More Air Flow for a Model 425 on a 327 SBC

[Marc]

[This topic was split from topic 2 300a mixers on 1 modelE evaporator]

Hi, Any update on your 2 x 300a impco set up? How is it working out?
I am myself interrested in a way to have more air flow enter my SBC. I have a 425 impco on a healthy 327ci and like most of us, I feel clearly the engine being limited @5000 rpm however the engine is capable of making strong power @ 6000 rpm.

Thanks for your feedback!

[storm]

Hi, Any update on your 2 x 300a impco set up? How is it working out?
I am myself interrested in a way to have more air flow enter my SBC. I have a 425 impco on a healthy 327ci and like most of us, I feel clearly the engine being limited @5000 rpm however the engine is capable of making strong power @ 6000 rpm.

Thanks for your feedback!

Can you give us the build specs on this engine and your fuel system because something isn't right here. I know that many people say their engine is running out of breath at 5000 rpm but unless your engine has 100% Volumetric Efficiency (highly doubtful for a 327) at 5000 rpm it isn't air it is running out of. If you have already done this in a separate thread please link to the thread.

[Marc]

Hi,
Sorry for my late answer.

327ci engine is fitted with:
Comp cam HE268 single pattern 218/218 duration at .050"
Edelbrock performer rpm vortec intake
Vortec heads with hardened seats and ss valves
Compression about 9,5:1 but never was really measured, this comes fro my calculation
Impco 425 straight
Tri Y headers
2,5" magna flow exhaust
Accel electronic ignition with box

It is been a while since I haven't driven the engine on gasoline.
It runs very smooth and is a very responsive funny engine, with a lot of mid range power.

I believe it should provide more power on top end.

Regards,
Marc

[storm]

Hi Marc
No need to apologise.

I wonder if this should be split of into its own thread?

Have you got a vacuum gauge fitted to the engine that is readable when you are driving?
Do you have O2 sensors fitted with a gauge to tell you what your Lambda or AFR readings are?

The 3 1/4 inch stroke means the 327 is a revvy engine but it still isn't a powerhouse for its size. The 302 and 350 are much better combinations for higher rpm power.
The Vortec heads are good, even in stock form, but they are designed for emissions compliance rather than performance. While they do perform, and much better than the heads the 327 would have come out with, they were never designed for the 327.

The cam will run to 5500 rpm but you will see an obvious decline in output past 5000 rpm regardless of fuel type
Intake manifold is a good choice for the heads, I'd suggest it would have a 6000rpm limit on a 327-350 and a 5500 rpm limit on a 383-400 and larger engines. The rated 6500 rpm limit would be for a 302 or smaller.

If you want to take advantage of the better flow of the vortec heads and rpm manifold I'd go a larger cam or a split ratio like 218 intake 228-232 exhaust to help the exhaust to pull in more intake charge. Just as an aside Edelbrock's own advertising says this about the cam you have "Good street performance w/ stock converter, choppy idle. Biggest cam for inboard/outboard boat engines. Good for ski, economy & some performance" In other words it is a mid range cam which is what you seem to be describing.

[Tom68]

Marc the 327 will be lacking up top as you've found with a 425, you don't state what Holley base you have it on but even a 600 base will outflow a 425 by way more than the implied 175 cfm deficit, actual deficit probably closer to 200+cfm..

Great size engine for factory heads etc that were available when it was produced, even better with the Vortec heads, but as you've found your combo is stifled by the 280hp mixer, maybe it's time for gasoline and a 750 Holley.

Reading Cam cards etc doesn't tell the whole story, having some over rev capacity in the lower gears can be handy for pulling the next gear when it's up in the power range, doesn't happen with restrictive cfm carburation though, engine lays down too much to go on with it. Switched my 327 from a Holley 600 vac sec to a 450 Holley Economaster back in the day, killed it, at 100 mph with the 600 it'd push you back in the eat with acceleration, with the 450 it was long wait to get to the reduced top speed.

P.S. I adapted the 450 to a 260Z, turned that into a tyre fryer. Datsun motor loved the accelerator pump shot as opposed to the Hitachi SU's variable venturi's.

[Marc]

Thank you Tom68 and Storm for your replies.

