Low performance offenhausen duel plane

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Jammer4jm
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Low performance offenhausen duel plane

Post #1 by Jammer4jm » Sun May 07, 2017 12:17 pm

[font=Comic Sans MS][/font]I've got a 85 f150 with a rebuilt 300 mild cam chevy 1.7 rockers offenhausen duel plane heholly 4 barrel 7452 and headers 3X2 to thrust glass packs. Had the head intakes and exhaust polished. Timing is set at 12 Dec with a recurve distributor.no vacume leaks.
So the problem is that I'm only getting about 10 miles a gallon and it doesn't have the increassbin power that I was expecting. It has a c6 Tran with 308 rear end gears.. also I have never seen any type of linkage to utilize the kick down cable and the cruzcontrol .I
Last edited by Jammer4jm on Fri May 19, 2017 2:53 pm, edited 1 time in total.

F-250 Restorer
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Re: Low performance offenhausen duel plane

Post #2 by F-250 Restorer » Tue May 09, 2017 11:11 am

Hello. Generally, if you want a response from readers, it is best to ask a question.

pmuller9
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Re: Low performance offenhausen duel plane

Post #3 by pmuller9 » Fri May 19, 2017 10:34 pm

What is the ignition timing at 3000 rpm with the vacuum advance disconnected?

Jammer4jm
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Re: Low performance offenhausen duel plane

Post #4 by Jammer4jm » Sun May 21, 2017 5:53 am

At 3000 it's at about 35 reg with the vacuum Advance disconnected with it connected its at about 45deg

Off road 200
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Re: Low performance offenhausen duel plane

Post #5 by Off road 200 » Sun May 28, 2017 10:25 pm

I couldn't find Holley 7452. How many CFM? Are you overcarbed? Next, 3.08 is a pretty long legged gear. I would look at replacing that first if you want performance.

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Re: Low performance offenhausen duel plane

Post #6 by guhfluh » Sat Jun 03, 2017 11:15 am

10mpg doesn't sound too far off for that combo, depending how rich the carb is running and how you're driving it. The C6 is a power robber and the carbs are slightly richer than needed from the factory. Stop and go city driving only and always getting into the power valve to keep up with traffic with the tall gears doesn't lead to good mileage. The carb could be boiling over on a hot soak also.
1967 F-250 Crew Cab 2wd, 300 6cyl, T-170/RTS/TOD 4-speed overdrive
240 head, Offy C, EFI exhaust manifolds, Comp 268H, mandrel 2.5-3" exhaust, Edelbrock 500, Pertronix ignitor and coil, recurved dizzy. 200whp/300wtq

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Re: Low performance offenhausen duel plane

Post #7 by 86f250straight6 » Wed Jul 05, 2017 7:33 pm

I think your biggest obstacle is the 3.08 gears. what size tire are you running?

in my opinion the i6 is kinda slow and was designed to not have enough power to break itself. I love the engine and it is reliable as heck but just dont expect too much out if it. Ive spent alot of time and money modifying the 300 (with 4.10 gears as well) even converted to fuel injection and honestly its just not what I was led to believe. my 03 dodge 5.9 cummins work truck could outrun/outpull it running on 3 cylinders. That said, the 300 still powers my main welding truck at 300k+ miles and it works well at this but not a highway runner, not a good choice to pull a heavy trailer thru the mountains, but it will move that welding rig around till the body and frame rust away around it. If you want a truck to race at the stoplights with id consider an engine swap or a new truck
I'm trying to build a truck that I can't break :bang:

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Re: Low performance offenhausen duel plane

Post #8 by Superbum » Fri Sep 01, 2017 4:47 pm

First off check and make sure your vacuum advance is working right. A borrowed vacuum pump and a timing light is all that's needed. Next use a vacuum gauge to manifold vacuum and test drive the truck noting when you put it under a good amount of load say 1/2 throttle but lower rpm so the secondarys don't kick in. That will give you an idea of what power valve you need to get for you're carb. When you change the power valve, which I'm pretty sure you'll have to, see what jets you are running. 56 to 58 is a good start point if I remember correctly. They are probably way too big. Don't worry about the secondary side until you have the primary side figured out. Don't worry about milage until you have the power thing figured out, then the milage will be easier.
10 mpg for a 300 build like your set up is not good. You should be able to get 14mpg with that stetup and you should feel a good amount of power.

