Tell me why I shouldn't go back to carbs

[gboone]

I have changed my induction on my 1970 Datsun SRL311 from 45 mm SK Racing Carburetors to TWM 45 mm 2900 Series throttle bodies. I have attached performance curves with optimized calibration for both setups on the same graph for easy comparison.

I have also attached a datalog from the most recent wide open street run starting in 1st gear through part of 4th gear.

After seeing that I've lost significant torque and HP (15% less for both) through most of the useable rpm range after changing from carburetors to EFI, I obviously have a major problem. I can even feel the loss of power and torque and my competetiveness has fallen off significantly in autocross. The dyno tuner spent several hours on a steady state dyno optimizing the tune for the EFI and has a reputation for being the best tuner in the state.

This situation is extremely disturbing considering I've spent over $4,000 on the conversion to EFI and 100's of hours of work into it to end up with poorer performance. I spared no expense to get the best performance (with the exception of MS).
So why shouldn't I just scrap this project and go back to carbs? I get sick thinking about flushing several thousand dollars and all those hours down the toilet, but losing performance is worse.

Roadster Carb VS EFI Performance Curves.xls

2011-08-04_19.59.12.msl

[2milehi22]

I have seen a loss of power when too big of a carburetor or a tunnel-ram intake was used (on V8s). It is possible that the ITB are too much for your engine at the lower RPM that you run. A manifold with a common plenum "could" run better with your setup, but a dyno would be the best (objective) proof.

[WASyL]

Answer is real simple, change tuner not fuel delivery system. It is true that You get equal power on carb and EFI. But EFI gives more more possibilities on fuel mileage, ease of tune etc.

I deal both EFI and Carbs and both are easy for me but i get mad when i have to change fuel jets especially with Holley type carbs.

best regards

[2milehi22]

Answer is real simple, change tuner not fuel delivery system. It is true that You get equal power on carb and EFI. But EFI gives more more possibilities on fuel mileage, ease of tune etc.

I deal both EFI and Carbs and both are easy for me but i get mad when i have to change fuel jets especially with Holley type carbs.

best regards

From OP - The dyno tuner spent several hours on a steady state dyno optimizing the tune for the EFI and has a reputation for being the best tuner in the state.

Looks like the OP got the best person for the job under controllable conditions.

[gboone]

From OP - The dyno tuner spent several hours on a steady state dyno optimizing the tune for the EFI and has a reputation for being the best tuner in the state.

Looks like the OP got the best person for the job under controllable conditions.[/quote]

I guess I could throw away another $400 on more dyno hours by someone less reputable. BTW, it's very difficult to find a dyno tuner around here who knows MS.

[WASyL]

Get Your self WBO2, read megamanual twice, some books or webpages how engine works and tune it by Yourself on street, it's cheaper, gain lots of knowleage and you know everything about your tune and engine. Then conform that with dyno. Dyno tunes are just to smooth the power and torque eventualy to fine tune spark map. Good start for ignition (even thoe they concern carbs it worsk same on EFI) http://ls1tech.com/forums/advanced-engi ... 101-a.html

Best regards

[trakkies]

Do you have the other data from the rolling road - like A/F ratio and ignition timing for both the carb and injection runs? Or a log from MS during its run? The torque being down more or less uniformly over the entire range is pretty significant.

[LT401Vette]

Other consideration... Is your intake manifold designed for your EFI conversion?
If you changed it from the one that you were using with the carb, it looks to be targeting a different operating range.

The tuning of the the intake runners is very important to WOT power. If for instance you use the same intake manifold, but now with injectors near the port, you now have dry gases in the intake runner compared to the much heavier fuel air mixture, thus you have changed the physics significantly. Typically a with a dry intake, the operating rpm range will increase over the same intake runners with the more dense wet mixture.

[Sack of Spuds]

What size chokes did you have in your 45 carbs?

Jumping up to 45 TBs could be a huge change and I note the TBs made more power at very high revs - I think they may just be too big for a low reving 2L engine.

I'm not familiar with SK carbs but the largest choke for a 45weber is 41mm, but the go down smaller. A chokeless 45 is 20% larger than a 41mm choked one, obviously if you had smaller chokes the difference would be greater.


Also worth asking - are the two graphs standardised for air temp and pressure - as I assume they mut have been done hours if not days or even weeks apart - atmospheric conditions could be exaggerating any power loss.