First track run - less than exciting....

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Rat Patrol

Rat Patrol

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Basically I can sum it up like this....the Modman intake SUCKS with a Vac sec carb

Here's the beans.....

The problem only occurrs when I flat nail the pedal to the floor...a quick progressive press shows no issue.

Now Ive been tuning AFB and Thermoquad carbs for 30 years...so I know what Im doing....this issue is intake related.

Ive been struggling to get a massive lean out which occurs when you nail it for the last three weeks....jetting, air door tension , rods and springs have had little effect.

Going with a massively bigger pump shooter and stronger step up springs helped....but didnt eradicate it.

So I go to the track...nail it and the car just sits there like its waiting to fart!

Then all of a sudden it launches clean....no stumble.

So my best 60ft was a pathetic 1.9....and that was with a progressive throttle input.

The other issue is mph is down...most guys with my stick are running 111-113 mph.....I ran 109 @ 12.6 ( without the launch issue it would have been closer to 12.3)


So Ill be trying an 850 double pumper shortly....I suspect that huge Modman plenum needs more fuel that the AVS can deliver...especially at launch transition.

Either way I dont think the Modman is a very good design...
 
No bog when I nail it in 2nd or 1st gear on a roll....the car lifts its nose and flies....but ..it does lean out very slightly when I plant it in 3rd.

As I mentioned...I think the Modman plenum ( which is huge btw) needs more fuel on transition than the Carter can supply.

I mainly posted this so folk know what they're buying if they go the carb route.....

Kinda wish Id fitted a scoop and run a drag pack intake now...
 
Rat Patrol said:
No bog when I nail it in 2nd or 1st gear on a roll....it does lean out very slightly when I plant it in 3rd.

As I mentioned...I think the Modman plenum ( which is huge btw) needs more fuel on transition than the Carter can supply.

I mainly posted this so folk know what they're buying if they go the carb route.....

Kinda wish Id fitted a scoop and run a drag pack intake now...
Click to expand...

hmm... well rusty may have some answers as he is a big TQ guy
 
Rat--been there done that with my XV,Had a custom 750 carb built by biggs and had the same problem,That carb flowed 840 cfm and we couldn't get the bog out no matter what we did,Had promax (Indiana) build me a 950 and was very pleased till we hit the track and it wouldn't come off the line (Launch) Changed squiter's(up 4 sizes) and didn't take it completely away but improved it greatly and is giving us 1.50-1.60 sixty ft times on drag radials.--Steve
 
Cheers Steve...thats great confirmation on what I suspected was the issue.

Any idea n the squirter size that worked out for you?
 
Steve - Was there a mph improvement in your car when you went to the bigger carb?

Ive been wondering if more carb would help the mph as well.....
 
Right now I'm in Florida on vac. with the family will get back to you on the squirter size,Here is another thing I've learned from Hemijoejr and his set-up--He has dual 500 carbs on his 5.7 build with his eddie intake and his motor is NOT stroked and his engine swallows all that gas up--Think about it !!!!!!!-I think a lot has to do with the short runners on our intakes as do you BUT we just have to make that up at the stab of the throttle.I really can't wait to try my Drag pack tunnel ram intake on the 6.1 we are about to build and see what the longer runners do. Steve
 
Keep something in mind, I have the eddy dual quad and the 2 500cfm 4bbls. I also have a 8" 4500 stahl converter and 4.56 or 4.88 in my car. When I had the 11" 2500 stahl converter in my car I had no bog at the line but the best 60' time I had was 2.00. That was with 4.10s and 4.88s with 0 tire spin. As soon as I changed the converter I went from 2.00 60' to a PB 60' of 1.66 with the DRs barely holding on with 4.10s and an 11.57 at 115.5. The new hemi has a huge runner volume with whatever head is on it compaired to a SB. I would absolutely try to work the bog out first but what size converter are you running rat?
 
