Chevrolet Nova Forum banner

21 - 40 of 46 Posts

·
Registered
Joined
·
1,896 Posts
David loved the Comp 270 Magnum cam on a street engine build that you have now but usually used a 108 LSA. You would probably pick up 15HP minimum just going with a Vortec head swap and probably another 15 also including a change to headers. You have a good combo to see significant results with almost any head change (195cc or smaller) in your 355 now and will respond well to any mild enhancement to get more air in and back out. Any choice you make should depend on how bad you want more performance. As Big Dave said, if you have a 2:73 rear gear, moving to a 3:42 or 3:73 would make a huge difference in the street light to street light thrill but cost mileage and higher freeway RPM. JMO Jack :yes:
 
  • Like
Reactions: nova1972x

·
Registered
Joined
·
144 Posts
Discussion Starter #22 (Edited)
Thank you for going over that with me. Sounds to me that maybe these motowns, or possibly other heads aren't the answer.It appears I have highjacked my own thread! Sorry everyone!

I had originally started it to see what other people here use on their SBCs, and after posting this thread, I heard about the motown heads from my friend he is selling, and got fixated on not wanting to miss out on what seems a good deal. Since my engine only has around 3000 miles on it, and since I thought the rest of my components are good, albeit mild performance would benefit from a head upgrade.

As for horsepower gain, I consider 40 HP (maybe 50 if I also install headers?) a good gain. I definitely am not looking for 400+ HP, because I know that would require essentially redoing the whole engine. I think my engine right now is probably around 325-330 HP, at the flywheel, but that is just an estimate from the engine shop who built it. So if a head upgrade wouldn't do that, be them motowns or others unless I make other changes to the engine, I agree, maybe I am better off not doing it.

The whole thing stems from me regretting the heads I have on there. Had I realized how bad they really are, I would have probably just bought new heads right off the bat. I paid almost as much rebuilding them, as if I were to have just bought new iron heads like Vortec or Torquer S/R. I feel like I made a huge mistake with them. When I heard about these motowns, I thought I could fix the weak link in my engine, but sounds more complex than that.
 

·
Registered
Joined
·
144 Posts
Discussion Starter #23
BTW, my last post must have been done before i refreshed the page and after Big Dave and Jackel were replying.

Thank you for the book referral Big Dave, particularly the second one.

Jackel, as for heads, probably will review the readings before I select a new head, if it even makes sense to do at all.
 

·
Registered
Joined
·
1,896 Posts
As for horsepower gain, I consider 40 HP (maybe 50 if I also install headers?) a good gain. I definitely am not looking for 400+ HP, because I know that would require I think my engine right now is probably around 325-330 HP, at the flywheel, but that is just an estimate from the engine shop who built it.
A 50HP gain would probably happen with a set of 180cc or 195cc AFR Heads and headers. That is pretty common. I would order the heads with 1:6 ratio rockers to get the lift up ON YOUR CAM to 5.013....

*
Dyno Test Criteria
Horsepower: 420
Torque: 450
Engine: 355
Heads: AFR 180cc
Compression: 9:5to1
Cam: Magnum 270 Comp
Holley 0-80779 750 cfm
MSD Distributor 34° Timing
Comp Cams 270 Magnum Hyd. Cam 224 @ .050 Dur *.470 Lift
1 5/8" Headers
93 Octane Pump Gas
AFR #5029
Engine built and dyno tested by AFR
 

·
Registered
Joined
·
144 Posts
Discussion Starter #26
Hi all. What I am about to say will probably have some of you shake your heads, and think I am crazy...I ended up buying the Motown heads. I bought them knowing full well they aren't the best choice for a smaller SBC, like a 355.

Here is the background on the heads:
The person I bought them from told me that the previous owner only had the heads on his race car for 12 races, then upgraded to aluminum heads. My friend is putting new springs on for me, but said that the heads otherwise are still like new. He also was the one who assembled them for the previous owner the first time, and had even ported them out a bit.

Here's the reasons why I bought them:
1. The person who sold them to me is a friend of mine that I have known since 2010, who owns a machine shop. He built my 355 in the first place. I know I can trust him, when he says the heads are good, and don't have anything wrong.

2. My friend is in a bit of a financial bind because he has some health issues, and though he didn't say it, I knew he could use the money. This was the biggest factor, because I wanted to help him in some way, because he has done A LOT to help me over the years, and had connections that saved me a lot of money, too. My car wouldn't be running if it wasn't for him.

