403 w/ 350 heads - Large or Small valves ?
September 30th, 2018, 10:50 AM
#1
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403 w/ 350 heads - Large or Small valves ?
I have a 403 for build-up. #7 heads.
I value expert opinions. It is fair to say I am a noob, but an educated one
My chief question is, for my desired application, should I modify to larger valves or keep the stock smaller size ??
I have seen on 3 sites, opinion that with smaller valves one can have just as much lower end power (hp/trq) and may fit the application. The explanation I interpreted was the larger valves, porting for more flow, & maximizing compression to the nth degree were for higher RPM motor applications.
Is there any sense to this?
I have a street cruiser (TA) and just want some bottom end acceleration for fun, balanced with a decent highway ride.
Being a 403, I am fine with peaking lower, prefer actually. (5000 rpm ? open for suggestion).
Will be going to a 3.08 or 3.23 gear (stock at 2.73)
example on build thoughts at minimum:
What are very rough anticipated incremental power gains for each individually in this combo?
1a. stock 403 w/ stock #7 heads [power gain]?
vs.
1b. stock 403 w/ modified #7 - larger valves, complete port job [power gain], estimated cost? ?
With Stock 403 & stock #7:
2. Cam. (moderate? want to learn sizing later) [ further power gain]?
3. Dual Exhaust [further power gain]?
4. Long Tube Headers (cheap non tuned 1-5/8" port 3" collector) [further power gain]?
I value expert opinions. It is fair to say I am a noob, but an educated one
My chief question is, for my desired application, should I modify to larger valves or keep the stock smaller size ??
I have seen on 3 sites, opinion that with smaller valves one can have just as much lower end power (hp/trq) and may fit the application. The explanation I interpreted was the larger valves, porting for more flow, & maximizing compression to the nth degree were for higher RPM motor applications.
Is there any sense to this?
I have a street cruiser (TA) and just want some bottom end acceleration for fun, balanced with a decent highway ride.
Being a 403, I am fine with peaking lower, prefer actually. (5000 rpm ? open for suggestion).
Will be going to a 3.08 or 3.23 gear (stock at 2.73)
example on build thoughts at minimum:
What are very rough anticipated incremental power gains for each individually in this combo?
1a. stock 403 w/ stock #7 heads [power gain]?
vs.
1b. stock 403 w/ modified #7 - larger valves, complete port job [power gain], estimated cost? ?
With Stock 403 & stock #7:
2. Cam. (moderate? want to learn sizing later) [ further power gain]?
3. Dual Exhaust [further power gain]?
4. Long Tube Headers (cheap non tuned 1-5/8" port 3" collector) [further power gain]?
Last edited by Via3d; September 30th, 2018 at 11:02 AM.
September 30th, 2018, 03:38 PM
#3
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Join Date: May 2013
Posts: 195
Probably a stock intake or the street dominator as those are the only ones that will fit under the hood. Even with street dominator you have to cut the intake. RPM has no chance fitting.
The 350 head swap and 3:23 gears will wake up your Trans Am. We have three of them 403 powered. You will need a decent cam and adjustable valve train. I’ve used a lot of early 350 heads various cams. I have left some leftovers from 2 builds.
If you talk to cutlassefi he will take care of you. Otherwise, you can see my builds on transamcounty.com
The 350 head swap and 3:23 gears will wake up your Trans Am. We have three of them 403 powered. You will need a decent cam and adjustable valve train. I’ve used a lot of early 350 heads various cams. I have left some leftovers from 2 builds.
If you talk to cutlassefi he will take care of you. Otherwise, you can see my builds on transamcounty.com
September 30th, 2018, 03:44 PM
#4
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Intake:
this is a Firebird and I want to keep scoop.
I was considering on intake, but I have read several times that the 3711 RPM will fit but doesnt make a difference over stock.
I guess anyone one match ported will help... can add a future date too... Open to suggestions.
I am not familiar with the Street Dominator, will it fit under lat e 70's Firebird scoops?
this is a Firebird and I want to keep scoop.
I was considering on intake, but I have read several times that the 3711 RPM will fit but doesnt make a difference over stock.
