Theoretical idle vacuum question

[My427stang]

Dual plane, all iron, 1.5/3.0 ring combo, 10.5:1 compression, torque plate honed 461 inch FE, 114 LSA hyd roller using a 291/236 lobe to ensure enough idle vacuum for brakes while balancing some top end for a steep factory gear

The intent right now is to run the cam at 106 ICL, how much difference in idle vacuum could their be by retarding it to 108 ICL...110 ICL? Thought is that the lobe is on the small side for the build, but I don't want retard it too far to further kill vacuum.

Old school days we used to advance the cam until cranking compression stopped climbing, then go back a step, but that was hardly precise. Not that this estimate is either, but how much does the earlier opening and closing intake affect idle vacuum?

I have plenty of room on the intake and exhaust side, and all tuning will be tight, only asking the ICL effects. I know we will eventually get into the "wrong cam discussion" but if you could entertain my general theory discussion first

Thoughts? Thanks in advance

[racin69z]

I would think with 460 inches a 291 236 lobe on a 114 la should build enough vacuum to operate power brakes just fine. Running the cam 8 degrees advanced seems like a lot to me. Advancing it that far will give you more cylinder pressure and with 10.5:1 that may be trouble. The only 460 I ever messed with had the flat top piston down in the hole about .060 if I remember correctly. If yours is that way you may have a detonation prone engine.

A long time ago I ran a 280 280 230 230 on a 110 lsa on a 106 icl and it had enough vacuum to operate brakes just fine in a little 350. With 110 more cubic inches you should be fine.

Does running the cam advanced have a large affect on vacuum? The overlap is ground into the cam and no amount of advance will take that out.

Lynn

[steve cowan]

my 383 sbc has 10.5:1 comp
178cc dart heads
comp 235* 242* @ 0.050'' 270-280
106 LSA 106 ICL 0.500'' lift SFT
rpm airgap 1'' open spacer,Reher and morrison 1'' tapered spacer and 1/2 shear plate
950 ultra hp carb
16'' vac @ idle
20'' at hwy cruise 3000-3200 @ 60mph
i will be doing some track testing with retard and adv cam timing in the next month or so,i am happy to give some of my results if you are interested.
there are alot of variables at the track as in performance gain/loss and i really dont expect much at all.

[My427stang]

I would think with 460 inches a 291 236 lobe on a 114 la should build enough vacuum to operate power brakes just fine. Running the cam 8 degrees advanced seems like a lot to me. Advancing it that far will give you more cylinder pressure and with 10.5:1 that may be trouble. The only 460 I ever messed with had the flat top piston down in the hole about .060 if I remember correctly. If yours is that way you may have a detonation prone engine.

A long time ago I ran a 280 280 230 230 on a 110 lsa on a 106 icl and it had enough vacuum to operate brakes just fine in a little 350. With 110 more cubic inches you should be fine.

Does running the cam advanced have a large affect on vacuum? The overlap is ground into the cam and no amount of advance will take that out.

Lynn

We should be fine with 10.5:1 on pump gas, it's not a 460, it's a stroked 428 Cobrajet FE at 461 cid with flat tops at .005 proud with a .051 gasket, so quench is at .046, about 20% quench pad ratio, polished CnC cut chambers. It's a pretty good match for intended fuel at 106, I just don't think I am going to get quite the RPM I want. I am not concerned with what I will have now for vacuum, question was to determine how much vacuum I would lose if I rolled the cam back to 108 or 110

As far as your last comment, yes the overlap is the significant contributor to vacuum. However, I have seen some smaller gains in idle vacuum by advancing cams before and wondering if anyone had an opinion of how much it may change if I roll the cam back

[rebelrouser]

I hate to keep posting this on all these similar questions, but get into the 21st century, lots of cheap engine software that will predict with reasonable accuracy things like cranking compression, proper advance curve, what octane fuel will be required. No need to guess about what will happen when you fire the engine up. I get lots of customers who want to bring me a pile of swap meet parts to assemble an engine, without a good engine program to put those parts into and see what going to happen, I don't know if I could build half the engines that I do. I use performance trends, and Wallace racing has tons of free calculators. If you take the time to do some measurements and input good numbers into the program, they are very accurate especially on a normally aspirated engine. I regularly get within 5% on a real dyno compared to the virtual one. And I set the timing to what the program wants and its usually within a degree or so of what the prediction says. On supercharged or turbo engines I have issues getting a good number on the actual cfm output of the charger, but if you can get the company to give you a good number it works as well. The other issue is the program assumes right air fuel ratio, so if there is an issue most of the time its the fuel system.