3. The heads were only $800, including my friend going through them to ensure they are good to go, and replacing the valve springs, retainers, etc.

4. I reasoned that these heads couldn't run any worse than the stock 1972 76CC heads I currently have.

5. I figured that if I changed my mind, I could probably sell these heads for the same or better money than what I bought them for, and use that money towards getting a set of Edelbrock performer RPMs or AFR.

6. I was able to pick up a polished Edelbrock performer RPM air gap intake manifold for $200, which would likely go well with the heads.

So, the reason I bring this all up: now that I have the Motown heads, should I use them or sell them?

I know that the 220CC runners are too big for a 355 compared to other performance heads mentioned in this thread, but compared to my 3973493 heads, will I lose any bottom end, if I keep the same cam? If yes, I will probably sell them. If no, I am thinking that I may just use them, knowing there are better heads out there.

I suppose another answer is to upgrade my short block to a 383, too, though that has less than 5000 miles. It's barely broken in.

So, if I haven't annoyed anyone here already, does anyone have any feedback on keeping or selling the heads?
 

·
Super Moderator
Joined
·
17,540 Posts
I don't know why anyone should be annoyed unless you beat them at the track. It is your engine build it the way you want.

I'm an engineer. I make decisions based upon scientific theory, including my experience in the field of engine building. Based upon this I can state that these heads will shift your power band up higher in the RPM range. They will also increase the area under the curve (make more power than stock heads).

Consider that every American made V8 from the smallest to the largest will make more power than needed to move your car (at a constant 45 mph on a level hard surfaced road it takes roughly 7 horsepower to overcome all resistance to maintain that speed). Now if you want to pass that moron driving at a constant 45 mph on a level hard surfaced road in front of you the amount of extra power under the hood and your gearing (gearing multiplies torque and torque powers acceleration) will determine if you have enough power available to pass him in the space available.

Your car's weight, air resistance and gearing are also factored into this with more modern cars having a slight advantage if they are a car (not an SUV) thanks to a slippery shape. Though modern cars are smaller in size for the most part (the new Mustang is about the size of your Nova) they weigh nearly twice as much (the 425 horsepower Hemi powered 2015 Challenger is pushing 5,000 pounds compared to your 3,400).

Once again it all comes back to physics or F=ma. I have in addition to engine simulators to play what if with motor building I also have racing simulators that can simulate what adding power to a given car will do to performance.

The drag racing simulator is pretty simple, as it is based upon a guestimation of available traction and F=ma. It is so simple that Moroso has a circular slide rule that can tell you your horse power from trap speed or algebraically any thing else about a drag car.

I also have a NASCAR simulator that factors in a lot more information about the car as air resistance is the biggest factor with these cars performance. It might surprise many people who claim how fast they drive their import cars that rotation velocity (V with R subscript) is under 140 mph for most car bodies. Exceed it and the car body will fly unless you do something to prevent air from getting under the car body (ground effects are not just for style).

I would build what you have and change the rear gear and add an automatic overdrive to reduce RPM on the interstate.

Big Dave
 

·
Registered
Joined
·
1,896 Posts
Try them out, you may like the result. The tune will be important with those big 220 runners. Advancing the cam a few degrees may help gain back more low RPM torque (Earlier Intake Close). Heck, the 1970 Boss 302 Mustang with WAY to big of heads responded well to a little cam duration reduction for better street driving back in the day so you may find that the 270 Magnum acceptable for your driving pleasure with the big heads. Jack
 

·
Registered
Joined
·
5,245 Posts
Generally speaking smaller cubic inch engines need to rpm to make larger heads function properly. Larger cubic inch engines just need a a bit more CC to take advantage of the cubes.
For example you have a little 283 with 220 heads you going to need to spin the rpms up to take advantage of the heads capability. A 400 with the same 220 heads wont need to spin as much rpm to take advantage of the heads capability. Some of the capability comes in cubic inches and its ability to fill the volume of the head. This is a general rule but not absolute.
 

·
Registered
Joined
·
144 Posts
Discussion Starter #30
Thank you Jackel. My friend plans of referring me to some people he knows who can tune an engine really well (I know basics, but these guys race). I will ask about advancing the cam. I feel that as long as I don't lose low end torque compared to what I have now, and I gain mid and upper rpm horsepower, I will be happy enough with those heads.

Do you think I can get away with the same cam, and 600 cfm carb, or should I changes those out too? My short block has good pistons, but it is a 2 bolt main. I didn't upgrade at the time, because I hadn't planned on going about 400 HP, and still don't I'd like to see a gain from where I am at around 325 HP, to something somewhere between 375 to 400 HP, max. No more than that. Is that doable?

