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Discussion Starter #1
Hey all... I have a nice 10:1 355 SBC with the Isky z25 cam, 1.43" Comp dual springs, 1.6 rockers, eddy performer rpm heads, Summit single plane intake, Holley HP750, etc... nice strong motor up to 6500 RPM, love it.


With my old ignition (Taylor HEI) and headers (Sanderson 1.5" primary shorties through 2.25" pipes), My AFR gauge seemed to be around 12.8:1 at WOT with 72/80 jets.


Well, Now I have Hooker Super Comp Fenderwell headers (1.75" Primary, tuned, 36" length, 10" collector) into 2.5" side pipes and race bullet mufflers. I also installed a Mallory 8360 Dizzy, 45K Volt MSD coil, and Mallory Hyfyre 6AL box). Now, the AFR gauge only drops to about 13.6:1 at WOT with the same jets, so I jetted up one size front and rear to 73/82 (I didn't have an 81 jet), and it still only drops to about 13.3:1 at WOT! It definitely pulls much stronger with this jetting... but that seems like it should be plenty for my set up. Should I go up to an 83 or 84 secondary jet?? The only other change I made was smoothing of the intake runner dividing walls in the intake to smooth up some rough edges in the casting, but left the rest of the intake alone for turbulence purposes.



This just seems like a lot of fuel in my opinion. What do y'all think?
 

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With a smaller cam, I would expect about #66 primary jets in a 355", but with a 240/240 cam, and lower vacuum, it prolly needs bigger ones... another problem may be too much cam for those heads... what cc are the intake ports? Is it 13.3 all the way from 2500 to 6500 RPMs?
 

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Discussion Starter #3
I went as low as 70 primary on the jets with the old headers and ignition, but it would go a litte too lean during cruise so I settled at 72. The intake runners are 185cc, and have had the first inch of the runner opened a little bit to port match a fel pro 1205 gasket.

It seems to hang at between 13.2:1 to 13.5:1 all the way through. I ran it from about 3000 up to 6500 and the richest it went was 13.2:1.

I pulled a plug and it looks a little rich, but I did a lot of low rpm driving back to the garage wit school zones and such, so it probably needs a smaller primary jet.
 

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I imagine that cam idles rather dirty, so that may be the plug reading... proper way to read plugs is new plugs, run engine at WOT for a ways on a deserted good road, shut engine down, tranny into neutral, coast to a stop, read plugs after it cools enough...

Some of the early Performer RPM heads were only about 165-170cc, 185cc isn't so bad...

Cruising A/F can be 15-16:1...
 

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Discussion Starter #5
At least I'm pretty sure they're 185cc... I bought them used off a friend and I can't find the part number on them and I never cc'd the runners, only the combustion chamber. All the new edelbrock performer rpm heads are 185cc or 195cc, so I assumed they were 185cc. I didn't know they made a smaller cc runner on older designs.

Yeah, I'm probably gonna jet back the primary. Cruise afr is between 13.9:1 and 14.5:1 right now.
 

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14.5 primary is not bad and 13;1 wfo WOULD BE GREAT.
nice cam choice,,,
idle should be decent and mid range power excellent?
 

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Discussion Starter #7
Oh yeah! I love the idle on this cam.. nice and choppy with the 108 LSA, but still manageable on the street. About 11in/hg idle vacuum at 1000 rpm on a 10:1 355 and from 2500 all the way to 6500 it is strong and snappy, especially with my small intake runners. Beat a 9:1 roller 308 sbf with ported heads and a 650 Double pumper in a 2700lb car. Has no problem spinning the tires all the way through first gear (th400 and 3.73 axle ratio).
 

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Hey all... I have a nice 10:1 355 SBC with the Isky z25 cam, 1.43" Comp dual springs, 1.6 rockers, eddy performer rpm heads, Summit single plane intake, Holley HP750, etc... nice strong motor up to 6500 RPM, love it.


With my old ignition (Taylor HEI) and headers (Sanderson 1.5" primary shorties through 2.25" pipes), My AFR gauge seemed to be around 12.8:1 at WOT with 72/80 jets.


