Correct ITB size for 3.7?
 

Looking to the experts here, Henry, Steve,TimT, RS Scotty and anyone else who knows.....

Building a 3.7 with 10.5 cr, twin plug, 1.75 headers and cam yet to be decided. Motor will run ITB's , what is the optimal size? Redline around 7500 or less depending on cam. 48mm, 50mm or ????

Cheers

Throttle valve sizing
 

There is not a great amount of information out there on this subject that I have found, but this is a quote from our source.....

What is the best throttle body diameter?
Factors influencing size are; Power output, RPM, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.

Choice of bore size is a balanced compromise resulting from the following;
1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.

Basic references for BHP per cylinder, assuming ca 120mm from butterfly to valve head and a max of 9,000 rpm are;
Up to 30 - 30mm, up to 33 - 32mm, up to 39 - 35mm, up to 46 - 38mm, up to 51 - 40mm, up to 56 - 42mm
Up to 65 - 45mm, up to 74 - 48mm, up to 80 - 50mm, up to 87 - 52mm, up to 93 - 54mm.
These power figures may be increased by up to 10% in a purpose - designed and well proportioned system.
As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa.
Lower revving engines and those with injectors placed before the butterfly will generally accept a larger body.

Very good info! Thanks!

I have run 48's on a 3.4 with good results but have not tried different sizes. It sounds like I could still run the 48's on a 3.7.

Cheers

Give Jae at Mirage Int'l a call. If I remember correctly, he has some motors (3.8's) running up to 55mm ITB's, and I know of one customer who went from 46's up to 50's and then 50-somethings.

Cheers,

Hi Jeff,

We have two engines here of that displacement. One is a true 3.758 and the other 3.8 plus. Each engine has a different ITB set up. The smaller engine, if you can call 3.7 plus small, has bigger 52mm TWM throttles (true independant throttles). We did the entire job here. It makes good power was but very hard to tune for light throttle applications during street driving. I would recommend that if the engine is going to see any street life at all the throttle size be big enough to make the power you are striving for but, small enough to give good drivability on the street.

Itb
 

I removed the RS air filter so you can see the size of the air horns and the throttle set up. THis picture is of the 3.758 liter engine.

Bill Rudtner

http://forums.pelicanparts.com/uploa...1229444616.jpg

Itb
 

Here is a picture of the bigger engine. These throttles are PMOs and are 50mm They are also one cast piece per side.

Look past the poor asthetics as we built the engine but, did not perfrom the install, wiring or fuel lines..

Bill Rudtner

http://forums.pelicanparts.com/uploa...1229444932.jpg

Jef,

Re-reading your original post I would either do the GT3 crank or the NASCAR bearing mod with 2" crank journals as your 7500 RPM requires either mod.

Bill Rudtner

Bill, thanks for the info. The top motor looks like Mike's..... Nice work.

I run Jenvey 48mm ITB's on the 3.4 and it has great light throttle driveability and was fairly easy to tune.

What does the nascar bearing mod run dollarwise?

Cheers

Private pm sent

Bill

Jeff,

I still haven't driven the car yet.

Bill has though, tell him about Ocean Parkway Bill:D

I'll pick it up soon!

3.8
 

1980 911 Stock gearbox. 8,000 rpm shift in each gear. 155 mph in 5th and still pulling on the Ocean Parkway. Our little South Shore test road.

nice!