302 SBF stumble

General engine tech -- Drag Racing to Circle Track

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Geoff2
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Re: 302 SBF stumble

Post by Geoff2 » Fri Jun 22, 2018 6:03 am

Stat,
Before you continue with your 'advice', you might want to read the original post. There you will find the word 'street' in the first line. Yes, street!

So this is not a drag car that leaves the line at 5000 rpm every time it is driven.

And the laws of physics have not changed because you think they have.

And on the subject of Holleys, double pumpers & vac secs. A 300 ci engine at 1500 rpm requires about 120 cfm, if at 100% VE, which it would be nowhere near at that low rpm. The actual cfm ingested will be MUCH lower. These figures come from Holley engineer Mike Urich, who worked on the design of these carbs....
A typical V8 engine consumes about 17-20 cfm at idle.
The extra acc pump on a DP pump is hardly a design 'feature', it is a crutch to fix a big dump of air entering the engine with no fuel mixed with it. The size of & delivery time of the pump shot is always going to be a compromise because the shot needs to vary for engine load/rpm conditions.
Some mech sec carbs are so well designed that they do not need sec acc pumps............

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Re: 302 SBF stumble

Post by cgarb » Fri Jun 22, 2018 1:31 pm

Actually it says street/strip it's a footbrake drag car that is tagged and sometime's gets driven to town or the gas station.

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Re: 302 SBF stumble

Post by novadude » Fri Jun 22, 2018 2:09 pm

cgarb wrote:
Fri Jun 22, 2018 1:31 pm
Actually it says street/strip it's a footbrake drag car that is tagged and sometime's gets driven to town or the gas station.

I guess street/strip means different things to different people. When you were talking about nailing it from a 20 mph roll in the OP, I assumed 90% street (since that never happens at the track).

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Re: 302 SBF stumble

Post by BigBro74 » Fri Jun 22, 2018 2:24 pm

C Garb- 2 questions(at least haha) what is the converter stall? And does this car still have duraspark ignition in place?

I always used the ford ignition on mine- but it did give problems Over time that we’re hard to track down. Seemed like the life of ignition components got less and less- probably parts store junk going overseas 🙄. That goofy coil connector too..... at times the car would fall off a bit and be hard to start (especially warm...) and an ignition tune up seemed to fix it. Now I am older and not as poor lol I woulda done something different for spark..........
Last edited by BigBro74 on Fri Jun 22, 2018 3:19 pm, edited 1 time in total.

statsystems
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Re: 302 SBF stumble

Post by statsystems » Fri Jun 22, 2018 2:27 pm

Geoff2 wrote:
Fri Jun 22, 2018 6:03 am
Stat,
Before you continue with your 'advice', you might want to read the original post. There you will find the word 'street' in the first line. Yes, street!

So this is not a drag car that leaves the line at 5000 rpm every time it is driven.

And the laws of physics have not changed because you think they have.

And on the subject of Holleys, double pumpers & vac secs. A 300 ci engine at 1500 rpm requires about 120 cfm, if at 100% VE, which it would be nowhere near at that low rpm. The actual cfm ingested will be MUCH lower. These figures come from Holley engineer Mike Urich, who worked on the design of these carbs....
A typical V8 engine consumes about 17-20 cfm at idle.
The extra acc pump on a DP pump is hardly a design 'feature', it is a crutch to fix a big dump of air entering the engine with no fuel mixed with it. The size of & delivery time of the pump shot is always going to be a compromise because the shot needs to vary for engine load/rpm conditions.
Some mech sec carbs are so well designed that they do not need sec acc pumps............

I read the OP very closely.

I stand by what I posted.

Show me a mechanical secondary carb without an accelerator pump on the secondaries and I'll show you another junk carb. The 450 Holley.

Take your carb math and apply it to most any street car you see today. Virtually ever engine would be over carved by that math. My street car uses a carb capable of 1000 CFM.

The math is wrong.

