Different rocker ratio between intake and exhaust?
- Thread starter Harte3
- Start date
The folks at www.reedcams.com must have thought it does some good because my Economaster cam purchased from them has slightly higher lift and a tad more duration on the exhaust side (using the stock rockers on both sides).
They said it was "to keep the heat down", probably wanting a quick blowdown of the burnt gases.
Chebbie rockers would give a similar effect.
Joe
They said it was "to keep the heat down", probably wanting a quick blowdown of the burnt gases.
Chebbie rockers would give a similar effect.
Joe
I have been told by many sources, that on a small block chevy run 1.6 on the intake & 1.5 on the exhaust. The cams that this combo was run on had about 10 more degrees on the exhaust duration anyway.
On the fomoco products??????? Bill
On the fomoco products??????? Bill
Ford heads have a reputation for having poor exhaust. They seem to run better with higher lift and longer duration for the exhaust. Alex.
It depends on the cam. Stock, its likely to be pretty marginal. All Fords are compromised on exhast flow. Any time better intakes are used, Ford often doesn't balanace that with better exhast flow. Example is 2V 250 Ford, 4V Cleveland, early 4V Hi Performance 302W and 4V 351W compared to the stock variants have no major exhast profile changes.
Same with later High Swirl heads, Ford is quite happy to have a wooping big intake with lots of shroud area (86 EFI 5.0 Mustang, 86-93 EFI 4.1 Falcon) and do nothing to the the exhast at all.
All info I've seen from Vizard, Car Craft and Hot Rod tends to focus on a 65 to 80% flow drop in cfm from intake to exhast, and thats just for maximum power.
For low speed torque and day to day driving, minimising the duration when both valves are open 30 thou off the valve seat is the way to gain drivability, and if you've got a very mild cam, you can gain quite a lot of extra urge by increasing the lifter ratio on the intake. If you increase the exhast lift, yu blead off cranking compression.
On an engine with a very small, restrictive carb, you can gain quite a lot of extra power by creating extra cold cranking compression by raising the intake lifter ratio, but leaving the exhast lifter ratio stock.
As the old saying goes, it's not what goes inside that makes you fat or thin, its what comes out your butt. Maintaining a restricted outflow, while increasing the input, you gain pressure and power to an extent.
Since most stock Fords are hard pressed to get even a 65% exhast flow. Adding intake lift might hurt power with a restrictive exhast. Adding high lift intake rockers and a header and dual out low restriction exhast will effectively gain good intake to exhast flow balance. The key is that exhast flow is the fifth stroke to an Otto cylce engine. If you increase air in, and improve the ability for each slug of exhast to scavange the combustion chamber, power and especially low speed torque will improve. Exhast lift gain is useless unless the backpressure of the exhast system can be reduced. A stock Ford 6 has at least 6 to 9 psi of backpressure. Add a V8 exhast and header, and its cut by half...3 to 4.5 psi is pretty easy. Add those high lift inlet rocker ratios, and you'll get benefit.
Same with later High Swirl heads, Ford is quite happy to have a wooping big intake with lots of shroud area (86 EFI 5.0 Mustang, 86-93 EFI 4.1 Falcon) and do nothing to the the exhast at all.
All info I've seen from Vizard, Car Craft and Hot Rod tends to focus on a 65 to 80% flow drop in cfm from intake to exhast, and thats just for maximum power.
For low speed torque and day to day driving, minimising the duration when both valves are open 30 thou off the valve seat is the way to gain drivability, and if you've got a very mild cam, you can gain quite a lot of extra urge by increasing the lifter ratio on the intake. If you increase the exhast lift, yu blead off cranking compression.
On an engine with a very small, restrictive carb, you can gain quite a lot of extra power by creating extra cold cranking compression by raising the intake lifter ratio, but leaving the exhast lifter ratio stock.
As the old saying goes, it's not what goes inside that makes you fat or thin, its what comes out your butt. Maintaining a restricted outflow, while increasing the input, you gain pressure and power to an extent.
Since most stock Fords are hard pressed to get even a 65% exhast flow. Adding intake lift might hurt power with a restrictive exhast. Adding high lift intake rockers and a header and dual out low restriction exhast will effectively gain good intake to exhast flow balance. The key is that exhast flow is the fifth stroke to an Otto cylce engine. If you increase air in, and improve the ability for each slug of exhast to scavange the combustion chamber, power and especially low speed torque will improve. Exhast lift gain is useless unless the backpressure of the exhast system can be reduced. A stock Ford 6 has at least 6 to 9 psi of backpressure. Add a V8 exhast and header, and its cut by half...3 to 4.5 psi is pretty easy. Add those high lift inlet rocker ratios, and you'll get benefit.
Thanks for the info. I picked up on the difference in ratios on another Forum but didn't understand the what and why of the difference. I did pick up on Vizard's discussion of the difference in Intake v Exhaust valve sizes...I think Exhaust is 75% of Intake and on the back pressure too...should be no more than 5lbs or the exhaust is considered restrictive.
Thanks again.
Thanks again.