[My427stang]

I hate to keep posting this on all these similar questions, but get into the 21st century, lots of cheap engine software that will predict with reasonable accuracy things like cranking compression, proper advance curve, what octane fuel will be required. No need to guess about what will happen when you fire the engine up. I get lots of customers who want to bring me a pile of swap meet parts to assemble an engine, without a good engine program to put those parts into and see what going to happen, I don't know if I could build half the engines that I do. I use performance trends, and Wallace racing has tons of free calculators. If you take the time to do some measurements and input good numbers into the program, they are very accurate especially on a normally aspirated engine. I regularly get within 5% on a real dyno compared to the virtual one. And I set the timing to what the program wants and its usually within a degree or so of what the prediction says. On supercharged or turbo engines I have issues getting a good number on the actual cfm output of the charger, but if you can get the company to give you a good number it works as well. The other issue is the program assumes right air fuel ratio, so if there is an issue most of the time its the fuel system.

Rebelrouser, your response missed the mark. I build big inch pump gas FEs, to include my own EFI 489 inch street motor which has got little more than oil changes since 2006. I have all the calculators, and not many are more careful in blueprinting than me. I also have zero issues with this or any other build running on pump gas. However, if you can tell me based on all the numbers what my idle vacuum will be with confidence, I am willing to provide all the values and willing to listen. This is a very unique build to meet a externally correct concours-driven resto on steroids. This is not a pile of swap meet parts in any way shape or form, which was likely clear from torque plate hone, hyd roller and metric ring pack on a 68 Cobra Jet....

Regardless, these calculators don't speak vacuum very well because they just don't have all the inputs. I didn't ask how to build, cam and time the motor, just if anyone had experience with a single change to cam indexing and how much effect on idle vacuum.

[travis]

I think a lot of it will depend on how well the intake side flows at low lifts. I have seen slightly reduced idle vacuum from advancing the cam (but still slightly stronger torque on the low end), but have also seen practically no difference as well.

For example...the very low $$$ engine in one of my trucks has about 9.0 compression, ported 1.78/1.46 E7TE heads, and a stock 351w FI cam that is 206/221@.050, on a 115 lsa/117 ica (2* retarded). Advancing the cam 4*, with no other changes, lost about 1” vacuum at idle that I couldn’t get back. I tried 8* advance out of curiosity and lost another 1/2 to 1”. It was a little rougher running right off idle but picked up noticeably by 1700-1800 rpms. This engine seems much more sensitive to any change than most I have seen.

[Tuner]

Where are the professional cam grinding experts who know for sure, beyond any shadow of a doubt, the Chevy '140 cam makes engines run backwards and it is impossible to win a race with one in your engine? Don't the cam grinding experts make cams that have to meet the intake vacuum rules enforced at some circle tracks? They should know the answer to the OP's question.

[novadude]

Where are the professional cam grinding experts who know for sure, beyond any shadow of a doubt, the Chevy '140 cam makes engines run backwards and it is impossible to win a race with one in your engine? Don't the cam grinding experts make cams that have to meet the intake vacuum rules enforced at some circle tracks? They should know the answer to the OP's question.

LOL.... I think that maybe the GM Engineers knew a thing or two about making a a cam that was dynamically stable at high rpm without killer spring loads that wear guides, etc. I also think that they knew how to make a cam that lived a long life with production tolerances on lifter bores, etc. I also think that maybe technology has progressed, but I do NOT believe that newer is ALWAYS better. I believe that the ‘140’ cam will deliver in the intended application. That’s probably because the older I get, the more I realize that GM knew what they were doing.

[My427stang]

I think a lot of it will depend on how well the intake side flows at low lifts. I have seen slightly reduced idle vacuum from advancing the cam (but still slightly stronger torque on the low end), but have also seen practically no difference as well.

For example...the very low $$$ engine in one of my trucks has about 9.0 compression, ported 1.78/1.46 E7TE heads, and a stock 351w FI cam that is 206/221@.050, on a 115 lsa/117 ica (2* retarded). Advancing the cam 4*, with no other changes, lost about 1” vacuum at idle that I couldn’t get back. I tried 8* advance out of curiosity and lost another 1/2 to 1”. It was a little rougher running right off idle but picked up noticeably by 1700-1800 rpms. This engine seems much more sensitive to any change than most I have seen.

