I just wanted to get opinions on this. I had my engine built professionally and it is getting dynoed this week. All along I thought the plan was to install a 750 Speed Demon on it, but the shop says the 650 SD will make more power and not bog.

Although I'm alot suprised, the guy at the shop said he dyno checks engines every day and he would not recommend a 750 for my combo. He says putting a 750 Speed Demon on it would be like putting an 850 Holley on it, because the SD's flow better???

So here's the brief engine combo:

SBC 400 (.040 over)
KB147 pistons (about 9.5:1 compression.)
Dart Iron eagle heads 70cc, 200cc runners, 2.02 intake, 1.6 exh.
Scat 4340 crank
Eagle SIR rods, 5.7
Comp Cams XE274H, Dur. 230/236 Gross lift 490/490
Comp Cam roller tip rockers, 1.5 ratio
Professional Products intake crosswind (RPM Airgap)
Proform mech advance HEI distributor

They said they'll put both a 650 and 750 on if I want, but they say the 650 will make more power - and I never heard of putting such a small carb on a 408 myself.

- Thanks for any replies!

 

I dont think the 750 will bog. My gosh, its going on a 400 not a 305! I would go with the 750. It should be fine. Let us know how things work out.

 

Why not take them up on their offer to dyno both? Don't pick the one with the highest peak torque/power, but rather the one with the most "area under the curve".

SD's are flow rated "wet", vs standard Holley dry. But, 100 cfm is really overstated - more like 25 cfm.

FWIW: With my 396, 1/4-mile MPH was the same with 750 Holley VS, 650 Holley DP, and the same DP with the 750 Proform main body. That's an indication that power is pretty much the same.

However, the best 1/4-mile times come with the last combo.

 

I have a 406 and when I called Demon Carbs and told them my 750 speed demon was always rich at idle they told me that a 650 mighty demon would actually work better. The speed demon will always be too rich at idle and too lean in the upper rpm range. My camshaft is 241 @ .050. The speed demons aren't made for such large camshafts. I was actually surprised because I also told them I was going to spray 175 hp shot of juice. You should actually call Demon and let them recommend a carb for your compression, camshaft, heads and intake. They will ask a series of questions to set you up with the right carb. Very helpful.

Also, five7kid...It's more than 25 cfm. Demons flow 10% more than Holley. A 650 will be more like 715 cfm. A 750 will be about 825 cfm.

 

Your engine is only going to use as much air and fuel as it needs. So a 750 will not be too big especially since it is going on a 400.

 

If that were true then why not put a 1050 on a 305? Too big a carb will cause sluggishness and will not provide the right air fuel mixture.

 

Here is some interesting reading on your subject, take a few minutes to check this out(its on a 383):

http://www.chevyhiperformance.com/te...8_0403_choose/

 

Thanks guys!

94-6spd - when you said "The speed demon will always be too rich at idle and too lean in the upper rpm range. " , I think that's exactly what the shop was telling me. They said the 750 would be too lean just like you're saying. Good idea about calling Demon and asking them. I'll do that tomorrow!!

My original plan was a Holley 750DP, but the shop basically said to get a Demon because it "outflows a Holley silly". But I assumed it would still be 750 cfm's.

five-7 - You can bet I'll have them test both the 650 and 750!! The guy already said he would do that if I want.

SweetS10v8 - Thanks for the link!!

 

Very interesting results on the dyno. I would really like to see the engine under load on a dragstrip to see which is quicker in a car. The results may be the same I don't know but also would like to see the part throttle drivability in the car.

 

The Demon website was pretty informative! But it basically says I could go with either the 650 OR 750 cfm Speed Demon!!!

The only difference was the cubic inch range....

For a Cu Inch range of 302-427, they say a 650 cfm will do...

And For a Cu Inch range of 393-477, they say a 750 cfm.....

Either for Cams with 240 duration or less.

So where does a 408 fall in?? Might be an interesting dyno!!!

Still gonna call them to see what they say. I'll let y'all know.

 

Well, just a little update....

I called the Demon tech support line, and the guy recommended the 650 cfm!! Guess the shop knows what they're talking about.

I still want to see what they both look like on the dyno though!

 

Sounds like they wnat to sell you a 650 demon.

As a general rule on a performance motor a 750cfm carb will make more power than a 650cfm carb.

Cause its bigger, flows more air and is less of a restriction.

But.... the fuel curve of that carb has to be right for
your motor.
Sounds like the problem is the out of the box calibration of the 750 demon.

I'd get them to prove ( that what dynos are for)
that their 650 carb will out power a holley 750 cfm carb or even an Edelbrock 750cfm carb that has correct jetting.

I'd put my money on both the holley and edelbrock 750cfm carbs over any 650cfm carb of any brand. (remember, correct jetting)
You have to run the jetting up and down on each carb to get it right. The dynos AFR numbers can be decieving.
try different jetting and believe the power and exhaust temp numbers.

 

F-BIRD'88 - No, the shop is not trying to sell me a 650 Demon!! They'll put on whatever carb I want. They are recommending a Demon however, based on thier experience. I spoke with the tech. rep. from Barry Grant, and they also recommend a 650 over a 750 for my application. I will be testing both a 650 and 750 on the dyno anyway, just to see what happens.

Holley's are also great carbs, I've had a few.

So far as Edelbrock carbs go, I had one of those also. A 650cfm if I recall. Never again!! By far the worse carb I ever had. Others like it, so it's just my opinion....

As the rep explained to me, there's alot more involved with carb selection than total flow. Primary metering must also be taken into consideration, along with the velocity of the air flow through the primary circuit. Not to mention intended RPM range of the engine, and cam/intake considerations.

I used to think sticking a big honkin carb on my car would solve problems back in the day, till I finally realized I was creating more problems. That's why I have carefully put together this combo from the oil pan up. I want everything to work together.

 

It is interesting to read that Chevy High Performance article linked earlier in this thread. I have that issue and they found not much difference at all between a 600 cfm Holley and even a 1000 cfm on a well built 383. Less than 10 hp or so from 600 cfm through several others, all the way out to 1000 cfm!

So I can imagine a 650 working very well.

Sounds like you've got a barn burner ready to go in the old Camaro!

 

Not saying a 650 demon won't work well for ya. But am saying a Holley #4779 750 double pumper will likely make more power and be just as responsive when dialed in. (jetting, accelerator pump cams and shooters.
If you have the chance try a edelbrock 750carb too.
#1407 with "out of the box jetting". (not a 600)
You may be suprised. These carbs are usually right on the money and only need accelerator pump tuning.
Float setting is critical.

If you give each carb a fair shot I think you'll find the results interesting.

Can't really speak on a 750cfm demon but have also heard of others having a hard time getting one dialed in.