Weak part throttle performance

[travis]

Since building this new engine for my heavy weight ‘83 F150 extended cab, I haven’t been very happy with its performance, especially in part throttle stop and go traffic situations. To recap...

.030 over 351w, 10:1 compression, promaxx 180cc heads (flow 262@.550”, 266@.600”), Jones hydraulic roller (264/268@.006, 218/222@.050, .544/.544 lift, 112 lsa, 109 ica), weiand 8016 dual plane, 650 vacuum secondary Edelbrock carb, 1 5/8” long tube headers, dual 2 1/2” exhaust. 2500 stall, 3.55 gears.

I built this thing for good low end and a super strong midrange...you know, the things that a 4900 pound truck NEEDS. And I thought I had accomplished that. Throttle response is absolutely amazing right off idle, but the truck feels sluggish at part throttle, and the fuel economy is absolutely terrible.

Today I put a fresh set of plugs in it and changed the oil, and went over the tune again. A/F’s are good, plugs look good, nothing seems out of whack. One thing I hadn’t checked was the converters flash stall. I tried a couple different scenarios...and I’m seeing around 3200 rpms flash, even at 3/4 throttle.

How much more cam do I need to need to make this right?

[My427stang]

What does the timing curve look like?

[turbo camino]

Try this...

intake manifold -> thin gasket -> thin 4-hole plate -> thin gasket -> 1/2" or 1" spacer -> thick gasket -> carb

The 4-hole plate should be no thicker than 1/16 (thinner is better), with nice sharp corners on the holes, no smoothing or radiusing. Bore of the holes does not seem to matter as long as they are the same size or larger than the carb throttle bores.

As to whether the big spacer should be divided or fully open, you'll have to experiment.

[lefty o]

What does the timing curve look like?

i second this. poor part throttle response, and poor fuel economy, ignition curve would be my first thing to dig into.

[peejay]

Not just timing curve, but also vacuum advance curve.

Tee in about six-eight feet of vacuum line and stick a vacuum gauge on the windshield and see how much vacuum the can is actually getting at part throttle. Then duplicate that with a vacuum pump and see just how much timing you are getting. You might be surprised at how little vacuum you are getting at the ported vacuum and also how little advance you are getting from that amount.

[travis]

The timing is 8* initial, 32* total in at 3000 rpms, with a 12* can connected to manifold vacuum.

I think the problem is the converter. It seems to be just blowing through the converter at low speeds. Throttle response on this thing is amazingly sharp, and WOT performance is pretty good considering the weight and mild gears. This converter is a Hughes heavy duty 10.5” (iirc) 2500 rated stall

[F-BIRD'88]

The high stall converter working against the BIG BIG truck weight ,3.55 and tall truck tires is going to feel sluggish
spark timing seems sluggish.

What is the trucks 60 mph steady cruise rpm?

This engine belongs in a lil notchback Mustang with 4.30 gears

[Geoff2]

351 cubes pushing 4900 lb is never going to be a sprinter...

And 4900 lb with 3.55 gears is never going to be an economy queen....

Timing curve, as others have suggested.

Also, check the float level.

A metering rod with a richer [ thinner ] power step might also help.

Advancing the cam 4* might help marginally.

[mag2555]

Peak torque for any NA motor takes place at a Intake flow rate of 350 fps and those 180 cc port volume heads with that cid motor are making that happen at too high a rpm and providing a narrower power band then could be had.

[barnym17]

I agree with vacuum advance tuning you would be surprised at how much it actually takes.

[My427stang]

With the short advertised on that cam, I don't think it will tolerate much more of a curve unless the chamber is pretty good, if it isn't good, then you likely need more total too. I actually would like to see the cam earlier, but I think you'll have issues with 10:1 compression if you are really there

However, if it is good and a tight quench, I'd likely bring the initial up to 12, keep the same total, and leave the rate alone for now, then sneak up on maybe 2800-ish if it will tolerate it.

Is your compression calculated from actual measurements or published parts values? Also, did you get to 32 total on a dyno for peak power? Did you degree the cam? Just to make sure, your 32 and 8 is mechanical only too right?

The converter isn't helping mileage, 2500 stall with a heavy vehicle could be more than that and inefficient, but should provide torque multiplication during acceleration unless is a BAD 2500 stall converter

My gut says that thing should be pretty strong, even at that weight, seems like something is wrong. Can you check idle vacuum? Should be pretty high with 49-ish degrees of overlap and a fresh engine, might give us an indicator

[travis]

Actual measured compression is 9.97-1. 17” idle vacuum at 700 rpms. Cam was degreed in. Quench is .044”. 8 initial/32 total mechanical, then another 12* with the vacuum can in play. I’ve tried both ported and manifold vacuum and moved the timing all over the place...this is where it feels best. It runs cooler and stronger with 30-32 total (without vacuum advance), but the idle quality starts diminishing pretty badly with less than 8* initial. It will run up to 40* total initial plus mechanical without pinging, but it is lazy and runs hot. A/F is 12.7-13@WOT, 14.5-14.8@3200 rpm cruise. It starts instantly, and will idle down to 450 rpms if I want it to.

It runs strong at WOT, especially from cruising speed. It jumps from 60mph to 110+ much faster than any old farm truck should, and will blow the tires off easily from a dead stop.

I don’t think the converter is bad, but I do think it is wrong for this application. And I wonder too if the heads are too big. I do think this thing would be an awesome daily driver in a 3200-3600 pound car.

[bigmike]

What rear end gears and how tall are the tires?

[panic]

Peak torque for any NA motor takes place at a Intake flow rate of 350 fps

??????????????

[travis]

What rear end gears and how tall are the tires?

3.55’s, approximately 29” tall tires (235/75-15’s)