Thread: Can I tune out this bog?

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Hi All,

With my new motor, I've been running into a problem bogging off the starting line maybe a third of the time. Cam comes in hard about 4000-4500, converter flashes to 4000 but I can't footbrake it much more than 3200.

When the trans is cool (160-170*) I rarely have a problem but hot lapping a couple rounds trans will get up to 180-190 and I'll start bogging off the line (and usually don't make it to the next round after that ).

I know the best solution is to get a looser converter but I'm wondering if I can tune it out with the carb. I was thinking a bigger pump shot or nozzles might help(?)

Here's the specs:

427++ aluminum head 10.6:1
.630/.630 255/264 @ .050 110LSA 104ICL
Single plane Holley strip dominator
850 Holley dp race (annular) carb (30cc pump, 6.5" power valve)
TH400 trans 4.56:1 gear 3340lb car loaded

Do I have a shot of tuning the bog out or will I just need to deal with the tq converter?

Thanks,

Chris

After a bit of investigating, I've found secondary pump is 50cc with a 30cc primary pump. The secondary squirter is a #31.

I think I will start experimenting with the squirter size to see if I can minimize the bogging... I'm not sure what range to try but I was thinking of jumping up to maybe 39-41 on the top end with a hollow screw...

Anyone have an opinion on what range I should try? I'm planning to go up until I see black smoke, then back off a couple sizes but it would help if I can narrow down the likely range rather than buying a huge set of squirters...

thanks again,

Chris

Skids, my gut instinct tells me you ought to fix the tranny problem instead of tuning your carb to cover it up. How will that make it run when your trans is cool? Do you have a good cooler installed? How is it bogging, do you mean it gets "spongy" off the line and your TC wont grab? (I am not an avid racer)

65cayne, thanks for the reply...

I agree, the best solution is to swap the converter out for something with a higher stall. I believe the reason it bogs more when hot is things get tighter when the fluid is hot and I can't get the revs up quite as high while keeping the car from creeping forward.

The bog acts like a lean out when the secondaries kick open and with the tighter trans the effective load on the engine is higher so the main circuit doesn't have time to kick in. I'll mash the gas and jump off the line only to fall flat for a second (okay 0.1 seconds) before the mains kick in and it's off to the races. I guess in short you could say the TC grabs too much and the motor can't keep up because it's not quite into it's power band.

Since it only happens when hot and feels like a lean out, my reasoning is I might be able to fill the "hole" in the fuel curve with more gas in the pump shot or something along those lines.

As a practical matter, if I can solve the problem with $30 and a screwdriver in a couple hours that's better than the >$600 and at least a day to swap the converter at a shop with the required tools and expertise.

A trans cooler might help also, but it will probably need a fan since it won't be getting much air driving to the staging lanes at 15mph.

I'll probably get a new converter at the end of the season if the evening boss will approve the funds

-Chris

Hi Denny,

Manifold vac is low (~5") at idle with 6.5" power valve. I just got a longer hose for my gauge today and was going to see what the cruise and mild acceleration vac are tomorrow. I've heard a couple different stories... the holley performance tuning book I'm reading says to pick power valve 1-2" less than vacuum under normal/mild acceleration, but I've read elsewhere to pick it below idle vac...

Is it not true a lean bog usually means power valve opens too late, or pump shot is too small?

Erik suggested I go to a 3.5 valve based on my low idle vac (thanks Erik!) but now I'm not sure what to do... maybe it will be clearer when I see vac under "normal" driving conditions.

Do you think it's worthwhile to fiddle with the pump shot for this problem?

I still have the stock nozzles for this carb even though the jets are 4 sizes bigger (and usually, you jet down about 3 sizes from sea level to Denver)

Thanks,

Chris

Yeah, I'm pretty much breaking all the rules running a DP with a 3300# car and an automatic... And yes it can be done but it ain't so easy as you pointed out and hence some of the problems I'm having now.

