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Author
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Topic: DESTROKING A SBF
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93mustang50
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Member # 6642
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posted
 
 
I have a 91c-4, my best friend wants to buy it, he wants to build the motor and turbo it. We are both ex GM guys(we both have seen the light, lol), and a setup that we did once on a chevy 350 was sweet. i was wondering if it is possible on sbf's? just to let you know a chevy 305,327, and 350 are the same block(minus 2 bolt and 4 bolt mains). We did a 305 crank with i think was a 5.2 inch rod, with a nice top end setup(carb setup). we put it in his 71 nova, then we took it a local dyno shop, the #'s were decent, but the kicker was it made 412hp at 7600rpms(that was a fun car), that was peak hp. He wants know if something like that is possible on a sbf?
[ November 11, 2006, 02:37 PM: Message edited by: 93mustang50 ]
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93GT 306 H/C/I supercharged
06 Cummins 6spd
90 GT 5sdp daily driver HCI
16 Hellcat Charger
91GT aod(sold)
88GT5spd(totaled)
Posts: 213 | From: Vacaville | Registered: Mar 2006
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kingmoochr
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Member # 5834
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posted
302 is about as small as you get on a ford (or 289) any larger crank is aftermarket. i dont think theres really anything you can do easily to get a 302 to rev that high, but it is possible, just go super lightweight and beef up the valvetrain. however, none of that would really be economical and you could probably make much more power for the money with a larger motor.
Posts: 881 | Registered: Jul 2005
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2stangs69-91
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Member # 1951
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posted
what about a 260 sbf not sure if the crank has less stroke I think it might.
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69 Mustang on hold
1991 LX hatch getting a make over
1994 F150 4X4 351
2006 Yamaha V-max 1200 Modded
Posts: 3711 | From: Redding | Registered: Oct 2002
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93mustang50
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Member # 6642
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posted
i what thinking more along the lines of a 351 with a smaller crank and longer rods. i forgot to add, when my friend and i did that chevy motor, we had to use pontiac 350 rods. we got the idea from a hot rod mag(AKA chevy motor mag). if a 351 block has the same length in crank space, as a 302,289, or even 260 could work,(with the exception of the rods) i dont see why it cant be done? any block, crank, and rod size info?
[ November 11, 2006, 03:14 PM: Message edited by: 93mustang50 ]
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93GT 306 H/C/I supercharged
06 Cummins 6spd
90 GT 5sdp daily driver HCI
16 Hellcat Charger
91GT aod(sold)
88GT5spd(totaled)
Posts: 213 | From: Vacaville | Registered: Mar 2006
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kingmoochr
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Member # 5834
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posted
351 crank specs should be the same as a 302. im not 100%, but the only external difference is the deck height. however, you are going to have to basically look into 351 conversions, as you will need modified exhaust and intake manifold as opposed to stock as well as other 351 parts
Posts: 881 | Registered: Jul 2005
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88bird
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Member # 1424
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posted
A 351 with a smaller crank and rods is a 302, they both use the same bore, just different stroke. The 351 block is heavier and can handle more horsepower, but for what you want, use a 302 and get ultra light pistons, ultra light 5.4" Ibeam rods and light weight crank, build a long rod 306, (302 .030 over) and use a high duration camshaft with moderate lift and rev the liv'n piss out of it. Use a 750 carb with a victor style manifold. The only limitation will be cam and valvesprings.
Pay attention to the gram weights of the parts, You will see a noticeable between manufactures.
My local machineist just finished up a 331. He used the lightest components he could find then lightened them even more. This engine would rev like nobodys business. It rev'ed like a 14to1 800hp sprint car engine, just exploded with rpm.
[ November 12, 2006, 12:43 AM: Message edited by: 88bird ]
Posts: 149 | From: Citrus Heights | Registered: Jun 2002
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1988Stang
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Member # 6280
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posted
This doesn't have to due with what you are trying to accomplish but since i know a lot about chevy stuff i thought i would let you know some facts about chevy's.
A 305 small block is a 5.0 liter with a bore size of 3.736 in.
A 327 small block is a 5.4 liter with a bore size of 4.000 and a crankshaft stroke of 3.250 in.
A 350 small block is a 5.7 liter with a bore size of 4.000 and a crankshaft stroke of 3.480.
The 305 motor is a totally different block than the 327 and 350.
So the 327 is a de-stroked 350, that is why they rev so fast and so high. A de-stroked stroker will usually always rev faster and usually higher than an up-stroked motor.
In a chevy world but with a ford motor, you could do this if some how you could put a 302 crankshaft in a 351 motor and de-stroke it to something like a 325 cubic inch. That would rev quick and with the right cam it might be able to rev high, i don't know because i haven't built nearly as many Ford motors as i have chevy's, so i haven't experimented(ck.sp.) with them yet. Good luck with the project.
