Bigger jets than typical?

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Brokejoebuilds
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Bigger jets than typical?

Post by Brokejoebuilds »

So I'm having a small issue that has me partially stumped. My nova was running great (breakdown on the combo later) and I decided to put a harger exaust figuring it was a restriction. I went from a 2.5, chambered muffler deal to 3in bullets with turn outs a foot or so before the rear. It immediately started popping out the exaust and I though ok she must be lean now. I finished porting an intake for it so I waited till the following weeked to swap the intake and do a jet change in one go. Swaped from a dual plane to a single plane. I open the carb up for the first time and its jetted 84 on all corners with a pv in front. Knowing my combo I thought it may have been overly rich instead and put 79 in the front and 81 in the back to see how things changed. It fot worst and killed power noticeably. I put 84 and 86 in the rear now and it improved a fair bit. With little to no popping out the exaust. Im going to bring it to the track and do final jetting there just think its kinda in the high side already. The combo
406 sbc
11.3 compression
260 at .050 straight pattern solid flat with .600 lift
Ported enginequest vortec heads flowing in the 270 range 2.02 1.60
Ported super victor intake
4781 850 carb
1 5/8 headers and a 3 in exaust. 1 3/4 coming soon

What you guys think?
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Re: Bigger jets than typical?

Post by mag2555 »

Popping from the exh with a exh system on the car many times is from excessive fuel getting in the exh system and then lighting off when the throttle is closed and the exh gas temp goes up.

Are your intake ports and manifold runners where you reworked them polished.

Is so you may have created a wet flow problem.

On a side note, 1 5/8” headers are small for a 406 cid motor if your looking to increasing levels power above 5400 rpm.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
Brokejoebuilds
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Re: Bigger jets than typical?

Post by Brokejoebuilds »

Thats what I though but as soon as I gave it more jet it went away and pulled to 6700 clean. I port and leave a bur finish. I agree on the headers they are a carry over from when it had a milder 355. Better headers will be on the way soon.
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Re: Bigger jets than typical?

Post by steve cowan »

Brokejoebuilds wrote: Wed Mar 22, 2023 5:14 pm So I'm having a small issue that has me partially stumped. My nova was running great (breakdown on the combo later) and I decided to put a harger exaust figuring it was a restriction. I went from a 2.5, chambered muffler deal to 3in bullets with turn outs a foot or so before the rear. It immediately started popping out the exaust and I though ok she must be lean now. I finished porting an intake for it so I waited till the following weeked to swap the intake and do a jet change in one go. Swaped from a dual plane to a single plane. I open the carb up for the first time and its jetted 84 on all corners with a pv in front. Knowing my combo I thought it may have been overly rich instead and put 79 in the front and 81 in the back to see how things changed. It fot worst and killed power noticeably. I put 84 and 86 in the rear now and it improved a fair bit. With little to no popping out the exaust. Im going to bring it to the track and do final jetting there just think its kinda in the high side already. The combo
406 sbc
11.3 compression
260 at .050 straight pattern solid flat with .600 lift
Ported enginequest vortec heads flowing in the 270 range 2.02 1.60
Ported super victor intake
4781 850 carb
1 5/8 headers and a 3 in exaust. 1 3/4 coming soon

What you guys think?
When you make things bigger as in porting velocity decreases and everything moves up in the rpm range,I don't believe you're jetting is anything out of sorts at all.less signal means more jet.
Remember you are dealing with atmospheric pressure at the carb and the open collectors.
1 5/8" header will support 600hp if built correctly.
Report back with your track testing if you can.
steve c
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Brokejoebuilds
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Re: Bigger jets than typical?

Post by Brokejoebuilds »

[/quote]
When you make things bigger as in porting velocity decreases and everything moves up in the rpm range,I don't believe you're jetting is anything out of sorts at all.less signal means more jet.
Remember you are dealing with atmospheric pressure at the carb and the open collectors.
1 5/8" header will support 600hp if built correctly.
Report back with your track testing if you can.
[/quote]

I agree, and I just tested the car when I left work and this thing is moving out well. But even ported I didnt cc it after I was finished but cant imagine it be more than 180cc. Idk im hoping to test it this Sunday if the weather permits. Regardless I'm happy with it, because such a simple combination really moves out for what it is.
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Re: Bigger jets than typical?

