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Discussion Starter · #1 · (Edited)
Hello all..stay safe during this Virus going around...So I have a lot of time on hand now, want to re-think my build. ....My parts on hand are new Comp cam H268 non roller specs are; lift @.50 = .218, Dur. = 268 for In. and Ex.., Val. lift is .454 w/ 1.5 rockets... Should I get 1.6 rockers for the exhaust side to help it out to make it .484 lift or use the new 1.5 rollers I have for the 400ci. The 350ci - 383, I have to buy cam. What cam specs for the 350-383 ci to buy?

Motors choice of 1985 350ci roller block 2bolt main, Stock bore, I can stroke it to 383 with a crank, piston,rod kit..
or 509 block 400 ci not bored 2 bolt main, can stroke it too. No pistons on order .. need to find out what cam to use first .

New cast heads in stock 200 SR2 64cc ported to 68cc, 355 gear, 2900 pounds. 2 Intakes both are RPM Performers.. Driven on the street, 10% at track. Auto Trans, both BM kits 350, 400 turbo with 10 inch converter, and a 700R4 to be rebuilt.

I like to use these heads and buy new heads for the other motor...looking to built motors for 89-90 pump gas with good torque / HP - looking for 360-375 hp and in the area of 425 torque... any idea or thoughts on this ?
 

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To a very large extent the cam establishes the peak horsepower. A good example is the 300 horse 327 and the 305 horse 350; both use the same cam. Now the 350 delivers a fatter torque curve. But the cam runs out of air sooner so while the power is up a little bit it happens at a few hundred rpm earlier. So if you want to improve the power on the 383 in proportion to a 350 with an XE268H you need to look at a cam that is proportionally longer in duration or roughly 10% more. That, however, is pretty simple you also need to consider lift which if the rest of the system is larger than what you find on a 350 such that the carb or injection can feed more air and the ports and perhapes valves support more flow to where adding lift can be used to soak up the component flow potential, then you can get the result with less duration.

Bogie
 

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Discussion Starter · #3 ·
Thank you for the quick answer...So something in the range of 222-229 duration and lift of 488-499 or so..Should I go with a split lift ( a higher lift for exhaust ) ?... I will pick the carb with the cam choice, but I do have a early holly injection setup that I can use up to 425 ci. that what holly told me, it bolts on top where the carb goes and I have two spacers, a open 2 inch and a divided 2 inch one that I can play with too.
 

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use the roller block with roller cam
better heads makes for better power 9:1-9.7:1 for 89 octane
425 pounds torque is high for a 350 hp 355 ci
usually higher lift goes on the intake side
what are your head flow numbers
how did you lose 4 CCs in the combustion chamber?
 

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You are focused too much on lift...it is the least important of all the various spec's for a cam.

Duration is more important, along with intake lobe centerline and lobe separation. Sure, you want as much lift as possible, but it is third or fourth on the list to consider when looking at cams.

Whether you need more duration on the exhaust side is entirely dependent on what the exhaust side of the head flows, and what your exhaust system consists of. A head with good exhaust port flow won't want much duration split(more duration on exhaust than intake) and can often not want any split at all.

On the SBC, 9 times out of 10 if you are using a split ratio rocker arm set you'll want to put the bigger ratio rocker arm on the intake side....more lift and the slight increase in duration at the valve help more there, as the incoming intake charge is harder to get into the cylinder than the exhaust is to get out.
Intake side has just atmospheric pressure pushing the incoming air in, the exhaust side has expanding exhaust pressure blowdown at high velocity and the rising piston to help push the exhaust gases out.

Putting the bigger ratio rocker on the exhaust side will help power above the HP peak rpm, but will usually hurt power below that....and with HP peaks being 5500 rpm and higher that lost power at every rpm below that hurts more than the upper gain helps.

I wasn't sure what head you have, you posted "200 SR2 64cc ported to 68cc" which seems to have confused a couple of terms....
Are they World Products 200cc intake port volume 64cc combustion chamber Sportsman II's with the chamber opened up to 68cc?? And you are implying some actual port work has also been done??
Or are they World Products S/R heads, ported to a 200cc intake port volume(up from 165-170cc as-cast) and the chamber volume opened up to 68cc volume??
 