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Re: Low performance offenhausen duel plane

Post #9 by MidSouth Motorhead » Tue Sep 05, 2017 2:10 am

Ever back-fired (even once)? Check the power valve. Holley power valve are notorious for rupturing at the slightest hiccup, which will cause it to run rich.

For clarity sake, are your running the Offy Dual Port 6019DP (primary and secondary have separate/siamesed runners)? That can be a real torque producer with the right carb. Best I've seen was an old Carter 400cfm AFB. 390-400 cfm is all you need pulling under 5,000 rpm. Keeping the carb cfm reasonable combined with the smaller primary runners (along with proper tuning) should give you great throttle response and low-rpm torque. I've seen that combo (+ header) in an E350 lwb van getting better fuel economy than you're reporting. It pulled a big boat like there was no tomorrow.

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Re: Low performance offenhausen duel plane

Post #10 by THE FRENCHTOWN FLYER » Tue Sep 05, 2017 8:54 am

Holley sells an anti-backfire kit that you can retrofit to older carbs that do not have a power valve check valve.
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Re: Low performance offenhausen duel plane

Post #11 by Superbum » Tue Sep 05, 2017 4:00 pm

Summit also has a kickdown cable kit for the c6 p/n 20039 witch could also help.

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Re: Low performance offenhausen duel plane

Post #12 by Superbum » Thu Sep 07, 2017 4:18 pm

Sorry I meant a jet size of 46-48.

Keystoner16
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Re: Low performance offenhausen duel plane

Post #13 by Keystoner16 » Sat Nov 11, 2017 9:27 am

Hello.
Slightly off topic, but what kind of compression readings do you have on your rebuilt 300?
Thanks.

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Re: Low performance offenhausen duel plane

Post #14 by old jupiter » Wed Nov 15, 2017 8:12 pm

If you have to work on your C-6 at any point, an outfit called Level 10 has long claimed that it's , , trofit kit with a rolling element thrust bearing will have a noticeable effect on fuel economy . . . .

-- old jupiter, old smitty, or seattle smitty, depending on what the site software wants to work with when I try to log in.

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Re: Low performance offenhausen duel plane

Post #15 by crgintx » Tue Nov 21, 2017 7:56 pm

If you're not doing some heavy towing, I'd build or buy an AOD and replace the C6. The overdrive will make a difference even in city driving.
I have an '86 F150 with an Offy DP with a 500 cfm Summit carb, Hedman Headers and Petronix ignition system in a '67 points type distributor with the TOD transmission and 3.08 rear gears. I have 235/70R15's instead of 215/78R15 for tires. Worst mpg tankfuls of just pure city driving are about 12 mpg. Pure highway driving mileage is about 18.5 mpg @ 75mph. I'd look into the Summit 500 cfm carb with a 12 inch air cleaner with a good 2.5-3.0" quality air filter. That's the largest air cleaner diameter you can fit on the engine without interference.

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Re: Low performance offenhausen duel plane

Post #16 by old jupiter » Thu Nov 23, 2017 5:47 pm

Well, if you want to upgrade an AOD, you can do that too. Check out a Canadian outfit, Lentech.

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Re: Low performance offenhausen duel plane

Post #17 by 1967 Tempest » Fri Jun 08, 2018 3:36 pm

My 300 runs great and is pretty quick for a 1 Barrel. I have 3.08's as well. I think as previously stated that there is a vacuum leak some where.

