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Intake Closing Point vs Cylinder Pressure

Old 12-20-2018, 01:38 PM
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Intake Closing Point vs Cylinder Pressure

Every engine is different, but has anyone seen any studies done on the optimal intake closing point to maximize peak engine power or torque? The general idea behind it can be seen here:

https://www.hotrod.com/articles/inta...g-performance/
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Old 12-20-2018, 01:55 PM
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Re: Intake Closing Point vs Cylinder Pressure

That's what goes on when you select a camshaft using the dynamic compression ratio. The intake valve closing point for the most part controls cylinder pressure. And that's why I say always choose and buy your camshaft last when building an engine for performance.

The reality is you optimize cylinder pressure for the octane of the fuel you use. That meaning use the intake valve closing point that keeps your cranking pressure below the 210psi to 220 psi range for premium fuel. Lower cranking pressure is leaving power on the table.

Good luck.
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Old 12-20-2018, 01:58 PM
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Re: Intake Closing Point vs Cylinder Pressure

Yeah lol race teams with unlimited budgets do this all the time

its just getting cam grinds and testing them. All the calculations in the world establish a baseline. A cam is made and tested. Results tabulated and new theories developed. New valve events are tried. Iteration iteration iteration then final result

eventually find the best for the application

you can also see it in modern day vvt motors. Variable cam timing advances or retards the cam centerline which changes all 4 valve events on an ls motor lets say, but the coyote ford 5.0 has a separate cam for intake and exhaust lobes so you can control 2 events separately. It works fantastic. Advance it at low rpm really makes low end power. Retard as rpm climbs make more top end
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Old 12-20-2018, 02:04 PM
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Re: Intake Closing Point vs Cylinder Pressure

And dynamic compression aka cranking doesnt necessarily mean much. Its very losely correlated with fuel octane, a guideline but not end all be all. Volumetric efficiency changes how much mass is trapped in the cylinder when its running, it could be much more or less mass than whats trapped in a cranking test lol meaning effective comp ratio is higher or lower than calculated by intake closing. Guideline but i wouldnt design around it
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Old 12-20-2018, 02:42 PM
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Re: Intake Closing Point vs Cylinder Pressure

Yea, so the cranking compression may not mean much at all once VE gets high since when the engine is running the effective compression is actually higher than static compression, and dynamic compression. Cranking compression would be a linear correlation to dynamic compression, I'd imagine. But once the engine is running, especially at high rpms, that all changes.

So the goal is to maximize cylinder pressure without causing detonation (through all rpm ranges), and tune the intake closing point to peak power, or just before. If that is known (say 80 ABDC), then I could tune my static CR to achieve 8.0:1 Dynamic CR since that seems to be an agreed number to prevent detonation?

I am trying to decide what the best method/theory would be to go in setting compression to cam before I am locked into either.

Cam manufacturers seem to not be on the same page when speaking with them. Any recommendations on who to use?
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Old 12-20-2018, 02:54 PM
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Re: Intake Closing Point vs Cylinder Pressure

There is no "one" "optimum" number for any of those things. Everything is a trade-off. Give up something you don't want or need to get something else.

Best you can do is, select results that give the compromise you can best accept. Trade off low-end grunt for high-end HP, trade off "max power" for pump gas compatibility, etc.

If you use Pat Kelley's DCR calculator, the one everybody talks about, you should generally choose a cam that gives somewhere in the low 8's of DCR, for a typical street build. Any lower than that, you have no low end; much higher, you have pinging problems that end up requiring race gas or other special accommodations.

Beyond that, there are any number of other cam properties you can manipulate to get the desired results for YOUR combo. Int/exh split is one: generally, the better the exh ports flow, the less split you need. Another is LSA: generally, the lower the LSA, the closer to the same RPM the peaks in torque & HP are, and the more of a peak they produce; higher LSA tends to give lower peaks but the HP peak moved upward in RPMs, kinds "smearing" the curves down and to the right with the area under the curve staying about the same; the "optimum" choice for YOUR situation depends on things like gearing and vehicle weight. And so forth.

