Inliners International Forums General Discussion Hi Performance Which Manifold?

Hi All,

Well I am now at a crossroads. I have collected many o' intake manifolds for the Chevy inline 6 but I am going to limit the question to the single quad unless there is a compelling event to use duals or triples.

This will be for a turbo'd 292, TBI, street use, 300-400HP, torque the same, 1965 C10 (or maybe C20).

I am looking for info/advice on performance specs, personal experiences, dyno numbers (same engine, different manifolds), RPM ranges, best fuel distribution, for the Clifford, Offy, and Aussie single carb intake manifolds. Maybe even just the answer to the question, "Which would you choose?"

I know this is a lot to ask. I am hoping someone has gone before me, tried these three options and have information regarding the outcome.

Thanks to everyone on here. You are a wealth of knowledge.

When it comes to intakes The Aussie Speed has better fuel distribution Hands down Because of the design of their intakes weather it be a single or a dual , along with their blower intakes. I can send you pics if you want to see their intake. I also have them.

Hi Larry,

I already have the Aussie Speed, the Offy and the Clifford Single carb manifolds. I also have the Clifford dual, the Offy 2x1 and 3x1, and a blower manifold.

But since this one, my first inline 6 full build, will be a turbo, I figured 1 4bbl would be the easiest.

I have quite a few options but I figured I would limit the build and the questions to the 3 single carb manifolds.

W/r/t "300-400HP": apples/oranges. The engine and turbo requirements will be very different.

What blower manifold?

Do you have the turbo now?

Blow through or draw through?

Hi Panic,

I have multiple turbos. It will be blow-through. The exhaust is an SPA. Though I do have other exhaust options.

I am aware of the differences in HP. That should not change which manifold other than the more airflow required, the less the Offy is in the running. But I figure the Offy is a poor choice for fuel distribution as well.

My question is, of the following 3 intakes (I have all three), which is the best intake for the job?

1) Aussie Speed single 4 barrel manifold
2) Clifford single 4 barrel (I have the dual also)
3) Offy single 4 barrel (I have the 2x1 and 3x1 as well)

I am looking for a compare and contrast, personal experience, rpm range in which the manifolds operate best, best manifold for fuel distribution, etc..

The above query is more about the manifolds, their strengths and weaknesses, their designed purpose, their specs, and what would you choose and why...vs my particular build.

when doing a blow-through I feel it would be much easier to keep it in Tune with one carb.

I have ran single carb, dual carb, All NA but the best set-up I have ran on my older 250 was my 3x4 which track wise showed to be the best set-up. I had thought about turning into a Blow-through set-up as I am re-making my 3x4 intake thicker to do so, But I sometimes sec guess how easy it may Or may not be to keep tuned.

Hi T6,

That's a bit much for a street use daily driver.Though it sounds fun...I am not adverse to multiple carbs, I rather enjoy tuning them, though I have never done it with EFI. I would think it would be easy. I have several of the GM Rochester TBIs both the 1 and 2 barrel and have a couple of Megasquirt MS3Xs. So I have the options to go with multiples and may do that on my next build which will probably be a 250.

Since this is my first GM inline 6 build I want to keep the plumbing simple hence winnowing down my options to one of the 3 afore written single 4 barrel manifolds.

Though it would be kinda cool to use the Offy 3x1 and plumb it with 3 GM tbi 300 series or 200 series. I know the newer GM TBIs are better but I already have the 200 and 300 series. The fuel distribution would be better but the plumbing would be more complex and troubleshooting would be a bit harder.

There is a bunch more stuff I 'could' do but I don't want to get too distracted.

Thanks again for your posts.

I think the tbi /s would much easier to deal with and keep in tune then carbs .