I was running a 750dp last year with a "tamer" cam (.540, ~240) and smaller motor (and an extra 100lbs), never had a bog but was running the same converter which was a better match to the cam. This is a race car that gets street driven so I can't expect it will run like a daily driver so I make some compromises. Thanks for the Holley tech tip on the power valve selection... It seems you all agree (great minds think alike!).

I think the risk I have now with running the 6.5 is I can be fouling plugs faster but dropping the power valve shouldn't cure the bogging problem(?)

Yes bog could also be too rich or pump timing. I'm not getting smoke out the exhaust so I don't think it's too rich. Pump does begin squirt on each primary and secondary as the plates begin to open... secondary opening is delayed quite a bit (maybe 30*?) from the primaries.

I think while staging on the TC at a bit over 3200rpm, the secondaries are probably not even cracked till I take off. Or maybe they are cracked when I don't get the bog(?)

As I said, I think the root is that I sometimes can't get the revs high enough for the cam and I'm trying to find if I can compensate to a degree with the carb.

I'd just like to know if I'm wasting my time with the carb or if I need to take more steps to cool the trans and/or get a new converter...

Thanks again for all the advice...

Chris

Originally Posted by skids72

Yeah, I'm pretty much breaking all the rules running a DP with a 3300# car and an automatic... And yes it can be done but it ain't so easy as you pointed out and hence some of the problems I'm having now.

I was running a 750dp last year with a "tamer" cam (.540, ~240) and smaller motor (and an extra 100lbs), never had a bog but was running the same converter which was a better match to the cam. This is a race car that gets street driven so I can't expect it will run like a daily driver so I make some compromises. Thanks for the Holley tech tip on the power valve selection... It seems you all agree (great minds think alike!).

I think the risk I have now with running the 6.5 is I can be fouling plugs faster but dropping the power valve shouldn't cure the bogging problem(?)

Yes bog could also be too rich or pump timing. I'm not getting smoke out the exhaust so I don't think it's too rich. Pump does begin squirt on each primary and secondary as the plates begin to open... secondary opening is delayed quite a bit (maybe 30*?) from the primaries.

I think while staging on the TC at a bit over 3200rpm, the secondaries are probably not even cracked till I take off. Or maybe they are cracked when I don't get the bog(?)

As I said, I think the root is that I sometimes can't get the revs high enough for the cam and I'm trying to find if I can compensate to a degree with the carb.

I'd just like to know if I'm wasting my time with the carb or if I need to take more steps to cool the trans and/or get a new converter...

Thanks again for all the advice...

Chris

Chris,this may be way off but how much total timing are you running?
At what rpm is it out by?

Granted, you're running a pretty big cam, but it the engine vacuum should be a little higher imo.

Is your vacuum gauge connected to full-time manifold vacuum?

Or to ported vacuum which would give you a low reading at a higher idle speed (and zero vacuum when idling 500 rpm or so)?

Originally Posted by C9x

Granted, you're running a pretty big cam, but it the engine vacuum should be a little higher imo.

Is your vacuum gauge connected to full-time manifold vacuum?

Or to ported vacuum which would give you a low reading at a higher idle speed (and zero vacuum when idling 500 rpm or so)?

a cam that has 255 degree's duration at .050 won't even begin to idle at 500 rpm's.

i have run cam s this big or bigger for many years and have had about 3 inchs of vac. i have all ways plug the power valves and go up about 10 jet sizes . and i run 10 on the crank .26 in the distributor all in below 3000 . have set up many cars this way it works just fine. for me here is some thing s i will add make sure you do not go past the idle transfer slots when you turn up the idle if this happens you will loose your ilde circuit you can open the back barrels to help this. and if you play with the squirters fined the holley tube discharge nozzel s .i have played with the cam ramps to for pump shot this i would do before the disharge nozzels if you are running a MSD dist the way to go is the red bushing for the cure and one lite sliver and one blue curve spring

Thanks for all the replies...