If there was "one" "optimum" cam, there wouldn't be about 10 million grinds available. Everybody would just buy the ONE and be done with it. But it just doesn't work that way.
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Old 12-20-2018, 03:11 PM
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Re: Intake Closing Point vs Cylinder Pressure

Yes - trade-offs. So my trade-off is low end torque for high end, say 7000-7500 power on 93 octane. Definitely not saying "what is the perfect performance cam for sbc"... thats why I prefaced with "Every engine is different". Now, using that DCR calculator, or the one on UEM pistons (both very similar) I can easily set DCR to 8:1. Where I am kind of stuck trying to decide what is better, 8:1 DCR, 11.5:1 SCR, and 80* Intake Closing Angle - OR - 8:1 DCR, 10.7:1 SCR, and 75* Intake Closing Angle. In my opinion, these should be the starting point of the cam design and work around that, no? Or knowing the IVC angle, I can set the compression to that. It seems like I would want closer to the 80* range?
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Old 12-20-2018, 03:12 PM
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Re: Intake Closing Point vs Cylinder Pressure

Also glad to have you guys sharing your knowledge
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Old 12-22-2018, 12:42 PM
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Re: Intake Closing Point vs Cylinder Pressure

Originally Posted by Orr89RocZ View Post
And dynamic compression aka cranking doesnt necessarily mean much. Its very losely correlated with fuel octane, a guideline but not end all be all. Volumetric efficiency changes how much mass is trapped in the cylinder when its running, it could be much more or less mass than whats trapped in a cranking test lol meaning effective comp ratio is higher or lower than calculated by intake closing. Guideline but i wouldnt design around it
Then what's your better idea? Please elaborate what a hobbiest can do on a hobbiest budget?
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Old 12-22-2018, 06:57 PM
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Re: Intake Closing Point vs Cylinder Pressure

For most hobbiest guys who arent gonna be generating super high ve, dcr and cranking compression can be used. Just saying its not the end of all be all number.

there are 9:1 comp circle track motors with cams designed to generate/trap high cyl pressure. They make good power and have torque. theres compromises

imo on 93 gas and a good aluminum head motor, 11:1 is fine with a cam thats gonna get you cranking compression 200 psi or less. Some guys have run higher just fine. My personal 383 was around 11:1 with a dcr of 8 i believe. 93 oct just fine, lots of power to 6800.

consider the whole induction system from cubic inches/ cylinder size, to valve size to port area to runner length to your rpm peak target. A well balanced system sized for typical street 6000’s rpm range,, 230 deg and .550-.600” lift on a 383 will get you a strong 6000-6300 rpm peak and can carry power abit past 6500-6600 depending on exhaust lobe. 100-200 rpm higher peak easily with a 350. Lsa’s typically end up 107-112 depending what you want. Better the heads and well balanced top end you can go wider. More stable smooth idle likes wider. All rules of thumb

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Old 12-22-2018, 07:03 PM
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Re: Intake Closing Point vs Cylinder Pressure

Originally Posted by brodysZ28 View Post
Yes - trade-offs. So my trade-off is low end torque for high end, say 7000-7500 power on 93 octane. Definitely not saying "what is the perfect performance cam for sbc"... thats why I prefaced with "Every engine is different". Now, using that DCR calculator, or the one on UEM pistons (both very similar) I can easily set DCR to 8:1. Where I am kind of stuck trying to decide what is better, 8:1 DCR, 11.5:1 SCR, and 80* Intake Closing Angle - OR - 8:1 DCR, 10.7:1 SCR, and 75* Intake Closing Angle. In my opinion, these should be the starting point of the cam design and work around that, no? Or knowing the IVC angle, I can set the compression to that. It seems like I would want closer to the 80* range?
maybe some more details on your setup is needed to understand what you are looking to do? If you believe intake lobe needs to close in the 75-80 range then i assume you considered your induction system, the cubic inches, and rpm target and determined how much duration you need and lift to support your rpm goal? I would figure duration and lift then set comp to what you feel comfortable with on pump gas...which leads us into dcr as a guideline again. 8-8.3 i suppose is a starting point, but as i mentioned earlier, i would generally keep most alum headed sbc’s at 11:1 or under, if cranking comp is calc’d under or around 200 psi
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Old 12-22-2018, 08:31 PM
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Re: Intake Closing Point vs Cylinder Pressure

I recently went through a learning curve while trying to select a cam for an engine I have. I read many articles about camshafts, discussions on forums, etc, etc. My conclusion: I'm not qualified to choose a cam without help. The topic is too complex and requires iteration of experiment. Effort and money I'm not willing to expend.

