BOTH the OFFY and the SY1 were DESIGNED for producing max power in the 3500rpm -7000rpm plus band in the 302 displacement SBC, used in the ORIGINAL TRANS AM RACE CARS, they work great on the larger SBC PROVIDED the cam, compression ratio, and CYLINDER HEAD FLOW RATES and gearing keep the engine in its effective opperational band which stays at or about 3000rpm- 7000rpm due to the design of the runners length,and cross sectional area.
since they are basically max effort race intake designs no effort was made to make the under 3000rpm performance work very well, and youll notice that if you install either intake, yet above about 3000-3500rpm with a 383, with 10.5:1-11:1 cpr and the cams listed below they come alive once the rpms build...if youve got the manual trans and correct rear gearing to keep the rpm band in the 3000rpm-7000rpm band.
BTW its kind of a waste to install a HYDROLIC cam as most hydrolic cams enter valve controll issues in the 6000rpm range,area, where the intakes pull very effectively, so youll want a solid lifter design that can reach 6500rpm plus to 7000rpm, too maximize the intakes potential, OR too install these intakes on heads that flow less than about 250 cfm at .500 lift as that would restrict the intakes flow potential.
both can support 450-500HP as they flow 270 cfm plus (UN PORTED)while the stock cross fire flows less than 190cfm,the differance in power is simply amazing IF you match the components correctly so ALL the components maximize that 3500rpm-7000rpm power band.
NOTICE the stock cross fire restricts power badly above about 4000rpm with its 190cfm flow,so they are totally differant types of intakes even thou thier design looks similar.
if you use a basic 383 engine size they produce even better power.
Ive found the cam that works best is usually the 114681 or 110921 CRANE solid lifter cams and a 4.11-4.56 rear gear ratio and a manual trans.
now Id point out that they both can be a P.I.T.A. to tune if your not good with carbs and the offy is designed for two small four barrel carbs, in the 450cfm-600cfm range, the SY1 for a single 780-850 cfm
YOU can ADAPT the bases to use the STOCK or modified TBI injection off the cross fire, but it requires an adapter plate to match the base,
theres a guy on this site than makes the correct adapter plates.
BTW the SY1 doesn,t hit the windshield wiper motor while the OFFY does, and both will require a small dia. distrib. the OFFY is still easy to buy off summit racing, the SY1 is only available USED
BRAND NEW LOWER WITH GASKET $344
dual quad lid $150 NEW
BRAND NEW LINKAGE $117
this link gives you good info
BTW Ive used BOTH intakes with CARBS several times ,on my cars and would not hesitate to do so again,and they look and run great once tunned correctly, with a conversion to TBI it easily something you can do, but it takes more work and while Ive worked on a couple cars that were modified that way, its not as easy as carbs, too use and tune, so while its a huge potential increase over the crossfire, the TBI, will require a bit more effort