Well, Now I have Hooker Super Comp Fenderwell headers (1.75" Primary, tuned, 36" length, 10" collector) into 2.5" side pipes and race bullet mufflers. I also installed a Mallory 8360 Dizzy, 45K Volt MSD coil, and Mallory Hyfyre 6AL box). Now, the AFR gauge only drops to about 13.6:1 at WOT with the same jets, so I jetted up one size front and rear to 73/82 (I didn't have an 81 jet), and it still only drops to about 13.3:1 at WOT! It definitely pulls much stronger with this jetting... but that seems like it should be plenty for my set up. Should I go up to an 83 or 84 secondary jet?? The only other change I made was smoothing of the intake runner dividing walls in the intake to smooth up some rough edges in the casting, but left the rest of the intake alone for turbulence purposes.



This just seems like a lot of fuel in my opinion. What do y'all think?
I think the 1.5 inch shorty Sanderson's were bottling the engine up and it did breath as deeply as it could have. The new headers are working well and working with your cam so a lot more air is going through the engine and that air needs fuel the previous jetting couldn't support.

Bogie
 

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Discussion Starter #10
I think the 1.5 inch shorty Sanderson's were bottling the engine up and it did breath as deeply as it could have. The new headers are working well and working with your cam so a lot more air is going through the engine and that air needs fuel the previous jetting couldn't support.

Bogie
Yeah that's more or less what I was thinking Bogie. I just didn't expect it to want so much fuel. With the 73/82 jets it's strong throughout the RPM range with part throttle and WOT. However I pulled the plugs to get a look at how it's running around town and it looks like I could drop back to a 71 on the primary to try and clean it up some, maybe even one step hotter of a plug. I think I'll compensate for the primary reduction with larger PVCRs so that way I keep the balls but don't foul plugs lol.

I guess I'm just concerned that this jetting is a lot for a 355 with a cam that only has about .495" lift with the 1.6 rockers and hot lash.. but then again it does have a lot of duration at .050" and a nice tight LSA.. not to mention these headers are tuned length so I'm sure they really help scavenging.

What jets would y'all expect out of a mildly warmed over small block like this?
 

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Bigger duration and tight LSA both lower vacuum and hurt MPG and require bigger jets to move the same amount of fuel...

Chevy used a widish 121 degree LSA and shortish 210/229 durations in the EFI LS7 427" engine to get 530 HP, 30 MPG cruise, and redline 7,000 RPMs power band...

You may need a Power Valve that stays closed till lower vacuum is reached so it isn't opening during idling and cruising... what number PV do you have now?

Secondary jets might need to be even bigger to achieve proper WOT A/F... is there a secondaries power valve?
 

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Discussion Starter #13
Lol those stock cams always make me laugh a little with their wide LSAs and short durations... but they sure do work wonders on a properly built factory engine.

I was considering going to a 6.5 pv. Right now I have a 7.5 pv, and idle vacuum in gear I'd at 8in/hg.. but it sure loves that 7.5 for part throttle. Getting on the highway if I throttle I right to the secondary opening point vacuum drops to 7in/hg and the afr gauge will drop into the low 13s. Nice a strong, real good torque.
 

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Yeah that's more or less what I was thinking Bogie. I just didn't expect it to want so much fuel. With the 73/82 jets it's strong throughout the RPM range with part throttle and WOT. However I pulled the plugs to get a look at how it's running around town and it looks like I could drop back to a 71 on the primary to try and clean it up some, maybe even one step hotter of a plug. I think I'll compensate for the primary reduction with larger PVCRs so that way I keep the balls but don't foul plugs lol.

I guess I'm just concerned that this jetting is a lot for a 355 with a cam that only has about .495" lift with the 1.6 rockers and hot lash.. but then again it does have a lot of duration at .050" and a nice tight LSA.. not to mention these headers are tuned length so I'm sure they really help scavenging.

What jets would y'all expect out of a mildly warmed over small block like this?