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Discussion Starter · #8 · (Edited)
Thanks for all the replys....Yes the head are World Products S/R heads, ported to a 200cc intake port volume(up from 165-170cc as-cast) and the chamber volume opened up by being ported to 68cc volume.. And I have 1.5 roller bearing pivot rockers I can use...

I do have two Isky full High rev harden face cams complete set up with and dog bone roller lifters, reverse dist. cam, and gear to gear timing gears, for older blocks ..But way too munch for street set up 510 lift, 310 Dur., 490 lift, 300 Duration....not munch gas mileage ...LOL...

For the 400 ci, The next step up cam is 280H , 480 lift, 230 Duration, and 110 LS...I do not want to go higher then this cam, so I want to keep gas ( 89 - 90 ) at a easy place to pick up when I travel for the long hall ( 250-450 miles or more )...So if I get this cam I will match pistons. in the are of 9.4 CR ...Head gasket need to be figure out too...
 

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Discussion Starter · #9 · (Edited)
To Tech guy thanks I had these parts....The cam 268H I have new I just want to build a motor around it...I could use the 400ci ...so what king of HP and torque with the cast iron heads and RPM intake, and 1.5 roller rockers. And using 89 pump gas..... what would the max pistons compression ratio be ? Can I use with 9.5 pistons in it...or is that too high for 89 ump gas ?

Now for the roller motor going to get a 383 stroker kit, with matching pistons and cam...SBC 383 Stroker Kit, Scat 9000 Cast Crankshaft 3.750 Stroke, 350 Main, 5.700 I-Beam Rods, Keith Black Hypereutectic -5cc Flat Top Pistons, 1-PC Rear Seal...And they do sell a matching cam for this set up..I will ask them what heads the motor needs for max performance...I think more likely a newer style head and better flow.
 

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Discussion Starter · #10 · (Edited)
This is what I base my choice for picking a cam and piston ratio's...It is important to be sure that the engine's compression ratio matches the recommended ratio for the cam you are selecting. Too little compression ratio (or too much duration) will cause the cylinder pressure to drop. This will lower the power output of the engine.
With too much compression ratio (or too little duration) the cylinder pressure will be too high, causing pre-ignition and detonation. This condition could severely damage engine components.

Static Compression Ratio (SCR) , Intake Valve Duration (degrees @ .050" lift) ,Power Range (RPM)

8.00:1 185º Idle-4,000
8.25:1 189º Idle-4300
8.50:1 194º 800-4,500
8.75:1 200º 900-4,600
9.00:1 204º 1,000-4,600
9.25:1 208º 1,200-5,200
9.50:1 212º 1,600-5,400
9.75:1 216º 1,800-5,600
10.00:1 221º 2,000-5,800
10.25:1 227º 2,400-6,200
10.50:1 233º 2,800-6,400
10.75:1 236º 3,000-6,800
11.00:1 240º 3,200-7,000
11.50:1 244º 3,400-7,200
12.00:1 248º 3,600-7,400

So that's why I pick using (218 Duration, 454 lift, 110 LS cam ) and why I want to use 1.6 rockers with this cam so to raise the lift to .484 with out having to buy ( 230 Dur., 480Lift, means 10.5 comp. ratio) I am not building a track car...just trying to max out the motor with that cam in my rpm area too...that bigger cam would mean 92 octane gas... thanks for all your input!!!
 

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Too little compression for the cam not only cuts power but increases the fuel burn for the power you get. GMPP's 290 horse crate a case in point. With no other changes a set of L31 heads, which admittedly flow way better, have better shaped chambers that add about 1.5 compression ratios suddenly make 60 to 70 more pound feets and poney powers. But there is a huge downside to cast iron heads as they force the use of less compression against the fuel's octane so many brands of aluminum heads will go another 20 units of measure on this short block.

So what you have going here is a receipe for less torque and power, for greater fuel burn, with a requiremnt for a higher octane fuel to do it.