As for my 3x4s Those where more set for track then street. As it was a my daily driver for a long time as far as the motor went. Then I de-sided to do more track then street. But by the same token Most people don't drive a motor with compression between 13.9 15.1 on the street with a 583/607 lift cam as a daily driver. I never did nail down the compression We just knew it was somewhere in between those numbers. when I ran the numbers with Glen Self. It ran on pump gas as a daily But Race fuel other wise (track)

For me, the hardest part of an engine built has always been being honest with myself about what the engine would really be asked to do. Blowing the doors off of a tri-year gasser on Friday night calls for different components than hauling a ton of hay up the hill on Monday. A turbo, EFI, & HEI can change some of that but not all. Honesty is the best policy.

Even without knowing the duration, that's a lot of cam (and compression) for street use. What was/is the duration?

Speaking of duration, as long as cam duration is less than 240*, are there any issues with charge robbing on the 2 outside intake ports?

Hi Beater,

That is exactly what I am trying to do. I am trying to gather information, in this case, for the manifolds. I am looking for people who have used the 3 manifolds I have listed and what their experiences were. If someone has seen data that compares them to see which would be considered best. Twisted 6 has already pointed out that the Aussie Speed would have better distribution because of it's design and looking at it, I can see they have done some unique things vs the other 2 manifolds. Yes, I am looking for subjective info but objective data would really help the cause. I haven't seen much objective data on these 3 manifolds.

I am looking for fun stoplight to stoplight fun. To bother my V8 buddies and have something for Hot August Nights that is not like every other. Maybe go to the Nugget and see what it can do on their 1/8 mile strip. I will tow from time to time but this will be a daily driver. I know it is and will always be a low rpm motor. I will never throw more than 1 atmosphere at it (probably not more than 10 psi). I am looking at a power band of 1500-4500. Maybe a hair lower. I have a few turbos on hand but am looking at using the one with the smaller turbine so it will spool up sooner.

Thanks for your response.

Rick, Are you a member of the Northern Nevada Chapter? Do we have breakfast together once a month?

There was some discussion here several years ago about Clifford vs Offenhauser manifolds. Try searching this. Just paste it in the search box.
Clifford vs Offenhauser rehashed, 292 4 Bbl

Leo's book references those 2 manifold, and minimally states what each should be used for...but nothing technical. There is only a pic of the Aussie Speed manifold.

A lot of what is out there is subjective and anecdotal. There doesn't seem to be any written specs or test results that I can compare. Anything that references/compares power, rpm, or air flow would help.

I'm on the email distribution and I'd really like to get down there. But I don't have any of my inlines running and don't feel I have anything to bring to the party.

I have all the bolt on goodies. Do we have someone up here that has access to an engine, a Dyno and a flow bench? If I can get my engine run stand put together, I would be able to get an engine running for testing.

Comments have been made to the effect that a manifold with a large plenum (Clifford) will have better peak power, but may be soft on response and torque; a smaller plenum (Offenhauser) the reverse.
However, in a conversation with a hot Corvair engine builder 40 years ago regarding higher CFM with a turbo, he pointed out that the turbo must first fill, then pressurize the entire intake tract before anything reaches the valves. Large plenum = lag. This is not related to spooling delay or turbo efficiency.

"I'm on the email distribution and I'd really like to get down there. But I don't have any of my inlines running and don't feel I have anything to bring to the party."

You should come for the "fun". Most of us don't drive our inlines to the meeting anyway. Mine has bee parked for almost a year. You might find the information you need from guys that don't use this site which is almost all of them. It is really a good group!

Panic,

Your 1st paragraph is what I've seen. With a V8, long thin runners vs short fat runners...we know what it does and it's been tested, documented, compared and contrasted. The vendors publish a lot of info on their websites.

Clifford's only claim with their single carb manifold is to 'hold on to the steering wheel'. Not a lot of empirical data there...and if you install their intake/exhaust kit it will 'increase power and torque 70%". 70% over what? It doesn't state 70% over stock or even which engine.

A guy that used to post here a lot was building a 250 turbo set up in a Camaro. He had a Clifford but wanted an Offenhauser so we traded. I don't know what happened to him or his project but I still have the Clifford.

