3 Carb manifold- fuel injection?

Moderator: Mod Squad

biggyr
Registered User
Posts: 14
Joined: Thu Feb 27, 2014 2:16 am

3 Carb manifold- fuel injection?

Post #1 by biggyr » Tue Jun 23, 2015 9:32 pm

Last year I bought one of those Offy 3 carb "manifolds" for my 250 six. You know the kind, they take 3 Holley single carbs and bolt on top of the stock log manifold. You mill the top of the log manifold then drill holes for the two extra carbs and also drill and tap mounting holes for the bolts that hold it all together.

Rather than futz around with sourcing NOS carbs and linkages why not just convert to Megasquirt with three independent throttle bodies? I would like to keep the old school 3 carb look but with fuel injection instead. Anybody have any thoughts on this?

The stock log manifold is really bad for airflow and fuel distribution. I think this would go a long way to evening out the cylinder flow mismatch. It doesn't make sense to me to just go with a single TBI setup with the poor design of the integral manifold.

John in Kalifornia

User avatar
rbohm
Assistant Admin
Posts: 5617
Joined: Fri Nov 08, 2002 4:00 am
Location: Tucson, AZ
Contact:

Re: 3 Carb manifold- fuel injection?

Post #2 by rbohm » Tue Jun 23, 2015 10:42 pm

the ford taurus from 86-89 with the 2.5l four cylinder engine has a throttle body fuel injection unit that bolts to the large log head in place of the carb. three of those are what i think you are looking for, and you can find them on ebay for about $100.
64 falcon
66 mustang
05 grand marquis

my mind is aglow with whiriling
transient nodes of thought
careening through a cosmic vapor
of invention

biggyr
Registered User
Posts: 14
Joined: Thu Feb 27, 2014 2:16 am

Re: 3 Carb manifold- fuel injection?

Post #3 by biggyr » Wed Jun 24, 2015 1:36 am

Thanks, that's what I was looking for. I think these can be had with integrated iac, tps and fuel pressure regulator which would be nice. Might be "overcarbed" and have a flat spot at off idle throttle. Might have to use a progressive linkage like with the carbs?

John in Kalifornia

User avatar
rbohm
Assistant Admin
Posts: 5617
Joined: Fri Nov 08, 2002 4:00 am
Location: Tucson, AZ
Contact:

Re: 3 Carb manifold- fuel injection?

Post #4 by rbohm » Wed Jun 24, 2015 5:01 am

they have an idle speed controller, not an idle air controller, does a similar thing but in a different way.
they also have an integrated fuel pressure regulator and a throttle position sensor. as for being over carbed, that is the beauty of efi. the computer controls the amount of fuel sent through the injector, and as such has a much tighter control over the air/fuel ratios, thus you can have all three throttles open at the same time, and the same rate, without over fueling the engine. in fact you may have issues if you do use a progressive linkage. dont try to treat the efi system like you would a carb, they operate in two different manners.
64 falcon
66 mustang
05 grand marquis

my mind is aglow with whiriling
transient nodes of thought
careening through a cosmic vapor
of invention

biggyr
Registered User
Posts: 14
Joined: Thu Feb 27, 2014 2:16 am

Re: 3 Carb manifold- fuel injection?

Post #5 by biggyr » Thu Jun 25, 2015 1:31 am

I see your point; the fuel flow rate is determined by the computer and not by the velocity of the air thru the carb. However, opening all three butterflys at once would casue a huge jump in airflow and abrupt throttle response. Maybe some throttle bodies from a motorcycle would be more appropriate.

John in Kalifornia

User avatar
rbohm
Assistant Admin
Posts: 5617
Joined: Fri Nov 08, 2002 4:00 am
Location: Tucson, AZ
Contact:

Re: 3 Carb manifold- fuel injection?

Post #6 by rbohm » Thu Jun 25, 2015 2:16 am

you are still thinking carbs, not fuel injection. true when you open the throttle bodies, you do get a jump in airflow, but not as big as you think. even with carbs you dont get a big initial jump in airflow as the engine is going to consume only as much air as it needs regardless of how wide open the throttles are. and dont forget that the throttle body opening in the same size as the large log carb throttle opening.

understand that air has weight to it, and it takes time for that weight to accelerate which means the ECU has time to increase the fuel flow to the engine as needed, and it is far better controlled than a carb is.

one more thing, carb run off vacuum, where as fuel injection runs off pressure. so you cant think carbs and try to translate that to fuel injection. there is a point where too large of a throttle body will have a negative effect on low end power, but you need to go much larger than the three throttle bodies the size of one barrel carbs.
64 falcon
66 mustang
05 grand marquis

my mind is aglow with whiriling
transient nodes of thought
careening through a cosmic vapor
of invention

biggyr
Registered User
Posts: 14
Joined: Thu Feb 27, 2014 2:16 am

Re: 3 Carb manifold- fuel injection?

Post #7 by biggyr » Tue Jun 30, 2015 3:42 pm

How about three throttle bodies off a motorcycle?

User avatar
rbohm
Assistant Admin
Posts: 5617
Joined: Fri Nov 08, 2002 4:00 am
Location: Tucson, AZ
Contact:

Re: 3 Carb manifold- fuel injection?

Post #8 by rbohm » Tue Jun 30, 2015 6:01 pm

they would work also, and they would have similar airflow characteristics as the tempo/taurus throttle bodies
64 falcon
66 mustang
05 grand marquis

my mind is aglow with whiriling
transient nodes of thought
careening through a cosmic vapor
of invention

Who is online

Users browsing this forum: No registered users and 2 guests