait for some of the ice to melt off the local roads first.

I'll go and check my diff gearing to see what is in there. I can't remember all of the "good" ideas I had as a teen, but I do recall some time in a ring and pinion shop back in the '70s!

Thanks for the suggestions!
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post #4 of 10 (permalink) Old 01-19-2016, 09:14 AM
 
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Several companies make a dual exhaust cross flow muffler for the '67-69 Fbody and '68-74 X body. The shell length of the muffler is usually 1" narrower than the version offered for early 2nd Gen Birds/Camaro's. I'm using a imposter crossflow muffler from Waldrons on my '72 T/A, the rest of the system I pieced together building my own head pipes beginning with sectioned J bend mandrel tubing, very similar to those used on 455 SD head pipes.

Going to repro long branch manifolds, there are mandrel bent head pipes you can buy from RARE or from PYPE's, these work well with the F body long branch manifolds, have installed the long branch version several times and they fit a lot better than their A body RA manifold mandrel bent head pipes.

On your '74 GTO, the original rear should be a 3.08, Pontiac wasnt offering high ratio rearend gears like 2.41's or 2.56's under the '74 GTO's.
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post #5 of 10 (permalink) Old 01-19-2016, 02:43 PM Thread Starter
 
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Originally Posted by Pinion head View Post
Several companies make a dual exhaust cross flow muffler for the '67-69 Fbody and '68-74 X body. The shell length of the muffler is usually 1" narrower than the version offered for early 2nd Gen Birds/Camaro's. I'm using a imposter crossflow muffler from Waldrons on my '72 T/A, the rest of the system I pieced together building my own head pipes beginning with sectioned J bend mandrel tubing, very similar to those used on 455 SD head pipes.

Going to repro long branch manifolds, there are mandrel bent head pipes you can buy from RARE or from PYPE's, these work well with the F body long branch manifolds, have installed the long branch version several times and they fit a lot better than their A body RA manifold mandrel bent head pipes.

On your '74 GTO, the original rear should be a 3.08, Pontiac wasnt offering high ratio rearend gears like 2.41's or 2.56's under the '74 GTO's.
Pinion,
Thanks for the inputs on the exhaust and the long branch manifolds. Good to hear from someone who has installed the long branch manifolds and has positive experience with their fitment.

You are correct on the rearend gears. Just checked the ratio yesterday and it is 3.08. I think I had it changed decades ago, but then eventually changed it back to original, again decades ago, but couldn't remember what I had put back in, so I checked it.
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post #6 of 10 (permalink) Old 01-21-2016, 09:01 PM
 
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From what I have read, even though you have the 350CI, it was comparable in performance to the earlier GTO's with the 400CI as the '74 GTO body was much lighter. So with a little tweaking you should have a nice pulling car. Bet a set of 3.55's would make a world of difference in acceleration, but you trade off some gas mileage and highway drivability.

Had a 350CI in a '67 Firebird when I was a kid. Have no idea what year the engine was. The heads had been worked, but I did not know what that meant back then. Had the 3-speed manual on the floor and single track rear. I had no problem smoking tires and ran it up to 6,000 RPM's because I just thought it should! That was part of my early learning curve on owning a fast tire burning car and liking it.
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post #7 of 10 (permalink) Old 01-22-2016, 12:18 AM Thread Starter
 
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Jim, thanks for the thoughts about the rear end ratio, you're probably right.

I've got some more engine work on the ignition, exhaust, and shaker hood scoop operation to do first, then some work on the headliner before I revisit the 3.08 gearing.
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post #8 of 10 (permalink) Old 01-22-2016, 01:43 PM
 
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Originally Posted by Red74Goat View Post
Jim, thanks for the thoughts about the rear end ratio, you're probably right.

I've got some more engine work on the ignition, exhaust, and shaker hood scoop operation to do first, then some work on the headliner before I revisit the 3.08 gearing.