So I changed my strategy and decided to look at engine builds similar to mine and see what works and what doesn't. I spent weeks searching the forums and found engines like mine with 23 different cams with dyno curves, many backed up with time slips. Basically I ran a large experiment without spending a dime of my own money.

One thing I noticed is all the cams that worked really well, whether big or small, had the same Intake Centerline Angle as installed in the engine, and had the same Dynamic Compression Ratio. Intake closing events varied. So ICA and DCR became a key characteristic that I hunted for since it was demonstrated what values my engine likes.

Then I started paying closer attention to torque curves, driving characteristics that people described, whether easy or difficult to tune.... anything that would help me narrow the list. I noticed one particular shop was exceptionally good at making killer torque curves with excellent throttle response and good drivability. So I picked up the phone and had that shop spec out a custom cam for me. The results ended up right where I figured it would be, based on my "23 cam test".

Anyway, that's how I chose a cam without having to know a whole lot about the subject.

Last edited by QwkTrip; 12-22-2018 at 09:02 PM.
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Old 12-23-2018, 09:14 AM
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Re: Intake Closing Point vs Cylinder Pressure

Thx QT. Nice report.
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Old 12-23-2018, 09:42 AM
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Re: Intake Closing Point vs Cylinder Pressure

Originally Posted by Orr89RocZ View Post


maybe some more details on your setup is needed to understand what you are looking to do? If you believe intake lobe needs to close in the 75-80 range then i assume you considered your induction system, the cubic inches, and rpm target and determined how much duration you need and lift to support your rpm goal? I would figure duration and lift then set comp to what you feel comfortable with on pump gas...which leads us into dcr as a guideline again. 8-8.3 i suppose is a starting point, but as i mentioned earlier, i would generally keep most alum headed sbc’s at 11:1 or under, if cranking comp is calc’d under or around 200 psi
Sure, The goal is to make the most power in the high rpm, say 4500 - 7500, and use 93 octane and a solid roller. The parts I have are: Dart SHP block that will be 4.155" bore, Liberty 4340 forged crank, Dart Pro1 215 (or used to be) heads worked over flowing [email protected], [email protected] and [email protected] , [email protected], jesel shaft rockers, matched super victor intake, Barry Grant 750 DP mechanical secondary carb, and Stainless Works 1 3/4 longtubes with a 3" dual exhaust and flow through mufflers. I would like to see 650 or so crank, as I see this as an attainable, but high goal to reach for.

I don't have pistons, rods and cam, so I am free right now to set CR and cam wherever I'd like. I would like to have a very good understanding of cam events and their effect on power in order to make the best decision, or know what NOT to do or listen to when calling cam grinders.

Originally Posted by QwkTrip View Post
I recently went through a learning curve while trying to select a cam for an engine I have. I read many articles about camshafts, discussions on forums, etc, etc. My conclusion: I'm not qualified to choose a cam without help. The topic is too complex and requires iteration of experiment. Effort and money I'm not willing to expend.

So I changed my strategy and decided to look at engine builds similar to mine and see what works and what doesn't. I spent weeks searching the forums and found engines like mine with 23 different cams with dyno curves, many backed up with time slips. Basically I ran a large experiment without spending a dime of my own money.

One thing I noticed is all the cams that worked really well, whether big or small, had the same Intake Centerline Angle as installed in the engine, and had the same Dynamic Compression Ratio. Intake closing events varied. So ICA and DCR became a key characteristic that I hunted for since it was demonstrated what values my engine likes.

Then I started paying closer attention to torque curves, driving characteristics that people described, whether easy or difficult to tune.... anything that would help me narrow the list. I noticed one particular shop was exceptionally good at making killer torque curves with excellent throttle response and good drivability. So I picked up the phone and had that shop spec out a custom cam for me. The results ended up right where I figured it would be, based on my "23 cam test".