Post by Tom68 »

Brokejoebuilds wrote: Wed Mar 22, 2023 5:14 pm I finished porting an intake for it so I waited till the following weeked to swap the intake and do a jet change in one go. Swaped from a dual plane to a single plane. I open the carb up for the first time and its jetted 84 on all corners with a pv in front. Knowing my combo I thought it may have been overly rich instead and put 79 in the front and 81 in the back to see how things changed. It fot worst and killed power noticeably. I put 84 and 86 in the rear now and it improved a fair bit.
So you took off a dual plane, leaned the jetting and put the carb back on with a single plane. Not surprised you had to change back, would have thought plus a bit.

Doesn't sound like enough split front to rear considering the rear has no PV, others will know more.

4781 should have had 80, 80, 6.5, 6.5.
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Re: Bigger jets than typical?

Post by HQM383 »

Brokejoebuilds wrote: Wed Mar 22, 2023 5:14 pm I put 84 and 86 in the rear now and it improved a fair bit. With little to no popping out the exaust.
The combo
406 sbc
11.3 compression
260 at .050 straight pattern solid flat with .600 lift
Ported enginequest vortec heads flowing in the 270 range 2.02 1.60
Ported super victor intake
4781 850 carb
1 5/8 headers and a 3 in exaust. 1 3/4 coming soon

What you guys think?
Improved a fair bit with what kind of driving? WOT, low speed cruise, highway steady state?

Those 4781 have .070” pvcr so as others have said split has the potential to be greater. By that I mean something like primary 80 and secondary 89-91. What rating # power valve are you using?
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
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Re: Bigger jets than typical?

Post by Brokejoebuilds »

[/quote]

So you took off a dual plane, leaned the jetting and put the carb back on with a single plane. Not surprised you had to change back, would have thought plus a bit.

Doesn't sound like enough split front to rear considering the rear has no PV, others will know more.

4781 should have had 80, 80, 6.5, 6.5.
[/quote]

So the popping started after I changed the exaust. I pulled the carb apart and found out it was jetted 84 squared rear pv blocked. I should of added that I bought this carb reman from a ebay seller that is what he does.
So after seeing the jetting I assumed it was overly rich and jetted down when I switched to a single plane. First time was 81 front kept the 84 in the rear. Tested it and it did more of the same jetted down one more time 79 81 and was wayyyyy worst. Then went up and the motor was super happy. I just didn't think it would be happy with so much jet with this combo.
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Re: Bigger jets than typical?

Post by Tom68 »

Brokejoebuilds wrote: Thu Mar 23, 2023 5:59 am


So the popping started after I changed the exaust. I pulled the carb apart and found out it was jetted 84 squared rear pv blocked. I should of added that I bought this carb reman from a ebay seller that is what he does.
So after seeing the jetting I assumed it was overly rich and jetted down when I switched to a single plane. First time was 81 front kept the 84 in the rear. Tested it and it did more of the same jetted down one more time 79 81 and was wayyyyy worst. Then went up and the motor was super happy. I just didn't think it would be happy with so much jet with this combo.
[/quote]

EBAY remanufactured with unknown history, so you wouldn't know if it has its original metering blocks then, sounds like you need to get some AF readings to be safe.
I'm ignoring the exhaust change, unless the jetting was right on the ragged edge changing exhaust shouldn't throw it over the edge.
Ignorance leads to confidence more often than knowledge does.
Nah, I'm not leaving myself out of the ignorant brigade....at times.
Brokejoebuilds
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Re: Bigger jets than typical?

Post by Brokejoebuilds »

[/quote]

Improved a fair bit with what kind of driving? WOT, low speed cruise, highway steady state?