Posts: 161 | From: Petaluma | Registered: Dec 2005
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kingmoochr
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Member # 5834
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posted
quote:
Originally posted by 88bird:
A 351 with a smaller crank and rods is a 302, they both use the same bore, just different stroke. The 351 block is heavier and can handle more horsepower, but for what you want, use a 302 and get ultra light pistons, ultra light 5.4" Ibeam rods and light weight crank, build a long rod 306, (302 .030 over) and use a high duration camshaft with moderate lift and rev the liv'n piss out of it. Use a 750 carb with a victor style manifold. The only limitation will be cam and valvesprings.
Pay attention to the gram weights of the parts, You will see a noticeable between manufactures.
My local machineist just finished up a 331. He used the lightest components he could find then lightened them even more. This engine would rev like nobodys business. It rev'ed like a 14to1 800hp sprint car engine, just exploded with rpm.
deck height of 351 is 9.5, deck height of 302 is 8.2. not the same block. however, im pretty sure if he tried to run a 302 crank in a 351 block, hed have to have custom rods made, there arent any interchanges that will work i dont think
[ November 12, 2006, 01:26 AM: Message edited by: kingmoochr ]
Posts: 881 | Registered: Jul 2005
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BlueOvalRacing
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Member # 1531
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posted
quote:
Originally posted by 88bird:
A 351 with a smaller crank and rods is a 302, they both use the same bore, just different stroke. The 351 block is heavier and can handle more horsepower, but for what you want, use a 302 and get ultra light pistons, ultra light 5.4" Ibeam rods and light weight crank, build a long rod 306, (302 .030 over) and use a high duration camshaft with moderate lift and rev the liv'n piss out of it. Use a 750 carb with a victor style manifold. The only limitation will be cam and valvesprings.
Pay attention to the gram weights of the parts, You will see a noticeable between manufactures.
My local machineist just finished up a 331. He used the lightest components he could find then lightened them even more. This engine would rev like nobodys business. It rev'ed like a 14to1 800hp sprint car engine, just exploded with rpm.
Ray? is that you? ![[Wink]](wink.gif)
All of you youngsters probably forgot the 221 small block, with a 3.5" bore but it shared the same crank as the 260 and 289 (2.87" stroke)engines. That's as destroked as you will find in a stock configuration. Hell, the 221 would actually breathe OK with a set of reworked stock heads. Hmmm, maybe even a set of Mondello ported hipo's.... Nevermind.
Don't forget about the deck height difference between the 302 and 351 engines. There would be no reason to destroke a 351 when the 302 is smaller & lighter and has the same bore size. A long rod 302 version might be the ticket.
93Pony? Opinions?
Posts: 1159 | From: Pioneer | Registered: Jul 2002
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93mustang50
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Member # 6642
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posted
this good info guys but,you are telling me things that i all ready know, and some or you are way off. It might be my fault, maybe i didnt ask the right questions. some of you have the right concept i was thinking of. 302 and 351 are not the same size block, you would know that if you have ever worked on them. ok here is the question can i take a 351 block and use a 302 block crank(260,289,302,maybe even a 331 crank) use a longer rod, and make a light weight high reving motor. this is just a project, there are very credible members on this forum, and i am hoping that some of them have already tried this or have seen it done.
[ November 12, 2006, 04:03 AM: Message edited by: 93mustang50 ]
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93GT 306 H/C/I supercharged
06 Cummins 6spd
90 GT 5sdp daily driver HCI
16 Hellcat Charger
91GT aod(sold)
88GT5spd(totaled)
Posts: 213 | From: Vacaville | Registered: Mar 2006
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93mustang50
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Member # 6642
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posted
I just called my buddy, i was wrong with the setup we did, here it is 400 block,327 crank(large journal crank,3.25 stoke,and crank spacers), 6 inch rod,and 4.165 bore. then boom a 353 high reving son-of-a-bitch.
[ November 12, 2006, 04:31 AM: Message edited by: 93mustang50 ]
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93GT 306 H/C/I supercharged
06 Cummins 6spd
90 GT 5sdp daily driver HCI
16 Hellcat Charger
91GT aod(sold)
88GT5spd(totaled)
Posts: 213 | From: Vacaville | Registered: Mar 2006
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88bird
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Member # 1424
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posted
To directly answer the question, 302 crank will not, fit due to main journal size.
Posts: 149 | From: Citrus Heights | Registered: Jun 2002
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93PONY
Mr. Valve Events
Member # 60
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posted
Couple of things to think about...specifically with reguards to an EFI SBF.
The intake manifolds are severely restrictive compared to a carburator setup. Just recently we had a 408 on the dyno that picked up 35hp from a spider intake w/elbo to a 4 barrel Accufab TB. 35hp from removing one 90 degree bend in the intake tract. Now imagine the typical 5.0 intake design with a 180 degree bend & a small plenum. By comparison high-rpm import 4-bangers have a plenum size comparable to the displacement of the engine. The 5.0 has less than half the plenum volume to cubes that a Honda has.
