Selecting Cam, getting that low rpm off-idle response?

Jonas Nordstrom

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One important part of your off idle response is your distributor .
Many times it is over looked but has as much effect as cam / carb selection.
myself I would point you to a thermoquad carb , a good fast curve in the distributor.
You want everything to hit at once , carb off idle response, a good tourqe style RV cam,
Going by your post of 650 rpm mash the gas.
Heck a good curve and thermoquad or any carb. tuned right is as important as the cam.
Tons of good advice on here, these guys will steer you right.

That is interesting. Ive been tuning lots of different carbs the last couple of years and that has now led me to the Thermoquad.

IMG_20220814_165211.jpg
 

Geoff 2

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Jonas,
Cam in post #23. No good, poor choice.

I suggest this:
[1] Thermoquad. Best 4bbl carb made for a performance engine.
[2] Another Vizard quote: the right cam costs the same as the wrong cam
[3] And another: I do not have an opinion, I have a dyno....
[4] And another: Getting the LSA [ cam] wrong by just 2* can typically cost 20 ft/lbs & 20 hp! DV claims he has tested over 19,000 cam combinations. Until I find somebody who has tested more, I will be taking his advice....
[5] Your heads are overkill for your application. But probably too expensive to switch heads so you will need to work around them.
[6] The TF heads have a muuuuuuuuuuch improved exh port. There is no need for a cam with extra exh duration. The exh port flow is much improved over stock. Extra exh duration reduces low end because of the increased overlap. You are wanting MORE low end. A lot of smart people are using cams with less exh duration these days. Jon Kaase, multiple winner of the EMC used a mild 246/238* @ 050 SFT cam to make 600+ hp from a 400 Ford engine. Did I mention the LSA? 98 lSA & 92 ICL. It was making 478 ft/lbs @ 2500!
[7] Yeah, you could get a stall CV. Then you get the slippage with it, the extra heat generated. From your original description, I believe the stock CV with a mild cam would give you what you are seeking.
[8] Go to the Isky Cams website, then to 'Tech Tips'. Is extra exh duration really necessary?
[9] I would urge you to buy DVs SB Chev book, SA-57, before getting a cam. There are three pages of cams listed & much more to be learned about cam selection in general. You will save much more in the long run than the cost of the book.....
 

RemCharger

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Jonas,
! DV claims he has tested over 19,000 cam combinations.
At first I thought this was silly, but no, I checked and if you did 1 cam test every day of the year, it would only take 52 years.
Hope the poor guy squeazed in a vacation here and there!
 

Geoff 2

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Vizard actually claims he has done 500,000 dyno pulls. About 8000 of the 19000+ pulls were done under contract at Crane cams. He probably had more than one engine available to test, along with helpers changing cams. So quite believable that multiple tests were done daily.

Relative to LSA. In Harold Bettes's Engine Airflow book. There is a LSA v stroke graph on p. 127. 3.75 stroke, 105 to 115 over 6 categories, but there is no reference that I can see to the categories. Confusing.
What you can see from the graph is that as stroke reduces, so does the LSA. And the 440 has a short stroke for a BB.
 

66 Sat

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Vizard actually claims he has done 500,000 dyno pulls. About 8000 of the 19000+ pulls were done under contract at Crane cams. He probably had more than one engine available to test, along with helpers changing cams. So quite believable that multiple tests were done daily.

Relative to LSA. In Harold Bettes's Engine Airflow book. There is a LSA v stroke graph on p. 127. 3.75 stroke, 105 to 115 over 6 categories, but there is no reference that I can see to the categories. Confusing.
What you can see from the graph is that as stroke reduces, so does the LSA. And the 440 has a short stroke for a BB.
500,000 dyno pulls sounds like a throwaway comment. To get some idea of how many that is, try 27 dyno pulls per day, every single day (including Christmas Day) for 51 years.
 

Jonas Nordstrom

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Why the preference of the thermoquad over something more conventional like a Holley?
Small primaries, large secondaries combo.
The younger cousin the Street Demon is a nice step on the way but the TQ has a lot more tricks up its sleeve.
 

Jonas Nordstrom

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Jonas,
Cam in post #23. No good, poor choice.

I suggest this:
[1] Thermoquad. Best 4bbl carb made for a performance engine.
[2] Another Vizard quote: the right cam costs the same as the wrong cam
[3] And another: I do not have an opinion, I have a dyno....
[4] And another: Getting the LSA [ cam] wrong by just 2* can typically cost 20 ft/lbs & 20 hp! DV claims he has tested over 19,000 cam combinations. Until I find somebody who has tested more, I will be taking his advice....
[5] Your heads are overkill for your application. But probably too expensive to switch heads so you will need to work around them.
[6] The TF heads have a muuuuuuuuuuch improved exh port. There is no need for a cam with extra exh duration. The exh port flow is much improved over stock. Extra exh duration reduces low end because of the increased overlap. You are wanting MORE low end. A lot of smart people are using cams with less exh duration these days. Jon Kaase, multiple winner of the EMC used a mild 246/238* @ 050 SFT cam to make 600+ hp from a 400 Ford engine. Did I mention the LSA? 98 lSA & 92 ICL. It was making 478 ft/lbs @ 2500!
[7] Yeah, you could get a stall CV. Then you get the slippage with it, the extra heat generated. From your original description, I believe the stock CV with a mild cam would give you what you are seeking.
[8] Go to the Isky Cams website, then to 'Tech Tips'. Is extra exh duration really necessary?
[9] I would urge you to buy DVs SB Chev book, SA-57, before getting a cam. There are three pages of cams listed & much more to be learned about cam selection in general. You will save much more in the long run than the cost of the book.....
#5 I wrote in post 13 that my heads order was canceled.
So choice of heads are up again on the table. :thumbsup:

