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Troubleshooting performance 350

 
Zaxxin
I love my Chevy Chevette!
Posts: 4
Joined: 04/07
Posted: 04/25/07 10:09 AM

I have a 350 bored 40 over with slight dish pistons.  I have a set of 195cc runner Canfield heads with 2.055/1.60 valves, 64 cc chambers, Performer RPM intake, Comp Cams Extreme Energy 274 Solid Lift cam, roller rockers, Comp Cams Magnum hardened pushrods correct length (stock + 0.150").

I am told the engine has about 9:1 or 9.5:1 compression.  I have a 2000 stall and 3.08 gears with 26" tall tires.

My problem is that the engine does not seem to make much power at all.  I currently have single exhaust with cast iron manifolds.  I know, great engine junk exhaust.  I am saving up for the swap.  I have a Single 2.5" Dynomax muffler but the Y pipe seems to be 2.25" pipe even for a slight distance after the 2 pipes combine then an adapter to the 2.5" pipe for the rest of the exhaust length.

Would a single exhaust make a good engine perform really badly at even low RPM's?

I have changed the fuel pump, changed the distributor, changed wires, tried a edelbrock 650, holley 600DP, holley 750Vac, set the timing to everything possible, and still very low output at below 4000 rpm.

Thanks for your coments.  

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bigcam406
My first time was in a Chevy
Posts: 206
Joined: 03/07
Posted: 04/25/07 08:04 PM

15/8" headers with a 2.5" dual exhaust system will solve the problem.friend had the same problem years ago,built a performance 350 similar to yours,couldnt wait to drive it,took it out for a drive and was almost in tears.following weekend i helped him put headers and dual exhaust on the beast, and he peed his pants after taking it around the block.remember to check the plugs and jetting in the carb after you install a free flow exhaust system,as they need to be recalibrated after changing the exhaust system.what kind of car is this motor in? Grin  

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Zaxxin
I love my Chevy Chevette!
Posts: 4
Joined: 04/07
Posted: 04/26/07 09:32 AM

84 Chevy truck.  Thanks for your input on the exhaust.  Im glad to hear that the exhaust makes that much difference.  

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oldbogie
I mow my lawn and find Chevys
Posts: 1195
Joined: 08/03
Posted: 04/26/07 03:06 PM

I think you have a mismatch between components and the rear gears.

You have big port volume heads with big valves and pretty warm cam. This caused the power band to move up the RPM range. Combine that with a tight converter and the engine just feels mushy. At the other end of the RPM the exhaust is too restrictive.

The root problem is that the gas velocities inside the ports aren’t high enough under 4000 RPM.

Things you can do is to run the rear axle ratio to 3.50s or 3.90s and spin the engine faster for a given road speed. Kiss the gas mileage good-by, but that will put the power band where it needs to be. A higher stall converter will do similar things, but again efficiency goes away and more tranny oil cooling is required.

Certainly down sizing the heads to something with about 170 cc ports and 1.9/1.5 valves would help even with the cam you have. Or trade out the cam, but that leaves the big heads so it'll still be more mushy than not. Or, and I like this one, you have the right heads and cam for a 383 or 406, a stroker crank would solve the problem.

You can fix some of this with the exhaust. Small tube headers say 1-1/2 inch shorties with long 2-1/2 collectors or 1-5/8s long tubes especially a Tri-Y configuration will make the exhaust pull harder which will fake the intake and cam a lot by speeding gas flows up. Hanging with the smaller carb especially with vacuum secondaries will also help as will putting some heat on the intake air. If you don't have exhaust heat crossover on the heads and intake, putting a factory snorkel air cleaner on the thing and warming air off the exhaust tube will help atomize the fuel and you'll get a better burn. This is always a fine walk between enough heat to vaporize the fuel and enough cool to prevent detonation. The big ports and valves prevent good fuel vaporization till the RPMs build, this is another problem with an over headed, over valved, over cammed engine, slow turning engine.

If the rockers are 1.6s going back to 1.5 would help.

With the gearing you have, the exhaust pipes and muffler doesn't strike me as all that bad. Making them larger will cut flow velocity with will make the engine more sluggish. I think the real low buck trick here in the short term is headers. The next is more gears unless you want to redo the heads and cam. As it stands now the engine is overbuilt for how it can be used with 3.08 gears. I prefer stiffer gears over a real loose converter, but that makes me a chorus of 1. I never put more stall in than is necessary to keep the engine from pulling the car at a stop such that you have to really clamp the brake to keep it from creeping. A high stall converter is like driving around slipping your clutch all the time, it wastes power, reduced fuel economy, and generates lots of heat. For a street machine it's just not where I like to go.

Bogie  

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Zaxxin
I love my Chevy Chevette!
Posts: 4
Joined: 04/07
Posted: 04/26/07 03:33 PM

Thanks Bogie,

I have no problem with the engine idle or with vaccum at a stop.  I notice I can hold 1st till about 60 MPH.  The reason I suspect the exhaust is that at about 75 MPH the engine almost will not accelerate the vehicle.  30-60MPH it sinks you in the seat but 60-75 is sluggish and above 75 is almost non-existant.  Of course you have the gear change and RPM drop but I would expect the thing to do more than 75.  The speed limit is 70 around Oklahoma and its hard for me to pass someone on the Highway.  The further I push the accelerator the more engine noise I hear but no acceleration (remember I have tried 3 carb's)  The transmission is not slipping, the engine does not miss, it just doesnt accelerate.

My 74 camaro runs 12.80's with the same type engine configuration (RHS Iron Heads) but with open headers and 4.11 gears and a Hydraulic cam.

I really appreciate the input.  I bought the heads expecting to put them on the Camaro but decided to upgrade the truck.  Took off some 305 heads, pulled out the 292H cam, put on  roller rockers Performer RPM intake, 274 Solid Cam, and have almost the exact same performance.  And now I cant even chirp the tires on take off...  

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Zaxxin
I love my Chevy Chevette!
Posts: 4
Joined: 04/07
Posted: 04/26/07 03:33 PM

when I mean engine noise, im not talking about detonation.  The engine timing is set to where it does not detonate about 34 total.  

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bigcam406
My first time was in a Chevy
Posts: 206
Joined: 03/07
Posted: 04/26/07 08:15 PM

freind has same combo in a 79 malibu,308 gear,2200 bm convertor,runs 13 flat at the track with the air on.as far as the heads and cam,i think they are matched pretty well.throw some headers on the truck and let that sucka breathe!!!! Grin  

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