Triple Carb Manifold Short or Long??

[jmortensen]

Some of the CP racers would mill their triple manifolds to angle the intake manifold up. Then they'd port the head to match. The idea is that the short side radius in the head is straightened this way, and it's done to improve flow rather than to stick a longer manifold or air horn on. There are pics of some of these manifolds in the How to Modify book.

[EricB]

that's a great end-on pic Alan... really highlights the angle difference...

jmortensen, if you mill the manifold to ease out that angle, then your carbs on the end of manifold are themselves even more tilted from an end-on view... I don't remember what the How To book said about those but doesn't that interfere to a certain extent with the way the float works inside the carbs??? or is it impervious to being tilted like that?

-e

[byunique]

I was looking for this answer myself, but I was motivated to check into it because of a stumbling issue I ran into as I increased the venturi size. For some background, my Mikuni 44's came with the Mikuni manifold, but I later changed it to a Cannon (long) manifold since I was told the long manifold makes more power. I never tried the Mikuni manifold, and just sold it on Ebay.

My motor is a pretty built 2.8, 9/1 compression, large cam, ported heads and I love to see power above 6500 rpms.

As I increased the venturi size to a 37 or larger I started to run into a big stumble if I accelerated hard, and that would be right off the line or punching it down the freeway. I could tell that my motor was getting lean at those moments, (per mixture meter..) and despite what changes I made to acceleration pump, jet size, etc. the issue was always there. It would be milder with 34's but would get worse with the 37's. Of course, this hurt my top-end power and I was always yearning to go to 39's but the stumble would be ridiculous at that size.

So, I had asked around about manifold length and was told a shorter manifold would help, but was never told it would cure the problem.

I hunted around on Ebay, and found a SK/Sanyo manifold which looked just like the long run Cannon (mine was 5.5 inches btw), but it was a lot shorter (3.5 inches I think).

see pics of shape/size

http://www.brandonyu.com/gallery/main.php?g2_view=core.ShowItem&g2_itemId=11681

After getting it setup, I was amayzed. The flat spots that I had disappeared, and there was a much tighter feel to the engine. It felt much more torquey all around. It ran great with the 37's and then I upped from 37's to 39's, and the high rpm range was extended further, but this time without ANY hesitation off the line or midrange.

I can basically roll on acceleration off a stop light and smoke the tires all the way across the intersection and pull strongly to 7000rpm. (a 4.44 helps with this too ) Yes, this basically cured my car of the huge flatspot and I was able to bump my timing a few degrees as well w/o pinging. The engine is no longer lean upon acceleration I believe it has to do with the velocity of the charge due to the shorter length. I also noticed the manifold is cut at a higher angle so, it tilts up more the Cannon, which also may help per the previous comment about that.

Yeah, a long manifold may be good generally, but didn't help in my case with larger venturies! The car is night and day different My advice is get something straight, but shorter than the standard 5.5" Cannnon.

[Steve Parmley]

When I talked to Dave Malvern several years ago he said he would get his race carbs level at all costs. He would remove as much vibration as possible by modyfing the Comp rubber isolaters and shield the carbs from the header heat. "No matter how much insulation you have now it is still not enough".

I have the short manifold (can't help it I was born that way) and those funky white plastic isolaters with the carbs at an off angle and it has been fine even with quite a bit of autocrossing. That said, if you know anyone selling a Datsun Comp manifold let me know since I was told by a racer more torque is available with that manifold.

Jon, my large ITG filter is a problem with Vacuum booster when I position the engine back to the firewall which is legal in XP SCCA auto-x or in FP if you run under the GT-2 in excess rule with accompanying weight penalty.

[jmortensen]

I was looking for this answer myself, but I was motivated to check into it because of a stumbling issue I ran into as I increased the venturi size. For some background, my Mikuni 44's came with the Mikuni manifold, but I later changed it to a Cannon (long) manifold since I was told the long manifold makes more power. I never tried the Mikuni manifold, and just sold it on Ebay.

My motor is a pretty built 2.8, 9/1 compression, large cam, ported heads and I love to see power above 6500 rpms.

As I increased the venturi size to a 37 or larger I started to run into a big stumble if I accelerated hard, and that would be right off the line or punching it down the freeway. I could tell that my motor was getting lean at those moments, (per mixture meter..) and despite what changes I made to acceleration pump, jet size, etc. the issue was always there. It would be milder with 34's but would get worse with the 37's. Of course, this hurt my top-end power and I was always yearning to go to 39's but the stumble would be ridiculous at that size.

