Been driving for a week. Running nicely. But. But. I feel like it should accelerate harder than it does. Not from a stop mind you, but while under way. Just to remind all it's a 92 Safari with sbc 350, edelbrock intake and 600 carb, dual exhaust, mild cam, 700r4, open diff. Could the stock rear diff be holding it back? The gearing in the trans? Honestly it doesn't feel like it down shifts out of OD and I've tried shifting myself, but the shift action is vague. Hard to tell if I'm getting out of OD. Ideas?
How is the tv cable set up? And what is the trans out of?
Oh, and how are you dealing with torque converter lock up?
Even if the TV cable needs a bit of fiddling, the stock diff is probably holding it back. Lunchbox lockers for 10 bolts are pretty cheap, and some slightly steeper rear gears could help acceleration from a roll by keeping RPMs closer to the meat of the torque curve.
Yah, what gear is in your diff? Prolly a tall (numerically low) gear in the diff, biased more towards fuel economy than acceleration? Re the trans, kickdown should be quite noticeable.
Vigo
UltimaDork
9/23/17 7:58 a.m.
So, you don't have a tach? I agree that shifting from 4-3 should be pretty noticeable!
Also, if those cylinder heads are circa 1992 as well you'd probably be shocked how little power you might be making. Engine Masters recently dyno'd a similar setup in good tune and iirc it was barely making 240chp.
Anyway, with stockish gearing and tire height I calculated that you should probably be taking 2nd to almost 70mph WOT so if you're not 'screaming' in the 50-70mph range you didn't downshift enough.
A malaise era 350 with a small 4 barrel will probably be about as quick as the injected 4.3 was, as long as it's tuned well.
That van should have a 3:55 or 3:73, probably. 3:08 would be pretty doggy.
I forget what year your 350 is and what heads it has, but there were many years there that the most hp you could get from the factory in a 350 was 180hp. The 4.3L that came out could have been the 150, 165, or 180 hp version. So its possible you didn't add much HP.
Can you refresh me on the 350 specs?
curtis73 said:
I forget what year your 350 is and what heads it has, but there were many years there that the most hp you could get from the factory in a 350 was 180hp. The 4.3L that came out could have been the 150, 165, or 180 hp version. So its possible you didn't add much HP.
Can you refresh me on the 350 specs?
The engine in it now was a crate motor and not the original from the 75 Stingray. I researched the numbers and it was made in Mexico as a generic replacement for almost any sbc from the 70s to the mid nineties IIRC. I dynoed it at one time while still in the Vette and it showed 185 at the rear wheels. We rebuilt it before putting it in the van including a mildly aggressive cam. It has truck exhaust headers, MagnaFlow high flow cats, and dual exhaust exiting in front of the rear tires on each side. The trans is the 700r4 that was in the van originally as is the diff. The TV cable is set correctly.
Dusterbd13 said:
Oh, and how are you dealing with torque converter lock up?
Um, not sure. Does the 700r4 have electric lock up? Guess I should look into that.
Yes, it automatically locks up.
If tv geometry and setup are right, and it still doesnt kick down, i dunno.
Are you getting all 3 shifts?
Sounds like the general replacement crate motor. There were a few different HP ratings over the years, but 200 was a pretty common one.
Upping the cam unfortunately probably added little or no HP and tanked the torque. The generic heads were likely the L83 heads - the 624 castings. Large chambers, dished pistons, 8.5:1 compression, small ports, little or no quench. More cam probably made things worse, hate to say.
Have you done a cranking compression test?
curtis73 said:
Sounds like the general replacement crate motor. There were a few different HP ratings over the years, but 200 was a pretty common one.
Upping the cam unfortunately probably added little or no HP and tanked the torque. The generic heads were likely the L83 heads - the 624 castings. Large chambers, dished pistons, 8.5:1 compression, small ports, little or no quench. More cam probably made things worse, hate to say.
Have you done a cranking compression test?
What you describe sounds like my situation. IIRC that numbers sounds familiar for the heads. Have not done a cranking compression test. What would this show me?
Dusterbd13 said:
Yes, it automatically locks up.
If tv geometry and setup are right, and it still doesnt kick down, i dunno.
Are you getting all 3 shifts?
Could the kick down just be real soft. Yes it shifts up through all the gears.
On my truck the kickdown is pretty noticeable. Do you have a tachometer? Additionally I know the servo does something with shift feel and kickdown. The big question though is in transmission related or horsepower related?
In reply to Dusterbd13 :
No tach. I really should add one.
Going from a Vette to a van, that sounds like something I would do (insert thumbs up).
Unrelated automotive wise but I just watched a guy on tv named Rob Van Dam.
A compression test won't show you the magic answer but it may point in the right direction. Those crate motors should have a cranking compression of about 180-190 psi. The small cam (suited for lower RPMs) would be more effective at trapping more air in the cylinder at cranking speeds. Its this dynamic that makes higher compression a necessity when using larger cams.