I guess anyone one match ported will help... can add a future date too... Open to suggestions.
I am not familiar with the Street Dominator, will it fit under lat e 70's Firebird scoops?
September 30th, 2018, 04:01 PM
#5
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Join Date: May 2013
Posts: 195
Holley street dominator not made for 20 yrs. They are single plane. What I did was used a performer before and had it ported paper thin. Everything works fine.
We do have the street dominator going on a build later this year and cutlassefi sold me all the parts for it and we cut the intake 3/8 of an inch and whacked off the 7degree angle from it too. I hope it will fit and if it does and it should I’ll post.
Not sure what you are doing, but I have a collection of 403 parts going back 30+ years for these trans ams. Intakes, heads, pistons ,cams, valve train. Just all depends on what you are doing. If you have the 7 heads send them to cutlassefi and be done
We do have the street dominator going on a build later this year and cutlassefi sold me all the parts for it and we cut the intake 3/8 of an inch and whacked off the 7degree angle from it too. I hope it will fit and if it does and it should I’ll post.
Not sure what you are doing, but I have a collection of 403 parts going back 30+ years for these trans ams. Intakes, heads, pistons ,cams, valve train. Just all depends on what you are doing. If you have the 7 heads send them to cutlassefi and be done
September 30th, 2018, 04:23 PM
#8
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Location: Phoenix, AZ
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That horsepower number is with a totally different rating method, so you can't directly compare them. A 403 will make significantly more torque due to the larger displacement.
September 30th, 2018, 09:57 PM
#11
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2" or 2.07" intake and 1.625" exhaust valves and at minimum use a bowl hog under the valves to open the bowl area under the valve. Talk to Cutlassefi about the cam. What pistons are you planning on?
October 1st, 2018, 08:02 AM
#12
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I was trying to determined the differences of expected gain between large (modified) and small (stock) valves for starters. Rational as you will, and the science behind it.
Compression will be raised regardless with use of #7 heads. With Small stock valves it seems logical I likely will experience torque and HP at the lower RPM curves.
I understand what optimum would be, and I know there are a few varying opinions on what should be done at a minimum, or what should be done since it is already tore down. I have a good idea what majority consensus should be as a minimum once case is open.
I am merely trying to get comparisons of expected gains for making informed decision on how to proceed, I have a couple 403 motors for build.
It is rough estimates and I know it varies on several factors, but there are nominal ballpark gains none the less, that should be known and expected.
Perhaps I need to start another thread.... and keep it to one item at a time.
Compression will be raised regardless with use of #7 heads. With Small stock valves it seems logical I likely will experience torque and HP at the lower RPM curves.
I understand what optimum would be, and I know there are a few varying opinions on what should be done at a minimum, or what should be done since it is already tore down. I have a good idea what majority consensus should be as a minimum once case is open.
I am merely trying to get comparisons of expected gains for making informed decision on how to proceed, I have a couple 403 motors for build.
It is rough estimates and I know it varies on several factors, but there are nominal ballpark gains none the less, that should be known and expected.
Perhaps I need to start another thread.... and keep it to one item at a time.
October 1st, 2018, 08:48 AM
#13
Old(s) Fart
Join Date: Mar 2007
Location: Northern VA
Posts: 47,311
I was trying to determined the differences of expected gain between large (modified) and small (stock) valves for starters. Rational as you will, and the science behind it.
Compression will be raised regardless with use of #7 heads. With Small stock valves it seems logical I likely will experience torque and HP at the lower RPM curves.
I understand what optimum would be, and I know there are a few varying opinions on what should be done at a minimum, or what should be done since it is already tore down. I have a good idea what majority consensus should be as a minimum once case is open.
I am merely trying to get comparisons of expected gains for making informed decision on how to proceed, I have a couple 403 motors for build.
It is rough estimates and I know it varies on several factors, but there are nominal ballpark gains none the less, that should be known and expected.
Perhaps I need to start another thread.... and keep it to one item at a time.
Compression will be raised regardless with use of #7 heads. With Small stock valves it seems logical I likely will experience torque and HP at the lower RPM curves.