Those 4781 have .070” pvcr so as others have said split has the potential to be greater. By that I mean something like primary 80 and secondary 89-91. What rating # power valve are you using?
[/quote]

So the problems I was having were all mostly wot. I have a 3.5 pv in the front and for the most part the car drives very well. At wot is when she wasnt happy at wot. My confusion is the fact it seems like it needs alot of jet for a combo that is mild compared to some of the others I built. This carb was a reman so Idk exactly what he changed. I'll pin some of the passages it later today and see.
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Re: Bigger jets than typical?

Post by Brokejoebuilds »

Tom68 wrote: Thu Mar 23, 2023 6:05 am
Brokejoebuilds wrote: Thu Mar 23, 2023 5:59 am


So the popping started after I changed the exaust. I pulled the carb apart and found out it was jetted 84 squared rear pv blocked. I should of added that I bought this carb reman from a ebay seller that is what he does.
So after seeing the jetting I assumed it was overly rich and jetted down when I switched to a single plane. First time was 81 front kept the 84 in the rear. Tested it and it did more of the same jetted down one more time 79 81 and was wayyyyy worst. Then went up and the motor was super happy. I just didn't think it would be happy with so much jet with this combo.
EBAY remanufactured with unknown history, so you wouldn't know if it has its original metering blocks then, sounds like you need to get some AF readings to be safe.
I'm ignoring the exhaust change, unless the jetting was right on the ragged edge changing exhaust shouldn't throw it over the edge.
[/quote]

Thats what I dont get. All the problems began after the exaust change. Literally the car would break up and pop out of the exaust, something it never did before. Only went down hill from there till I jetted it back up
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Re: Bigger jets than typical?

Post by BILL-C »

Make sure you are at least a little rich at WOT before heading to track. It is much easier to take a little jet out at track because you are a little too rich than put new pistons in to replace the ones you detonated from being too lean. Bring extra spark plugs also. You can't read used dirty ones.
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Re: Bigger jets than typical?

Post by HQM383 »

Brokejoebuilds wrote: Thu Mar 23, 2023 6:09 am So the problems I was having were all mostly wot. I have a 3.5 pv in the front and for the most part the car drives very well. At wot is when she wasnt happy at wot. My confusion is the fact it seems like it needs alot of jet for a combo that is mild compared to some of the others I built. This carb was a reman so Idk exactly what he changed. I'll pin some of the passages it later today and see.
Pin gauging would be a good idea to know what if any mods have been done beyond factory by the ebay guy. you will be more informed and understanding of future changes.

From the factory:

4781 850cfm specs

PMAB .026"
SMAB .025"
PIAB .076"
SIAB .055"
P & S IJ .035"
E-bleeds 2 x .026" or .028" in each main well
Kill bleed P&S .028"
PVCR P&S .070"
Jets 80 both ends
6.5 pv both ends.
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
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Re: Bigger jets than typical?

Post by 1980RS »

mag2555 wrote: Wed Mar 22, 2023 5:58 pm Popping from the exh with a exh system on the car many times is from excessive fuel getting in the exh system and then lighting off when the throttle is closed and the exh gas temp goes up.

Are your intake ports and manifold runners where you reworked them polished.

Is so you may have created a wet flow problem.

On a side note, 1 5/8” headers are small for a 406 cid motor if your looking to increasing levels power above 5400 rpm.
Ha Ha, that's funny. My 406 was quicker and faster with the 1 5/8" headers vs the 1 3/4" headers. The ET slip proved that. 11.92 with the small headers 11.06 with the bigger Hooker super comp headers. With headers bigger tubes are not always better.
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Re: Bigger jets than typical?

Post by rgalajda »

Your problem is that this carb originally had front and rear power valves. So if you run without the rear power valve then you must compensate with larger rear jets. If original was 80 front and 80 rear , with rear power valve, then aprox equivalent would be 80 front and 86 rear without rear power valve. Of course this is dependant on the size of the rear PVCR and your combo.

I assume you don't use AFR gauge.
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