I guess its one of those things I won't know until I try, huh?
 

·
Registered
Joined
·
144 Posts
Discussion Starter #31
Generally speaking smaller cubic inch engines need to rpm to make larger heads function properly. Larger cubic inch engines just need a a bit more CC to take advantage of the cubes.
For example you have a little 283 with 220 heads you going to need to spin the rpms up to take advantage of the heads capability. A 400 with the same 220 heads wont need to spin as much rpm to take advantage of the heads capability. Some of the capability comes in cubic inches and its ability to fill the volume of the head. This is a general rule but not absolute.
Thanks for articulating and confirming what I suspected, and in a clear, concise way. From what I have read, I basically have heads that flow almost like stock BBC heads, or not that far off from them, and the 355 is normally too small.

I know they won't flow as efficiently as a head better designed for a 350 like an AFR, but my hope at this point is have some boost over my current heads, and at a minimum not lose any lower torque.
 

·
Registered
Joined
·
1,896 Posts
Head flow numbers for the 493 casting number:
70/58
125/108
175/135
204/141
205/142
206/142
They are all done at .400 lift, anything HP wise above that lift has to come by RPM. They were made for low end torque during the initial stages of the smog era with a 400ci sbc. That said, you are going to loose some torque down very low in the RPM range with the 220's but by 2,500 you will be about even. I doubt your current combo makes 325HP with those heads. Keep the Comp cam, it is a great all around cam and small enough for the new heads. Any more cam the engine will fall flat when you leave from a stop. I would put your $$$$ into a set of 1.6 rockers to gain more lift with the new mega 220 heads and make sure they have the right springs for that Magnum cam. With the 1.6's no loss of low end torque but increased mid-range and above. Keep the 600 carb and tune it for the head/cam combo. It sure isn't perfect but it should be fun especially mid-range and above and also get you near the HP you are looking for. JMO Jack
 

·
Registered
Joined
·
144 Posts
Discussion Starter #33 (Edited)
I don't know why anyone should be annoyed unless you beat them at the track. It is your engine build it the way you want.

I'm an engineer. I make decisions based upon scientific theory, including my experience in the field of engine building. Based upon this I can state that these heads will shift your power band up higher in the RPM range. They will also increase the area under the curve (make more power than stock heads).

Consider that every American made V8 from the smallest to the largest will make more power than needed to move your car (at a constant 45 mph on a level hard surfaced road it takes roughly 7 horsepower to overcome all resistance to maintain that speed). Now if you want to pass that moron driving at a constant 45 mph on a level hard surfaced road in front of you the amount of extra power under the hood and your gearing (gearing multiplies torque and torque powers acceleration) will determine if you have enough power available to pass him in the space available.

Your car's weight, air resistance and gearing are also factored into this with more modern cars having a slight advantage if they are a car (not an SUV) thanks to a slippery shape. Though modern cars are smaller in size for the most part (the new Mustang is about the size of your Nova) they weigh nearly twice as much (the 425 horsepower Hemi powered 2015 Challenger is pushing 5,000 pounds compared to your 3,400).

Once again it all comes back to physics or F=ma. I have in addition to engine simulators to play what if with motor building I also have racing simulators that can simulate what adding power to a given car will do to performance.

The drag racing simulator is pretty simple, as it is based upon a guestimation of available traction and F=ma. It is so simple that Moroso has a circular slide rule that can tell you your horse power from trap speed or algebraically any thing else about a drag car.

I also have a NASCAR simulator that factors in a lot more information about the car as air resistance is the biggest factor with these cars performance. It might surprise many people who claim how fast they drive their import cars that rotation velocity (V with R subscript) is under 140 mph for most car bodies. Exceed it and the car body will fly unless you do something to prevent air from getting under the car body (ground effects are not just for style).

I would build what you have and change the rear gear and add an automatic overdrive to reduce RPM on the interstate.

Big Dave
Thank you very much for the detailed response, Dave. I just didn't want to seem like one of those guys that asks everyone a question, and then just does the opposite, anyway. :)

Do you think 3.42 rear gears with posi combined with a 700R4 would be sufficient for my case, then? Right now I have 3.42s with a TH350, and a 2500 stall. I don't have the exact gear ratio for both, but I seem to remember a 700R4 having a fairly significant "lower" first gear than the TH350.
 