I understand what optimum would be, and I know there are a few varying opinions on what should be done at a minimum, or what should be done since it is already tore down. I have a good idea what majority consensus should be as a minimum once case is open.
I am merely trying to get comparisons of expected gains for making informed decision on how to proceed, I have a couple 403 motors for build.
It is rough estimates and I know it varies on several factors, but there are nominal ballpark gains none the less, that should be known and expected.
Perhaps I need to start another thread.... and keep it to one item at a time.
To your valve question, I'll repeat what I always say here (and apologies to regular readers who are sick of hearing this). If we were talking about a 400 cu in big block Olds, would we even be having this discussion? A 403 small block moves just as much air as a 400 big block. Would you put SMALLER valves in a BBO? Why would you even consider it here? I can't envision any scenario where the small valves would be a better choice. I'd go all the way to 2.070/1.625. Of course I'd normally say to use BBO heads on a 403, but I understand that you need to run the stock SBO intake on a T/A, so you're limited in this build.
October 1st, 2018, 09:28 AM
#14
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Posts: 54
Joe,
I can appreciate the "system engineering problem" statement
I understand your input, and agree, regarding optimum sizing - go with the larger valves. But my question was not this at all. If I sound frustrated, I apologize for my frustration. in advance.
I am interested in the science behind power curves, peaking at lower RPMS... I have read several posts on several sites this was briefly stated in argument of small valve vs. large, but did not go into detail thoroughly for me to assess validity.
Further, as I understand it, this starts with heads (hence valves) and then on to cam. Of course gearing will change from stock to realize this potential and of course dual exhaust to let her breathe.
In all fairness:
Doing the MAX option, is not always the BEST option. It may be the OPTIMUM option if BUDGET is not of concern. Optimum is perfection but we live in an imperfect world.
This is a real life scenario... Budget ! Time! Scope ! A safe but ballsy 403 street cruiser. It is not a Drag car or a $3,000+ motor.
This Fall, I will have 2 - .403's to build....and 3 total 403 motors in all. Plus a 330.
Motor 1. I will take the stock motor out of a low mileage car and put in on the shelf. Freshen it up down the road.
Motor 2. have another I will build up performance style.... porting, valves, bore, pistons, 350 heads,
Motor 3. AND for the time being, to enjoy my ride, I am putting a stock 403 in, and considering putting the stock #7 heads I have on it. Dual exhaust is coming next, then a gear ratio change. This is BUDGET, TIME, SCOPE.
SO for motor #3, Compression alone gives a performance gain or not ???? How much ??? this is the small valve vs large valve contention #1.
Contention #2 is Motor 2. What do I want out of the ride? A ballsy 403 street cruiser, noticeably low end torque, yet good highway manners, sub-par mileage. I think I want peak at 5000 RPM and as much reasonable torque to that point. Being I am not winding the motor out,, and I am interested more in torque than high revving RPM's.... doesn't small valves achieve equal gain or more in the lower ranges?
Respectfully looking to learn and get input for my new hobby, ~ J
I can appreciate the "system engineering problem" statement
I understand your input, and agree, regarding optimum sizing - go with the larger valves. But my question was not this at all. If I sound frustrated, I apologize for my frustration. in advance.
I am interested in the science behind power curves, peaking at lower RPMS... I have read several posts on several sites this was briefly stated in argument of small valve vs. large, but did not go into detail thoroughly for me to assess validity.
Further, as I understand it, this starts with heads (hence valves) and then on to cam. Of course gearing will change from stock to realize this potential and of course dual exhaust to let her breathe.
In all fairness:
Doing the MAX option, is not always the BEST option. It may be the OPTIMUM option if BUDGET is not of concern. Optimum is perfection but we live in an imperfect world.
This is a real life scenario... Budget ! Time! Scope ! A safe but ballsy 403 street cruiser. It is not a Drag car or a $3,000+ motor.
This Fall, I will have 2 - .403's to build....and 3 total 403 motors in all. Plus a 330.
Motor 1. I will take the stock motor out of a low mileage car and put in on the shelf. Freshen it up down the road.