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Re: 302 SBF stumble

Post by cgarb » Sat Jun 23, 2018 1:46 am

Converter stalls 3000 and the ignition is a factory duraspark distributor, that the weights and springs are modified to curve where we wanted it, hooked to an MSD 6A ignition. The car was proven, not saying something in the ignition couldn't have went bad though. I'll post up what I find.

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Re: 302 SBF stumble

Post by Geoff2 » Sat Jun 23, 2018 6:22 am

No Stat, the maths & physics are not wrong. They don't change. What HAS changed is VE. It has increased over the decades with more knowledge & better parts, not with changes to physics.
All Carter 4bbl & QJ carbs are mechanical sec carbs & do NOT have sec acc pumps because of the superior design. I have seen dyno tests where a 800 QJ equalled or bettered a 850 Holley DP. Refer to the website www.maxperforormanceinc.com for actual HP#s. And the QJ got a LOT better mileage too....

If Holleys are so good, why is every man & his dog making 'better' versions??

But none of this changes the FACT that the OPs 302 engined car is to be street driven & is never going to be able to use 600 cfm at the low rpms that street cars frequent.

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Re: 302 SBF stumble

Post by cgarb » Sun Jun 24, 2018 6:44 pm

I worked on it today, the valvesprings were only 100lbs on the seat and 250 open. I replaced them with a set that were 135 and 325 open. The valvetrain sounds much better now. We just finished it up and started it before it started raining this afternoon. It still has the hesitation, but that really got worse with the replacement carb, he's going to put the original 650 back on and give it a try once the road drys up some.

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Re: 302 SBF stumble

Post by statsystems » Sun Jun 24, 2018 6:52 pm

Geoff2 wrote:
Sat Jun 23, 2018 6:22 am
No Stat, the maths & physics are not wrong. They don't change. What HAS changed is VE. It has increased over the decades with more knowledge & better parts, not with changes to physics.
All Carter 4bbl & QJ carbs are mechanical sec carbs & do NOT have sec acc pumps because of the superior design. I have seen dyno tests where a 800 QJ equalled or bettered a 850 Holley DP. Refer to the website www.maxperforormanceinc.com for actual HP#s. And the QJ got a LOT better mileage too....

If Holleys are so good, why is every man & his dog making 'better' versions??

But none of this changes the FACT that the OPs 302 engined car is to be street driven & is never going to be able to use 600 cfm at the low rpms that street cars frequent.


I didn't say anything about physics. I said the MATH is wrong. By that stupid math that has been published since before I was born is wrong. If it's not wrong then every single engine I know of is over carbed. Any engine running a tunnel ram is over carbed. Any engine with a single 1250 on it is over carbed.

Again, the math used to figure what CFM carb to use is WRONG.

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Re: 302 SBF stumble

Post by statsystems » Sun Jun 24, 2018 6:55 pm

Geoff2 wrote:
Sat Jun 23, 2018 6:22 am
No Stat, the maths & physics are not wrong. They don't change. What HAS changed is VE. It has increased over the decades with more knowledge & better parts, not with changes to physics.
All Carter 4bbl & QJ carbs are mechanical sec carbs & do NOT have sec acc pumps because of the superior design. I have seen dyno tests where a 800 QJ equalled or bettered a 850 Holley DP. Refer to the website www.maxperforormanceinc.com for actual HP#s. And the QJ got a LOT better mileage too....

If Holleys are so good, why is every man & his dog making 'better' versions??

But none of this changes the FACT that the OPs 302 engined car is to be street driven & is never going to be able to use 600 cfm at the low rpms that street cars frequent.

Just re-read your post.

Seriously? You want to compare an air valve secondary to what the Holley style carbs use? Then go ahead. But you KNOW how the air valve secondary carbs work. They may have mechanical secondaries, but the air valve isn't open until air flow is up enough to make the upper blades open.

Rediculous argument.

