ok, so i USED to have a chart showing proper exhaust size based on horsepower output on NA cars, and im wondering if throttle bodies can be rated the same way
so 4 questions
NA cars: 1)throttle body size vs HP output 2) exhaust size vs hp output
FI cars: 1)throttle body size vs HP output 2) exhaust size vs hp output
who can give me guidelines ?
On a car with EFI as opposed to a carb, the TB really can’t be “too big”. The incremental gain decreases as it gets larger however and there is a point at which making it bigger stops producing gains. That would be where it becomes considerably larger than the next smallest restriction in the intake tract. As a simple example, the stock LT1 SBC intake is designed to use a TB with two circular bores. The stock TB is 48mm*2. The openings in the manifold are 52mm. Adding a 52mm TB will produce some gain, though it’s quite small on a stock motor. 58mm TB’s are also sold. Adding one of these produces no gains unless the manifold is ported to at least 58mm.
OTOH, the headers can be too big, for rather obvious reasons. This is true on an NA setup because NA depends more on exhaust tuning to achieve a high VE. The effect is less pronouned on a forced induction setup.
I converted my BBC race motor from a carb to EFI and am using a 2,000cfm TB on a fairly mild (7,000rpm) 60 over 427. That’s more than twice as much flow as you could get away with in a carb. This illustrates my point about “it can’t bee too big”. Consequent to this change, there was an ~5% increase in RWHP.
what kind of gain would you expect to see on a Lt1 with the TB if it was port matched to the intake?
mods would be heads/cam/exhaust? ect…
5-10 rwhp with the upper end of the range for a 58mm with a ported intake. 52mm does not need any modification to the intake.