Motor 2. have another I will build up performance style.... porting, valves, bore, pistons, 350 heads,
Motor 3. AND for the time being, to enjoy my ride, I am putting a stock 403 in, and considering putting the stock #7 heads I have on it. Dual exhaust is coming next, then a gear ratio change. This is BUDGET, TIME, SCOPE.
SO for motor #3, Compression alone gives a performance gain or not ???? How much ??? this is the small valve vs large valve contention #1.
Contention #2 is Motor 2. What do I want out of the ride? A ballsy 403 street cruiser, noticeably low end torque, yet good highway manners, sub-par mileage. I think I want peak at 5000 RPM and as much reasonable torque to that point. Being I am not winding the motor out,, and I am interested more in torque than high revving RPM's.... doesn't small valves achieve equal gain or more in the lower ranges?
Respectfully looking to learn and get input for my new hobby, ~ J
October 1st, 2018, 09:37 AM
#15
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Join Date: Mar 2007
Location: Northern VA
Posts: 47,311
Airflow in and out of the motor under the intended use conditions is what you care about. There is no "one thing" that helps or hurts this. Airflow will be limited by the weak link in the chain. It might be the ports, it might be the bowl, it might be the valve diameter, it might be the cam, it might be the intake, etc, etc. Keep in mind that valve diameter is not as important as valve curtain area, which is the cylindrical area opening formed as the valve opens. Of course, what matters is the integrated curtain area through the entire valve opening event - the "area under the curve". Changing the cam lobe ramp profile and total lift will change the integrated curtain area for a given valve size. Obviously, smaller valves require more aggressive ramps (thus more shock to the valvetrain) and more lift to get the same integrated curtain area. Conversely, larger valves let you achieve the same airflow with a less aggressive (more street-friendly) cam. And yes, once you optimize the integrated curtain area, the next bottleneck in the port will become the limiting factor. Also keep in mind that moving air OUT of the engine is just as important as moving it in.
October 2nd, 2018, 06:29 AM
#16
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Via, no one does back to back builds with detailed evaluation like you're talking about. It's expensive and time consuming, and in the end doesn't really matter. You can check out the Engine Masters series on youtube - that's about as close as you'll get to anything like this. They did some good header tests for general questions ("Does denting a header really hurt power?"), but you're talking about super specific things.
IMO valve size is relatively low on the priority list, well after the cam, exhaust system, head selection, exhaust port work (if OEM heads). Heck, I'd worry about zero decking the block before the valve size. Now, if the heads are bare and you have the cash (or need to replace seats anyway or valves are bent anyway), then might as well throw the largest valves you can in there.
IMO valve size is relatively low on the priority list, well after the cam, exhaust system, head selection, exhaust port work (if OEM heads). Heck, I'd worry about zero decking the block before the valve size. Now, if the heads are bare and you have the cash (or need to replace seats anyway or valves are bent anyway), then might as well throw the largest valves you can in there.
October 2nd, 2018, 09:20 AM
#17
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Oddball, definitely, the builds will not be one at a time. if you crack one open and do as much as you can, all you can, one and done if you can. It's a different boat for people.
LOL, the 'ol dent the exhaust video....
Thanks for input! good stuff Oddball. Absolutely if block out or deeper in the block, I would go cam as a priority. Also dual exhaust is an easy upgrade but an expense none the less.
Interesting input re: decking the block before valves size, but I think I understand. I assume to increase compression (nominal gain vs. valves at this point for this applied scenario)
Does this also have to do with quench (which I don't fully understand) ?
In my scenario, for a Street Cruiser, anything over stock power will be more fun right? And I will avoid placebos.
I will build up another block separately, but meanwhile will the car on the road running with a 2nd engine, minimalist approach.... Regardless of engine, Dual exhaust, gear, rebuild carb is already going to happen.
So engine in place, call it bolt on if you want, reasonable improvement in order, all relatively at same time.
1. Dual exhaust - 5, 10, 15, 20 HP, I have seen it all said - <$600 - 2.5" manifolds back, new straight through mufflers (realizing 2.25" will suffice up to XXhp, but future motor will need 2.5"
2. 350 heads (stock #7) - 15 25, 30, 40 HP ? - $100 - stock as is
3. Gear - 3.08 or 3.23 - $ ?? - feel the torque to ground after all, more important on a street machine IMO
4. Rebuild Carb - none expected atm -<$100 - kit and misc; 800cfm version; - I have no expectation but consistency here, until airflow is addressed on the 2nd motor build then XX hp may be realized.