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Re: 302 SBF stumble

Post by 427dart » Sun Jun 24, 2018 7:02 pm

Well my '85 LX would run very well with it's TUNED factory 585 Holley. With a 4.11 gear and 5 spd. I could run the short yellow spring without the check ball. Secondaries would open with no bog in 1st or second gear and it would run 12.50's @108.5.
I tried a new 750 DP and it only picked up .01 of a MPH thru the 1/4 The E.T. stayed about the same.
Intake was the Wienad Xcellerator single plane...cam was B302 and the iron GT-40 heads... this back in 1994.

Have to remember look here for suggestions BUT build your car the way you want!

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Re: 302 SBF stumble

Post by 427dart » Sun Jun 24, 2018 7:09 pm

statsystems wrote:
Sun Jun 24, 2018 6:55 pm
Geoff2 wrote:
Sat Jun 23, 2018 6:22 am
No Stat, the maths & physics are not wrong. They don't change. What HAS changed is VE. It has increased over the decades with more knowledge & better parts, not with changes to physics.
All Carter 4bbl & QJ carbs are mechanical sec carbs & do NOT have sec acc pumps because of the superior design. I have seen dyno tests where a 800 QJ equalled or bettered a 850 Holley DP. Refer to the website www.maxperforormanceinc.com for actual HP#s. And the QJ got a LOT better mileage too....

If Holleys are so good, why is every man & his dog making 'better' versions??

But none of this changes the FACT that the OPs 302 engined car is to be street driven & is never going to be able to use 600 cfm at the low rpms that street cars frequent.

Just re-read your post.

Seriously? You want to compare an air valve secondary to what the Holley style carbs use? Then go ahead. But you KNOW how the air valve secondary carbs work. They may have mechanical secondaries, but the air valve isn't open until air flow is up enough to make the upper blades open.

Rediculous argument.
What... you never saw a Holley with AVS? :mrgreen: Worked fairly well too! :mrgreen:
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Re: 302 SBF stumble

Post by cjperformance » Sun Jun 24, 2018 7:11 pm

427dart wrote:
Sun Jun 24, 2018 7:02 pm
Well my '85 LX would run very well with it's TUNED factory 585 Holley. With a 4.11 gear and 5 spd. I could run the short yellow spring without the check ball. Secondaries would open with no bog in 1st or second gear and it would run 12.50's @108.5.
I tried a new 750 DP and it only picked up .01 of a MPH thru the 1/4 The E.T. stayed about the same.
Intake was the Wienad Xcellerator single plane...cam was B302 and the iron GT-40 heads... this back in 1994.

Have to remember look here for suggestions BUT build your car the way you want!
They are an excellent carb when set up well. Hard to get here in Australia so I have not used one for probably 10/12 years but always liked them.
Craig.

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Re: 302 SBF stumble

Post by statsystems » Sun Jun 24, 2018 7:35 pm

427dart wrote:
Sun Jun 24, 2018 7:02 pm
Well my '85 LX would run very well with it's TUNED factory 585 Holley. With a 4.11 gear and 5 spd. I could run the short yellow spring without the check ball. Secondaries would open with no bog in 1st or second gear and it would run 12.50's @108.5.
I tried a new 750 DP and it only picked up .01 of a MPH thru the 1/4 The E.T. stayed about the same.
Intake was the Wienad Xcellerator single plane...cam was B302 and the iron GT-40 heads... this back in 1994.

Have to remember look here for suggestions BUT build your car the way you want!


Well by your logic a 500 tuned carb would have been faster yet.

Was the 750 "tuned"?

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Re: 302 SBF stumble

Post by 427dart » Sun Jun 24, 2018 7:51 pm

Throttle response on the street was much better than the big 750. I ran the 750 right out the box after checking the basics.
When I say the 585 was tuned...it was de-tuned of it's smog settings and rejetted accordingly.
The 585 Holley with it's annular boosters in the primary ran really well at the little 302's power level.

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