CID/RPM/CFM/HP

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skinny z
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CID/RPM/CFM/HP

Post by skinny z » Wed Oct 25, 2017 11:08 am

There is a lot posted here and elsewhere regarding this topic. Calculators too. I'm trying to make sense of it or perhaps just as importantly, gather it all together.
Here's the scoop:
Subject: Gen 1 SBC. Currently 355 CID. Contemplating moving up to 385 CID
I have a set of cylinder heads. Freshly ported with verified flow numbers.
Intake: 253 CFM @ .500". 170 cc before porting. 2.02" valve. Specifics beyond the advertised port volume I don't have.
I have seen and witnessed 355 cubic inches produce 440 HP with heads less than these (Vortec heads). Dyno engine (with the usual lack of accessories) 224@.050", .500", 108 LSA,104 ICL, decent dyno headers. Peak HP @ 6200 RPM.
Now my question is: to what extent should I be able to produce the same HP/CI with the above mentioned heads? Is the CFM going to limit RPM to the point where HP would also be limited? Is 500 HP an unreasonable expectation? For this application, max RPM would be 6500. Assume a cam spec'd to suit. Something along the lines of 282 ADV. 106 LSA,102 LCA.
Thanks in advance.

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Re: CID/RPM/CFM/HP

Post by mag2555 » Wed Oct 25, 2017 1:00 pm

Without getting into a ton of details I will tell you this, if your running only .500" lift on heads that flow a touch over 250 cfm@.500" then your hp made will go more in hand with your average flow numbers @ .350" to .400" lift.
Assuming something like 235 cfm your hp will be more like 440 to 450 if you have 10.5 comp.

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Re: CID/RPM/CFM/HP

Post by skinny z » Wed Oct 25, 2017 1:12 pm

Interesting point and something that I neglected to include in the "lack of details" section.
While I haven't selected a cam for a 385, the cam currently installed in the 355 (being used with the heads in question) is Comps XR288HR with 1.6 ratio rockers. All in all a terrible match for the 10.2:1 CR and 355 CID but lift is .550"/.570". (288/292, 110 LSA, 106 ICL)
Any new cam would keep lift values close to that. Flow bench data shows the CFM dips slightly at .600" to 246. Can't say what .550" flow is.
Thanks for insight and the prompt to add additional information.
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Re: CID/RPM/CFM/HP

Post by mag2555 » Wed Oct 25, 2017 2:08 pm

There could be a few reasons for that flow drop between .500" and .600" , a quick quess on my part would be chamber shrouding in that lift range.
Remember a ideal chamber will have very close to the same amount of valve edge to chamber wall clearance as the amount of valve lift atleast on the spark plug wall side in terms of 23 degree head.

You have a good average 80% Exh to int ratio up to .500" with out factoring in 3 or 4 % more with a header so a single parttern Cam may serve you better and keep your cranking compression up a bit more too!

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Re: CID/RPM/CFM/HP

Post by mag2555 » Wed Oct 25, 2017 2:18 pm

Tomorrow I will post up dyno sheet that may interest you.

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Re: CID/RPM/CFM/HP

Post by skinny z » Wed Oct 25, 2017 2:21 pm

I know one thing for certain. There's nothing more that will be done to these heads. They are what they are and I only hope that the thrashing this engine has taken isn't reflected in the condition of the heads. They are fresh, maybe 3000 miles. The shortblock, not so much. At over 15% leakdown on the [i]best[/i] cylinders, it's due for a replacement.
Now that said, this is the intent of this thread. To examine the potential of more cubic inches with regards to these heads and how they stand. Some dyno simulations have shown more or less what I expected and that's equal (or somewhat more) power made at lower RPMs. Truth in, once I factor in a cam that's of a better specification than the 288 that's in the 355, there's more power at less RPM for sure. I'm looking for 50 HP at equal RPM and would like some additional support in that regard.
To sum it up, if the current 355 makes 425 HP at 6000, then my target is 475 (+/-) at 6000. (Cam selection notwithstanding). I'm doubtful that 250 CFM heads will support that much HP.

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Re: CID/RPM/CFM/HP

Post by skinny z » Wed Oct 25, 2017 2:21 pm

[quote=mag2555 post_id=694638 time=1508955520 user_id=15428]
Tomorrow I will post up dyno sheet that may interest you.
[/quote]

Excellent.

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Re: CID/RPM/CFM/HP

Post by mag2555 » Thu Oct 26, 2017 12:19 pm

Here's a dyno sheet from a 364 cid I did for a guys van with focus on the power band to work with his 3.23,gears.
The heads where Trick flows that I cleaned up to flow 256 @.550" lift and I think it was 190 on the Exh.

The Cam was a Crower 220/226. 462"/ 470" 112 LS advanced 3 degrees.
Compression was 10.25.
The Carb was a 675 B grant and the big key part was the Holley single plane street Dominator with a well belended Pelum.

Notice it topped 400 hp already by 4750 rpm.

A test with a Victor spl Manifold produced 8 more hp above 5000 rpm, but lost 20 ft lbs below 4000 rpm!

The 3.5" collector likely made for smaller average numbers then what could have been had with a 3" version also.

Note also that it was starting to get fat above 5000 so a bit more Carb tuning might have made 4 more hp.
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Re: CID/RPM/CFM/HP

Post by skinny z » Thu Oct 26, 2017 2:10 pm

What are the engine specs for that? Bore, stroke, rod length. Is that a hydraulic roller? Do you have advertised specs? I'd like to do a little reverse engineering. My guess is that cranking compression was north of 200 PSI.
Putting that cam into my current 355, (10.2:1 SCR with iron heads) might be a little hard to manage for a daily driven vehicle. I've been there and done that and detonation was difficult to control with a full timing curve even with good engine management. It did however make for a lot of torque. Witness my personal best 60' times.
That said, (and back to the intent of this thread) how would that transfer to 385 CID with head CFM as posted? I'm looking for peak power more in the 6000-6200 RPM range. Peak torque in the low to mid 4000's. Certainly with more cam but are the heads up to it?