Other motor build: 1. Cam first & foremost, 2. porting of #4 heads, 3. timing chain 4. decking, 5. larger valves, 6. 30 bore, 10 under crank, 7. intake - not sure on getting another intake (keeping scoop).
And I am attempting to develop power gain and budget expectations for these.
--------------------------------------------------------------
~J
LOL, the 'ol dent the exhaust video....
Thanks for input! good stuff Oddball. Absolutely if block out or deeper in the block, I would go cam as a priority. Also dual exhaust is an easy upgrade but an expense none the less.
Interesting input re: decking the block before valves size, but I think I understand. I assume to increase compression (nominal gain vs. valves at this point for this applied scenario)
Does this also have to do with quench (which I don't fully understand) ?
In my scenario, for a Street Cruiser, anything over stock power will be more fun right? And I will avoid placebos.
I will build up another block separately, but meanwhile will the car on the road running with a 2nd engine, minimalist approach.... Regardless of engine, Dual exhaust, gear, rebuild carb is already going to happen.
So engine in place, call it bolt on if you want, reasonable improvement in order, all relatively at same time.
1. Dual exhaust - 5, 10, 15, 20 HP, I have seen it all said - <$600 - 2.5" manifolds back, new straight through mufflers (realizing 2.25" will suffice up to XXhp, but future motor will need 2.5"
2. 350 heads (stock #7) - 15 25, 30, 40 HP ? - $100 - stock as is
3. Gear - 3.08 or 3.23 - $ ?? - feel the torque to ground after all, more important on a street machine IMO
4. Rebuild Carb - none expected atm -<$100 - kit and misc; 800cfm version; - I have no expectation but consistency here, until airflow is addressed on the 2nd motor build then XX hp may be realized.
Other motor build: 1. Cam first & foremost, 2. porting of #4 heads, 3. timing chain 4. decking, 5. larger valves, 6. 30 bore, 10 under crank, 7. intake - not sure on getting another intake (keeping scoop).
And I am attempting to develop power gain and budget expectations for these.
--------------------------------------------------------------
~J
October 2nd, 2018, 11:08 AM
#18
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I can see a major case of M.A.W. coming on here. When you take off the intake and heads, you'll think I "might as well" put a new timing set while I'm this far. Next if any neoprene is missing, I "might as well" pull the oil pan and clean the screen/change the oil pump. "Might as well" check the bearings/change the camshaft yada yada.
October 2nd, 2018, 02:13 PM
#20
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I’ve followed this thread a bit.
Bottom line is you make max tq wherever you’re filling the cyl most efficiently, whether it’s at 2000rpm or 5000, doesn’t matter.
With the big bore of the 403 I wouldn’t hesitate to put the big intake valve in it, maybe even a 2.125, they’re available. Leave the exhaust at 1.62. Add a single pattern cam with a tight lobe sep and you’ll have all the low end tq you’ll need.
You have a crappy intake so all you can hope to do is add some flow and volume wherever possible. No pun intended, it ain’t Rocket science.
Jmo.
Bottom line is you make max tq wherever you’re filling the cyl most efficiently, whether it’s at 2000rpm or 5000, doesn’t matter.
With the big bore of the 403 I wouldn’t hesitate to put the big intake valve in it, maybe even a 2.125, they’re available. Leave the exhaust at 1.62. Add a single pattern cam with a tight lobe sep and you’ll have all the low end tq you’ll need.
You have a crappy intake so all you can hope to do is add some flow and volume wherever possible. No pun intended, it ain’t Rocket science.
Jmo.
October 3rd, 2018, 06:08 AM
#21
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Location: Plano, TX
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For me, cutting the deck is more about getting the quench height correct than the compression ratio. The compression doesn't change a whole lot for that kind of movement (maybe half a point?), but I've had plenty of fun fighting detonation, so my voodoo is to get a tight quench height.
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September 20th, 2013 09:17 PM
