Small Block Chevy Question

[Chaplin]

Originally posted by Bruce Edwards


JP,

It is my opinion that doubles are best on the track and not the street. They are over kill for most engine set ups. Think about it most folks never drive more than 75 or 80 and most of the time is spent driving around town at slow speed and low rpm. For this type of set up a vacum second is ideal. When you do want it is ready to go but only opens as fast as the engine needs it. Better fuel use and smoother change from the front to all fours.
If you are running at 120+ all the time and the carb never goes back to the front barrells use the double other wise save your money.
To me it is a no brainer. 20+ years of building cars I had to learn something!
By the way thanks for the nice words.

We do agree. That is exactly what I've been saying. Too much can be overkill for the type of driving that most of us do, which is what I was trying to explain in my last post.

[RallySnake]

Bruce,

I have run a bunch of carbs on my 356 ci, Brodix headed engine over the years. The best "off the line" power was generated with a 650 DP, so far. I used a new Holley Avenger 670 on the last setup, and it's doing well but doesn't make the power that the double pumper did. It's great at pulling from 2,200 rpm in fifth gear on the freeway at 70 mph. Do you think a 750 vacuum secondary would work better in this application? And, how about the Avenger 770?

Paul

[Bruce Edwards]

Paul,

Can you give me more info on your engine? What kind of gear are you running in the rear?
The black car in my picture gallery is pushing 400+ from a 30 over 351W. It is running a 750cfm and cruises at 70 in fifth turning 1250 rpm with the AC on. If you drop it into third gear and step on it it will tear your head off.

[RallySnake]

Bruce,

Can you give me more info on your engine?
>It's a 350 bored .30 over with TRW forged pistons. I'm running the Edlebrock Performer RPM cam: 296/300, 234/238 @ .050. lift .539/.548, lobe separation 112. I have found that Rhodes lifters smooth out the bottom end a lot.
What kind of gear are you running in the rear?
>It's a Jag 3.31 rear end. The Tremecs 5th gear is a .68 overdrive.

The black car in my picture gallery is pushing 400+ from a 30 over 351W. It is running a 750cfm and cruises at 70 in fifth turning 1250 rpm with the AC on. If you drop it into third gear and step on it it will tear your head off.

>Sounds perfect! Do you recommend the Avenger carbs?

Paul

[Bruce Edwards]

Chaplin,

I guess we were both going in circles to get to the same point. I have seen way to many overcarbed engines. I had a stack of carbs for a time from ones I removed from customers cars to install the correct size. After all these years I wish I still had some of those old carbs.

[flipper35]

Just two comments. Some hemi engines with a cross ram intake were getting 102% - 103% ve. I hope I can find the article again. They also installed freeze plugs as reliefs in case of a backfire through the intake.

Mopar recommends that the Magnum 300hp 360cid use a 600cfm and the Magnum 380 360cid use a 750cfm. Justa6 can tell you what a properly set up large carb can do an a properly set up small engine.

I agree most people over carb their engine, but mine runs much stronger wth a 750 Holley over a 600 Holley. It also had better bottom end with a Carter AFB over the 600 Holley.

[Bruce Edwards]

Paul,

Sorry it took me a bit to get back to you on this. You did not say, or I missed it, what intake you are running. If you are matching the cam and intake even Edelbrock recommends a 750 or even a 800 cfm.
As I recall the lifters you have are metered to bleed down at low speed to smooth out the low end. What the lifter does is drop the lift at low speed so the car will have more vacuum at idle and low rpm. As the engine speeds up the lifters do nat have as much time to bleed down and the lift goes up.
With out seeing the car and only going by what you have told me I would say that you should see a bit of improvement with a larger carb.
I have not played with the Avenger carb as yet so I will leave that to someone that has.

[MidOHasp]

Bruce,

Well, the money wasting (as it were) has already happened. The thing runs like a real beast, and I've never had a throttle response issue. At least not when I put my foot down hard.

I was reading in another post (and maybe I should start a new post for this question) about some low-load hesitation.

I'm currently experiencing that. Car sits and idles like a pissed off dump truck, just how it should. Give it some gas and it hesitates a little, sounds like a dead miss.

Get out on the road and with very mild throttle (1/8 or less) it hesitates as the revs build...sounds like ignition failure, like the whole thing's turning on and off repeatedly and quickly. If you stomp it, this all goes away (as far as I remember...but it's been a while since the sun was out and it wasn't freezing) and it pulls immediately to 6K where I lift, straighten the car, grab second and do it again.

Some of the suggestions I saw seemed to focus around lean front jets. I know that on the dyno, we saw some pretty hot cylinder temps and we went up one jet size. Temps maybe came down 20 degrees but not what we were looking for and we abandoned that route.

Could another jet size possibly help the hesitation and the temps? I'll be damned if I remember what jets are in that motor now, but I can do some looking around one of these nights and let you know the specs of the motor and stuff, maybe that will help.

JP

[Bruce Edwards]

JP,

Ok let me see if I have this right. It idles alright but has a miss at low cruise speed? If so that is a sign of a lean mix. Does the car have a manual choke? If so what you can so is drive the car to what ever speed the miss starts. Slowly pull on the choke a small amont at a time. If the miss goes away and the engine starts to smooth out, you can hear it when it is right, the jets are to small. If it starts to break down worse it is to rich and you need to back off the jet size.
Now if I did not understand and it misses as you are putting your foot down you have an accelerator pump issue. Those are a bit harder to sort out.
Let me know which it is and I will do my best to walk you through it.

