Need help with parts selections.

MiracleMax · 12-28-2000, 11:26 AM

>What do you think?
Hmmm... that depends? For a carburated engine with iron heads. a 10.7:1 motor is right at the edge. On hot days it'll ping (from my experience, My brother has a 10.5:1 carbed SB and for pump gas its on the edge) The reason being is the inexact nature of fuel/air distribution with a carb engine. All things being equal an injected engine can get away with more compression because fuel metering is more precise. For an aluminum headed motor this should be no sweat. Since its reccomended that you go a full point higher to recover some lost heat in the chamber. Interestingly on two identical engines all things being equal (everything including weight)with the only difference being the material from which the heads are constructed , the iron head engine will run faster since it leaves more heat in the combustion space. However the exact limit on compression is hard to target! Induction/Exhaust, Engine Tolerances, Head Design/Construction, and Piston Design/Construction all effect this. Thats why somebody can come along and say BS! my motor can only run on pump gas with a 7:1 comp ratio

>What head would you use and why?
Personally I am a fan of the AFR heads. Good flow with high velocity has a coupla benefits. The biggest being better torque and and fuel economy. sounds wierd on a performance engine, but heads that flow the same with bigger ports or slightly more with bigger ports tend to slow down the air/fuel mixture. This being the case torque is lower because less fuel and air is being rammed into the engine, plus the mixture can seperate with slower movement raising .BSFC numbers and in turn lowering fuel economy. And in turn a less homogenous mixture in the chamber can increase the chance for detonation.

> roller cam (need number)
Really the best advice here is to figure your whole combonation from the air cleaner to the exhaust, trans type, gear ratio and driving style. Then contact a cam company. I typically provide as much information as I can (including head flow numbers). The reason being, Rod to Stroke Ratio, Cylinder Head Flow, Compression, etc. has a big effect on cam choice. For example a 347 vs. a 351; The 347 will actually be more volumetrically efficent at lower RPMs due to the smaller R/S ratio since the piston speed is higher as it is being yanked away from the chamber. Conversely a 351 will be a better high speed motor since it has more dwell time at TDC due to a longer R/S ratio. In any event the two engines will react differently to the same camshaft installed in each motor. However if I was going to pick a cam based on the what you are building, I'd go with a 35-442-8 Comp cams magnum hyd roller retrofit

All things being equal to my first try with the wiz bang miracle machine;

750 cfm carb
Perfromer RPM Intake
GT40p Heads
10:1 comp
1-3/4 long tubes
35-442-8 Comp Cams Magnum Hyd Roller

HP: 390 @ 5500 RPM
TQ: 428 @ 4500 RPM

With the AFR 185 cc heads
HP: 470 @ 6000 RPM
TQ: 465 @ 4500 RPM

(Remeber these sim dyno's are useful for broad comparisons; 1. because trash in, equals trash out so modeling the engine accurately is important and in this case its not to hard. 2. These programs have to make a few assumptions and the results are always the best possible power for a combination)

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12-28-2000, 11:26 AM	#5
MiracleMax Registered Member Join Date: Apr 1999 Location: Hayes, Va, USA Posts: 798	>What do you think? Hmmm... that depends? For a carburated engine with iron heads. a 10.7:1 motor is right at the edge. On hot days it'll ping (from my experience, My brother has a 10.5:1 carbed SB and for pump gas its on the edge) The reason being is the inexact nature of fuel/air distribution with a carb engine. All things being equal an injected engine can get away with more compression because fuel metering is more precise. For an aluminum headed motor this should be no sweat. Since its reccomended that you go a full point higher to recover some lost heat in the chamber. Interestingly on two identical engines all things being equal (everything including weight)with the only difference being the material from which the heads are constructed , the iron head engine will run faster since it leaves more heat in the combustion space. However the exact limit on compression is hard to target! Induction/Exhaust, Engine Tolerances, Head Design/Construction, and Piston Design/Construction all effect this. Thats why somebody can come along and say BS! my motor can only run on pump gas with a 7:1 comp ratio >What head would you use and why? Personally I am a fan of the AFR heads. Good flow with high velocity has a coupla benefits. The biggest being better torque and and fuel economy. sounds wierd on a performance engine, but heads that flow the same with bigger ports or slightly more with bigger ports tend to slow down the air/fuel mixture. This being the case torque is lower because less fuel and air is being rammed into the engine, plus the mixture can seperate with slower movement raising .BSFC numbers and in turn lowering fuel economy. And in turn a less homogenous mixture in the chamber can increase the chance for detonation. > roller cam (need number) Really the best advice here is to figure your whole combonation from the air cleaner to the exhaust, trans type, gear ratio and driving style. Then contact a cam company. I typically provide as much information as I can (including head flow numbers). The reason being, Rod to Stroke Ratio, Cylinder Head Flow, Compression, etc. has a big effect on cam choice. For example a 347 vs. a 351; The 347 will actually be more volumetrically efficent at lower RPMs due to the smaller R/S ratio since the piston speed is higher as it is being yanked away from the chamber. Conversely a 351 will be a better high speed motor since it has more dwell time at TDC due to a longer R/S ratio. In any event the two engines will react differently to the same camshaft installed in each motor. However if I was going to pick a cam based on the what you are building, I'd go with a 35-442-8 Comp cams magnum hyd roller retrofit All things being equal to my first try with the wiz bang miracle machine; 750 cfm carb Perfromer RPM Intake GT40p Heads 10:1 comp 1-3/4 long tubes 35-442-8 Comp Cams Magnum Hyd Roller HP: 390 @ 5500 RPM TQ: 428 @ 4500 RPM With the AFR 185 cc heads HP: 470 @ 6000 RPM TQ: 465 @ 4500 RPM (Remeber these sim dyno's are useful for broad comparisons; 1. because trash in, equals trash out so modeling the engine accurately is important and in this case its not to hard. 2. These programs have to make a few assumptions and the results are always the best possible power for a combination)