Vacuum question!

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Chrippa

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Hi!

I have a question about vacuum! I just got help from a friend to measure the vacuum on the engine.
The thing is we got 15 inches per mercury! I think thats alot and i wonder what may cause it!?

The engine is a 340 stroked to 416cui. The cam is a comp cams mechanical cam with 280/300 in/ex, 239 and 242 @ .50. The lift is 534/540 with 1.5 rockers. The lobe separation is 112 degrees with 4 degrees advance ground in!
The valves are 2.08/1.65.
The distributor is a mopar performance with a MSD 6AL box. Timimg is set 18 degrees to 1200 rpm and then from 1200 to 1500 is increases from 18 to 38 degrees! The carb is a Holley 750 which has been properly adjusted! Im about to test with a Carter Competition 750, thats why we started mesuring!

The rest of the engine is a mopar 4" crank. Mopar M1 intake, not ported! Mopar commando heads stage1 that are converted to open chamber heads. The pistons are KB with -26.5 cc´s. TTI step headers with 2½" all the way back!

Did i forget something?
Well thats about it! Is my cam to gentle or what could it be thats causing this? Shouldnt the vacuum be like under 10 inches per mercury?

Someone have a good idea?

Thanks in advance!
Chris
 
Well, the cam is mild enuff for those numbers and it is a 400+ cube engine...It all looks right to me. You could go bigger on the cam and still have a decent vaccum.
112 on a Comp Cam? What cam is that. There mostly on 10.
 
The cam is V 9215 Part # 2-000-5. I cant find it at comp cams page so i guess they dont make it anymore?

How much vacuum is, so to say, wanted in a streetable car? Im not so good at this as i only have begun to understand engine buildning, but im trying to learn!
How much agressive cam would be wise to go for ? How would the gain look like? Would there be any big difference ? :)

My friend who helped me measure told me that 15 ipm is more than a stock 340 had in vac. ! Is the vacuum mostly becuase of the cam or is it more to that the engine is stroked and has small intake channels for the displacement ?
 
Chrippa said:
The cam is V 9215 Part # 2-000-5. I cant find it at comp cams page so i guess they dont make it anymore?

How much vacuum is, so to say, wanted in a streetable car? Im not so good at this as i only have begun to understand engine buildning, but im trying to learn!
How much agressive cam would be wise to go for ? How would the gain look like? Would there be any big difference ? :)

My friend who helped me measure told me that 15 ipm is more than a stock 340 had in vac. ! Is the vacuum mostly becuase of the cam or is it more to that the engine is stroked and has small intake channels for the displacement ?
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OMG, so many questions! LOL.

I don't know what there making these days. My catolog is old and the new on-line one I can not seem to open up. (PDF's)
Vaccum, more the merry!
More aggressive cam? Depends on a number of things starting with what you can live with and your car. The weight, converter current, rear axle ratio, tire size etc.....
The gain or differences depend on the next cam chossen. Theres allways give and take with each choice. Lose of torque here, gain of HP there thing for starters.
I don't know what the stock 340 had for vaccum, but, the cam starts the ball rolling as well as the size of the motor and how it's configured and cam timing and type of cam.
Small intake runners do not add vaccum, but will aid in the speed of which the fuel and air run down into the cylinder. AKA, Velocity.
 
At idle in gear (auto) what is the vacuum reading and @ what rpm?
 
Im not really sure what it was when the car was in gear as my friend did the measurments but i think it was 12-13 ipm at 1000 rpm. What should it be ?
 
I would be more concerned about that 38 total degrees of timing than the vacuum. Most small blocks require around 35 degrees. Unless you have had the engine on a dyno and this combo wants 38 I would stay on the safe side at 35.
 
dgc333 said:
I would be more concerned about that 38 total degrees of timing than the vacuum. Most small blocks require around 35 degrees. Unless you have had the engine on a dyno and this combo wants 38 I would stay on the safe side at 35.
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could these 38 degrees be fuelrelated? me and chrippa is from the same country and my 340 also seem to like 38 degrees, just asking because the thought crossed my mind..
 
Ceretianly could be the fuel. The slower burning the furel is the more timing you will need. To get maximum power you need to have maximum cylinder pressure around 20 degrees ATDC, so you need to start combustion early enough for that to happen.

Also, the more efficient you cyliner head combustion chamber the less timing you need. The new magnum heads require less timing for max power, normally less than 35 degrees.
 
Those numbers are fine for a 4" arm. I'd expect to see a slightly less number with a better carb. 750 is just plain too small for a long stroke like that. In "std" Holley cfms, I'd say at least an 850 on it, in Demon, Avenger Holley, or "wet flowed" the number should be 800 or larger. The cam is mild, but perfect IMO. If the heads were milled for the open chamber, the 38° number may be ok. If the smaller chambers are run with pump gas (at least the 10% ethanol and 92-93 ocvtane numbers we have over here) and properly designed quench, the low 30s will be the highest you need. A recent big block with Indy SRs likes a max of 28° total. I had to make a bushing for the MSD for that...lol. Like Dave said...you want max pressure at about 20° after TDC in crank degrees. Lower octanes burn faster.
 