Sounds like a plan. If you decide to go into the engine and play (maybe change your timing chain & gears at the same time) around a little, I might go with a Competition Cams grind having a 110 degree LSA which will work well with your factory compression and wake up some HP by raising the Dynamic Compression of the engine. The CC 110 LSA cams work best on engines under the 9 to 1 compression range. The engine pulls strong at your lower RPM's. Had the XE274H cam in my '72 400CI build with its 8.0 compression, and maybe a little less with the aftermarket cast pistons I used, and it pulled like a freight train. But, when it hit about 5,500-5,600 RPM's, that was it and you could feel the power drop off - which is fine in all respects, but I liked to buzz my engine to 6K on occasion.

Just for fun, you might drop an email to CC and see what they recommend in a cam/kit and then compare it to your stock cam specs. You could also go with the next bigger hydraulic cam recommendation for your engine and use Rhoads lifters. The Rhoads lifters drop the lift/duration of the cam due to their bleed down rates at lowe RPM's, but pump back up around 2,500 RPM's to provide the cams full lift potential. This gives a bigger cam a smooth idle and drivability like a stock cam at lower RPM's and the advantage of the bigger cam specs above 2,500 RPM's where your engine should be pulling when you go wide open throttle. Used a set in my brother's Mopar 360 SixPack build and they seemed to do what is claimed, idles like a factory stock cam, good vacuum. Just my opinion and something to consider if you want to go with a bigger cam. Never used them personally or in a Pontiac, but did on my brothers car.
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post #9 of 10 (permalink) Old 01-23-2016, 01:05 PM
 
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In the Pure Stocks ranks, there is a fellow by the name of Jody Traylor(sp). posts on PY board, threefifdee, and on v8buick Pure Stock forums. Campaigned a '74 GTO into the high 13's and a '68 Firebird 350 HO. Jody's currently finishing up a '72 HO GTO for Pure Stock drags. Jody would be a great resource to contact on helping get a relatively stock '74 GTO to really get up and go.

Not sure if you are aquainted with Pure Stock build rules, the cars are run at the races on repro bias ply or radials in specific sizes. The engine rules do not allow major deviations, though CR is allowed a deviation of being raised 1.5 points. engine builds often mirror to a degree what an owner would do in building for NHRA stock class. there is no adding other cyl heads, porting, gasket matching, have to run the ex manifolds the vehicle was equipped with. Pontiac engine builds with optimized 066, 067, 068, 744 cams are limited to gross valve lift at the rocker of .424. Cams often times are not totally stock out the box Melling replacements, but are ground on different centerlines.

Exhaust systems.. In all the stock tire class muscle car racing classes going back to when the NMCA had their first stock muscle car clases, one of the largest contributor to lowering ET's has been a strong flowing "scienced out" exhaust system. Mandrel bent 2 1/2" headpipes, x pipe crossovers were first introduced in the Buick racer camp, then Torque Tech, and finally PYPES, and RARE systems. 2 1/2" head pipes with mandrel bends followed by a freeflowing muffler(s) and tailpipes also utilizing mandrel bends are today, the NORM. Its quite common to knock off 3/4 to near a full second off 1/4 mile times through a properly built exhaust system. Local muffler shop bent, or ordered head pipes with a kink at the down tube, flat spots, and more kinks going back, combined with poor flowing mufflers, all restrict exhaust flow. If I was serious about building a Pontiac X body like the '74 GTO, would look into alternatives to adding 45 lbs of weight with the long branch manifolds. Have installed them on numerous 400 Firebird restorations and have an orig set on a '69 428 HO GP. The longbranch manifolds look cool, but on a 350 Pontiac, doubt one would see a performance gain with added weight... then there is the expense.
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post #10 of 10 (permalink) Old 01-24-2016, 10:08 PM Thread Starter
 
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Pinion,
Thanks for the suggestion of contacting Jody Traylor, I'll give that a try, as well as checking out the Pure Stock forums, sounds interesting.

The exhaust part is a challenge, as I would like to drop the headers soon for a couple of reasons, including the problem with having to disconnect the backdrive linkage from the TH350 up to the column PRND2L indicators and the park lockout. Still a work in progress just considering the possible options. Perhaps Jody will have some insight regarding good exhaust options for the 74.
Thanks.
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