# Carbs vs ITBs a comparison.



## Prof315 (Jan 5, 2009)

First off let me say I have nothing against carbs. Weber DCOEs are an amazing feat of engineering especially when you consider they were invented over 60 years ago.

That being said I got the opportunity to do a true back to back comparison between Carbs and ITBs on an engine dyno a few weeks ago. The results surprised me and surprised the motor builder even more.

We were tuning a couple of old Datsun A14s that were fully race prepped for SCCA GT Lite. 1.4 liters, 12.8 to 1 static compression, custom cams, big valves, lots of head work done on a flow bench, long rods, dry sump oiling and billet aluminum roller rockers. These are pushrod motors with counterflow heads that are redlining at 8800 rpms.

Since we were dialing in a new, custom 36-1 crank trigger wheel, the engine builder wanted to go with carbs initially as he had a set of 45mm DCOEs tuned for these motors and ready to go. He also wanted to use them for baseline power numbers since he knew where we should be.

After getting the ignition set up and the timing curve correct, (we used the MS3 that was to control the ITBs as an ignition only controller to reduce variables) We made several pulls and got 168 bhp and 114 ft/lb of torque consistently. Then it was time to replace the carbs with ITBs. We used a set of 42mm GSXR 1000 ITBs and their matching injectors. After getting the fuel curve right, we again made several pulls and jaws started dropping. The ITBs made 193bhp and 127ft/lb torque!!!

After some discussion, the engine builder (who is THE expert on these old Datsun motors) said that while he could make similar peak numbers with carbs, the motor would be un-drivable as it would have a usable powerband of less than 1000 rpms. 

So there you have it. As good of a real world comparison bewteen carbs and ITBs as I could do.

EFI RULES!!!!


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## ps2375 (Aug 13, 2003)

Nothing like modern trickery to get the most out of a motor. If you have the graphs, those would be cool to see.


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## Turbo3 (Sep 15, 2005)

Not trying to be a dick or anything. But we all know that ITBs will make more power and have better drivability vs carbs cuz of the tune ability of EFI. But thank you for your time testing. Vortex has been lacking these things for a long time


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## dougkehl (Nov 30, 2011)

Quite interesting to see just how much more efficient ITB's are due to EFI than even the most well tuned side drafts. I'd like to see some graphs too if possible!


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## Prof315 (Jan 5, 2009)

Unfortunately I don't have graphs available. But basically what happened was that the carbs go rich above 8000 rpms and so the power flattens out and starts to drop. As I stated in the original post you can fix this with bigger chokes but only at the cost of a much narrower powerband.


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## Turbo3 (Sep 15, 2005)

Damn it...I wanted to see a graph


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## candm (Apr 27, 2003)

one of the main things for the webers is there adjustability for engine size, compression, etc. typically, if you have an engine with mild to medium cams and compression, you can attain full power, as well as, good streetability. onc you go above that range, by installing larger chokes to get the flow needed, you will definitely lose streetability down low.
i would just say that once you had the proper chokes installed, and then matching jets and emulsion tubes, that if you had been able to reach the maximum rpm on those motors, you certainly would have made more horsepower with the webers than you did.
im not saying you would of made as much as properly tuned efi, or had the steetability ( allthough a small motor with a peak power at 8800rpm isnt exactly something to drive around town ), but unfortunately, without the proper chokes and matching jets, it wasnt exactly a fair comparison for peak horsepower.


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## Prof315 (Jan 5, 2009)

candm said:


> one of the main things for the webers is there adjustability for engine size, compression, etc. typically, if you have an engine with mild to medium cams and compression, you can attain full power, as well as, good streetability. onc you go above that range, by installing larger chokes to get the flow needed, you will definitely lose streetability down low.
> i would just say that once you had the proper chokes installed, and then matching jets and emulsion tubes, that if you had been able to reach the maximum rpm on those motors, you certainly would have made more horsepower with the webers than you did.
> im not saying you would of made as much as properly tuned efi, or had the steetability ( allthough a small motor with a peak power at 8800rpm isnt exactly something to drive around town ), but unfortunately, without the proper chokes and matching jets, it wasnt exactly a fair comparison for peak horsepower.


But we weren't trying to compare peak power. The motors in question are purpose built race motors designed to operate in a specific rpm range matching the gearing of the car they were built for. The chokes, jets and emulsion tubes were sized and tuned for the specific application and operating range we were using by somebody who has built and tuned more winning Porche and Datsun/Nissan racing engines than anybody in the world for the last 20 years. And HE was impressed!


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## candm (Apr 27, 2003)

Prof315 said:


> But we weren't trying to compare peak power. The motors in question are purpose built race motors designed to operate in a specific rpm range matching the gearing of the car they were built for. The chokes, jets and emulsion tubes were sized and tuned for the specific application and operating range we were using by somebody who has built and tuned more winning Porche and Datsun/Nissan racing engines than anybody in the world for the last 20 years. And HE was impressed!


:thumbup:


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