Hey guys, need a quick hand to calculate an engines airfolw requirements. Basically am trying to decide how big a TB I need for my BBC project.

 

Enginecalc (http://www.breakherthewang.com/enginecalc) can calculate an engines airflow requirements assuming 100% VE at a given rpm. So you can punch in the engines bore stroke and cylinder number then punch in say 8000 for the rpm (assuming you want to shift at 8000 rpm) and it will tell you how much air you would need at 8000 rpm I believe it defaults to 1.5 inches of mercury.

 

Wow, that's a pretty cool program. I can't download it because I'm at work but I will look into it when I get home. If you have the time the information is:

bore: 4.56
stroke: 4.25
cylinders: 8
rpm: 7000 approx.

 

1377.43 CFM @1.5 In. H.G.
You probably want to go at least 20% higher (1652.916) than this number due to variances in the way the calculation works and because the goal of a high power engine is to exceed 100% VE (Though this will happen at your torque peak, not your HP peak).

 

Thanks a ton, that basically tells me that I can't use a 58mm Monoblade and now I need to find out what a LSx style 90mm TB flows. Thanks again!

Edit: Just checked TPIS 90mm TB's and they flow 1500 cfm. Next question I guess would be with that CFM requirement I'm assuming the intake and heads are a restriction and I would never need that much CFM's? And that would only be at WOT at 7K it would obviously be less at lower RPM's.

 

Use a big one. Unlike a carb, there is no issue with going too big. I am using a 2,000cfm TB on my BBC. The motor is 470ci and turns 7,000rpm, so it's far bigger than "needed" and there are no issues associated with it.

Rich

 

I thought if I went too big then I might run into tip in issues that the LT1 guys were running into when doing a single plane conversion.

Are you running a 4bbl setup on yours or an elbow and monoblade set up? If a 4bbl then what did you do for a cold air setup? Right now I am going to use an elbow and route it to the stock WS6 ram air intake.

 

It's a 4bbl type with a progressive linkage - no problems with throttle response. Not in a 4th gen., it's in my race car.

rich

 

Thanks guys, I came across a decent deal on a new Comtech TB that I went ahead and picked up. Hopefully everything will work out.

On a side note though is what I said about the intake being a restriction true? You can get a 2,000 cfm TB, but if your intake can't flow that does it matter?

 

Well, the gains get incrementally smaller as the as the TB gets larger. Remember, the "standard" for carb flow is at 1.5" of vacuum. That's because less than that and the carb won't meter right. But that doens't mean there won't be more hp if the flow is greater. Just that with a carb, it might not work right. The intake side is just like exhaust systems - the more flow the better. The traditional carb rating scheme gives a size that leaves maybe a percent or so of peak power on the table. The implication is that using as big as TB as practical is the right way to go.

Rich