5K Single 40Mm Dcom Weber

[donk]

Hi guys, hope someone can explain .

 

I have a rebuilt 5k with the following been done.

 

bored 20"

bottem end fully balanced

Wade 446b cam

std 3k big port head (shaved 60")

extractors

distributor recurved

180 holley (32/36)

 

when we had it on the dyno we had 91hp @ rear wheels

 

we have since upgraded to a single 40mm DCOM weber 32mm chokes (remanufactured & looks like new) & redline manifold (new) purchased through a very reliable company.

 

& now we have 87hp @ the rear & lost power right through the range. (same dyno used ).

 

the guy from the dyno thinks the carby is too big & is going to try smaller chokes 30mm & 28mm, but my question is how do others use twin 40mm with no problems & do they actually make more power ?

 

Thanks for any insight into this problem as I have just spent $800 for no gain in power.

[altezzaclub]

That'd damm good power!

 

What is the compression ratio? ..and the air/fuel ratios? I assume he had it jetted correctly.

 

Wade list that cam as 295deg, which to me is a large duration, so it should haul at the top end with the Weber. The only thing I can think of is what your mechanic said, the extra air volume is going in at decreased velocity and not filling the cylinders efficiently.

 

It will be interesting to see if the smaller chokes give more power.

 

Do DCOE make more power.. probably not, but they look sexy and sound great, so everyone uses them! Richard here is using a 40DCOE with 33mm chokes on his 4K, but he's got no comparison except for the stock Aisan.

[donk]

comp ratio is around 11:1 with 200psi compresion I think he said , but I will check today & confirm. The smaller chokes should be here today so I will let you know what we end up with.

[donk]

We tried the 30mm & 28mm chokes today & found the 28mm were better. At the end of the day & after alot different jet combinations we walked away with 93hp with a small loss in power down low but a little better up top. So we gained an extra 2 hp over the 180 holley & spent over $1000. Maybe when we get the head work done & flow more air the weber might be better.

Can any one tell me that are running twin webers if they tried a single weber first & then the twins & were they better in hp than the single?

[snot35]

28mm sounds kinda small for a single? I've had a single 45mm weber on a 1600 before and I reckon the chokes were around 32mm and it hadn't had the work done that you have.

 

I would have thought 28mm would be the right size for each choke in a set of twins for a drive able 5K, bigger if you want to rev it. Remember with the 32/36 one cylinder can more or less breathe through both chokes (26mm and 27mm from memory) when the plates are fully open. With the shared single Weber you've got cylinders adjacent in firing order breathing through one choke, 28mm in this case. It's not ideal. Twins are good because they have one choke for each cylinder which is far less taxing and is good for torque.

 

I don't think better airflow will help a lot for a single.

[ke70dave]

Out of curiousity were all these HP figures at the same RPM?

[donk]

same RPM starts to drop off after 7500. When I pay him next week he should give me the dyno sheets with the two setups & I will post it on here.

Has anyone got coclusive evidence that twins are better ( I mean actually had the two set ups on a dyno) because I was sure the sidedraught was heaps better when we put it on. Its one thing to sound & feel more responsive when free reving , but put it on a dyno and you find the truth.

[altezzaclub]

Its one thing to sound & feel more responsive when free reving , but put it on a dyno and you find the truth.

 

Yep! Most interesting.... I would reckon a 33 would out-perform a 28 too, but there is always more to it than just the size.

 

Seeing we've been pumping water around the farm I'm thinking of a home-made engine dyno... it doesn't matter what it says in absolute terms, its just for comparisons exactly as you did.

[snot35]

Given the number of race and rally engines all sporting one throttle per cylinder for decades, I think you'd be pretty safe knowing twins would be better.