Cam selection specs are still a maze to me but the replies here are awesome.
Learning a lot.
 

Jonas Nordstrom

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Just to complete the driveline, rear tires are 28".

IMG_20220818_071658.jpg
 

gtxrt

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Geoff 2, i agree with you 100% on the extra EX duration it hurts bottom end. what the extra EX does it will keep you your HP from dropping off after peak RPM HP which i don't think helps that much on a street build which you don't want to rev that high anyways. what i do disagree is putting a tight 106 LSA on a street engine. say you put a 218-224 on a 110 LSA now put it on 106 LSA you have a rough street idle, you say to lower the duration to make up for that, to do so you would have to go 210-216 duration to get the same overlap and idle. with that cam you will get more detonation and less mid range and top end power on the 106 LSA. it all about compromising.
 

Geoff 2

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Gt,
I suggest you re-read post #24. agree it is a compromise. The trick is to make the right compromise.....
Maaaany engines use tight LSA cams & short duration, just not American V8s. This is a time to learn from others. The humble Mini Minor had factory cams with 107 LSA, 230* adv duration. They didn't detonate. Many performance cams for those engines, including factory perf cams, were on 102 LSA.
To say you would get less mid range & top end with 106 LSA is nonsense. I have seen enough cam tests to know that rarely, if ever, does wide LSA outperform narrow LSA. Look at road race engines that need tq coming out of corners & then have to accelerate to max rpm. They are always tight LSA cams.
Richard Holdener recently on you tube did a cam test on a LS engine, three different LSA's. Look it up. Pretty sure the tightest was 108 & it made more power everywhere, even with EFI.

Another example: a specific test to compare 106, 108 & 110 LSA on a 350 Chev. Identical cams ground by Isky for the test. The 110 made................3hp more. Buuuuuuuuuuuut it gave up 19 hp through the mid range. I know which cam I would be using......
More tests, in DVs BB Chev book. 468 & 572 BB engines. Both were tested with 107 & 112 LSA cams; in each case the 107 outperformed the 112 everywhere.
Sig Erson & DV worked together in the 1960s. Erson was a smart bloke, ahead of his time; he says this in his cam catalog: "A cam with closer the lobe centers will always produce more power in the midrange than a cam using the same profile & wide lobe center, & in many applications will produce more power all through the range depending on many variables such as the induction system, rod angularity & flow capacity of the ports."
 

Geoff 2

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Jonas,
I would NOT use the Street Demon carb. It is a crude copy of the TQ [ the best ] & it is poorly made. I have used two. One worked ok on a 350 Chev. The other on a 400 Pontiac had a flat spot when the secs opened that could not be eliminated. The owner rang Holley from Australia but when the H 'tech' ran out of options on his computer screen, he said sorry, don't know....
Owner refitted his QJ.
 

gtxrt

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Geoff 2 i said the 106 lsa with 210-216 is going to give up upper mid range and top end to the 218-224 110 lsa. your comparing 106 to 110 with the same duration and am comparing them with the same over lap so you need less duration on the 106 to get the same over lap.
 

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Hi Jonas, I like the the Holleys and the Thermoquads; GTX John could probably tell you about performance with the Thermoquads. As for the a street cam, I like the Lunati Voodoos. And as for compression, my 6 pac GTX runs about 190 psi compression with the stock-like cam the previous owner installed, and with my recurved and limited distributor, it works on pump gas and pops pretty good, though it is a 4spd car. If your compression is a little too high, just buy some thicker Cometic gaskets, they make about whatever you might need.
 

khryslerkid

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I really like the Summit 6401cam in my 440. Just under 10.1 compression. I've ran 89 octane with no problems. Normally run 92 octane. Does require a stiffer spring and looser converter. I'm running a stock converter with 3.23 gears and it responds great. There's also the milder 6400.

Summit 6401
Basic Operating RPM Range:
2,000-5,500
Intake Duration at 050 inch Lift:
224
Exhaust Duration at 050 inch Lift:
234
Duration at 050 inch Lift:
224 int./234 exh.
Advertised Intake Duration:
298
Advertised Exhaust Duration:
303
Advertised Duration:
298 int./303 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:
0.466 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:
0.488 in.
Valve Lift with Factory Rocker Arm Ratio:
0.466 int./0.488 exh.
Lobe Separation (degrees):
114

Summit Racing SUM-6401 Summit Racing™ Classic Camshafts | Summit Racing
 
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