The headers and the tight LSA on the cam are working together. A step hotter on the plug sounds like a good idea to start with as the cam's overlap is allowing the intake to start sooner will tend to cool the plug more than usual.

It sounds like you're in the range but need to work out the details of timing advance, plug heat range, mixture ratios. It's something of a PIA as you have to play these things back and forth to hit the optimum solution. Then the weather will change and so will the needed tune.

At some point a wide band AFR can help aim you at whether the carb needs main jet, air correction, emulsion jet changes to nail the mixture down in specific places. Reading the plugs in my experience is more a generalization and will change depending on how the engine was last used. The idea of making a run and shutting off at the peak is good for making runs at full throttle but doesn't tell you much about the mixture cruising around town. It would be an incomplete interpretation for a street/strip engine to assume that if it looks good on the top end it's good everywhere else.

Bogie
 

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my 4779-2 was jetted at 72/82, but that can change with the weather.You are close,,,,,
Your heads may not have a good intake/exhaust flow ratio so the tuning might be tougher.If your exhaust flow is less than 75% of the intake side your cam is an issue.
when you are a little closer to tune,run a lash loop and see if that helps you a little?
 

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I was considering going to a 6.5 pv. Right now I have a 7.5 pv, and idle vacuum in gear I'd at 8in/hg.. but it sure loves that 7.5 for part throttle.
Maybe you could trade the rich cruising PV for a different pump cam profile? (assuming you do some cruising)
.
 

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Discussion Starter #17
Well vinnie, I'm not sure if the exhaust is an issue or not.. but I do know with these headers it is way stronger than it used to be.. I broke em loose when I had a 15 roll going and bounced it off the rev limiter.. never did that before lol. What do you mean by lash loop?
 

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Discussion Starter #18
I was considering going to a 6.5 pv. Right now I have a 7.5 pv, and idle vacuum in gear I'd at 8in/hg.. but it sure loves that 7.5 for part throttle.
Maybe you could trade the rich cruising PV for a different pump cam profile? (assuming you do some cruising)
.
That's not a bad idea bogie.. I could probably live with a 6.5. It's just nice having the extra fuel come in without having to crack the secondaries.

With the 71 primaries it runs much cleaner. I went from an ngk bkr7e to a bkr6e plug and it seems to like this plug/jetting.

I also backed out a smidge of timing from the vacuum advance, it's now set to 48° with full mechanical advance and vacuum.

Seems to run very well... I do need to tweak the primary high speed bleeds. I went from a .036" to a .038" and it richened up a tiny bit at cruise.. only about .2 or .3 AFR. I think I'll try a .034" bleed and see if that moves the primary fuel curve up a bit, I think it's overlapping with my idle circuit.
 

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a lash loop is changing the valve lash up or down about .002 from stock. If you tighten the lash and the engine perks up then you have not enough cam and vise versa.You have to do the exhaust and intake independently.If you tighten the exhaust lash 2 thou and it helps then you need more breathing on that side, That could mean more duration and or freer exhaust system

you get the idea?

your cam has no split so your int/ex ratio should be around 78-80%.
If your heads are weak on the exhaust side then a split cam is usually the fix
 

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That's not a bad idea bogie.. I could probably live with a 6.5. It's just nice having the extra fuel come in without having to crack the secondaries.
Actually, it's Buzz (or Randy), not "Bogie"... but, anyway, may want to check your vacuum at normal cruise MPH range and get the power valve lower than that... I assume you don't want the PV open while cruising for better MPG and clean plugs, but ready to open whenever you tip into more throttle...

Headers/free flowing exhaust system definitely make a different... in an extreme case, the 1960's Chevy L88 427" engine on engine dyno made 375 HP with its stock iron manifolds/duals kink bent exhaust system/stock mufflers... but 560 HP with open headers... nearly a 200 HP difference! ... And a good mandrel bent 3" dual exhaust system with X-crossover and free flowing mufflers takes away almost none of that HP, and may even deliver slightly more torque...
.
 
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