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Topic Review (Newest First)
11-04-2015 07:43 AM
yellow88monte I agree with the stock jetting that is on the 625 street demon is just a little rich for a mild small block 350 which was verified by a tail pipe installed O2 sensor. Strangely though, the porcelin on my spark plugs were always stark white. When I switched to a Holley Street Avenger my plugs got more color and my gas mileage didn't get any worse. (Actually got a little better on cruising.) there are fewer hassles on the 625 street demon and I would highly recommend the carb to anyone who wants a reliable carb that seems to perform the same reliable way day after day.... season after season. In the quarter mile, the e.t.'s I experienced with only a carb change to a 670 street avenger were within a tenth of a second and 1.2 mph. (Holley was a little faster)
11-03-2015 01:16 PM
JohnTN
Success in Tuning Street Demon 625 Carb for Better Mileage

I have posted earlier on disappointing experience with gas mileage on the Street Demon carb. But I have made some positive changes.

The Street Demon stock primary rod was 60/52, stock jet was 78. I switched to leaner primary rods of 62/54 and leaner jet 76. Theoretically, the leaner rod/jet combination should flow 22% less fuel on the primary lean step and 21% less fuel on the primary rich step (rich step comes into play with low vacuum/moderate to fast acceleration).

Over the weekend our street rod club took a 202 mile color cruise, 75% state highways (40-55 mph with occasional red lights in small towns), 25% interstate at 60 mph. The car only used 9.5 gallons which equates to 21.3 mpg (compared to 15-16 mpg in stock form)! There was no audible pinging or lean surging at cruise. 21 compared to 16 is better than 30% improvement. I am happily surprised at the extent of improvement.

So, I believe the stock Street Demon 625 is calibrated for a higher performance small block than my mild build and was too rich out of the box. And, performance is better than the reman Qjet (not professionally tuned).

John
10-17-2015 09:10 AM
yellow88monte I ran my 625 street demon at the track , then two weeks later ran an aluminum Holley 670 street avenger with the 4 corner idle with no other changes. The car sounds better...is 2 mph faster and 1 tenth quicker in the quarter mile, 1 mpg better gas mileage on the highway with the 670 Holley street avenger. I'm sold on the Holley.
04-24-2015 11:29 AM
yellow88monte
Adjusting the floats every other day on Holley carbs

Sorry, my mistake BuzzLOL. To clarify the float setting statement I made earlier I meant to say that the best way I found to achieve top performance in the 600 Holley carb I used to own was to adjust the fuel level to the bottom of the hole because when the barometric pressure raises or lowers as it does so often in Florida I found that I could feel the responsiveness of my Holley carb fluctuate with the barometric pressure. It was an easy fix to adjust the fuel level and it gave me the satisfaction of being able to tune my engine for the conditions. With the new 625 street demon, I don't seem to notice any difference in the performance of it on any given day as long as the outside air temps are steady. I do feel however that although the 625 street demon responds well to part throttle changes, I feel the itlacks power unless I "kick in" the four barrel. Thanks again for your input. I was happy to hear your input.
04-12-2015 12:40 PM
BuzzLOL
Quote:
Originally Posted by yellow88monte View Post
I think the Street Demon does everything well without the constant hassle of adjusting the floats most every day like on most Holleys.

. I must be doing something wrong, I only check the floats on my Holleys about every 20 years...
04-12-2015 11:53 AM
yellow88monte
fuel mileage of street demon