Timing I adjust depending on the fuel... when running 100 octane I idle at 14* total 35* all in at 2500, this is for racing and I would probably turn up closer to 40* with 110 but I haven't run straight 110 yet. When running 91 octane, 11* initial 32* total. I added a timing controller (knob) to my MSD system so I can adjust the timing easily since I am running different fuels at different times, also I may reduce timing a few degrees at startup to turn over easier. I'm on the black bushing in the dist with one heavy and one light silver spring...

No vac advance (which I regret now because it would run cooler on the street and probably get better mileage)... As I told Erik before, I'm idling (very lumpy) at 800 but will try to turn it up to 1100 and see how the vac changes. I've also heard the suggestion to drill small holes in the butterflies so the idle circuit is working at higher revs, but I don't think this cam is so aggressive I need to go there yet.

Pat, I'm running the straight squirters #31 right now, are you suggesting I go to the kind with the tubes sticking out? Also I have the big annular straight boosters instead of the small downleg variety. I was thinking the tube nozzles might cause better vaporization of the fuel if it's shot into the boosters instead of down the hole like the other squirters(?)

Also, from the symptoms, do you think the pump cam needs to be sooner or bigger or both? I think I have the brown cam in the #1 position on the secondaries (but I'll have to double check that)...

I have the bushings and springs you mention for the ignition curve... that should give more advance +28* in just over 3000... I'm worried about how this will run on 91 octane because that would have to put my idle timing only at 4* to keep me safely out of detonation?

Thanks for all of the advice all of you... this board is great!

Chris

Okay, just got back from a spin...

Vac at 1100 rpm in 1st gear reads right at 4"... cruise varies between 7-11+" depending on speed and gear and "normal" acceleration is around 6". Perhaps with the right power valve and some fine tuning of the idle circuit I could bring up the idle vacuum a little so looks like I'll need about 2.5" power valve (this is the smallest they make) or a 3.5" as a number of you suggested...

When I open up the throttle vacuum goes to 2-3" depending on the conditions so I would think 3.5" valve would be better (if not 4.5"?)... I don't want to go too low for fear of risking a high speed lean out in the 1/4 mile(?)

Since I do put down a lot of street miles (drive 30 miles each way to the strip and back), I'd like to not plug the power valves if at all possible.

Pump cams are brown in #2 mounting position on the secondary and what looks like "tan" in #2 position on primary (although I don't know of a "tan" holley pump cam? Maybe it's orange or dirty white?)

Also... I took another look at the boosters and if I go with tube nozzles it would actually squirt at the side of the booster since they are mounted much higher....

Thanks for all your help guys...

Chris

BTW: since this is high altitude it is not unusual to get 1-2" less vacuum for a particular application than you would near sea level

My Holley carburetor handbook states that discharge nozzles with tubes will better direct the flow and that's how I'd go.

that is why i said plug it.. small 2.5 may work but i have plug them and go up on the jets . no i would not drill holes in the butterfly .just open the back barrels up there is a small set screw on the end of the rear shaft to open them up and yes i am taking about tube nozzles you may not have to use bigger nozzles but go down on size if you run the tube type. on the MSD i think you could go two the sliver bushing? one size bigger up from the red will let you run more on the crank

I'll try the silver bushing in the dist but that still puts me at 8* on the crank when I'm running the pump gas... Are you saying I'm running too retarded and this contributes to the bog? I've dialed the timing in for the various fuels I use based on the heat pattern I see on the ground strap of the plugs... when running 100 octane (what I usually run at the races) I'm running 35-36* total timing.... come to think of it though sometimes I'll start the day with a 75-25 or so mix of 100 and 91 and run 35* advance, but later in the day I add straight 100 and leave the advance where it's at. Would a few points (say from effectively 96 to 99) octane at the same spark lead be playing a role in this?

I had the tube type nozzles on my old 750 and they squirt right into the center of the downleg boosters. The boosters on this 0-9380 carb are the large annular variety and sit up so much higher in the venturis the tubes would be aimed toward the bottom of the booster so I'm not so sure the tube nozzles will help as much in this case(?)... Here's a pic: