another M-1 vs Airgap question

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crackedback said:
I'd put a 750 DP on it and toss the vacuum stuff to the curb.

My friend had a 360 built similar with a slightly smaller cam, iron heads with a 2500 convertor and 2.76 gear. It got 20 mpg on the highway and about 15 around town.

With that gear and convertor, the engine will eat up every bit of a 750DP you can give it.
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If you are real good a tuning carbs you can likely get a 750DP to work acceptbaly on a 360 but to use all the capacity of a 750 cfm carb on a 360 you would need to be turning in excess of 7000 rpm, its simple physics. A more properly sized carb will plain simpley work better and be easier to tune in the operating range the car is going to be used.

The Street Avenger carbs are wet flow rated like Demon carbs so the 770 that has been mentioned would have a much higher rating if it was tested and rated like the older Holley carbs such as the 750DP, something on the order of 810 cfm. The 670 Street Avenger is much more an appropriate size for a medium built 360.

Carbs and cams are the one area where the bigger is better mentality slips in and its very hard to resist it. But the reality is you may pick up a .1 or .2 in the 1/4 with a 750 compared to something in the 650 range but you will definately have better low and mid range response and with the smaller carb.

If the brag factor of having a big carb is important then maybe look at a spread bore, something like a Thermoquad. This carb has tiny primaries that will provide excellent response an driveability and huge secondaries for the top end. I believe the 340's came with a Thermoquad that was rated at 800cfm. This worked very well because of those tiny primaries and the secondaries would never fully open any away.
 
Well said dgc. Then again, Cole, you can brag about going fast with less. The HP difference with a bigger carb is more likely to be only 10 HP on his mild build up. If your driving the street more than the track, I'd look at a 650/670 carb.

Install a vacuum gauge and if it doesn't bottom out when floored down the track, then you can use a larger carb. Otherwise, your good. You don't really want more than a 1 inch of vacuum when floored. Zero is best, 1 inch is max.

Food for thought.

A buddy of mine was dynoing his 360. Mello street build. 10 - 1 Edel. headed and intake. Tried a 650 carb to start. It pulled 1.5 inchs of vacuum at wide open throttle. Dyno man said it needs a bigger carb. OK, we slap on a 750, tune it in and got 10 HP more.

Big whoop! Driveabilty took a dive and was a ***** to tune out and truly never got it to perfrom better than the 650 on the street. It wasn't as snappy or fast down low. 10 HP don't show at the track. You'll lose more time by leaveing a tenth late or not hitting the hammer well or spinning tires a tad.
 
Cole,seeing as your going more towards a street/strip car,i think your setup will like the bigger carb,it sounds like your more concerned with kicking some butt then driveability or gas mileage..silly street cars:toothy10::toothy10:
 
My 340 is running low 12's turning 6200 - 6300 rpm with a 650 DP. I originally thought (and was told) this engine would need a 750 cfm carb to really wake it up, but I started the season off using the 650 DP I had in hand. A few month's into the season, I bought a 750 DP , put it on the car and it went 2 tenth's slower!!!

I switched back to the 650 and car went .2 faster!! I rejetted teh 750 and tried later in the season, with similar results!!

My advice is to use the smaller carb and try out a bigger carb when you have the funds (or opportunity to borrow someone's carb).

Bob
 
stroked 340 said:
Cole,seeing as your going more towards a street/strip car,i think your setup will like the bigger carb,it sounds like your more concerned with kicking some butt then driveability or gas mileage..silly street cars:toothy10::toothy10:
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i think we're on the right page :)
but to be safe, whats considered bad driveability for street?
i dont have much vacuum now, as i have to run the car for a little for it to heat up to stay running and sometimes i have to throw it in neutral at stop lights to give it gas for it to keep running. and my brake pedal is as hard as a rock when cold. what other negatives are we talking about
 
VE's are great along with some other ratios and computations. I'll take track results over VE every time. According to the VE, my 360 shouldn't have been able to use a 950, but it did??? Ran much better with a 950 than anything smaller and picked even more with a dominator on it. According to 100% VE it shouldn't have needed more than a 750 or so.

Vac secondary carbs are great for street driving and that's about it, IMO. To get the most out of the engine, run a DP on it. Looser vertor, more gear = DP, which is what he's putting together.

We had a BB on the dyno the other day. It made about 20 more HP from 3000-4500 and about 10 more from 4500-5800 using a DP over a vacuum sec carb. The torque curve with the VS carb was sad even with the lightest spring, the DP stomped a mudhole in it.

Some will use math, other will use what they've done in the past and worked. Pick your poison.

One thing I do agree with... it's ALL in the tune.
 
If you were getting that much difference between a vacuum secondary carb and a mechanical secondary carb then the vacuum secondary carb wasn't tuned properly. Same if it ran better with a 950 the smaller carbs were not tuned properly.

And putting a 950 cfm carb on a 360 that won't be turning more than 6000 rpm is ludicrous. Even 800 HP NASCAR Sprint Cup engines turning 9000 rpm only run an 850 cfm carb.