[Bruce Edwards]

I did some looking on the Avenger carbs and boy is my face red. Ok so I did not remember that the avenger was just a polished Holley. So shoot me.

[RallySnake]

Bruce,

It's a little different than most Holleys in that there are no removeable jets. The metering plates are fixed and it has vacuum secondaries. All I can adjust is the electric choke (rich/lean) and I can replace the vacuum spring.

Paul

[Bruce Edwards]

Paul,

Say what?? The primary side has jets. The secondary side you have to change the plate. Most adjustments can be made at the primary side as you are running from that side of the carb 90% of the time.

[Bruce Robles]

rmb
It may be your cam timing. Most American Cars of the vintage of your engine were designed by the factory to run retarded. If you have a new cam that is timed on TDC and the wrong cam sprocket (your original type) and the wrong distributor curve, the thing will never work, regardless of what Carb you have on it. Sounds like you should get the specs on the cam, sprocket, and your distributor and call someone who has a clue what they are doing.

Or you could do the right thing and put a Ford in your car!

Good Luck!

[Bruce Robles]

Bruce Edwards,
You are spot on about the vacuum secondaries verses mechanical. All Cobras, & Shelby Mustangs built both for street and road course's came with vacuum secondaries.
We ran a GT 350R with the 715cfm carb with vac. secondaries. Same as on my 66 street car. We ran a test with a 750 double pumper on the race car and all it did was drink more fuel. The engine can only use what it can use, and that's it.
We could light up the rear wheels with vacuun carb idling with the clutch all the way out, mash down on the gas and sit there in the smoke. Couldn't do it with the double pumper though, it would just try to drive away like it was in to high a gear, slowly.
On the clock there was no noticable difference in lap times between the 2 carbs just fuel consumption, the double pumper was thirsty, but it didn't give any real benefit.

[Bruce Edwards]

Bruce,

Thanks for being with me on this. I guess the CobraClub is in trouble now that two Bruce's are on the same side.
I am sure with a name like Bruce you have to be a very intelligent and honest person.

[Bruce Robles]

Bruce Edwards:
Since we are on opposit ends of the country we will have plenty of _____space in between sending interesting post. You can take the East, I'll take the West and we'll see if we can straighten the mess out!
It's true about people named Bruce being smart though, with a name like our's we can't afford to be wrong!

Cheer's
Bruce Robles

[rmb_cobra]

Bruce (either one will do)....
The cam that was put in was a Lunati 00012:
HYDRAULIC, Fair idle. Good street street/strip cam. Needs headers, gear and converter.
2500-6500 RPM Range
Adv In/Exh 300° 300°
At .050 in/exhaust 246° 246°
At valve in/exhaust .515" .515"
at lobe in/exhaust .343" .343"
Lobe sep int c/l 108° Sep 104° Int C/L
Opens/closes @ .050 19°BTC 47°ABC / 55°BBC 11°BTC 00012
Lifters 71817

The web site for specs is: http://www.holley.com/HiOctn/ProdLin...CC/Chevy4.html

The big question: is this the right one? Looks like it to me. Cam and new lifters was all that was installed....
My only Cobra progress was to run it a bit last weekend, drove it around the block. I have been travelling for work most of the week and had to replace my daily driver last weekend. AACH. cars. gotta lov 'um.

Thanks again for the help.

[Bruce Edwards]

rmb_cobra ,

Did you ever hook a light to it or try to back off the timing? The carb change is going to depend on what you took out as far as cam goes. Need a starting point.
Try some of the things I and the other Bruce was talking about first and see if anything changes.
Then we can try some other things.
After all you have two of us on the case now, how can you go wrong??

[Bruce Robles]

RMB-Cobra

According to your post you changed the cam and lifters only. I went to the web site you posted and found your cam. Go back to the site and look where it says timing chain onthe chart with your cam. It say's 93099. If you go all the way to the bottom of the page and click where it say's products, go to internal engine parts and find timing sets.

You will see Chains set 93099. It is a full roller chain and sprocket set with 3 different Keyways. These timing sets can be retarded 4 degrees or advance 4 degrees. Your problem is either you used your stock timing chain and sprocket, with will not work, or your mechanic used the right one but couldn't figure out which keyway to use thus the timing is not correct.
If you put your timing set back in or replaced it with a new like one, take it out, buy the right one from Lanuti (93099) call thier tech. line to get the right keyway figured out, and install it.
You can do this yourself with a Chevy easier that with a Ford. Rent a puller for your damper, remove you timing cover and have at it. Make sure your motor is at TDC before you start and be careful not to move the crank while removing the damper. Mark the new Sprockets with white paint where your marks line up so you don't get confused and also so you can see them better since your not removing the engine. It should got like stink when your done!

Good Luck

[Bruce Robles]

RMB

After more research I think you should buy the whole valve train kit for your cam.
The specs call for #84146 Aluminum Roller rockers for 3/8" studs. they are 1.5 ratio which maybe ok but the problem is that your rockers and the length of your push rods might not be right for the cam and lifters. The kit has all the compatable parts you need, no quessing! Call the TechLine and find out!