That was interesting, I would have expected it to go the other way, with the advance, especially with such late intake valve timing to start and maybe just diminish the gains at some point.

[rebelrouser]

I hate to keep posting this on all these similar questions, but get into the 21st century, lots of cheap engine software that will predict with reasonable accuracy things like cranking compression, proper advance curve, what octane fuel will be required. No need to guess about what will happen when you fire the engine up. I get lots of customers who want to bring me a pile of swap meet parts to assemble an engine, without a good engine program to put those parts into and see what going to happen, I don't know if I could build half the engines that I do. I use performance trends, and Wallace racing has tons of free calculators. If you take the time to do some measurements and input good numbers into the program, they are very accurate especially on a normally aspirated engine. I regularly get within 5% on a real dyno compared to the virtual one. And I set the timing to what the program wants and its usually within a degree or so of what the prediction says. On supercharged or turbo engines I have issues getting a good number on the actual cfm output of the charger, but if you can get the company to give you a good number it works as well. The other issue is the program assumes right air fuel ratio, so if there is an issue most of the time its the fuel system.

Rebelrouser, your response missed the mark. I build big inch pump gas FEs, to include my own EFI 489 inch street motor which has got little more than oil changes since 2006. I have all the calculators, and not many are more careful in blueprinting than me. I also have zero issues with this or any other build running on pump gas. However, if you can tell me based on all the numbers what my idle vacuum will be with confidence, I am willing to provide all the values and willing to listen. This is a very unique build to meet a externally correct concours-driven resto on steroids. This is not a pile of swap meet parts in any way shape or form, which was likely clear from torque plate hone, hyd roller and metric ring pack on a 68 Cobra Jet....

Regardless, these calculators don't speak vacuum very well because they just don't have all the inputs. I didn't ask how to build, cam and time the motor, just if anyone had experience with a single change to cam indexing and how much effect on idle vacuum.

I have a lot of customers with muscle car restorations. 90% say build the engine stock but I want it to have just enough cam to lope at idle because it sounds mean. I plug in the numbers, and the program calculates idle vacuum, cranking compression, timing curve and octane requirements. If you take the time to input flow bench numbers, actual runner lengths for the manifolds, measure the true compression ratio, etc. The program hits right on the money within 5% for engine out put and the other variables. I know for example about 16 inches of idle vacuum you start to get a little lope. So I select a camshaft that makes 16inches of vacuum and advance the cam enough to make good cranking compression, set the distributor up with the timing curve for pump gas, and it works. simple as that. You may not get some of the free calculators to work that good, and many do not take the time to put good numbers into the software, but in my experience is works great. I use the program to teach my engines class about how an engine works, so we play with all the variables and the students get to see how all these factors affect the output of the engine. Lots of modern cars use variable cam timing and variable intake runner technology, I plug in the measurements and the students quickly understand how you can make 300HP out of a v-6 and still get 30 miles per gallon. My son is an engineer and when he was going to college one of his professors was doing research on combustion chamber burning and direct injection. It was kind of neat they had a one cylinder engine with a clear glass cylinder to do time lapse photography.They had access to a very expensive engine program they were using with their research. My son ran my race car engine on that program, and their was only 5 HP difference between it and the performance trends engine analyzer software. Like I said a customer brings me a pile of parts and I can make some very accurate estimates on how it will all work together in just a few minutes.

[travis]

I think a lot of it will depend on how well the intake side flows at low lifts. I have seen slightly reduced idle vacuum from advancing the cam (but still slightly stronger torque on the low end), but have also seen practically no difference as well.

For example...the very low $$$ engine in one of my trucks has about 9.0 compression, ported 1.78/1.46 E7TE heads, and a stock 351w FI cam that is 206/221@.050, on a 115 lsa/117 ica (2* retarded). Advancing the cam 4*, with no other changes, lost about 1” vacuum at idle that I couldn’t get back. I tried 8* advance out of curiosity and lost another 1/2 to 1”. It was a little rougher running right off idle but picked up noticeably by 1700-1800 rpms. This engine seems much more sensitive to any change than most I have seen.

That was interesting, I would have expected it to go the other way, with the advance, especially with such late intake valve timing to start and maybe just diminish the gains at some point.