Cole, driveability in my book is having the ability to smoothly pull away from a stop without feathering the the throttle to get past the bog or to get the engine to accept throttle without wanting to stall. Its the ability to be cruising along at 2000 rpm and being able to give it gas and smoothly accelerate. Its the ability to have instant response to throttle changes. Driveability is not having the car smell like gas is pouring out onto the ground at anything below WOT. As much as everyone says they don't care about gas mileage, when you want to go out for an hour or two cruise and you burn a tank of gas that gets old real fast. Being able to make it 2 or 3 miles to a cruise night is not driveability.

There is no reason that you can't build a 450 HP 360 that can be driven as a daily driver year round and the best way to do it is with a carb that is properly sized for the application and something larger than 750 cfm is not it.
 
The vacuum carb ran right on and had good A/F ratios throughout the pulls. The difference was due to the delay in secondary opening that caused the hp drop. Go look at the final opening points according to holley regarding each spring, CI and rpm. It's an eye opener!

You can "tune" a 750 dp to get good mileage. This poster has 4.10 gears/3000+ stall, I doubt mileage is at the top of the wants list. A 360 w/ a 750 DP that got 20mpg on the highway... hmmmm....

About fuel pouring out of the tailpipes, buy an HP carb and learn how to tune it. Holleys, even the street carbs, aren't known for their quality air bleeds/emulsions from the factory at the low end. They are set up on the safe/richer side than anything close to exact. If you got one that's perfect for your combo out of the box, you got lucky. He's running a 3000 stall convertor which allows for a slightly different carb than a 2000 stall stock piece which ties up the engine before it's in the meat of the range.

The 850 carbs that cup cars use may start as 850 rated carbs, they don't flow that when they get tossed on the engine.

I won't argue with you Dave. You have your opinion and others have a differing that has been proven to work even though the math says it's too big.

Between the two choices the poster asked about... RUN THE 750 DP

To the OP, learn to tune the carb. Holleys out of the box aren't a bolt on piece and need plenty of fiddling with to make right for each application. An out of the box, hit it right on the mark is a 1 in a million shot. Get the right squirter, pump volume, cam, power valve, jetting, etc. on it and it will run fine, with NO BOGGING. Ignition timing plays a big role in this as well. Learn to read plugs and get/use/borrower an A/F meter that you can use to log data while driving. They are all tools that will help in the long run.

It all in the tune...
 
Cole,take it from someone who drives his car on the street and actually DOES race it quite a bit during the season..you'll be fine with the 750!!!!
 
I think he's with you and goin to a 750.
 
crackedback said:
To the OP, learn to tune the carb. Holleys out of the box aren't a bolt on piece and need plenty of fiddling with to make right for each application. An out of the box, hit it right on the mark is a 1 in a million shot. Get the right squirter, pump volume, cam, power valve, jetting, etc. on it and it will run fine, with NO BOGGING. Ignition timing plays a big role in this as well. Learn to read plugs and get/use/borrower an A/F meter that you can use to log data while driving. They are all tools that will help in the long run.

It all in the tune...
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Any aftermarket general purpose carb is going to need some level of tuning to dial it in for the specific application. Doesn't matter if its Holley, Demon or Edelbrock. Going by the manufacturers recommendations for the application will get you the closest to being ready out of the box.

If you are talented at tuning a carb you can get most any carb to run decently. But for a neophyte putting their first engine together quick and simple is much better. Having to touch every circuit in the carb to get it to run right is a good way to discourage someone.

Vacuum secondary carbs are easier to get right than mechanical secondary carbs and smaller carbs are easier than bigger carbs.
 
ColeTrickle74 said:
carb will be either a 750dp or a 770 street avenger
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Hmmmmm I see two choices the OP asked about... a 750 DP and 770 avenger... don't see anything smaller than a 750.

So what's with the talk about smaller carbs.

Have a great day everyone.
 
Lots of good info and suggestions, thanks! I think I will just pull the 650 off my 318 and run it for a week or two and then try a 750dp and see what happens. I might as well use what I got in front of me.

Hah! Yeah I dunno how the smaller carb got in there, but I'm not gonna refuse any advice :)
 
Ok just went outside to double check the carb, looks like its a 600 carter afb, not a 650. I think I will just order a dp....
 
It could be, I'm not sure. I was having a tough time finding results for the numbers on it. 9605s is whats on it.
 
crackedback said:
Hmmmmm I see two choices the OP asked about... a 750 DP and 770 avenger... don't see anything smaller than a 750.

So what's with the talk about smaller carbs.

Have a great day everyone.
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ColeTrickle74 said:
After reading every thread on the m1sp and the air gap I havent really found anything regarding nitrous use. Heres what I got so far:

360 .30 over with kb107s, stock rods and crank
eddy heads
519/524 lift 294 int./306 exh cam
3000k converter and 4.10 gears with a 727 auto
carb will be either a 750dp or a 770 street avenger
It will be a 100 or 125 shot on street driven 74 dart with trips to the track.

I have no preference between the air gap or the m-1sp, any tips would be greatly appreciated, thanks!
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The smaller carb talk is gear towards street driving. Clearly this set up will perform best with a bigger carb than a 750, but it woin't be the best thing on the street and the carb would have to be spot on.

If this was track only or a Pro Street like set up and he's willing to live with a larger carb for max power, I'd be suggesting a Holley HP 950.:toothy7::-\"
 
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