Mikuni carbs vs. injected ITBs

[Kevin Johnson]

In 2012 we dyno tested a stock MGB with SU carbs. We then removed the SUs, and installed a pair of 42 mm HSR Mikunis. With no other changes to the MGB, we were surprised to get a 20% increase in power,

I have raced with Parker and he is a nice honest guy. The HP claims that I see are the 20% for the stock , little SU on the MGB. Increasing the area by 30% and using a much cleaner intake track could well make the 15HP increase. Also the stock SU may have been old with worn jets /needles etc. 55 WHP to 70WHP

If the carbs being replaced were clapped-out junk, then the claim is deceptive on that score. I wasted some of the best years of my young life trying to make those buzzing little anvils go faster. No one will ever convince me replacing two properly tuned SUs with two Mikuni on the same intake on a stock MGB will give a 20% horsepower increase.

US Market
Year Model Max Bhp Max Torque
1962 MGB 94 @ 5500 107 @ 3500
1973 MGB 79 @ 5350 94 @ 3000

I think the claim is merely that proper performance-tuned Mikunis will improve a later MGB with emissions tuned factory SUs. Amazingly enough the above difference is 15hp.

[enigma57]

Without getting off into the ITB/EFI versus carburettor aspect...... The Mikuni HSR 42 sliding valve carburettor was my first choice for the inline 6 engine I am building just now. There is insufficient clearance in my engine bay for sidedraught carbs of any kind, though. So am going with downdraught Webers for this build.

Best regards,

Harry

[Momus]

Keihin FCR carburetors are much nicer than the mikunis, but the largest you can get is 41mm

I concur. I've used the Mikunis and the Keihin FCR's win hands down. If only there was a 48 or 50 mm version.

[enigma57]

Have no personal experience with the Keihin carbs, but will make note of this. In theory, (3) 39mm - 41mm sliding valve carbs suited to the Harley Davidson 96 cu.in. twin cam engine should work quite well on my engine. Only issue is clearance to master cylinder with sidedraught carbs.

Thanks for the heads up,

Harry

[Truckedup]

Keihin FCR carburetors are much nicer than the mikunis, but the largest you can get is 41mm

I concur. I've used the Mikunis and the Keihin FCR's win hands down. If only there was a 48 or 50 mm version.

What makes you say the FCR's are far better?

[german4inline]

I think on the MGBs with relativly low power is not that big advantage with Mikunis, because the bifurcated intake manifolds keeps the engine low on power, at least compared to an ITB-layout. On the bifurcation there would be some wetting be present from the fuel anyway. Unfortunatly, John Parker doesn´t state the power increase of his Volve race engine over the Webers.
Whats the downside of the Mikunis, that one can´t position them close to the engine with sloping inlet ports, maybe there´s some trouble with the floater level.....at least I suppose. And they are not cheap, costs similar to ITBs.

Apart from topic: Does anyone experience big differences in spray quality with several injectors on ITBs? Big differences in output / throttle response whilst keeping the inj. rate equal? I would like best small pico injectors, weight saving is always an issue. Which would be best?

[Momus]

Keihin FCR carburetors are much nicer than the mikunis, but the largest you can get is 41mm

I concur. I've used the Mikunis and the Keihin FCR's win hands down. If only there was a 48 or 50 mm version.

What makes you say the FCR's are far better?

The roller slide being unaffected by vacuum makes them easy to closely modulate and they seem, in a difficult to quantify way, more responsive and forgiving.

[Protech Racing]

My Microsquirted race cars use the Bosch 24#, 4 hole injectors (Mustang/Chevy) and make about 40HP per cylinder. I have had no failures.
I can post my suppliers name. He balances them as a set of 4. Cheap ish also about $150per set of 4 .

[hoodeng]

A well matched and tuned EFI would be very hard to beat with any carburetor regardless of manufacturer.

A 20% gain is horsepower just by changing carburetor does not tell me the new carb is a miracle worker, it tells me the carbs the engine was tested with for the baseline horsepower run were incorrectly sized,poorly tuned or just junk to start with.
Some OEM carbs that were installed on early vehicles were a compromise of price, durability and more so in the later carburetor evolution, emission compliance, which in the early days of compliant emission carbs had us see some of the worst versions of trying to mix fuel and air. They were as good as they could be with the task they were given for the price they were made for.