I blame the cam, because that’s the only major difference between this engine and a hundred others that was kind of similar. 15* exhaust split, plus a wide lsa...it’s already opening the exhaust valve too early IMO for a low compression engine. I think advancing it is just aggravating the problem.

[My427stang]

I have a lot of customers with muscle car restorations. 90% say build the engine stock but I want it to have just enough cam to lope at idle because it sounds mean. I plug in the numbers, and the program calculates idle vacuum, cranking compression, timing curve and octane requirements. If you take the time to input flow bench numbers, actual runner lengths for the manifolds, measure the true compression ratio, etc. The program hits right on the money within 5% for engine out put and the other variables. I know for example about 16 inches of idle vacuum you start to get a little lope. So I select a camshaft that makes 16inches of vacuum and advance the cam enough to make good cranking compression, set the distributor up with the timing curve for pump gas, and it works. simple as that. You may not get some of the free calculators to work that good, and many do not take the time to put good numbers into the software, but in my experience is works great. I use the program to teach my engines class about how an engine works, so we play with all the variables and the students get to see how all these factors affect the output of the engine. Lots of modern cars use variable cam timing and variable intake runner technology, I plug in the measurements and the students quickly understand how you can make 300HP out of a v-6 and still get 30 miles per gallon. My son is an engineer and when he was going to college one of his professors was doing research on combustion chamber burning and direct injection. It was kind of neat they had a one cylinder engine with a clear glass cylinder to do time lapse photography.They had access to a very expensive engine program they were using with their research. My son ran my race car engine on that program, and their was only 5 HP difference between it and the performance trends engine analyzer software. Like I said a customer brings me a pile of parts and I can make some very accurate estimates on how it will all work together in just a few minutes.

I appreciate the follow up, I plugged it into EA and got 12 or 14 inches depending on adv or .050 inputs, the issue I have with the version of EA I have is that I have relatively limited choices for cam design and don't know the assumptions for either. I wish they had the ability to enter an entier lobe, (adv, .050, .100, .200) I think it would better understand, or at least if I could find EA's assumptions for the lobe design. Regardless, at this point, when I advance or retard the cam on EA I get the same number. So I would say it is not quite precise enough to give me what I am looking for for this specific question

I am very pro-simulator, don't get me wrong, but I haven't seen them line up on the dyno as well as you. As far as the 1 cylinder, I would be interested in the 5 hp as a percentage of total, if it was a 10 hp motor, 5 hp is 50% wrong. I generally see EA at about 10% off.

As far as modern cars and their capability, I wish I had variable cam timing on an FE, even if it was only intake centerline. One of my customers has a 2011 5.0 and it's a blast, spread the centers and advance at idle, and then let it get rowdy at RPM. Some guys even program in the "ghost cam" where they narrow LSA and make it thump. I'd rather make it sound and act stock and make more power later, but everyone has his own technique LOL. Unfortunately, I can't do any of that with this old FE

In the end, I am going to spend a little more time on the intake manifold to make sure I have it working well with the heads, porting iron is no fun....

[My427stang]

I blame the cam, because that’s the only major difference between this engine and a hundred others that was kind of similar. 15* exhaust split, plus a wide lsa...it’s already opening the exhaust valve too early IMO for a low compression engine. I think advancing it is just aggravating the problem.

Thanks for the follow up, these are a set of CnC ported CJ heads, flow is pretty good and I am planning a single pattern with a wide split. Right now, we are looking at 291/235 113/106 .600 lift HR, however my gut says the engine wants about 5 degrees more .050 to get the peak RPM where I want it at 5600-5700.

Nobody makes a lobe with a soft closing that will keep my overlap in check, while keeping at as close to .600 as I can, so the thought is to retard the cam above to buy back a little RPM. The car will be fine with any loss in torque (factory 4.30 geared resto Mustang), but I need to watch idle vacuum to ensure power brakes work (again, big dollar street driven resto, so no options for vacuum cans or deleting power brakes, etc)

I am likely thinking too much about it, ring seal should be very good, overlap is at 65 degrees, port volume is small for a set of modified CJ heads, and the intake really needs minor work to support the 300 cfm heads, but want to make sure I get him all the RPM we can with an externally stock engine (except for period correct headers)

[My427stang]

FYI, typo on my last and can't correct it, meant wide LSA not wide split, generally I use split to discuss the difference between intake and exhaust lobe duration. This is a single pattern cam