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Re: CID/RPM/CFM/HP

Post by mag2555 » Thu Oct 26, 2017 6:00 pm

4.060" Bore
3.500" stroke
Stock rod CtC.
The Cam was just Hydro, no Hydro roller.
That 3 degree installed advance does nothing for the hp made above 5500!

If I recall the Cam was a custom order.

I don't know what the cylinder cranking comp was.

I wanted the guy to Mill the heads to net 11 to 1 comp , but he was more then happy the way it turned out.

There are tons of great aftermarket Aluminum heads out there, something like picking up a used set of Brodix track 1 heads may not Brake the Bank and should get you darn close to 500 hp with your 385 cid , and if doing that it would be 11 to 1 comp I would shoot for!

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Re: CID/RPM/CFM/HP

Post by skinny z » Fri Oct 27, 2017 12:07 am

I agree 100% with the aluminium heads and 11:1. I had a chance for a set of wasted Brodix heads but for whatever reason, I declined. Perhaps my stubbornness to get this iron headed lump to be respectable clouded my judgement. That said, I'm dedicated to moving forward with these heads and will make the most of it. I doubt that my 500 HP target is reachable but considering that the current 355 has gone a corrected 12.4 @ 109 in a 3650 lb chassis, 400CHP and 1.1-1.2 HP/CI is the result. Make that 385 CID and maybe 450?
The target is 11's in a chassis that isn't optimized for the 1/4 mile. Suspension and gearing are more tuned to a cruiser but still, it doesn't seem unreasonable.
I appreciate your input and thanks.

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Re: CID/RPM/CFM/HP

Post by F-BIRD'88 » Fri Oct 27, 2017 3:18 am

If you superchatge what you got you can and will hit 11.'s
reliabily. You can do it with a 355.

Lower the cr.. Not hard
Torqstorm blower. You will spend LESS and get the result
in street trim. Your cam and heads are fine for this..

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Re: CID/RPM/CFM/HP

Post by F-BIRD'88 » Fri Oct 27, 2017 5:47 am

current 355 has gone a corrected 12.4 @ 109

This is BS.... When did they start handing out "corrected" ET slips at the track?
If you actually want to run 11's, get real. Same as looking at engine dyno tests with a sizable power correction factor. Look at the raw uncorrected numbers. 'cause that is the conditions YOU will be operating under. Not the conditions speced by the correction factor standard.
You will never operate under these conditions.

What did the car actually run? ET MPH That is the real performance. That is the real engine power.
That is what matters. If you actually want to get that car in the 11's in street trim.

A 383 SBC will make about 35 more HP than a 355 SBC if when you swap the top end from motor to motor.

if you really want to get 11 sec performance from that car in street trim plan on needing to add a REAL 30%
More engine power to what you got now.
You can easily and reliably do that by supercharging it. And get a practical 30% REAL net power gain result.

You can get REAL 11sec performance from a N/A 355 or 383 using those heads but it will require real changes to just about everything ELSE. Add up the real cost of all, to get there.

Plan on needing the real net power to get the car to a 118 MPH in the 1/4
to get a realistic 11sec et in street trim.

Start by looking at what you really got . What did it really run? uncorrected

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Re: CID/RPM/CFM/HP

Post by Cougar5.0 » Fri Oct 27, 2017 9:56 am

This all sounds familiar. 3630lb luxobarge; bolted a Kenne Bell onto a low 13's 306 - 11.9 @ 118 MPH.

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Re: CID/RPM/CFM/HP

Post by skinny z » Fri Oct 27, 2017 10:05 am

I use corrected ET and MPH as I've raced at various tracks at various altitudes and it helps it keeping track of engine performance. The 12.4 at sea level equals my 12.6 at 660' which equates to 12.9 at 4100'. As a matter of fact, when I first raced at a higher DA, I was certain that engine was broken as the time slips had gone out the window. Factoring in DA made sense of it all. That's why I use corrected ET. That's also why the NHRA uses altitude correction factors.
As for supercharging...not this engine. Perhaps the LS but forced induction is definitely not in the cards. I wouldn't rule out NO2 but I don't wish to enter into that debate either.
But that's not my point. Nor is it the point of this thread.
The question is:
skinny z wrote:
Wed Oct 25, 2017 11:08 am
Subject: Gen 1 SBC. Currently 355 CID. Contemplating moving up to 385 CID
I have a set of cylinder heads.
Intake: 253 CFM @ .500"
I have seen and witnessed 355 cubic inches produce 440 HP with heads less than these (Vortec heads).
Now my question is: to what extent should I be able to produce the same HP/CI with the above mentioned heads? Is the CFM going to limit RPM to the point where HP would also be limited? Is 500 HP an unreasonable expectation? For this application, max RPM would be 6500. Assume a cam spec'd to suit. Something along the lines of 282 ADV. 106 LSA,102 LCA.
Thanks in advance.
The point of an 11 second time slip came up secondarily. I'm fully aware that more stall, shorter gears and a suspension that's more in line with 1/4 mile performance would have gotten me there will the earlier engine which was arguably less than what I currently have. But it's not the chassis nor is it supercharging that I'm interested in at this time.
Thanks just the same though.
skinny z wrote:
Fri Oct 27, 2017 12:07 am
The target is 11's in a chassis that isn't optimized for the 1/4 mile. Suspension and gearing are more tuned to a cruiser but still, it doesn't seem unreasonable.
Perhaps it IS unreasonable...

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