That said, both the Mikuni HSR 42 to 48mm and Keihin FCR [H] 35 to 41mm are excellent carburetors and are comparable in tune ability and quality. I have used the Mikuni by the 100's and the FCR by a much lesser extent [due to availability] both are cylinder capacity dictated, in a V2 pushrod engine a 42 works very well from 38cuin/cyl up to 50cu in, and will comfortably make 58hp per cylinder at 6300rpm on a 50/cyl street engine, much over 50cu in/cyl and you are in the 45 HSR territory, we have used the 48 on 62 cu in/cyl with excellent results, these carbs are jetted to supply somewhere near 13lb of air to a pound of fuel over their running range, big variances in jetting over what is supplies should be investigated, usually an exhaust system anomaly or even a badly ported head or radical cam timing can make a carb do things that are not the norm, big changes in main jets is usually caused by something else behaving as the main jet, like the fuel supply.

Cheers.

[Truckedup]

On my vintage Triumph street and land speed racers I have had excellent results with Mikuni TM flatslides. The 34 MM's are about 135 bucks each and work quite nicely with leaner mixutures and sharp throttle response.. On the track they don't seem sensitive to changes in air density...As always, no matter how good the carb, proper tuning is most important...My 900 Ducati has the stock Mikuni CV 38mm down draft flatslides. Many guys switch to 39 or 41mm down draft Keihin FCR's, but they are $1000 plus a set...On the street, after some tuning ,the Mikuni CV's work quite nicely..On a road course track the Keihins do have an advantage...

[Dan Timberlake]

[
US Market
Year Model Max Bhp Max Torque
1962 MGB 94 @ 5500 107 @ 3500
1973 MGB 79 @ 5350 94 @ 3000

I think the claim is merely that proper performance-tuned Mikunis will improve a later MGB with emissions tuned factory SUs. Amazingly enough the above difference is 15hp.

1973 may have been subject to the "net" HP ratings, in addition to emissions tuning.
https://www.hagerty.com/articles-videos ... horsepower

[Kevin Johnson]

[
US Market
Year Model Max Bhp Max Torque
1962 MGB 94 @ 5500 107 @ 3500
1973 MGB 79 @ 5350 94 @ 3000

I think the claim is merely that proper performance-tuned Mikunis will improve a later MGB with emissions tuned factory SUs. Amazingly enough the above difference is 15hp.

1973 may have been subject to the "net" HP ratings, in addition to emissions tuning.
https://www.hagerty.com/articles-videos ... horsepower

Yes, I saw that SAE net was comparable to the Brake hp used in the UK. I initially wanted to compare to the even lower power ratings of later US Market cars but they were no longer using SU carbs. I only had personal experience with SU carbs on the Series 1 XJ6 that I owned.

https://en.wikipedia.org/wiki/Horsepowe ... horsepower
https://www.mgexp.com/article/performance.html

[dfarr67]

I have raced with Parker and he is a nice honest guy. The HP claims that I see are the 20% for the stock , little SU on the MGB. Increasing the area by 30% and using a much cleaner intake track could well make the 15HP increase. Also the stock SU may have been old with worn jets /needles etc.

55 WHP to 70WHP

Most of the time ,the EFI does spray the fuel near the valve. While the carb sprays it further upstream. Might be more power .

The old injected cars sprayed the fuel into the top of the venturi tubes or the Kinsler style , part way down, but still, well above the valve.
If I could run Microsquirt /EFI on my vintage Toyota instead of the twin Webers, I would . Modern programmable , density compensating. I do expect the Webers to make competitive power tho.
IMHO you are further ahead using something that parts are not 6 days away for .

Another option. https://controls.is/shop/ecu/lpc4

[Truckedup]

Yes, I saw that SAE net was comparable to the Brake hp used in the UK. I initially wanted to compare to the even lower power ratings of later US Market cars but they were no longer using SU carbs. I only had personal experience with SU carbs on the Series 1 XJ6 that I owned.

73 MG may be the last year of SU's for the USA? In the USA it would have been the new new HP ratings..When did they go to rubber bumpers? Those had a single Solex carb? I had a 64 MGB with the 3 main bearing engine..A little bit more compression , Crane street cam and a better exhaust.. It ran well for what it was but probably no more than 110 HP on a good day...

[Truckedup]

Yes, I saw that SAE net was comparable to the Brake hp used in the UK. I initially wanted to compare to the even lower power ratings of later US Market cars but they were no longer using SU carbs. I only had personal experience with SU carbs on the Series 1 XJ6 that I owned.

73 MG may be the last year of SU's for the USA? In the USA it would have been the new net HP ratings..When did they go to rubber bumpers? Those had a single Solex carb? I had a 64 MGB with the 3 main bearing engine..A little bit more compression , Crane street cam and a better exhaust.. It ran well for what it was but probably no more than 110 HP on a good day...