·
Registered
Joined
·
144 Posts
Discussion Starter #35
Head flow numbers for the 493 casting number:
70/58
125/108
175/135
204/141
205/142
206/142
They are all done at .400 lift, anything HP wise above that lift has to come by RPM. They were made for low end torque during the initial stages of the smog era with a 400ci sbc. That said, you are going to loose some torque down very low in the RPM range with the 220's but by 2,500 you will be about even. I doubt your current combo makes 325HP with those heads. Keep the Comp cam, it is a great all around cam and small enough for the new heads. Any more cam the engine will fall flat when you leave from a stop. I would put your $$$$ into a set of 1.6 rockers to gain more lift with the new mega 220 heads and make sure they have the right springs for that Magnum cam. With the 1.6's no loss of low end torque but increased mid-range and above. Keep the 600 carb and tune it for the head/cam combo. It sure isn't perfect but it should be fun especially mid-range and above and also get you near the HP you are looking for. JMO Jack
Thanks again Jack. So, if I understand right, adding the 1.6 rockers will help compensate for any loss of lower end torque with the new head, then? I was considering upgrading to full roller rockers, anyway (I have 1.5 roller-tipped comp cams rockers). I currently have forged flat top pistons. Any danger of the valves hitting the piston, if I did go with 1.6 rockers?

As an aside, the way I came up with the number of 325 HP for my engine, was a guesstimate, based on the 290 HP version of the Good Wrench 350 having similar flowing heads, but dished pistons VS my flat top for an increase of compression to 9 to 1 vs 8.5 to 1, a 3 angle valve job, and a better cam (comp 270). I never dynoed it, so it could be a high, or "optimistic" estimation.
 

·
Super Moderator
Joined
·
7,383 Posts
Pretty much, something to remember too is the 700 has a wider spread between 1st and second gear.

350= 1.52

700= 1.62
 

·
Registered
Joined
·
1,896 Posts
Thanks again Jack. So, if I understand right, adding the 1.6 rockers will help compensate for any loss of lower end torque with the new head, then? I was considering upgrading to full roller rockers, anyway (I have 1.5 roller-tipped comp cams rockers). I currently have forged flat top pistons. Any danger of the valves hitting the piston, if I did go with 1.6 rockers?.
No, the 1.6 rockers won't help with torque loss moving to the 220's but it will get you to a better spot in the flow curve of the 220 heads and produce more power mid-range and above. Your current heads are all done flowing air at .400 lift. Your new 220's keep flowing more air to .600 lift and you want to take advantage of that extra flow. Your Magnum cam has a lift of .480 with 1.5 rockers but with the 1.6 rockers it will increase to .515. That is free power and you see that increase in lift flow on the way up the cam lobe and on the way down! This is basically how the lift curve changes with the addition of 1.6 rockers.


Now the 7004R will bring back the low end feel and quickness because of the lower gear ratio (Higher Numerically) in first. Jack
 

·
Registered
Joined
·
144 Posts
Discussion Starter #39
I am sold on the 1.6 rockers, then. But do you think it will be an issue of the valves hitting the pistons? When I had my engine rebuilt, I used flat top forged TRW pistons. They do have valve relieves on them, but when I asked for it to be rebuilt, at the time, I had planned on a cam with less than .500 lift.
 

·
Super Moderator
Joined
·
17,540 Posts
I am sold on the 1.6 rockers, then. But do you think it will be an issue of the valves hitting the pistons? When I had my engine rebuilt, I used flat top forged TRW pistons. They do have valve relieves on them, but when I asked for it to be rebuilt, at the time, I had planned on a cam with less than .500 lift.
Most pistons have plenty of clearance at that valve lift value.

You can check your actual clearance with the heads still on the car by bringing the piston to TDC and removing one valve spring to replace it with a checker spring. Works best with compressed air pushing the valve closed. With the checker spring attached to the valve and a dial indicator zeroed out on the top of the keeper push the valve down until it hits the piston. Subtract your lift at the valve and then subtract another 0.100 of an inch and that is your total clearance with an ample margin for weak springs. With stiff springs that prevent any chance of valve float you can run it down to 0.060 before fly cutting a piston for more clearance.

The intake valve is closest to the piston at twelve degrees after TDC and the exhaust is closest to the piston at twelve degrees before TDC but there isn't a lot of lift on most cams at that point, but to be sure it should be measured there to be sure. This is also assuming you stabbed in the cam straight up when you built the motor.

Big Dave
 
21 - 40 of 46 Posts
Top