So, I had asked around about manifold length and was told a shorter manifold would help, but was never told it would cure the problem.

I hunted around on Ebay, and found a SK/Sanyo manifold which looked just like the long run Cannon (mine was 5.5 inches btw), but it was a lot shorter (3.5 inches I think).

see pics of shape/size

http://www.brandonyu.com/gallery/main.php?g2_view=core.ShowItem&g2_itemId=11681

After getting it setup, I was amayzed. The flat spots that I had disappeared, and there was a much tighter feel to the engine. It felt much more torquey all around. It ran great with the 37's and then I upped from 37's to 39's, and the high rpm range was extended further, but this time without ANY hesitation off the line or midrange.

I can basically roll on acceleration off a stop light and smoke the tires all the way across the intersection and pull strongly to 7000rpm. (a 4.44 helps with this too ) Yes, this basically cured my car of the huge flatspot and I was able to bump my timing a few degrees as well w/o pinging. The engine is no longer lean upon acceleration I believe it has to do with the velocity of the charge due to the shorter length. I also noticed the manifold is cut at a higher angle so, it tilts up more the Cannon, which also may help per the previous comment about that.

Yeah, a long manifold may be good generally, but didn't help in my case with larger venturies! The car is night and day different My advice is get something straight, but shorter than the standard 5.5" Cannnon.

Hmm... interesting. I have the stumble too with 37mm chokes on 44mm Mikunis, but I was able to *almost* completely eliminate it through jetting. Kept going bigger and bigger and bigger on the pilot. Bigger I went, the less it stumbled. I tried bigger pump nozzles, but that didn't have anywhere near the beneficial effect of the bigger pilots. Your comment makes me want to try a different manifold and see the difference though. Malvern was one of the people who told me the manifold choice was the right one, the other was Rebello FWIW.

[jmortensen]

I guess another idea would be to put an airbox on and see what difference that makes. In my non-engineer mind that would kinda group the vacuum from all the runners to get a more steady stream of air moving.

[EricB]

hmmmm very interesting indeed...

i also went to bigger pilots to cure a cough/spit problem i had when i accelerated just so... actually was jon's advice i followed... sure i did notice my MPG take a small hit and it runs pretty fat at idle but hey can't win all the battles right? my venturis are still the stock size my 44s came with - never changed them... all of this is very interesting... Brandon thanks for sharing your impressions...

-e

[jmortensen]

What manifold are you running Eric?

[byunique]

Glad I can share my experience with this EricB. The first time I really thought this would work is when I saw a car that Scott Performance had put together and it was at the Motorsport Show '05. It was a Cannon Manifold, but it was cut shorter, and thats what prompted me to do it.

http://www.brandonyu.com/gallery/main.php?g2_view=core.ShowItem&g2_itemId=11813

I thought about cutting mine up, but it would be quite $$$ . So, I am really glad I found that manifold on Ebay. I don't think the short straight shot manifolds are plentiful.

[EricB]

you know i'm not quite sure...

i just landed in tokyo last night and am still all out of sorts...

hmmm either i have a TWM or a Mikuni manifold - I honestly don't remember right now... sorry...

-e

[Ron Carter]

I have run the Mikuni Manifold on my old street car and it always went like heck from idle all the way to redline. That being said, On my BRE Z I run the Nissan Comp / Interpart manifold on the race car which is a balance between the Mikuni and the Cannon and it works very well...no flat spots and good top RPM.

The rule of thumb I always heard was Mikuni Manifold for the street. Cannon for Auto-X (better torque). Nissan comp for roadracing.

[cafebaltimore]

Has anyone ever mounted the long runner Mikuni Manifold in their car? Do you have any pictures.

As best as I could tell, there were only two people in the thread who have even seen one of these manifolds in person (Ron Carter; though I think I may have misread, and Gav240z, who found one on eBay). I have seen hundreds of Cannon long runner manifolds, and a few other brands as well, but I have never seen another Mikuni stamped long runner intake manifold.

I have always understood that the short runner Mikuni manifold was only intended to allow clearance between the carbs and the brake master cylinder (when used with brake master vac).