I was looking at Holleys 570 SA carb for when I remove all of the smog stuff. I also have 31" tires in the rear. I would also like to see if the PCV valve is stuck. :rolflmao:

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Re: Low performance offenhausen duel plane

Post #18 by J.R. » Tue Jul 03, 2018 1:41 am

old jupiter wrote:If you have to work on your C-6 at any point, an outfit called Level 10 has long claimed that it's , , trofit kit with a rolling element thrust bearing will have a noticeable effect on fuel economy . . . .

-- old jupiter, old smitty, or seattle smitty, depending on what the site software wants to work with when I try to log in.


Don't know if the OP still visits this forum, but if a C6 has to come apart, in addition to the Torrington roller thrust Seattle Smitty mentioned, a shop can re-assemble the C6 using an E4OD's 1st & 2nd gearset. That's a 2.71 first & 1.54 second, roughly 10% & 5.5% lower respectively, compared to the C6's factory 2.46 first & 1.46 second. Might even cost less than replacing that slow-off-the-line 3.08 rear end ratio!

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Re: Low performance offenhausen duel plane

Post #19 by 1bigsixdude » Sun Aug 19, 2018 9:38 am

Amazing how many people mount the carb wrong on the DP Offy front to rear vs. "sideways" with primaries facing engine. I have a DP Offy on my 300 with a custom ground cam I had Rubin at Chet Herbert do up for me (does all my cams), running Cleveland 1.73 stock rockers, 4gc Rochester 385cfm carb.& fi exhaust, trans is manual 4spd od top shifter with 4.10 gears in a 8.8. 20 mpg, HEI dist. It runs nice. 77 f100 LWB 85 f150 drivetrain.

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Re: Low performance offenhausen duel plane

Post #20 by Backwoodswalker375 » Mon Aug 20, 2018 2:59 pm

If.memory serves me good today. The small quad gc was 486 cfm. Perfect size for your build. I have seen carbs mounted with primaries toward engine side and fender side both. They seem to run the same. This is by "seat of pants dyno". I like the C myself.

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Re: Low performance offenhausen duel plane

Post #21 by broncr » Mon Aug 20, 2018 11:40 pm

Summit has a pretty cool CFM calculator. https://www.summitracing.com/newsandeve ... calculator

Guys were over-carbing engines when I first started racing, in the early 70's. I beat a lot of them by correctly sizing the carb - anywhere from 50 to 250 cfm less than all the "bigger" is better - (if you don't mind BOG) - crowd.

You need to rev your 300 to about 5000 RPM to "need" a 450cfm carb.
'82 FSP Bronco. Just about every mod ever mentioned. ( Too much to list - or remember...)

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Re: Low performance offenhausen duel plane

Post #22 by THE FRENCHTOWN FLYER » Tue Aug 21, 2018 8:15 am

X2 ^^^
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Re: Low performance offenhausen duel plane

Post #23 by broncr » Thu Aug 23, 2018 4:21 pm

The 450 will work if you can manage to keep your foot (well) out of it below 5000 RPM. With a 4 bbl quad, you are only running on ~1/2 the CFM until the secondaries open up. That means you are effectively running a pretty well matched 225 CFM for lower RPM's - good for up to ~2500 RPM. But if you romp on it at low RPM's, you're just asking for relatively poor performance ( BOG!).

I've got (had/will soon have again) a pretty tight 300, and the 390 Holley is a bit too much for it. It calculates OK for a 4K racing motor, which is the most I ever (almost never) run on the street. By playing with the vacuum secondaries and only going past 1/2 pedal at over ~2000 RPM it gives a pretty crisp response. Unfortunately the 390 appears to be the smallest 4 BBL commonly available (?)\

A 2BBL 350 is also pretty optimally sized for most 300's, but requires some pedaling finesse - nothing one can't get used to.

Like I said - the CFM calculator, linked above, is a very nice tool for determining carb sizing. Be realistic about what RPMs you are actually spinning.
'82 FSP Bronco. Just about every mod ever mentioned. ( Too much to list - or remember...)

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