As far as more lift versus more duration, this is a struggle between flow capabilities of the induction and exhaust systems. If you look at modern engines the trend is to shorter durations with much higher lifts using what in GenI SBC days is way and gone far too much flow capability in the heads. This of course is something you can get away with using timed fuel injection. This is also a function of roller cams that when you look at lift as an area under a curve, the roller can operate with lobe lift rates and profiles that greatly enlarge the atea under the curve. This becomes a reason why 1.6 rockers or 1.7's if you really want to hang out on the edges of power production and part wear with he Gen I and II motors produce more power, assuming the induction system from its openng to the atmosphere to the inlet valve has a high enough instant flow capability. So among the stuff you need to know is what the head ports actually flow not how big they are. The AFR 195 for example will out gun anybody elses head till they get up around 220 to 230 cc ports.

Alumnum heads on otherwise identical engines can be run at least a ratio higher on compression which if the cam is too long winded to optimize the compression on a modern chambered iron head it is typical to see gains of 20 to 50 horespower depending on how under optimized the compression is for the cam used. However, you uave to be carefull when swingng this paragraph around as there are notable examples such as the aluminum Flotec head that does no better than the iron L31 they copy. That largely being a port shape, size, and location in 3D space issue as that head uses a port really common to pre 1987 heads so conventional intakes can be used with out gasket sealing issues. In these cases this is mostly fixable with a high speed grinder and a box of burrs.

So like I said earlier on you have got to look at this in the context of assembling a system. Just buying performance parts doesn't assure an optimzed result unless you just got lucky.

Bogie
 

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Discussion Starter · #14 · (Edited)
To answer the question of the chart I have, it a general chart for all types of motors....you can go down one duration and keep the same piston ratio, anything more you need to change your compression too.

So what I am hearing is that I should go with new style aluminum heads because the flow is better out of the box and the way old style cast heads have to deal with compression and fuel....

So can some one tell me what I can use those new old style cast heads 200 flow, 68 cc ported , 2.02 with 3 cut valves/seats 3 for ? And how to build a 400 ci good torque motor for them ? I could stroke it I guess, but I want to use those heads.. Any ideas ? Thanks
 

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You guys are making this complicated.
The 268H can is better suited and designed to a bit bigger than stock but very streetable with stock convertor on a 350ci engine. Think RV type cam for the old folks here.
I personally think it wouldn't be enough on a 400ci engine.

I'd use the SR heads on the 400ci, and a cam somewhere near the 270ish seat duration. Call whatever you favorite cam company is and ask for something. They will figure out the details for you.

For the 350ci, I'd use the 268 and heads with around 180-190cc.

Aluminum or steel, it's not going to matter much on a street car at 9-10cr and pump gas.
 

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old style cast heads
Let me help you out a little here. You are using the word "cast" to describe iron heads. Please understand that both iron heads and aluminum heads are "cast" heads. They are both produced by the process of casting the metal into a mold and then machining them to the final product. So, to prevent confusing your readers, you can refer to heads as either "iron" or "aluminum". ...:thumbup:
 

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Discussion Starter · #17 ·
Thank you Tech and Johnsongrass1.... So you said to get a 270 cam for the 400ci and use my iron heads...could I just change my rocker to 1.6 and would make it a .484 lift , lift @.50 is 218, Intake / exhaust is 268 duration ( very close to 270 ) and when using the 1.6 rockers longer ramp time for the valve to be open for gas and exhaust ?........I have to order pistons ( trying to stay under 10.0 CR ) so near 9.5-9.75 ?
 

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NO.
Read above.
Lift isn't as important as duration and looking at lift and ignoring the rest of the cam design isn't a good way to this.
Changing the rockers 1.5 or 1.6, does have it's effects however not enough to matter for what your doing.
 

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Discussion Starter · #19 ·
thank you.... Ok then I will start to look for something near the 270 duration and keep my new 1.5 roller bearing rockers I have.
 

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Vd=0.7854B2S, CR=Vd+Vc/Vc
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Check out Comps 270 Magnum, its 224* with .470" lift, that cam pulls like a mule in my 9.75:1 383 Vortec, idle is sweet with Erson gear drive.
 
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