When I bought the Monte in January of 2014, it had a 650 Mighty Demon on it. Rather than rebuild this high performance carb, I bought the street demon to be able to cruise around more economically. I'm pretty sure that I did the right thing as far as fuel mileage goes.
I think the Street Demon does everything well without the constant hassle of adjusting the floats most every day like on most Holleys. I have no doubt that I can get more performance from an 670 Avenger or 650 Speed Demon or 650 Quick Fuel carb, but at the expense of fuel mileage I'm sure.
It's like one of the other guys said on here which I thought makes a lot of sense.... basically to just "enjoy the car".
I have 294 RWHP and 312 #'s of torque at the rear wheels on a stock drive train. That's gonna have to be good enough unless I want to start breaking my ****. I'm sure I can hold my own with any mustang on the street and that's good enough for me. Besides, there will always be faster and slower cars than yours or mine. Maybe some day I'll buy another carb, but my Street Demon runs pretty darn good and the throttle response is always amazing. I'd bet that it performs better than a 600Edelbrock or 650 Edelbrock AVS series. For $312 bucks, you can't really go wrong with this carb unless you plan on racing. Even then, I'd give it a shot.
04-12-2015 10:57 AM
BuzzLOL
Quote:
Originally Posted by yellow88monte View Post
I just wanted to add that on the dyno my Monte with the 625 street demon registered 12.5-1 AFR at cruising speed. Ideally, I'd like to see that number at 13.5-1 AFR. I have since played with a leaner setting with my primary fuel rods and richened up the rear holley style jets in the secondaries to achieve closer to a 12-1 AFR at WOT.

. I thought cruising was best at 14.7 - 16... and WOT power at 12.7-13...
.
04-12-2015 10:27 AM
JohnTN Yellowmonte,

Do you have any fuel mileage data before installing the Street Demon?

John
04-12-2015 08:17 AM
yellow88monte
To JohnTN

I just wanted to add that on the dyno my Monte with the 625 street demon registered 12.5-1 AFR at cruising speed. Ideally, I'd like to see that number at 13.5-1 AFR. I have since played with a leaner setting with my primary fuel rods and richened up the rear holley style jets in the secondaries to achieve closer to a 12-1 AFR at WOT. I'll post again when I get it back on the dyno to check my power numbers after removing the choke butterfly on the primaries.
I checked the gas mileage on my last trip and registered about 16 mpg. But you must remember that I have 3.73 gears, 200r4 trans., edelbrock performer rpm air gap, headers, dual exhaust, etc. so there is quite the volume of air moving around that the motor must be happy with too.
Like most older cars, there probably are other improvements where money would be better spent than my carb. Such as replacing my bench seat with bucket seats to improve the general "feel" of driving. As I drive my family car, I'm spoiled at how nice the form fitting seats are and I would feel safer if I didn't feel like I had to lean into the corners etc... lol
Thanks again everyone for the advice.
04-12-2015 12:41 AM
yellow88monte
Thanks everyone for your advice and point of views.

Thanks to everyone for the feedback to my first post on here. You guys rock.
04-11-2015 09:33 PM
JohnTN I just put a 625 cfm Street Demon on my mild Olds 350 with 2004R transmission, 3:42 rear gear to replace a 10 year old remanufactured QJet. The QJet gave good service an averaged 18 mpg over 25,000 miles until last fall when it started flooding at startup and leaking at the air horn gasket.

The SD has better throttle response and snappier when blipping the throttle. Also, starts reliably. But I have had two issues which may be sorted out.

1. The secondary opening seemed to have a "catch" in it initially. Felt like the secondary throttle blades were sticking and it took noticieable extra foot pressure on the throttle pedal to get through this resistance (more than just the extra spring resistance on the secondary throttle shaft). This significantly affected driveability of the car. Now after about 400 miles on the carb this catch seems to have gone away, maybe whatever was hanging up has "wore in".

2. I have a leak at the filler tube to my gas tank so I can't fill the tank above 3/4 full. So my estimate of fuel mileage is just that - an estimate. On the first 160 trip on secondary highways and interstate I estimated I got 14-15 mpg versus 18 or so I would have gotten on the QJet. After that drive, I changed the primary metering rods from stock to one step leaner. On a 155 mile drive last weekend, all on state highways, I estimate I got 19 mpg. Hard to believe one step leaner in metering rods would make this big a difference. My method of estimating is based on the gas gauge moving from 1/8 of scale to 3/4 scale when I added 10 gallons of gas before the trip. After 155 miles the gauge had only dropped back to 1/4 of scale. So, I used less than 10 gallons and estimated the mileage assuming I burned 8 gallons.

