[Rich-L79]

Some of you may recall my earlier thread concerning bolting a Performer RPM on my stock-ish SB400 and wondering if it would be too much manifold:
http://www.chevelles.com/forums/showthread.php?t=109202

Well, after lots of too-cold-to-work-in-the-garage weather and problems getting the right fuel line, I was finally able to get it running tonight. Outside it's freezing rain with a possibility of snow so I didn't get to drive it yet, but at least it's running again. No fluid leaks or vacuum leaks which is a nice side benefit. I still have to adjust the choke rod (won't come off medium fast idle by itself), determine if manifold or ported vacuum are best (using manifold now, had it on ported before) and get the timing checked and if needed reset (seems fine by ear).

The project started out to just retune the carb (thanks Mr. T!) and ended up with a manifold swap too. Since the manifold was gotten at a bargain price this has been a nice project in the end. I also ended up with a braided steel fuel line along the way which provides better peace of mind. It now idles like a dream, even in gear, something it certainly wouldn't do before. It ran great off-idle, but just wouldn't idle smoothly at a reasonably low idle speed setting.

Anyway, what I did notice about the manifold swap has me confused. I know it's hard to tell while it is sitting as opposed to driving, but the throttle response is incredibly snappy now. I would have thought a larger manifold would have softened the throttle response.

Also, it SOUNDS different. The muffler sound is still the same, but the engine seems to have a bit more of a rip to it at all speeds. The exhaust note appears more smooth and mellow, with slightly less cackle but noticeably more snort (if that makes any sense). Previously, aside from the exhaust sound the engine noise was predominently the sound of the fan moving air and the Q-Jet making it's patented wonderful sucking sound. Now it sounds more mechanical and the air moving sounds are much more subdued. Does this make any sense with only an intake manifold change?

It also seems to have more top end which makes some sense, but I really thought the small heads would still limit the top end but the old stock manifold must have been holding it back too.

Regardless, I like the results thus far. I can't wait to drive it and see if improvement is found there too.

Maybe I should bolt on some bigger heads some day.....

 

[Guest]

I figure she is just breathing better is all!

pdq67

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[70_chevelle]

The additional timing is most likely why it sounds different and is snappier.

Lee

 

[Slowpoke70]

I had a similar experience switching from a mid-70s Camaro Z28 manifold (EGR intake, had a huge hole in the plenum divider, some kind of EGR bar went through it originally, I guess) to the Holley 300-36 manifold I'm using now.

Throttle response is much better now, it will actually idle below 900rpm in gear (other intake just wouldn't) and the sound is noticibly different.

 

[bulb122]

I'll second the thought that the additional idle timing is what's causing it to run better at idle, and act more responsive.

About the heads....I don't know what you have, but... I had some 1.70/1.50 307 heads on my 350. It ran pretty good, I really liked it, it was peppy enough. It ran a best of 14.1 @100mph. Then I swapped heads for some bone stock 2.20/1.60 castings, and WOW. 13.54 @104 mph. It just felt sooooo much better....I wish I would went all the way for some aftermarkte heads, or at least tried some porting work on the 2.02's. I bet they are STILL the limiting factor...

 

[Cameano]

On my old '69 Nova, I swapped out a Holley 300-38 for a Torquer II, and it brought the TEMP problem under control. With the Holley, I'd get a temp creep, up to 210 in traffic. Swapped intake only, temp was rock solid at 180. Odd, but true.

 

[Slowpoke70]

On my old '69 Nova, I swapped out a Holley 300-38 for a Torquer II, and it brought the TEMP problem under control. With the Holley, I'd get a temp creep, up to 210 in traffic. Swapped intake only, temp was rock solid at 180. Odd, but true.

Funny, I experience the same thing going from the GM to the Holley intake. Everything else was all the same. I thought it was just coincidence and that I had dislodged something in the upper hose. Maybe it was the intake.

 

[marooned]

Hey Rich,

Welcome to my world.... It's called air flow. Your are building an air pump... you just made it better. :hurray:

 

[Rich-L79]

I just wish it'd warm up a bit outside so I can take it for a test drive! I hope the low end torque hasn't been affected too much as I still need this thing to haul the Chevelle. It's always run on the warm side, I wonder if having an aluminum intake in place of a cast iron intake will help that somewhat? I have the biggest radiator in it I could find along with an engine oil cooler and a trans cooler in addition to the one in the radiator. It makes me nervous when the temp gauge hits 210 or more.

Oh, and this one has the small 1.72 valve heads. Good truck heads.

This experience also makes me wonder if I should go back and work on the carb on the Chevelle a bit. It has always had below average off-idle throttle response and I've always assumed this is a combination of the cam and a huge intake manifold given the size of the engine. Once it hits about 3000 RPMs you'd better know what direction you want to go, but before that it's kind of lazy. Always has been.

 

[Tom Mobley]

Rich,

Isn't the engine in your Chevelle like bone-stock 327/350? the manifold is really a lazy piece, lots of runner volume, 40 year old technology, no velocity at low RPM. That manifold + 40 years of development is the Perf RPM Q-jet you have on your truck.

Those engines are just a touch lazy on the bottom. Modern cam and manifold wil help a lot, but then it wouldn't be same. Maybe after a bit you could send me the Chevelle carb, I'll give it a little tune-up too.

 

[Rich-L79]

Yep, bone stock L79 right down to a GM cam, original carb jetting/power valves and stock exhaust manifolds. The only alterations I've allowed myself have been to change the mechanical advance springs to get the advance all in by 3000 and I've installed Pertronix in place of the points. And yes, the manifold is HUGE for a 327. I guess that's why it can rip to 7000 RPM without trying too hard. Though the power drops off pretty steep after 6000, it sure sounds sweet!

 

[Tom Mobley]

I had a 69 Z/28 with the 302. it's still the only street car I've ever owned that would go to 7K anytime. Sounded like a Yamaha up there. Had that car in like '75 - '76, still remember that sound like it was yesterday.

What carb does your Chevelle have on it? Got a Holley list number?

Went a checked out your page again, those are some nice pics of a very nice car. Nice detail pics on the engine.

I wish I could find a pair of those cool 327 valve covers. I remember one time a friend had a all-chevy wrecking yard. There was a pick-up bed full of Chevy valve covers. nobody cared about them them, we fed the whole bed with the V/Cs into a shredder. Ouch.

 

[Rich-L79]

I do have a spare set of original L79 valve covers but I'm going to hang onto them.

If I recall correctly the Holley is LIST 3043 but that's just from memory. The carb was completely refurbished by Holley and other than the float levels I ran it as is right out of the box. I've probably fiddled with the idle air screws but that's about it. I have been meaning to try a softer spring in the vacuum advance ever since I recurved the distributor to get the two back in sync again but I just haven't gotten around to it.