hemijoejr said:
Keep something in mind, I have the eddy dual quad and the 2 500cfm 4bbls. I also have a 8" 4500 stahl converter and 4.56 or 4.88 in my car. When I had the 11" 2500 stahl converter in my car I had no bog at the line but the best 60' time I had was 2.00. That was with 4.10s and 4.88s with 0 tire spin. As soon as I changed the converter I went from 2.00 60' to a PB 60' of 1.66 with the DRs barely holding on with 4.10s and an 11.57 at 115.5. The new hemi has a huge runner volume with whatever head is on it compaired to a SB. I would absolutely try to work the bog out first but what size converter are you running rat?
Click to expand...

thats where im coming from, im used to gen II's with mag crossrams so its as close to apples to apples as i can get.

we run the original style 770's on the crossram and there are no issues anywhere, from a 426" motor to a 605 we have built. he may be correct that little carb just isn't keeping up with the volume...
 
hemijoejr said:
Keep something in mind, I have the eddy dual quad and the 2 500cfm 4bbls. I also have a 8" 4500 stahl converter and 4.56 or 4.88 in my car. When I had the 11" 2500 stahl converter in my car I had no bog at the line but the best 60' time I had was 2.00. That was with 4.10s and 4.88s with 0 tire spin. As soon as I changed the converter I went from 2.00 60' to a PB 60' of 1.66 with the DRs barely holding on with 4.10s and an 11.57 at 115.5. The new hemi has a huge runner volume with whatever head is on it compaired to a SB. I would absolutely try to work the bog out first but what size converter are you running rat?
Click to expand...

Hi Joe...thx for chiming in.... Its a 3800 converter the lean out is far worse when I stall it up....probably because it uses up some of the pump shooter travel when you stall up.


I really appreciate the feedback guys...at least I know what Im dealing with.


Here's a thought......did anyone experiment with a divider in the intake plenum?

Dors anyone think a partial divider might help?
 
do you have a holley 850 you can throw on it, maybe with that as it is or adding a 50cc pump would fix your issue
 
Rat Patrol said:
Hi Joe...thx for chiming in.... Its a 3800 converter the lean out is far worse when I stall it up....probably because it uses up some of the pump shooter travel when you stall up.


I really appreciate the feedback guys...at least I know what Im dealing with.


Here's a thought......did anyone experiment with a divider in the intake plenum?

Dors anyone think a partial divider might help?
Click to expand...

Due to the difference in the "K" value I was curious to know what the physical size is. You can have an 11" converter stall at 3800 rpm behind a 640 CID 950hp beast and the same 11" converter will only stall at 1800 behind a stock 273. My six pack was sort of like that. I think youre issue as you know is purely vacuum signal to the carb and a good Holley should be able to be tuned through it. I have had my dealings with Carters and Eddys. There are just some things that can be done with a Holley that can't be done with a Carter/Eddy. We have one of the most modified Carter AVS carbs on my friends F.A.S.T. Dart and it was a total bear!! I mean bored out for bigger throttle plated and completely modified and reworked within class rules.
 
yeah I agree...I've always been a Carter man..but the tuneability of the Holley style has got to be a help in this issue. ..and I'm not concerned about mileage..so may as well suck it and see :)
 
I had the same issue with my carb setup, glad to see I wasn't crazy. I only had a little 525 cfm AFB on mine (carryover from the 318 that was in the car). Best carb I've ever dealt with when it was on the 318. You could fire that engine up and drive it right away without having to wait for any kind of warmup, regardless of the weather. However, I had the same issues with quick full throttle stabs making the car fall on its face. Flat to the floor would kill it, but lift just a touch and it would take off like a rocket. Quick rolls were okay as well. Messed with several jets, rods, and springs, but was never very happy. Started looking for a mechanical secondary carb without much luck. Ended up trading some leftover pistons for a Holley 650, but I'll give you a word of caution now. The Holley I got would not work on the ModMan stock. The linkage on the passenger side hit the top plate during travel. You'd have to run a spacer to give you the clearance you need, on my setup as least. Not sure if all Holleys are the same, but you might try to borrow one before you buy one to test fit.
 
Cheers Map63...I will be borrowing one and if necessary Ill run a spacer woithout the air cleaner just to see if it fixes the issue.

If is does..then at least I have one option - and will look at dual 500 Carters as another option.
 
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