I have a 2 carb intake on my 270 with a small port head. I am looking for better top end performance.

I had larger valves installed, some porting and smoothed transition / alignment rings when putting the big port intake on the small port head.

I get good low end response and it pulls good until about 4000 rpm when it starts gasping for air (or at least that is what I think the problem is)

I put smaller venturies in the Zenith carbs at the outset because I figured it would be too much carburation with the two carbs on a small port head.

I am going to start by putting in the next size larger venturies.

At some point, I'd like to put on a 3rd carburetor and I'd like to find a 3 carb Nicson for this (all other speed / dress up items are also Nicson and I'd like them to match).

Or - should I stick with the two carb setup and switch to a big port head?

Questions question - anyone with experience with GMC sixes with these kinds of issues?

Compression is 9:1 with forged pistons, 1.84 intakes 1.6 exhausts with a Delta reground cam (profile 254S), dual cast exhaust manifolds with plenty of breathing through exhaust system. I am still running 6V so it has a pertronix with 0.6 Ohm 40kv coil running 0.045" spark gap.

Where is the cam installed (ICL)?
What's your total spark advance?
Even if Pertronix says .045", I'd narrow the gap to .030" and try it just to be sure.
Is 4,000 the limit in all gears, or only high gear?
How big are the venturis now, and what size is going next?

In my opinion you did everything correct for an excellant street engine. The use of a big port intake on a small port head gives you exactly what you really want which is the most low end torque you can get. A small port head will probably get you 4500 to 4800 rpm with some more carburation but at some point your going to give up low end driveability. The Nicson cast iron exhaust are really not that good. They come out of the head and make a direct 90 degree flat corner. I ran them once at El Mirage for the heck of it and lost 35 mph.
You don't say which small port head you have; if it's one of the last ones with the quenched area around the valve the addition of a 302 or "H" head will loose you compression and you will probably run out of intake air with only 2 single barrels. Good Luck

This would appear to be a good 270 street engine, with plenty of useful torque in the low and mid-range. For more carburetion you might try two Holley-Webers (with adapters for your manifold), which are progressive two-barrels and should work really well for you at low speed while adding some more fuel at the top end when you might need it. That would allow you to keep your present manifold and your basic setup as you have it. Tom Langdon sells these carburetors and the adapters, and he might have some other recommendations that would help your performance.

A large-port 270H or 302 head is more useful on a bored-out 302 than on a 270. Your low speed performance from the 270 would suffer, without much gain at the top end.

God's Peace to you.

d
Inliner #1450

Your setup looks just right to me, everything as it should be for an old Jimmy. My suggestion is; the 6 volt Pertronics still may not be giving you the best spark for the higher revs. Why not try 12 volts, you could isolate the ignition system for some testing. Is there a Nicson side cover over on the other side?

Tim

Thanks for the feedback -

Panic - every gear (except OD) runs out around 4100 rpm or so, it will rev higher, but I am definitely on the downhill side of the torque curve. Not sure what ICL means. Total advance . . . I cant remember exactly, I did check it and it seemed like it was around 37 or 39 degrees and all in around 2000 - 2300 rpm, it has been a few months since I checked and my memory isn't that great. Same story with the venturies - they are 2 sizes smaller than a stock 270 Zenith but I don't remember the size off the top of my head.

I did start with 0.030" gap on the plugs and then increased to 0.040 and then to 0.045 - each time I increased I got a little bit better starting / idle / acceleration so I left them at 0.045"

I have a stock head from a '57 270 engine - casting number 2194819

I have a small cap HEI that is set up for this motor and I am planning on giving it a shot in a few weeks -

I tried hard to keep this truck like you would see it in the '50s as far as speed equipment goes (including keeping the 6V) - I was hoping to see if carburetion would extend the power curve just a little more. I am running a 3.73 rear-end ratio with an A833 GM variant with 0.73 OD for forth gear. After all this work, I just want to make sure I am getting every pony I can out of it.

Tim - it has a stock lifter cover on it - I borrowed a friends Nicson side cover (chevy) to see if I could copy the style and morph it into a GMC version (making a mold for casting) - but time is always in short supply and I haven't made the progress that i would have liked by now.

That looks very nice.

Where did you come up with your MY6?

So far so good.
I'm not sure where the spark should be - part of it is how much turbulence you have and that is partially quench. Did you measure piston to head clearance @ TDC?
If your cranking compression is high (above 170?) the cam may be advanced beyond the mfg. suggestion, and retarding it 2-4° will pick up the top end in general.
Have you eliminated the obvious: are both throttles actually vertical when the linkage bottoms out?
The air filters look big enough not to be the problem.
Have you tried backing off to 7/8 throttle - is it better or worse?
How much valve spring tension?

can, you tell us the cam specs. i believe that is your limiting factor.
p.s.
nice looking engine!
tom

Pictured above are the cam specs - the valves are sbc valves and weren't quite as long as the stock valves, I shaved the bottom of the rocker arm stands to correct geometry.

Not sure if the photo comes through or not - based on what others have posted.

Ken:
I think they were asking about the cam lift for intake and exhaust, as well as the duration of those events. You indicated the profile earlier but not lift or duration.

Thanks:
Paul...aka xerxes

your link does not work, try this.

http://lh4.ggpht.com/_kSlgcyTaucw/Sdu_ni8WAsI/AAAAAAAAADM/u_J0l0kzV9Q/camspecs.jpg

looking at your cam #'s. i would say the duration should support 4500-5000. the lift may be the limiting factor. not letting enough air in. probably in combo with the head.

what does the idle sound like? maybe a video to be put on youtube? tom

I have never used the Google photo hosting before so not sure if this works from everyone elses perspective or not.

Link to cam specs

I'll try to get some video tonight - I live in the sticks so I won't be able to upload until I get to work tomorrow.