This is in regards to my '85 chevy C20 with a BBB 462. '76 Block and Intake '72 Heads - 3 angle valve job, no port work Speed Pro Hypereutectic Pistons - 9:1 Q-jet carb Stock HEI dist. Straightline Performance Cam, 220/235 @ .050", .473/.476 lift, 112 lsa, 108 lc. (4 degrees advance ground in). Cam was degreed and installed "straight up". Actual Calculated Compression is 8.98:1 Now, to my atrocious exhaust system. I'm running stock manifolds, into a stock '76 electra Y-pipe (slightly less than 2" ID), a custom "cross over" pipe to mate the passenger side outlet Y-pipe to the truck's driver's side mounted catalytic converter..... converter is a stock '76 unit. After the cat, there's a 2.25" pipe to the generic parts store muffler, then finally the 2" OD tailpipe that exits behind the driver's rear wheel. I'm still in the process of tuning in my q-jet and spark curves (total base + mechanical is currently 20 deg.), but the low end torque of the motor is great, it will incinerate the tires on the truck, but from mid-range on up, the motor feels like it is strangled. I'd swear this super-restrictive exhaust must be holding me back 75HP or more, but I've converted a 350 from a similar exhaust to headers and duals and I doubt it was more than a 15-20HP increase. What I want to know, is it possible to build any decent performance, while having the stock exhaust? I actually like the quiet ride, since this truck is my daily driver, and it also adds to the sleeper effect even more so :Brow: Anyone want to speculate how much HP and TQ I'm giving up compared to a set of long tube headers and a good dual 2.5" system? Thanks!!!
75 HP Maybe if you go with 2.5 dual, X pipe & headers, I'd say 75 increase is reasonable. Was that 350 worn out? I put 2.5 dual & headers on one in the 80's and had a hard time keeping that tire from spinning (yeah, ol peg legger).:laugh: o No:
Fix your ignition advance before you worry about the exhaust. You're losing LOTS of power from having too little advance. You need "about" 35 degrees total advance, plus another 10-15 of vacuum advance. The engine may run cooler, too. You are limited by law to a single cat, in the original location. The OEM GM cats of that era were highly restrictive pellet-type converters. I'd add headers if available to fit the chassis, a large diameter Y pipe, and a 3" aftermarket cat, which will be monolithic instead of pellet. Continue with 3" pipe into a performance muffler of your choice.
Re: 75 HP Dan - The 350 was a stock 2bbl motor with 120k on the clock at swap time, and still runs good to this day with 140k. It would incinerate the pass. rear tire with the 2.56 peg leg both before and after the headers. I'm basing motor condition mainly on power output, the silent valvetrain, and compression #'s -- 167 to 180 psi across all 8 cyls. with the motor cold.
Trying to sort out the timing has been a nagging problem. The vacuum advance unit I am using right now is from the '85 305 that I took out of the truck, because the unit on the 455 was seized. I think the unit I am using is giving me too much advance. I think an adjustable vacuum advance unit, as well as different weights/springs will be in order to get this where I need it. I need lots of vacuum advance at part throttle, since this motor has EGR installed. Trying to get the right amount of advance at part throttle, plus 34-36 degrees at WOT with all the mechanical advance in, AND stay out of detonation is tricky. I have to be very careful about detonation, since I used hyper piston in my build.
EGR is itself a TERIFFIC detonation supressor. If it pings at part throttle, I bet the EGR is blocked or broken. Stick your fingers under the EGR valve, so you can push up on the diaprhagm. The engine should run like crap when you do this. Means the exhaust passages aren't plugged. When you release the diaprhragm, the idle should smooth right out. Indicates the EGR valve seats properly. Add vacuum to the EGR: Again, engine runs like crap. Indicates the diaprhagm isn't torn. Check EGR hose at the valve for vacuum when the engine is fully warmed up: Should have 0 vacuum at idle (ported source) and as you open the throttle, you should have strong vacuum, indicating the hoses aren't block with a BB, and the thermovacuum switch is OK. Sometimes there is a delay valve in the vacuum supply, to make a gradual application of EGR. You can then verify the whole vacuum system by putting your fingers under the EGR valve, to feel the diaprhagm move when you open the throttle from idle with the engine at normal running temperature.
When I first installed the stage1 463 in my Convert. it had the original '69 exhaust manifolds and original 2 1/8" dual exhaust. Switching nothing else but Headers, mandrel bent 2.5" duals picked up 9/10th of a second in the 1/4 mile !! Thats Pretty HUGE.....
Schurkey - Thanks for you input. The EGR valve is brand new, and I've made sure that it is working, however, the timing was still too far advanced. I'm guesstimating on exactly how much vacuum advance I'm getting from the 305 vacuum canister, but it appears to be close to 30 degrees. Somehow the dial-back knob on my timing light disappeared :af: so it's tough to tell exactly what number I've got it dialed to. According to the '76 service manual, the base timing is supposed to be set at 12 BTDC, total mechanical should be 12 @ 3500, 13-18 at 4400+, and total vacuum should be 22-25 @ 14 In. Hg. I'm getting 12 degrees in before 3500, but with the incorrect vacuum advance unit, it looks that I could have been seeing advance as high as 54 degrees. That's at least 5 more degrees than the stock application, and my compression is 1.4 points higher than what it was rated stock. (7.6 vs 9.0) If I run the vacuum advance without the EGR valve hooked up, it will rattle like marbles in a can at almost any throttle position where there's enough vacuum available to operate the canister. I'm running my EGR off of ported vacuum, running through the EGR/TCC/Purge TVS that I pulled from the 305. It prevents EGR from operating until the motor is warm enough. I originally had my vacuum advance on ported vacuum as well, but due to poor idle quality, I moved it to manifold vacuum. I drove around with a vacuum gauge, and noted that - other than ported vacuum having no reading when the throttle is closed, both ported and manifold vacuum gave me the same readings, so using manifold vacuum for vacuum advance seemed like a wise choice. I suppose another solution to detonation supression would be to use a larger diameter orifice washer in the replacement EGR valve, but I don't want to cause the motor to buck/surge during part throttle operation due to too high % of EGR............ Ahh, the compromises for an emissions legal setup :blast: :laugh:
I think I'd rig an "inspection time" system and an "Other 364" system. Carbuerated engines suffer so much from the emission compromises of the 70s. Seems to me that, even if you get it exactly right, you will have a 190 HP wheezer that makes good torque in the 1200 to 1500 range and thats it.
WOW! :eek2: Now that's the kind of gains I'm looking for!! :Brow: On that note, elsewhere on the board I've found that the MT Super Scavengers fit the truck chassis... I also heard that they have a "unique" sound to them. Can anyone attest to that? Anyone have a sound clip?? Anyone going to BPG Nats have a set on their car that I could hear ? :laugh: I need to get this truck running good, a buddy of mine has a '79 K5 blazer, full time 4wd, and a 383 stroker, with edelbrock alum. heads, edelbrock pro-flow multi port EFI, and a comp cam. TH350 tranny, and 3.73's front and back, exhaust is mid length headers, and 2.5 duals thru flowmaster mufflers. In High range, it runs 15.2's at 93 MPH, but in low range, it'll click off the 1/8th mile in 9.2!! (not sure of 1/8 MPH) Anyways, he's been bugging me to "run 'em", and I can't let my Buick powered Chevy get edged out by a Chevy powered Chevy! :Brow:
Larry Hymowitz (LARRY70GS) will have his 1970 Stage1 at Salem and He is running the MT Super Scavengers. Here's a sound clip.....its one of those http://itnmedia.com/alan/3amigos.avi
