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Old 04-07-2012, 02:34 AM
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350 underperforms for build

I've got a HO 350 (stock 350 HP) from a '69 vette with 202 heads quenched, drilled studs, plates, etc. It's got an Edel 600 carb and Holly Street Dominator intake. It's running an MSD and HEI ignition setup. It's got a performance piston kit and original pink rods put back, block bored 30 over, four bolt main, nitrided crank, and mild upgraded cam. All told, machinest said about 400HP. I've got long tube headers and two 2.25 pipes out rear. YES, 2.25 pipes. But it seem like the exhaust out the back is struggling and farting as if too restricted on the way out. When I first ran it, I had it dumping at the axle and it opened up and roared far easier, breathing well. Some of the bend around the rear axle of the exhaust looks like it accordions to 1.75 inches at corners. Should I give up on this exhaust running from the Flowmasters back and go back to the old way and dump to the axle again or is there another way to run it and keep flow? It was rumbling in the cab and loud as hell with it dumping at the axle but I miss the easy roar and quick response, performance. It seems to bog it down. It's fouling a plug a little more charcoal-like than i'd like and I got in and checked the compression on this 5k miles motor and it was 150-160 all around. I was expecting wheelies by now. Thoughts?

Thanks ...

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Old 04-07-2012, 04:47 AM
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Whack the tailpipes off and install turn downs. You can angle them towards the ground or to the sides. Either way it'll still get a good deep exhaust tone and it'll reduce some of the cabin noise. I doubt if it's making 400 horses. On a good day it's likely making around 325 in real world figures. There could be several other reasons why it don't have any power, If the exhaust makes no change then the next thing I would suspect is the tune isn't correct. Also keep in mind, these cars were avalible with big blocks in that year model, and to copy the exhaust system of the big block for use on your small block should be easy for any competent exhaust shop and would be more than adequite as far as flow goes.
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Old 04-07-2012, 07:36 AM
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Your engine is not making 400hp. stock heads, mild cam, 600 edlebrock carb, 2-1/4 exhaust all need to go to make that kind of power.

what gear ratio and stall (if automatic) do you have? This will determine how you can build the engine.

You can lose 50 hp in a poor tune. How much ignition timing?
idle timing?
max mechanical advance with out the vacuum advance?
How much is the vacuum advance timing?

Is that a single plane intake? if so, not good for a mild cam, stock stall, and low numeric gears.

FYI, a H pipe after the differential (right before the mufflers) will stop the low rpm drone in a vette.
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Old 04-07-2012, 10:09 AM
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What gear ratio you got?
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Old 04-07-2012, 12:05 PM
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As far as the engine HP goes, the Vette motor is an early L49 or L-something that is supposed to be 350HP stock, so that is my baseline info. When i had it at dumps at axle the wheels spun in second gear easy. It's running a 700R transmission (hard shifting stage 3 build), 1500rpm torque converter and the rear end needs to go for sure, a 3.08. I'm trying to break it!!

I talked to Edel techs on phone and they told me a 600 cfm would take me to around max 400 HP. I have a 650 cfm as well but swapped down because it ran worse.

I'm tuned at about 12 degrees advanced at 900 rpm. I don't check any other timing. I don't know about the headers other than they are some beefy mothers glazed. The intake is a Holly Street Dominator. It's tuned well and there are no vacuum leaks.

I think I'm the victim of a crappy exhaust shop nearby for most part. Thanks for all the tips. DOubleVision, I'm getting my sawzall ready!!
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Old 04-07-2012, 12:54 PM
How fast is fast enough?
 
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You're not the victim of a crappy exhaust, you're the victim of a crappy engine. You're probably at 325hp with everything dialed in, since its not your probably at about 290hp, maybe less.
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Old 04-07-2012, 01:17 PM
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you work a Ron's Muffler shop by chance?
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Old 04-07-2012, 01:43 PM
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Got a casting number for the heads?
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Old 04-07-2012, 04:22 PM
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Whacked the pipes, dump and burn 'em

Hey, all you tuning aficionados. I went with my hunch and DoubleVision's vote to whack the pipes and dump at the axle. Left my tire treads on the street and it ran like a raped ape. Plugs were like brand new again after previously somewhat black on a few. Freeway, shot like a rocket without hesitation. Rumbles mean, rides like hell.
Thanks for the suggestions even though most of you were wrong this time.
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Old 04-07-2012, 05:18 PM
How fast is fast enough?
 
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Smh...
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Old 04-08-2012, 06:28 PM
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350 choked exhaust system, part II

Hey,
I posted a few days ago about my SBC 350 built for about 400HP that was exhibiting bad exhaust setup running conditions. Solution was to cut the rear portion and have it dump at axle. Did waaaay better, cleaned the plugs and ran far better off line and on freeway.However, I went to measure the interior diameter of that rear exhaust portion and found it was 2 inches, not 2.25. So that got me wondering about the length from the collectors to the Flowmasters, which turned out to be 2.25 interior diameter, not 2.5 inches. My exhaust shop knew my engine specs and said they had it under control. Apparently, they just wanted to sell the pipes they had available! Anyway, should I go back and have a 2.75-inch inner diameter from the collectors back to the Flows or maybe even 3.0? It runs pretty good now but it's making me wonder if I could rip 'em even more and get more umph ... Thanks.
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Old 04-08-2012, 07:45 PM
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I'm glad you think you got it all figured out.
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Old 04-08-2012, 09:49 PM
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You have two threads covering the same issue on the same car, read the board guidelines about double posting in multiple forums, it makes sense the same holds true for multiple posts in the same forum, please stay with one thread per issue. When you have a reply or post something, it bumps you to the top of the forum. I've merged them together here.
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Old 04-08-2012, 10:17 PM
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2.25 exhaust on my SBC ??

So my exhaust runs 2.25 inches from the collectors to the Flows. It runs much better now chopped to dump at axle. Should I go bigger though, 2.5 to 3.0 before Flows?
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Old 04-09-2012, 08:26 AM
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Summit & Jegs have these tapered pipes that are 8in. long, their 3 in. down to 2 1/2.Weld on another 4 inches of 3 in. pipe that will connect to the collector.That will make them 12 in. long, then connect the 2 1/2 in. end to your exhaust pipes.This adds to your low end torque & the exhaust gases don't bump into such a sharp angle at the collector.Sorta acts like a expansion chamber on a two stroke engine.In other words it will pull the fuel/air charge into the cylinder faster.I don't think they make them going down to 2 1/4 but you might take a look.And a cross over wouldn't hurt either,as close to the collectors as poss-able,and about 1/2 inch smaller id than your exhaust pipes.It works rather well....
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