View Full Version : logically matching parts to an engine intended use

01-23-03, 10:10 PM
logically matching parts to an engine intended use
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) http://www.babcox.com/editorial/ar/elements/30228b.gif 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 now lets follow the rules and build an engine suitable for the average street strip car 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



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.

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

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