If the secondary sticking problem has gone away and I can get fuel mileage similar to the QJet then I will be very happy with the change. Like I said, I think the car is more responsive now. And the secondaries opening do have a nice sound similar to the howl of the QJet.

Would like to hear any feed back from others who have tried the Street Demon.

John
04-11-2015 08:54 AM
eric32 From my experience with edelbrock carbs which is very close to that street demon by going over to a holley has always given me better results overall and still being in the same cfm range give or take about 25 cfm.

The street demon like an edelbrock or a quadrajet has smaller primary bores and will always give better signal in city driving compared to a square bore holley.

Bottom line flow is flow and the engine will take what it wants as far as power overall. I have just liked the better overall performance of a holley style carb compared to others. If it runs good and your happy with it just run it and enjoy it. I stopped trying to do this and that with mine a few years ago and now just enjoy it and take it for a ride and not worry about every last detail. Its matched good and runs good and performs good and I enjoy my rides for what they are.

My mild 350 is similar to your build but with just a little more cam and head size but I run a plain old holley 600 vacuum secondary and for a short time ran a quick fuel 680 vacuum secondary and get 19mpg on my setups and they run and have a very snappy throttle response. As far as power at the top end of things going to a bigger say 650cfm carb really won't make much of a difference. If you go with a classic 750 cfm vacuum secondary holley style carb then it might make more on the top end but your talking less then 10 horsepower at the most.

That's why I use a 650 ish range on both of my 350 builds. Don't buy a new carb if you really don't have too and if your happy with how it runs but do what makes you happy and what will satisfy you the most for your build. That's the nice thing to hotrodding as you always have options
04-10-2015 03:05 PM
BuzzLOL
Quote:
Originally Posted by yellow88monte View Post
I put the new 625 street demon with polymer fuel bowl on my '88 Monte Carlo. On the chassis dyno I got 292 rwhp and 312 ft. lbs. torque. did I leave performance on the table by trading mid and upper range performance for great throttle response I wonder if I would be happier with a 650

. From the power you're getting, looks like the engine might like a little bigger carb... the time to ask was when it was on the dyno... check to see if about 1/2" of vacuum at WOT full RPMs... or the vacuum was significantly higher because the carb. was an airflow restriction...
.
04-10-2015 02:54 PM
yellow88monte
my experience with the new 625 Street Demon

For everyone who has questions about the performance of the new 625 street demon I hope this report helps others make a better informed decision about their carb purchases.
I put the new 625 street demon with polymer fuel bowl on my '88 Monte Carlo a year ago and like it. Here are my specs:
355 sbc, vortec heads, 9.4 comp.ratio, comp cams xe 262 flat tappet cam, performer rpm air gap intake, 200r4 trans, 3.73 gears, Hooker 1 5/8 super comp headers with x-pipe/ 2 1/2" exhaust, dual flowmasters.
The 625 street demon was pretty close out of the box and I had no issures with setting the idle etc. On the chassis dyno I got 292 rwhp and 312 ft. lbs. torque. A/F with stock metering rods was steady at 13:1 all the way from 3500 to 6000 rpm. Initially, there was a bad bogging situation when I floored the gas pedal from 50 mph but was easy to remedy with tightening the screw/nut assembly for the secondary air valve door so it wouldn't open so quickly. The throttle response is incredibly good at any rpm.
My only question is...did I leave performance on the table by trading mid and upper range performance for great throttle response due to the smallish 1 3/8 triple stack primary venturis? I wonder if I would be happier with a 625 road demon jr, Quick Fuel 650 SS or 650 Speed Demon (all with vacuum secondaries).
Any input from experienced carb guys would be appreciated. Have fun everyone.
02-26-2014 09:50 AM
abyars Is it still working great for you?

Does anyone have anymore reviews of this carb?
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