Thx for all your answers!
I think i figured the vacuum thing out! I looked more at the advertised duration than at the @.050. And then also that the idle rpm is so high, i tested to lower it to about 700-800rpm and the vac. dropped to 4-5 ipm! :)


moper said:
Those numbers are fine for a 4" arm. I'd expect to see a slightly less number with a better carb. 750 is just plain too small for a long stroke like that. In "std" Holley cfms, I'd say at least an 850 on it, in Demon, Avenger Holley, or "wet flowed" the number should be 800 or larger. The cam is mild, but perfect IMO. If the heads were milled for the open chamber, the 38° number may be ok. If the smaller chambers are run with pump gas (at least the 10% ethanol and 92-93 ocvtane numbers we have over here) and properly designed quench, the low 30s will be the highest you need. A recent big block with Indy SRs likes a max of 28° total. I had to make a bushing for the MSD for that...lol. Like Dave said...you want max pressure at about 20° after TDC in crank degrees. Lower octanes burn faster.
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Would there be a performance gain worth the money to go with a Demon 800-850 ?
Would there be any big performance gain in switching the cam for something more agressive like Hughes HTL4652AS ? Or would that sacrifice alot of streetability? I dont find it annoying to ride with the cam i have today, is there a big change to switch to something more aggressive ? :)
And also how much gain is to ride with a mechanical cam vs. a hydralic! I read different opinions every where!?

I dont know the ethanol mix in our fuel but i think its about 10%! But i always go with Shell 99 octane and add octane booster! And it seems to like higher timing! I just ordered a MSD Billet dist. instead of my Chrysler dist! Its going to be fun to test :)
 
Comp cam; 239 and 242 @ .50. The lift is 534/540 with 1.5 rockers.
Huges cam; HTL4652, 246/252 @ .050 w/.555/.572 lift on a 106 w/1.5 rockers

Via duration @ .050 numbers, maybe a 500 rpm increase of RPM. Loose at the bottom, gain at the top.
When ever you increase a cams duration, you loose power at the bottom and gain it on top. Theres also a need for more stall once you start jumping up in duration. Jump 10*'s and theres a new converter in your future. I don't know about this 7* jump though.

Mech. vs. Hyd. differences are basicly, a higher RPM ceiling with Mech.'s cams.

There be a few HP more in the Huges cam. Is it worth the switch? I don't know. I have done them alot just for fun. Just to see. A waste of money for some, fun for me.
 
Just like others have said, The 4" stroke will eat up a good amount of duration. And like Moper said. That carb is kinda small. I run a 830cfm on my stock stroke 340.
 
The Carter can be modified, mildly. Add on's help. But an 830-850 it will not be. If your not shy of trying something.......
The boosters are aluminum, they can be made smaller (Outside of the booster only) with a dremel. Make the round part smaller on the outside and the arm/leg skinny. all around. sides, top and botton. The idea, less restriction.
Do not screw it up for those parts are not avaiable seperate. You'll need another carb to replace them. (Parts carb)
The throttle blades can be made thinner.
A K&N stub stack and extreme filter set up help alot.
 
Being very general here... The Demon will make more power. How much more, and whether or not it's $$ worth it, is up to you and your tuning skills. My experience with Demons is limited, but based on that, I'll keep it limited. I would prefer using a Holley Avenger, those I know dont come factory with crud left over from machining in the metering blocks and bowls. I've hear BG took care of that, but I also found them (once "fixed) to be very finicky. In almost all cases they need work Out of the box to run their best. That's not to say they cant run..Just you have to keep messing with them, and when you're done, and the weather changes, you're back to square one. (so good at being adjusted, that they will need constant adjustment to match conditions..EFI does all that for you...) My main point was, the longer stroke will create a much stronger signal at the boosters and under the plates. So, a larger carb will not act at all like over carbing a stock stroke engine. In fact, in many instances, the air bleeds and passages in the larger carbs are sized properly for an engine moving the air the 416/408 can. So idle can be better, and power be much better. I dont think changing the cam right now makes sense. You're leaving some on the table power-wise, and until it's all set up right, you dont know what you're missing, and you have to go all thru the same steps anyway. Do it now, see if it keeps you smiling. If it doesnt, and iot cant be gotten any better by you or a pro, then change the cam. I agree with Rumble tho, a change that large in cams make effect other parts of the driveline.
 