Anyway, that's how I chose a cam without having to know a whole lot about the subject.
Thanks for adding your experience. As I am going through this, I am basically following your footsteps it seems. I've started a list of a few builds and overall notes of CR and proven combos. Here is a link to some proven CR's that run on pump gas: http://purplesagetradingpost.com/sum...comb-sm-1.html

Care to share more of your engine combo? CR and cam? Also, what shop was it?

I've called a few cam grinders and you can really get different recommendations from different guys. Some said that static CR should be 9.5 to 1 to produce the most power in order to have the ideal timing. One recommended a 80 degree intake closing angle. One said forget about everything because I need 2" headers not 1 3/4 which will shut the motor off at 4500 rpm. So really, it is me who needs to be intelligent enough to at least know what to look for and to understand if I am getting good or bad advice. Once I feel comfortable, then I can call a place and spec a cam with them, generally knowing what I want. I am not there yet, but getting there!
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Old 12-23-2018, 12:52 PM
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Re: Intake Closing Point vs Cylinder Pressure

Originally Posted by brodysZ28 View Post
Care to share more of your engine combo? CR and cam? Also, what shop was it?
Tony Mamo at https://www.mamomotorsports.com/

I don't think any of my build examples will be helpful to you. My engine is a LS7 with EFI style longitudinal intake and it wants different valve events than a SBC engine.
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Old 12-24-2018, 10:39 AM
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Re: Intake Closing Point vs Cylinder Pressure

Ohh yup. I didn't realize it was a LS. Heard a lot about him though and intake/head porting.
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Old 12-24-2018, 10:57 AM
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Re: Intake Closing Point vs Cylinder Pressure

What size valves in the head? What stroke crank? Solid roller or hyd roller?

Might need alot more carb than that. 1 7-8 header be better but 1 3/4 could work for now

i can run it thru my cam software to see where it ends up

else you can likely talk to bullet cams, straub technologies, or Jones racing cams.
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Old 12-24-2018, 04:37 PM
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Re: Intake Closing Point vs Cylinder Pressure

Originally Posted by Orr89RocZ View Post
What size valves in the head? What stroke crank? Solid roller or hyd roller?

Might need alot more carb than that. 1 7-8 header be better but 1 3/4 could work for now

i can run it thru my cam software to see where it ends up

else you can likely talk to bullet cams, straub technologies, or Jones racing cams.
2.125 and 1.6 valves. It's a 4" stroke and 4.155 bore. I will go with a solid roller and high end direct oil axle style lifters. Yea the carb may need to go. I have it already so figured I'd try. Same with headers. Oh yea? I would be interested to see what the software says. What software? Thanks for the recommendations. I will give them a call!
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Old 12-24-2018, 08:20 PM
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Re: Intake Closing Point vs Cylinder Pressure

Controlled inductions software and pipemax stuff

any idea what head flows at .500 and .600?
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Old 12-24-2018, 08:24 PM
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Re: Intake Closing Point vs Cylinder Pressure

Originally Posted by Orr89RocZ View Post
Controlled inductions software and pipemax stuff

any idea what head flows at .500 and .600?
I don't. All I was given was 0.700 and 0.800.
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Old 12-24-2018, 09:07 PM
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Re: Intake Closing Point vs Cylinder Pressure

Well they seem similar in flow to my old speier 245’s so using similar data and making a few assumptions

cam around a 300/309 at .020” advertised, 269-276 at .050 188/190 at .200
.421/.410” lobe lift. Assuming 1.6 rockers and .020/.022 lash, net lift of .653/.634. Lsa 111. Install on a 108.

should get you close to a 6800 rpm peak and carry power to well over 7000. Get you close to 650 hp i’d say

more drag race only,
303/313 at .020
274/280 at .050
.453/.441 lobe lift
.704/.683 with 1.6 rockers
111 lsa again

but you gotta watch cam to rod clearance at 4” stroke. Base circle gets small so with shaft rockers go more ratio, 1.7-1.75 if you can


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Old 12-24-2018, 09:16 PM
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Re: Intake Closing Point vs Cylinder Pressure

Pipemax thinks the higher lift above .660 is ideal for that rpm range. 650 hp def doable based on head air flow if manifold and carb support it

intake close at 62.5 deg based on .050” duration specs

1 7/8 header ideal. Could even go 2”

assuming 11.5:1 comp, cranking compression would be between 180-200 depending on starter rpm. May be ok on 93.

