SY-1 intake manifold experience

Recently I installed a SY-1 manifold on my small block and wanted to share my experience and possibly gain some knowledge.

I’ve read a lot of the posts on the SY-1 but still had a few carb questions and was wondering if anyone who had experience with these manifolds, like Dave @SY1, could help me out with what I’ve got going on.

My car is a 68 Camaro and I’ve recently changed a few cosmetic and mechanical things in an effort to pay homage to the vintage trans am cars. Mechanical tach, SY1 manifold, vintage style kirkey seats etc...

Here are a few details of the car:

68 Camaro
T56 6 speed
4.10 True trac rear end

Small block 383
SY-1 unmodified intake manifold
10.5:1
Solid roller 248/252 duration @.050 ~.600 lift
RHS 200cc aluminum heads
MSD Ignition
1 ¾ headers
2.5 mandrel bent magna flow exhaust

750 holley double pumper
Proform annular main body
Proform billet throttle body
71 jets and power valve up front
84 jets and no power valve in the rear

A little history on the engine. It’s been in the car for 15 years but with only a few thousand miles on it. Originally I had an edelbrock air gap and the carb had a non annular hp main body, same jets as now. It ran great and would easily pull to 7200 rpm. Only issue was a tiny flat spot at low rpm that I didn’t get around to tuning out and it never felt very responsive free reving (most likely a accelerator pump issue that I didn’t fix).

Fast forward to this year and the only thing I changed was the SY1 manifold and annular main body. My thought was that the annular main body could help with the supposed fuel distribution issues of the sy1 by atomizing the fuel better. After reading how horrible these manifolds are on the street I was very pleased by how well it ran right away. Throttle response was much better and so was low end power. Cruising 6th gear at 1800 rpm was effortless compared to the previous set up. The only real change I felt was that the car stopped pulling as hard at higher rpm and at 6600 rpm it wouldn’t pull much more. I didn’t expect that outcome and thought that it would want to pull more than the air gap at higher rpms. I then wanted to see if possibly it was a main body issue so I put the down leg booster hp main body back on and found it also didn’t want to pull much beyond 6600. I did notice with the hp main body that it was much less responsive at lower rpm compared to the annular. Back went on the annular.


I’ve seen that the carb recommendation for this manifold was the 830 or 850 holley, and that was on smaller engines. Do you think I could be so undersized carb wise (750cfm) for this larger plenum manifold compared to the previous manifold and that’s why it stopped pulling as hard or is it just an old technology manifold?

Also, I’ve seen the recommendations for stagger jetting to correct fuel distribution issues but in the write up it says for manifolds with mixture fixes. Is it worth stagger jetting even without the mixture fixes like the damns they would sometimes weld in?

Last question, do you know if they’re running progressive linkage, or front and rear power valves with the stagger jet recommendations?


The engine runs really well now just a little down on power above 5000. Street manners are actually better than ever but I’d like to get as much out of it as I can primarily for the longevity of the engine by not running cylinders lean.

Thanks so much for any help!!!

Chris

Hi Chris. I have had a couple SY1s and Offy copies of the GM xram. Will try to answer what I can.

Yes larger plenums, and the SY1 is pretty large, require more accelerator pump, Holley agrees with this. Annular booster is always a great choice. The magazines comparing carbs on mild street SBC 350 and as the cfm increases the peak power numbers peak and start to drop when then finally get to the 750. They then install an 830 and are shocked they get the best numbers. Look at the Offy carb selection chart below and see what you think. To me if you want to spin to 7200 on a 383 this chart doesn't support it, but I would seek out the opinion of the guys running 383 with similar builds as to what works for them.

Offy carb selection chart

When you removed the annular proform main body to try the old down leg body did you also go back to original throttle base plate or retain your new one? If you retained the new one then that points at the new base plate setup accel pump cam not being heavy (aggresive), enough for the new larger plenum size intake and/or the adjustment of the throttle blades exposing too much transition slot as. Make sure the throttle blades, all 4, are closed enough to only expose .030-.060 of the transfer slots, .040 is what to shoot for. If the slots are exposed greater than this the engine is idling mostly via the transfer circuit and not the idle circuit. A sign of this is having mixture screws that have very little effect when turned in and in some cases aren't able to stall the engine out when turned all the way in. High primary idle speeds will expose too much transfer slot also. High duration cams sometimes require more idle speed and expose too much transfer slot. The fix to this is a single .070-.080 hole in each throttle blade onthe IDLE DISCHARGE HOLE SIDE of the blades. I would go small .070. Some don't believe in this mod, but Holley has a video on how to do it. My NASCAR 830 annular has it and it works great. The holes will allow additional air past the blades allowing you to adjust the idle screw to properly set the transition slot to .030 if needed.

Also the new throttle base plate probably came with accelerator pump cams. What color and what position are they clocked?

Holley makes 9 different cams. Each cam has at least 2 different possible mounting holes. Usually #1 hole is most commonly used. But position 2 retains the same cam profile but moves the pump lever up the cam ramp a little for a quicker shot. Red and pink are common.