moper said:
Being very general here... The Demon will make more power. How much more, and whether or not it's $$ worth it, is up to you and your tuning skills. My experience with Demons is limited, but based on that, I'll keep it limited. I would prefer using a Holley Avenger, those I know dont come factory with crud left over from machining in the metering blocks and bowls. I've hear BG took care of that, but I also found them (once "fixed) to be very finicky. In almost all cases they need work Out of the box to run their best. That's not to say they cant run..Just you have to keep messing with them, and when you're done, and the weather changes, you're back to square one. (so good at being adjusted, that they will need constant adjustment to match conditions..EFI does all that for you...) My main point was, the longer stroke will create a much stronger signal at the boosters and under the plates. So, a larger carb will not act at all like over carbing a stock stroke engine. In fact, in many instances, the air bleeds and passages in the larger carbs are sized properly for an engine moving the air the 416/408 can. So idle can be better, and power be much better. I dont think changing the cam right now makes sense. You're leaving some on the table power-wise, and until it's all set up right, you dont know what you're missing, and you have to go all thru the same steps anyway. Do it now, see if it keeps you smiling. If it doesnt, and iot cant be gotten any better by you or a pro, then change the cam. I agree with Rumble tho, a change that large in cams make effect other parts of the driveline.
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But for the carb, does the demon perform better than the avenger but the demon is more work to tune? Me and my father are thinking of investing in a lambda meter for tuning the carb! Would this save alot of hassle when working with a demon or would it be wiser to go for a avenger right away ? :)
And IF going with a demon what would be best to go for? A speed demon or a mighty demon ?

Does anyone use a Demon? Please share what you think of them!

One more question! I might try that thing that rumblefish said about modidy the Carter that i have! But when checking the vacuum to set up the carb what would be wisest: To measure at my current idle rpm which is about 1000rpm or lower the rpm to about 800 rpm and do the check there? As i had about 12-13 ipm when in idle and in drive @1000rpm and then when i lowered to 700-800 rpm it lowered to about 5 ipm. Maybe a dumb question but i dont know the outcome of either ! :)

Thanks to all of you answering my questions! What would i do without this board! :thumbup:
 
Me and my father are thinking of investing in a lambda meter for tuning the carb!
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Great idea. I did it. It has helped me alot and makes it quicker for me to tune up the carb. Just a simple thing I picked up years ago from Edelbrock. Now theres much better systems out there.
Though I have no probelm with the Edelbrock A/F meter.

I also use a vaccum gauge next to it and take a look at it when accelerating to see the ratio apon when the vaccum spring for the rods are suposed to over come the vaccum to provided more gas. (When the needle rises out of the jet, enrichment.) Just to try and dial it in better.

Tune to the idle your going to leave it at. If your engine idles at 1000 rpm, then set it up from there. Though that seems a bit high for an idle.....If it makes it easier for you, then do it.
 
The demons I've worked with are all race demons. They are finicky with idle mixtures staying steady (the idle hunts between rpms), and they were perpetually rich. They needed air bleed changes. Easy, except nobody stocks Demon air bleeds but places a day or more away. So, now tuning is a week worth of adjust, order, install, adjust, order. I can get holley stuf (and have some) locally. Nevermind the physical issues I found. If you have an LM1, I think it wil drive you crazy tuning. Because you'll get it perfect one day, then the temps will drop 10°, and it will feel all off again. It is cool to see how it all works together tho.

I regard to idle speed and vaccum, that's a heck of a drop for 300 rpm. I would verify the timing curve first. The better heads, I'd be aiming for 18° initial, and around 14° or so centrifical. You shouldnt need much more than 32-33° total in any case. Then proceed to set the carb. You cant do the carb, then timing. Because the timing will change what the engine wants for fuel.
 
Is there any drawback in having a high idle ( except for the noise ) ? More wear on components etc? I like having a little high as it tends to send the hole car in wobble when standing still with lower idle rpm!

The lambda meter seems to be very interesting as a tool to setup the carb, read about in Mopar muscle and then some on the web and it sure seem to make things easier!

Moper: What is a LM1 ? :)
Could it be so that the race demons are more sensitive to changes than the say speed demons and mighty demons? Maybe i should ask around? :)
Its so happens my friend has spent half his life tuning and working on Carters and a other friend run demons on his race hemi so i have helpful sources there, but no one i know well knows Holleys!

I have realized that the more you read, there are more things to keep track off, and you get more confused! One should really not learn anything more, but then there would be no challenge :mrgreen:
 
I have to agree with the meter driving you crazy. But then again, it's a carb. A confounded thing design to keep you crazy.
Tune it on a nice day. Warm between 70 -76 degrees F.
When it gets colder, it'll go lean, warmer, it'll go richer. You'll have to find a happy medium.
 
An LM1 is a very sensitive O2 sensor, connected to a gage, to say whther the collector gasses are rich or lean. IMO, to do it right, you need one in each bank, running simultaneously. Some places make kits like that, where you have one readout, and toggle between the two sensors. They are a cool toy to play with. The Demons by design react differnetly than a comparable Holley. They ar the same basic design, but taken to a further extent. My best advice is to buy a book on the subject, then grab some tools and go do it. Ignore what people tell you as opinion. Find it out for yourself what turnign a screw too far does. Learn what a smooth idle with a lumpy cam really means. It's the fastest way to learn for me. A fast idle doesnt wear parts. It does (at least to me) get irritating, can attract police, and wastes fuel. I'm finicky about idle..lol.
 
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