So if you put a cam that is too large for your compression, it would show up as low cranking pressure. Its nearly impossible to guess what your combo would make, but if you are getting less than about 160 psi with that setup, its a clear case of big cam, small compression.
Its all a dance. (and forgive me if you already know this). Airflow in and out of an engine is a balance of flow, velocity, and time. Big cams work better at higher RPMs because airflow isn't proportional to RPM. Airflow in the ports peaks at about 300 mph. For small ports, that means it maxes out its speed at a lower RPM. So you need a cam that is smaller to trap more air inside the cylinders which makes a lot of low end grunt but wheezes out at higher RPMs. If you add port size, that peak velocity will happen at a higher RPM. The secret to bigger cams is that the air is still flowing at about 300 mph but the pistons are moving much faster. If you think of the airflow as a rubber band getting pulled through the valves, keeping the valve open a little after the piston reaches the bottom means that inertia will keep flowing air in even after the piston starts to rise. Therefore longer duration cams are suited for higher RPMs because a big cam at lower RPMs holds the valve open too long and pushes that intake air backwards. That's why big cams have choppy idle and poor vacuum.
So, adding cam without adding compression and flow means you have removed its ability to trap charge in the cylinders at low RPM, and not added any additional flow to match the high RPM sweet spot of the cam.
To expand on what Curtis is saying, the ideal for making good power with a wide powerband is heads that are worked over carefully to get as much flow as possible without needing giant ports. The smaller ports help at lower RPM, but the better they're done, the more / faster they can flow, so they'll work better at high RPM.
For the cam in that combo, you want not too much duration (to keep the idle reasonable and good low end power) but lots of lift. As much (and as fast) as you can get and as much as the heads can take advantage of. A cheater lobe that dwells at peak lift for a few degrees over the top can help too. Higher ratio rockers can help with getting more lift and faster lift rates.
Done right, you won't quite make as much power as a radical combo that barely idles, but you can get surprisingly close. And it'll have a heck of a lot more power everywhere that's not the top of the rev range. Kinda like LS motor theory. Good but not huge heads, mild cam but lots of power.
Curtis is correct on the hp. The OP might even have a bit less hp now. But that's peak so it's almost irrelevant. Almost any 350 will have a lot more meat under the curve than the injected 4.3. If both are operating at the rpm where the peak hp is rated, then the 4.3 will be just as fast or faster. In the real world we are seldom at that particular rpm which is why the 350 "should" leave the 4.3 for dead.
While its true that its "easy to make HP with a SBC," that doesn't mean what most people think it means. Most SBCs are very well engineered for what they do. They make 180 hp because of the large chambers, dished pistons, tiny cams, small head ports, and small exhaust ports. So there isn't any bolt-on that is the magic restriction. Most SBCs require a complete re-do to get decent numbers.
Its likely that this crate motor has 11cc or 17cc dishes in the pistons, they're installed a little bit "in the hole" and the heads have 76cc chambers. That means about 8.5:1. The 624 heads flow terribly. So its all wonderfully matched for 180 hp. Find some 64cc chamber heads and do a little port work. The obvious answer is Vortec heads and get the matching intake. That will bump you up to 9:1 or so and get the increased flow to match. One of my standard SBC recipes is Vortec heads with flat-tops for 9.5:1 with a Melling 22221 grind cam (a marine cam with 197/206 duration) for roller lifters or their 22121 for flat tappet. (202/207 duration.) Use retainers on the heads that will allow .550" lift and use 1.7:1 rockers. That is a drop-dead reliable 275-290 hp assembly. Smooth idle, 87 octane. I would imagine that combo with your dished pistons should still be in the 260 hp area.
So whatchyall are sayin is I may need to rethink what I've done. Maybe start over? Regardless it's running and driving and will be put up for the winter.
Thanks for the input. Makes a lot of sense.
Could it be too that you might be confusing power with throttle response? You've got a carb now and most carbs (unless they're a tiny little 1 barrel) don't offer anywhere near the throttle response of an injected engine like you had.
Well, yes and no CJ. It depends on from what to what. His 4.3L was likely a TBI with notoriously awful throttle response, and if the 350 has a Qjet, it is still better throttle response than most EFI out there. From his initial description it sounds like what he was talking about was foot to the floor.
vwcorvette: I would look at maybe a head swap. There are old-school heads out there that will do better than the 624s, but the real bang-for-buck comes from Vortecs. The problem is that the bucks start to get spent on the Vortec-specific carb intake you need to go with it. But, if you scour around you'll find one for under $100 pretty easily. Look to the marine world. Hundreds of thousands of Vortecs were put in boats with a carb and they used a copy of the ZZ4 intake. I picked up an entire marine engine - pan to carb and with manifolds - for $50 from a Marina. It was a warranty replacement. I scavenged the carb and intake for a project. The heads were junk (cracked) but the shortblock was pristine so I sold it for $250.