But i would lean toward 11:1 or so.

900 or so cfm carb be good. 1000 wouldnt hurt

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Old 12-24-2018, 10:00 PM
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Re: Intake Closing Point vs Cylinder Pressure

The solid rollers seem to carry power really well, like 1000 rpm within 20 hp or so. So yea 6800 would be great. Now going to the more drag race only, what am I losing for driveability? Just low end power or the extra lift killing springs? I think over 0.660 would be smart to do. Surprised that the 62.5 is all it is looking for, although I haven't found any real engines with data including intake close. On a comp cams cam card is the ABDC @ 0.050"? Makes sense with the header size. I have a do have a dominator adapter, so I will have options with the carb.
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Old 12-25-2018, 09:24 AM
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Re: Intake Closing Point vs Cylinder Pressure

Drag race lobe might be tougher on springs. Depends which lobe type you go with if you have comp grind it. Bigger duration would be abit less driveable but both cams are pretty decent in size.
Going up in rocker ratio will help keep lobe size down. I assumed a 5.85” rod to get a decent piston in there.

comp cams usually are .050 specs but some are .006. Mine was .050” on last few

on the seat duration, the 62.5 becomes more like 89
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Old 12-26-2018, 08:26 AM
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Re: Intake Closing Point vs Cylinder Pressure

I won't want to be changing springs every year, so I will factor that into the decision. Not as concerned with idle/low end power. Most 4.125/4.155 bore, 4" stroke are actually 6" rods. doesn't leave much room for the piston, but on the brighter side... its lighter lol.

So, either cam, the right choice would be to push my SCR up to 11.5 using a 15cc dish. Depending on what my actual chamber volume comes out to.

Using Pat Kelly DCR Calculator:
With the drag race cam: 11.55 SCR and 8.11 DCR
With the 1st cam............11.55 SCR and 8.24 DCR

Now, it seems like the DCR has a higher role in detonation, so 11.5 should be fine right? I've reaf of a few engines that run high SCR as long as DCR is in the appropriate zone for pump gas... sketchy though to push the limit.
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Old 12-26-2018, 09:18 AM
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Re: Intake Closing Point vs Cylinder Pressure


Yeah that would be abit less tuning window. You would need to keep it cool, get the right heat range plug as always, and keep an eye on timing as it might become pretty sensitive quickly. Also may want to run it much richer than normal during part throttle stuff.

11:1 would be safer imo. And still get the power you seek. In fact you probably can get there with a 260 deg cam. Peak closer to 6300-6500. 434 is alot if cubes and you have the airflow to make closer to 680-700

good spring like a pac, psi, or manley nextek they will last a little while. Comp has some lobes that will last a decent while too. Rx lower lift 420-430 lobes are good.
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Old 12-26-2018, 09:56 AM
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Re: Intake Closing Point vs Cylinder Pressure

But if I go 11:1 I should scale back the cam to get the 8.1:1 DCR, correct? Heard of Tool Room springs? The heads were assembled with those and XcelDyne valves too. Seems like he used good stuff.
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Old 12-26-2018, 10:30 AM
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Re: Intake Closing Point vs Cylinder Pressure

No. I would not shoot for dcr as a design point
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Old 01-22-2019, 07:54 AM
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Car: 1983 Camaro Z28
Engine: 383
Transmission: TKO 600
Axle/Gears: Strange S60
Re: Intake Closing Point vs Cylinder Pressure

Shooting for high compression obviously increases the risk of detonation. Looking into the J&S Safeguard ignition box for the higher spark energy and automatic timing retard/detonation sensing. Anyone else interested in the group buy?

https://www.thirdgen.org/forums/grou...safeguard.html
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