I haven't spent a lot of time comparing intake manifolds. I'm sure there are some differences in hp numbers but how much, really? I've always been of the opinion that most of that is just dyno numbers. My guess is that you could swap from one manifold to the other, drive them back to back, and would be hard pressed to tell the difference. If you are setting a land speed record, or a national et record it may be worth getting down to that detail. Just my 2 cents worth. I'd run the one that I could get the best deal on.

Beater,

By chance was that guy from Canada?

6IE,

Yes, I'm looking for Dyno numbers...but it's more than that. I'm looking for RPM range. I looking at making peak TQ lower in the RPM range. With V8s you can definitely affect seat of the pants testing and which RPMs you can feel it at. You also have the Vendors specs that help you choose. There is a considerable difference between a single plane, dual plane, tunnel ram, cross ram, etc. The other thing I am looking for is better fuel distribution. The design of the Aussie Speed is supposed to improve fuel distribution. The design is different than all the rest. But if I want fuel distribution, I can use my Offy 3x1 but then the plumbing more complex. I want to try and make the build simple. I do not want to run any lean cylinders under boost hence looking for the manifold with the best distribution.

I already own all 3. I own 7 different aftermarket manifolds. I am glad you are volunteering my time to try all of them :-). I am hoping someone here has already been through this and has advice and/or numbers to give me a starting point.

6IE,

Yes, I'm looking for Dyno numbers...but it's more than that. I'm looking for RPM range. I looking at making peak TQ lower in the RPM range. With V8s you can definitely affect seat of the pants testing and which RPMs you can feel it at. You also have the Vendors specs that help you choose. There is a considerable difference between a single plane, dual plane, tunnel ram, cross ram, etc. The other thing I am looking for is better fuel distribution. The design of the Aussie Speed is supposed to improve fuel distribution. The design is different than all the rest. But if I want fuel distribution, I can use my Offy 3x1 but then the plumbing more complex. I want to try and make the build simple. I do not want to run any lean cylinders under boost hence looking for the manifold with the best distribution.

I already own all 3. I own 7 different aftermarket manifolds. I am glad you are volunteering my time to try all of them :-). I am hoping someone here has already been through this and has advice and/or numbers to give me a starting point.

I've been wondering something similar for a while.

My thought was to start out with Megasquirt running (2) 1-bbl TBI's on a 2x1 Offy. Now it has shifted to running Clifford (bought cheap) with 4.3 TBI set up.

The issue for me is fuel distribution. I have read the Aussie is better than the others.

Next upgrade would be turbo coming in about 2500 rpm and making max 8psi, redline around 5000 rpm. Was looking at the Marshall intake (I think Twisted6 handles these), individual runner EFI intake, perfect for turbo package with EFI. While the cylinder head is not divided it still puts the fuel in close to the valve. For a low RPM torque puller, I would use 1.8 intakes 1.6 exhaust and just polish up the head ports, perhaps a slight shaping of the boss. Not sure how much the lumps would gain me, head is efficient at low RPOM and the turbo would cover the midrange.

Boost changes fuel distribution.
You won't know what you have until you try it.

If you use the divided port EFI Marshall intake and then divide the ports in the head also, lumps are not necessary because you regain port velocity that is lost by removing the bolt bosses in the head to install the lumps. Installing the lumps with the ports divided will make the port window/cross section too small to give you much RPM range for performance or added benefits.

Hmm. I was mostly after a plenum with better injector placement than a conventional 4bbl manifold. My thought was having the injectors at the ports would give better fuel distribution than centralized TBI.
I think a port divider could be made, but previous efforts seem to have run into a 'too thin to not bend under boost pressure' condition. Of course, they were looking to go way past my low boost low rpm torque project.
The approach of welding up the ports and studding from the bottom looked interesting until I figured out how much slop was in my Bridgeport..
At this point it looks like just sticking with the TBI is my best bet. Unless I want to cast my own injector stacks... and I no longer work in the foundry.

Well, the Marshall EFI intake fits all of your criteria perfectly. Further dividing the ports inside the cylinder head is not required, but can further enhance your performance range if you choose to do it. Making dividers is not difficult so dont get tripped up too much on over engineering or over thinking them.