Holley accel pump cams

You said it has always been a littly lazy down low. If thats off idle a transition is circuit is often to blame. Check floats set properly, check your primary power valve is 2-3" below your lowest steady vacuum read on a gage. If all good install one size smaller squirter nozzles and drive car to see if any change. The pump cam and squirter control how much fuel and how quickly its pumped transitioning from idle to metering circuit. You now have a a very large plenum, healthy cam and probably what worked well before isnt enough fuel now. That may explain falling off accelerating through 6600, when it used to easily go to 7200 with the old intake and carb. They do make a 50cc accel pump, but not sute you you need that. If you do opt for it Holley says the brown cam is usually used for that.

I don't think you need stagger jetting. The racing Chevrolet wanted this for was the 1970 Trans-Am SCCA sanctioned and those were limited to 305 cid 67-69. I dont know what 1970 would have been. But the SY1 and the GM 2x4 both were know for having issues keeping fuel atomized at the torque box corners. But this was on a 302. You move a lot more air at a lot lower piston speeds than a 302. Just think of this as your "air pump" is a lot more efficient than a 3" stroke engine. The 302 had to spin really high to make the same power you can make at 1500 rpm lower. Engine masters dyno tested a 383 sbc comparing Vic Jr, Wieand tunnel ram and Edelbrock STR10 crossram (Eddys version of the GM). There was no fuel puddling with the 383 and the cross ram actually had the best A/F ratios, better than the tunnel ram and tq/hp graph and numbers almost identical to the tunnel ram. You shouldn't need the fuel dams. The other side note here is the 383 made excellent numbers and was all in by 6000. But you may have more cam than the mule did.

Holley site should have links to their youtube videos on the transition and idle circuits that are helpful. I hope this is helpful. You will probably get a lot of advice on your issues, keep an open mind to any posts offering fixes. These are just my approach. Others may differ. Its a very cool intake and GM actually assigned a prototype part # stamped on the lids Smokey was given to work with.

Also FYI you still have an air gap intake. Smokey Yunick designed the air gap. You can see it at the base in this old pic of a 331 I had long ago. Cheers!

Thanks for the very informative and interesting response!! Really appreciate it.

Both primary and secondary transfer slots are jest barely exposed and my idle quality is great.

I’m pretty sure my accelerator pumps are set up where they need to be. 30 cc squirters, 35 nozzle primary 31 secondary and both pink cams are flipped over to accelerate the pump enrichment. I dont have any flat spots anymore and it’s pretty snappy.

When I put the hp main body back on I did have the new billet base plate. There definitely was room for tuning on the accelerator pump side but I was just testing if there was noticeable full throttle improvement and I didn’t feel any so I put the annular back on. It really just felt smoother everywhere with the annular.

You wrote that the bigger plenums generally require more accelerator pump.

Would you say that the same goes for CFM on this manifold?

I’m wondering how much I’m leaving in the table with my 750 vs 830 vs 850. If it’s only 10hp then I could live with that but if it was noticeable, say 25 hp or more then I’d want to try out the bigger carbs.

Earlier today I pulled my carb off and decided to try 1:1 linkage on my base plate, add a power valve to the secondaries and try the stagger jetting as recommended for the 850 but with more comparable jet sizes to what I was already running. 72,78,75,72. My thought was that if there is a fuel distribution issue it would be all the time so why not expose all four corners throughout every phase of throttle.

Now that you gave me a little info on the stagger jetting and it’s importance for the smaller stroke motor only I’m not as confident that this set up will work out lol.

My plan is to get the carb back on tomorrow and see how it runs. I’ll come back with my findings. Hopefully my results are positive.

Thanks again!
Chris

Took the car out today.

Changes I made:

Stagger jetting. Primary- driver/ pass 72/79, secondary- driver/ pass 76/72, 5.0 power valves front and rear.

eliminated the progressive linkage and went 1:1

converted the main body and secondary metering plate to accept a power valve.

This is the best the car has ever felt, smoother all around and pulled harder than the previous set up. Shifted at 7200 and felt great. Mechanical jones tach says 6900 but my digital shift light showed 7200 on the recall.

Aside from the increased power and better feel I noticed two differences.

1. power comes in sooner because of the 1:1
2. It now has a sort of whistle turbo type sound when you roll into the throttle at lower rpm.

The car runs really well but I’m definitely curious on how it would run with a bigger carb.

I wonder how a 830 style base plate would do with the 1050 cfm proform annular? Maybe 900 cfm ?

Chris glad you made progress. Never used 1:1 myself. I read a lot of guys run 1:1 on track & switch to progressive for street. From what I have read the 1:1 is much more responsive but fuel economy suffers and plugs may foul on street driven car. I was going towards you needed more fuel and/longer shot. A 35 squirter is pretty large and thought it might not be giving long enough shot which could be squirter and/or pump cam, but easy to swap smaller squiters in to see if issue is in that system, then work on a correction. My 830 only uses 29 and PVs front & rear. Adding a rear PV was a good call. Maybe the stagger jetting did help too? I know on the Engine Masters 383 crossram they tried staggered but best AFRs & power #s with all 67 jets. I know comparing the 2 intake designs is apples to oranges but both had distribution issues with smaller displacement sbc.

Anyway if you ever go back to progressive would be interesting to try the likely smaller with the progressive linkage. Maybe motor will go to 7200 without the 1:1, and improve fuel economy too. Glad you are happy with the new found extra power. Its a cool set up!