What all this means is that it's VERY difficult to get RPM out of an EFI 5.0/5.8....reguardless of stroke. With a blower it's much easier....as RPM's increase so does boost & therefore HP. But N/A....the SBF EFI intake is too damn restrictive.
99% of the EFI 5.0/5.8's make peak power under 6500rpm. Given this limitation, it's best to build a motor that takes full advantage of a 6500rpm ceiling.
Sure, you could de-stroke a 302, but if you're limited to 6500rpm with an EFI setup, a 347 limited to the same RPM will run all over any destroked EFI 302. Typical gains from a 3.0 stroke to 3.4 stroke with similar velocity through the heads/intake/exhaust (yes, the 3.4 stroke will have larger heads/intake/cam/exhaust) is 75+ft/lbs of torque. It's not uncomon to see 400+RWTQ out of a truely streetable N/A 347 these days. The most I've seen out of a 306 is 340RWTQ....let alone a de-stroked 302. Given that HP = (TQ/5252) * RPM, the old adage fits the bill.... There is no replacement for displacement....at least when you're limiting yourself to EFI.
I did a few projects shooting for 7000+RPM out of an EFI SBF. The first was an off-set ground stock 302 crankshaft, custom pistons, SBC rod journals with a 5.4" rod. Made 319ci. Added a set of fully race-ported AFR185's, an RPM2 intake, 75MM TB, 1 3/4 longtubes, 3" exhaust, huge camshaft. Given the specs of the motor I was expecting peak power at 6800rpm & shiftpoints of 7300-7400. With peak TQ around 4500 & 360+ft/lbs, HP should have been right around 400RW at 6800. Things didn't quite work out....The motor made peak TQ at 4500 & hit 368ft/lbs, but HP peaked at 6400rpm & only netted 388RW. If the motor was able to rev higher I would have hit my goal....but the intake with it's 180 degree bend & small plenum volume really restricted the airflow at high RPM....resulting in a loss of power.
The next project was a 347 with a similar goal of 7000+RPM. This time I used fully ported VicJr's, a fully ported VicEFI intake, even larger camshaft & the same exhaust. TQ peaked at 388RW & HP at 422 @6400rpm....yet again, limited by the intake. The next step I took was to elnarge the plenum by 50%. This netted an increase in 10ft/lbs of TQ & HP till about 6000rpm, where the gains dropped to 1hp. 398TQ/423HP. Thinking the camshaft was too small & designed another larger profile & installed it. Power increased on the top end by 10hp, but peak RPM did not increase. The next step was to turn the EFI manifold around & mount the TB on the plenum cover....efectively eliminating the 180 degree bend in the intake design. After weeks of test-fitting, figuring out the fuel rails, etc I ran into problems on the dyno. It seemed the cover I had fabbed up had a bad weld that I could not find. A/F shot to 18:1 at WOT every time. I swapped back to the original configuration & ran the motor to a best of 395/442RW....again peaking no higher than 6400rpm.
I have stopped pursuing more RPM with this particular setup as it still uses a stock production block & I was afraid of cracking it. Although I was very close to buying a SuperVic EFI manifold & a 4-barrel Acufab TB, then converting to speed-density....or simply going Carburated.
Lots of investigation of N/A 306/331/347 dyno graphs on the net resulted in a curious observation. On every graph I saw not one made peak TQ past 4500rpm with a typical SBF EFI style manifold (non-carburator/spyder style). Not one! Haven't found one yet that can make peak TQ above 4500rpm. If this is the limiting factor in the EFI intake.....there is no point in building a motor that is intended to make peak TQ above 4500.
In other words, Either destroke it & go carburated, or better yet, stroke it & go carburated. ![[Smile]](smile.gif)
The restrictions of the EFI intake sucks for making power at RPM.
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www.advancedenginedevelopment.com
SCT dealer
Dynotuning
(916)715-7569
Posts: 4266 | From: Fair Oaks, CA | Registered: Nov 2000
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93PONY
Mr. Valve Events
Member # 60
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posted
Oh....& to throw another wrench in the mix..
Turbos slighly lower the RPM band of a motor compared to N/A. Although the boost does wonders for pushing past the restrictions in the intake, boost does not increase with RPM & therefore turbos do not increase the RPM band of the motor. Most dynos before/after turbo install result in a loss of 100-200rpm peak hp to peak hp.
With a 331 & VicJr's I shift my turbo stang at 6000rpm. My wife's N/A 347 with VicJr's is shifted at 6600. Although she runs the VicEFi intake & I run a ported Cobra....even if I ran the Vic, I'd probably only pick up 400rpm.
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www.advancedenginedevelopment.com
SCT dealer
Dynotuning
(916)715-7569
Posts: 4266 | From: Fair Oaks, CA | Registered: Nov 2000
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93mustang50
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Member # 6642
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posted
thanx 93PONY, all that helps a lot, ya i am not a fan of efi, but i am not from the south, i still have to do smog back home in az, but if i do pursue this proeject of destroking of sbf it will be carburated.
Posts: 213 | From: Vacaville | Registered: Mar 2006
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