Whats the equation for determining the proper injector size?

 

HP=Injector size x 2 x #cylinders x %DC...


let me know if I am wrong anyone

 

I was told by Doug Fugal owner of Instant Injection out of cal. that the formula was .46 X HP div. by # cyl. = injector size on naturaly asperated engins use .5 on forced induction but it depends on who you talk to about setting them up some people rune more injector with less presure and some run less injector with more presure and all depends on how the chip is burned from what I understand I am going through this process with my own setup right now

 

Its reletive to volumetric efficiency. I believe that if you engine is rated around 90-100% VE then the formula is .5lb of fuel/hp on NA engines. Then divide by the # of cylinders and that is the injector rating. Ex: HP X.5 /8. Don't hold me to it though...

 

http://www.smokemup.com/auto_math/fuel_injector.php

http://www.grapeaperacing.com/GrapeA...linjectors.cfm

http://users.erols.com/srweiss/calcpchg.htm

dont forget the intake port airflow , cam timing, overlap, exhaust flow and compression ratio must also match the hp levels wanted
http://users.erols.com/srweiss/calchpaf.htm

Ok I get lots of questions on how to build a good street engine and how to find and match the correct parts, now Im going to be refering to your average hot street combos built on a budget with easily available parts.

first the math
(1)youll be limited to about 1.2 hp per cid on engine size
(2) YOULL BE LIMITER to about 6400rpm with HYDROLIC flat tappet cams and about 7500rpm with SOLID LIFTER flat tappet cams
(3)piston speeds that exceed 4000fpm usually lead to trouble
(4)dynamic compression ratios of over about 8.3:1-8.5:1 with aluminum heads or about 7.8:1-8:1 with iron heads can cause detonation problems
(5)the formula for matching POTENTIAL HP to INTAKE PORT FLOW is (.257 x port flow at max cam lift x 8(3 of CYLINDERs)= POTENTIAL hp
(6)[image]http://www.babcox.com/editorial/ar/elements/30228b.gif[/image] look closely at the duration used for each MATCHING rpm range. ALSO KEEP IN MIND THE DCR AND OVERLAP MUST MATCH look here these are the valve timeing overlap ranges that are most likely to work correctly
trucks/good mileage towing 10-35 degs overlap
daily driven low rpm performance 30-55degs overlap
hot street performance 50-75 degs overlap
oval track racing 70-95degs overlap
dragster/comp eliminator engines 90-115 degs overlap
but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range

OK EXAMPLE, now lets follow the rules and build an engine suitable for the average street strip car ( CARBURATED)and lets set the goals at pushing a 3400 lb car to 12.5 seconds in the 1/4 mile with a 3.08 rear gear, and TH350 (JUST TO SCREW THINGS UP BECAUSE USEING A 4.11 RATIO MAKES THINGS TO EASY) 26.5" tires and an auto transmission to simulate an average late 70s car.
well the first thing we are going to need to know is how much hp /tq will we need useing this quick guide

http://www.prestage.com/carmath/calc_ETMPH.asp

we find that we need about 375 rear wheel hp or 469 flywheel hp to easily run very low 12s
those 3.08 rear gears and 26.5" tires we will only be spinning about 4900rpm in the lights so well need an engine with lots of mid range tq, the 469 flywheel hp says we will require heads that flow at least about 230-240 cfm at about .48-.499 lift if we figure that we must build a hydrolic cammed torque monster that has very high intake port speeds for good volumetric efficiency in the mid rpm ranges to get the tq curve correct for those crappy 3.08 rear gears, the cam rpm chart shows we will need a cam in the 230@.050 duration range with about a 50-75 degree overlap and we need about 390-406 cid mimimum to get 469hp, REMEMBER WE ARE ONLY SPINNING 4900rpm in the lights so an engine that makes most of its hp at far higher rpms is a waste, that combined with the low rpm range would make a 383-406 the first choices here.
http://www.prestage.com/carmath/calc_gears.asp

so what we wind up with is a 406 with a 10.5:1 static compression, a comp cams #12-246-3 cam with 190cc AFR heads as the smallest port heads that flow enough air at that low duration http://www.compcams.com/information/...umber=12-246-3
open headers 1 5/8" full length,at the track, an 850 carb and a dual plane intake. a 3000rpm stall speed, shifts at about 5500rpm

now theres other combos and ways of getting there but you get the idea about how the parts should all be working towards the goal and within the restrictions , drop a cam like the crane cams #114681 and your hp goes up but above the rpm range that you ideally need it in altho the results are still good, a 3.73-4.11 rear gear would make the change to the solid lifter crane cam a far better choice

if your useing EFI youll want to use a cam with a wider average LSA and lower overlap and duration in most cases

now just for info

vortec heads dont flow enough air at the max stock lift of about .470 while in the rpm ranges and cam lift durations needed and limit the dynamic compression ratio because iron holds heat (DETONATION)

cams with less than about 220 duration don,t tend to breath well in the rpm and duration ranges where youll need the peak tq 4500-4700rpm

intake ports much larger than 195cc tend to lack low rpm velocity/ cylinder filling efficiency

the cams with less than a 110 lsa would work fine with the long stroke and large duration but idle will be quite rough

almost any carb from 650-850 cfm will work but the vacuum secondary 750-800 cfm carbs are the best match

due to the rpm range 1 5/8" headers will produce slightly better low rpm tq that best matches the 3.08 rear gear

a dual plane intakes rpm range best matches the intender airflow speeds and rpm ranges

cylinder pressure kept at near max during the lower rpm ranges helps low and mid range tq