Bog/stumble off the line

by JF74chevelle Mon May 15, 2017 1:12 am

Here's my set up right now. 350 882 heads, unknown cam from last owner, EPS intake, 1406 600cfm edelbrock muncie 4-speed with highway 2.73s.
The problem is if I dump the clutch to launch it the motor bogs bad and almost wants to stall. I put the 1487 accelerator pump in, put the pink step up springs in to match my vacuum and I moved the linkage to the top hole the the accelerator pump lever. If I am already rolling and mash it to the floor it doesn't bog. Do I need a bigger squirter size or jet change?

by Joe73 Mon May 15, 2017 6:22 am

What rpms are you dumping it at? Too low could cause bog due to lack of torque multiplication with 2.73 gears. Also, street tires or drag radials?

by bracketchev1221 Mon May 15, 2017 6:58 am

Assuming that the rpm is up when you dump the clutch, it is not an accelerator pump issue. What transmission is in it, because a high first gear like a 2.20, is not going to like a 2.73 rear gear.

by Joe73 Mon May 15, 2017 7:10 am

Agree with Ray. 2.20 first gear ratio for a m21. M20 will be 2.52. multiply either by your gear ratio. 9 is the number your shooting for for a "fun" to drive car. But either trans will only give you a 1:1 final drive ratio so not so much fun on the highway when you go with lower gears.

by JF74chevelle Mon May 15, 2017 9:07 am

I have an m20 2.52 first gear. And I'm on BFG t/a

by Joe73 Mon May 15, 2017 9:13 am

Rpm before dump?

by JF74chevelle Mon May 15, 2017 9:41 am

Wasn't very high of an RPM I didn't really pay attention, maybe I'll try it today. What RPM should I be dumping at?

by JF74chevelle Mon May 15, 2017 9:53 am

Joe73 wrote:Rpm before dump?

The more I research the more it seems like I need to ditch the 2.73s soon if I'm going to be doing any hole shots. 3.42s with a trutrac is next in line but I have all my money saved for and engine build

by JF74chevelle Mon May 15, 2017 9:56 am

Would you guys go posi or trutrac?

by bracketchev1221 Mon May 15, 2017 10:47 am

I like the Eaton limited slip differentials. It is nothing special but it is what GM used in their rears when they had the posi-trac options. As far as dumping the clutch, I would try high rpm. Tire spin is your friend, bogging equals broken parts. Try sneaking up on it until it spins. Another thing is try a roll, get like a 20 mph roll going and push the clutch in, wind it up and let it fly.

by Joe73 Mon May 15, 2017 10:47 am

I like the Eaton posi unit. And the Yukon posi is pretty much the same thing with good reviews.

With a manual trans, holeshots are not an issue at all. You should have it up to at least 2000-2500 and then let it loose and punch it at the same time. That should work nicely.
My car came stock with 3.42 and a stick. 3.42 x 2.52 = 8.61. Pretty close to the 9 number for fun to drive. But since all muncies and TH350 are 1:1 final ratio, not much fun on the highway at all. Need an overdrive.
Your 2.73 x 2.52 = 6.8. You have to decide how much gear you could live with especially if its a daily driver.
My current setup is 4.10 x 2.52 = 10.3. Very fun to drive and lauch. Under normal driving I take off from a light in 4th gear. But forget about getting on the highway.

by JF74chevelle Mon May 15, 2017 11:06 am

Joe73 wrote:I like the Eaton posi unit. And the Yukon posi is pretty much the same thing with good reviews.

With a manual trans, holeshots are not an issue at all. You should have it up to at least 2000-2500 and then let it loose and punch it at the same time. That should work nicely.
My car came stock with 3.42 and a stick. 3.42 x 2.52 = 8.61. Pretty close to the 9 number for fun to drive. But since all muncies and TH350 are 1:1 final ratio, not much fun on the highway at all. Need an overdrive.
Your 2.73 x 2.52 = 6.8. You have to decide how much gear you could live with especially if its a daily driver.
My current setup is 4.10 x 2.52 = 10.3. Very fun to drive and lauch. Under normal driving I take off from a light in 4th gear. But forget about getting on the highway.

I'd really like to go with a 3.73 rear gear but I think it would take its toll on the motor while cruising freeway

by Joe73 Mon May 15, 2017 11:56 am

Just think it through. You dont want to change ratios. Easier to change to a 5 speed (or a 200r4) and have even better performance. With either of those trannies you would get an even lower 1 st gear for even better launches and holeshots and have an overdrive to the highway.
I always recommend a 3.73 gear with a 200r4. To me, best all around setup.

by REVINKEVIN / Mr. 600+HP Mon May 15, 2017 12:04 pm

Hey. I had that transmission in my 1970 Camaro, warm 350, and .373's. You should stay away from that if you ever get on the highway. My car would be at 3600 rpm at 75 mph. I know you have been doing alot on this project so the funds are tight. Instead of spending $800 on a Tru-Trac and new gears, install, and whatever else comes along I purchased a Tremec TKO 500 and sold my Muncie. The first gear in the TKO is like a 3.35 or so. It could get your car moving with that rear gear you have now. 75mph with your exact combo and this trans would be around 1500 or so rpm. And, it's rated for 500lbs of torque. Food for thought.

by JF74chevelle Mon May 15, 2017 12:14 pm

I just swapped to a four speed Muncie and I'd never go back to an auto or to anyone gears, a four speed is perfect for this car. Built with a NASCAR stock car theme so that's why I like the four speed so much. And you can't beat the whine of the gears

by Joe73 Mon May 15, 2017 12:19 pm

I'm with Kevin. Look at these numbers 3.35 x 2.73 = 9.1. But the Tremec's 5th gear ratios are in the .64 to .82 range. So in 5th gear on hills I bet it would bog and would need up and down shifting to keep the revs up.

But if it were a diesel, then we would have a combo. LOL
My stock diesel makes 660 ft lbs of torque at only 1600 rpm. I tow a 4,000 pound boat in the hilly hudson valley of new york and the truck never comes out of 6th gear.

by Joe73 Mon May 15, 2017 12:20 pm

That muncie shouldnt be whining!! Unless you have a set of M22 straight cut gears in it.

by JF74chevelle Mon May 15, 2017 1:42 pm

Joe73 wrote:That muncie shouldnt be whining!! Unless you have a set of M22 straight cut gears in it.

It has straighter cut gears when I rebuilt it, doesn't whine like a rockcrusher

by Joe73 Mon May 15, 2017 2:29 pm

Were any gears replaced?

by JF74chevelle Mon May 15, 2017 2:57 pm

Joe73 wrote:Were any gears replaced?

All except reverse

by JF74chevelle Mon May 15, 2017 3:31 pm

Joe73 wrote:Were any gears replaced?

I reality I don't even know if he put 2.52 first back in it when it was rebuilt

by Joe73 Mon May 15, 2017 4:20 pm

If all gears were replaced with NEW, including the cluster, then there should not be any whining. If there is any NEW gear meshing with an old gear you will get whining. The whining will decrease overtime to only slightly noticeable. Its because of different wear patterns.

by JF74chevelle Mon May 15, 2017 5:09 pm

I think that is what it is then, because the noise has gone down the more miles I put on it

by JF74chevelle Mon May 15, 2017 6:42 pm

Anyways, do you guys believe that the boggling is due to the 2.73s in the back with then requires a lot of RPM to get it holeshoted smoothly, even just driving normally letting off the clutch smooth it tries to bog.

by Joe73 Mon May 15, 2017 6:54 pm

More rpms will give you a good burnout. Is your carb mechanical or vacuum secondaries? What is your total timing, not what your running at idle?

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