Factory Five Racing Forum banner
1 - 12 of 12 Posts

·
Premium Member
Joined
·
972 Posts
Discussion Starter · #1 ·
The engine in my MK3.1 came from my donor 88 Mustang. The Mustang was built to be a hot rod by a guy I never got to meet for a kid that had limited resources. The kid that had the Mustang was out of a job, out of money, living at home with lots of parental pressure to get rid of the Mustang. He ran an ad and made me a good price on the car.

A few years later I had a local performance shop go thru the engine. He noted the 302 HO block was punched and stroked to 347 with a reasonably strong cam. etc. It had good internals and he thought it would last me a long time. He did note some issue with the way the engine was balanced (unmatched flywheel and balancer I think) and fixed that. The engine runs pretty strong and makes the Cobra scoot along.

I was encouraged by this site during the Cobra build to swap the single plane intake with a dual plane intake. Good advice which I ignored. The engine had what turned out to be a junk Holley that the above performance guy replaced with the QuickFuel 680 with vacuum secondaries. The Cobra has an automatic trans with a 3:08 rear end. It was too violent for me with the 355 and shift kit.

So here I am many years later dealing with starting problems after a long winter sit (another post is open on this) wondering if I should consider making a switch to a dual plane intake while I am working on my starting and running problems or leave things alone. I'm doing very little driving now. I think I drove like 300 miles last year.

Any good suggestions one way or the other will be appreciated. Jim
 

·
Registered
Joined
·
223 Posts
Why switch? It ran well or so it seems from your writeup. Maybe just going through the carb to replace rubber parts and clean it out, bad gas might be the problem. The plugs, cap, rotor, new fuel filter (cheap parts) and fresh gas. Then give it a try. These are all good things to do and not "make work, waste money" maintenance. Might want to adjust the valves and check the timing too. This is all tune-up stuff. Then if it doesn't run well your problems are other than the manifold.

If it runs well then you can make a better guess as to the benefits of changing the manifold. Basically, the dual plane manifold will move your power curve lower a few hundred RPM which might help, might not, depending on your driving style.

I'm considering going the other way. Started with a Performer RPM - dual plane, currently running a Vic Jr - single plane but have a Victor - bigger single plane, which will move the power curve up a few hundred RPM, got plenty of bottom and can live with less, might like more top end power. Kinda like traction control for a 427w.

Jim
 

·
Too Cheap to paint!
Joined
·
6,542 Posts
The dual plane is much better suited for torque and lower RPM's. probably a good match for the stroker.

The single plane is more for higher RPM's, and constant throttle opening.

Check your fuel pump.

I vote dual plane
 
  • Like
Reactions: Lickity-Split

·
Registered
Joined
·
68 Posts
Gotta drive it more! I drove 300+ miles last weekend and another 100ish today! Odo is at 9971. Hoping to get that photo op 9999 and 10000 this weekend. :)
 

·
Junior Charter Member
Joined
·
239 Posts
In 1969 I bought a new Z28. M21 (2.20 first gear), 3.73 rear. The engine was very happy running to 7,000 rpm before shifting. Power started to drop off around 7,200. So as 25 year olds tend to do I decided it would be a good idea to switch to a Tarantula SP intake. Car ran GREAT above 3,000 rpm. Spun up real fast. Around town it sucked. With the M21 normal driving was impaired. 10 years later I reinstalled the DP manifold and fell in love with the car all over again. With your choice of rear end ratios I highly recommend the DP intake.

Krusty
 

·
Charter Member
Joined
·
1,112 Posts
I would limit changes to one thing at a time. Get it running first, and drive it a little. Then change to a DP if you still feel like making more changes. If you attempt to change the intake manifold before getting it to run again, before fixing the original problem, then you have multiple things to think about as the source of why it won't start.
 
  • Like
Reactions: Greg_M

·
Premium Member
Joined
·
972 Posts
Discussion Starter · #8 ·
Got the Cobra running this weekend. It is OK, but has issues. My 2022 First Start thread provides details. During that process I got thinking about changing the intake. The dual plane would probably be a nice improvement, but I've decided I'll leave well enough alone. As Jim1855 noted earlier in this thread "why bother". Good advice that I'll take. I've got another thread still going on the 2022 Starting Issue and maybe will have another depending my research here on the new power steering driving issues that came up in my maiden drive to town Saturday. Thanks for giving me your thoughts. Jim
 

·
Registered
Joined
·
223 Posts
A good video, lots of reasonable information and I have experienced much of what is shown from many miles and lots of dyno runs.
What's not mentioned is drivability and this is what I think many owners are after or at least the huge switch to EFI seems to indicate this.
Carb size matters and will have a large effect on how an engine drives. Smaller carbs will have a higher air velocity and run smoother in the lower RPM range. Big carbs make upper RPM power and it can be a huge tradeoff. The video shows a 950cfm Holley for the 347, wouldn't be my choice. From my experience a 940 was too much carb for my 427w until about 5,500 and up, sang wonderfully at 7,000 but most won't be running in that range. I lost more power under 5,500 than I gained over 5,500 compared to a 780. The video charts show this happening but with manifold and spacer changes.
The balance between engine components is art with a bit of science thrown in, well, actually a lot of science but the math is daunting at best. All the components, compression, displacement, manifolds, headers, carbs and cams need to "play" well together. Get one way out of whack and the whole system faulters.
Jim
 

·
Registered
Joined
·
136 Posts
If you love to drag race the car and run between 5000-7200 rpm the single plane is your jam! If you drive the car on the street and like to launch between lights and drive around most of its life, a good dual plane is great for that. Carb size matters too! Just think of how you use the car and go from there. Drive the car! Then need it just like we do :D
 

·
Registered
Joined
·
2 Posts
Yesterday on the Motor Trend channel (Road Kill Show) there was a full half-hour devoted to comparing single & dual plane intakes on a 383 Chevy hooked to a dyno. They also added tests on 1" carb spaces. One was open like the single plane intakes. The other was 4 holer. Some of the graphic displays on the dyno were pretty full as they began comparing all of the tests against each other. I had a 1" spacer on my single plane when I bought the donor. Interesting but nothing to contradict the advice on this thread.

Think I had problems with hood clearance using the spacer with the Cobra oval air filer. I went looking for the spacer after the TV episode, but couldn't find it. I do rarely throw stuff away after a decade.
 
1 - 12 of 12 Posts
Top