I spent some time recently to read again old treads on this forum expecially from bigblockmopar 360 ci and Kuchen vapor injected 2CV. Also Hal bigblock twin 425 impco set up is awesome.
I like to learn what others are experiencing, it seems bigblockmopar never completed his vapor injection system?
Through the readings I learnt about the restriction plate on the vapor inlet of the 425 mixer : I just removed this little plate this evening. Maybe it will help…

Storm I have indeed vacum and AFR gauges installed, I did this to tune the advance curve correctly upon first installation of the propane mixer set up.

It is winter time here so I don't have any plans to test the camaro soon, however I will note and report to you the readings of my gauges at high revs. (no dataloggers)

Tom68, when i got the camaro 20 years ago it was equiped with a Holley 650 DP, a blast to drive at wot but difficult to get smooth drivability at low rpm and no stable idle.. Today I am more than happy to run a clean old car easy to tune and drive and on top of that economical. The camaro is only a hobby car I drive on rare occasion nevertheless I like the simplicity and easy maintenance of propane. The engine runs fantastic and I am aware the high rev zone is not so much important.

Still I remain interested to learnt how/if we can improve our system (for the fun of it). Therefore I started to ask about the return of experience on the dual 300a mixers installation… Last year Storm you told me about a test bench part of his plans, Did you complete it?

Thanks,

Marc

[Marc]

Oh yes my Holley base plate : it is a 600.

[storm]

Hi Marc.
For my test bench I ordered the vacuum motors at the time I posted, I received them just before Christmas. Covid has really caused alot of problems and sometimes getting parts is extremely slow. I am on holiday until the end of January and have been building a few things for my house but need to get back into my workshop to set it up for the year ahead. I have ordered some of the plywood and am waiting for it to be delivered next week, we have local supplies in Australia so it should get here.

With regards to BBM and his Vapour Injection I haven't seen anything new but @kikkegek has converted his Suburban to VSI and is using a Megasquirt ECU.

[Marc]

Thanks Storm for the feedback, I didn't found any info on Kikkegek conversion, would love to learn about it...
By the way, my intake is a performer, not a perfomer RPM.

I calculated the CFM based on your formula and yes to my surprise the mixer is correct for the engine up to 5500 without doubt.

I will definitely check my gauges at hi revs and see if all is right.

Best regards,

Marc

[Marc]

Hi guys,
Hi Storm,

I took some time to learn about my heads and camshaft theory.
I found a very nice u tube course on camshaft : very interesting !
Vortec cylinder heads max exhaust flow : 128 cfm 28 hg devided by max intake flows 195 cfm equal to 0,64. Under 0,75 it is recommend to select a cam with a dual pattern to help the exhaust side.
Also a simple formula provides approximate theoretic cylinder head max HP per cylinder, based on max intake flow : 195 x 0,26 , x 8 cylinders = 405 HP, to be multiplied by 0,64 = 260 HP.
So it could well be that my engine is not performing well above 4500 - 5000 RPM because of that wrong camshaft I have?

Another question I have here :
If I were to realize a camshaft swap (like the comp cam XE268 or a similar pattern) + modify my head with springs to deal with the additional lift + port my intake and remove the plate at the plenium --> to gain more HP output at all RPM range, would it be reasonable to do that keeping an impco 425 as mixer?
I mean increasing engine performance will ask for more flow : a highest pressure drop created by passing more air flow (more that the advertised 450 cfm) through the mixer wouldn't hurt to much the engine performance?
What is finally the limiting factor and is it interesting to do the engine mods discussed above in my case for my engine?

Lastly, I discussed with propane equipment dealer in the Nederland they develop very high efficient computer control (mapping) ignition system, and they advertise that the mapping + more spark intensity (3 x more), all together are adding + 15 % more power to the engine. The responsible guy explains that there is more to power than flow and the impco 425 will generate great power even above its cfm figures. he told me he reaches 370 - 400 HP on a 350 ci with a single impco and their ignition controlled system.
I want to go visit them and learn more.

Already your thoughts will be appreciated as it is not clear to me to define the limiting factor and to know what to believe.
Usually with gasoline people (US) install at least a 600 cfm carb on a 327 ci and don't play around with ignition curve mapping. I have not yet understood when cfm will be restricting and how ignition system can really boost performance, considering I believed I have already a good system : MSD 2 baster coil, electronic distributor, very low resistance spark plug wires.


Thanks,

Marc

[storm]

I'm writing this up this way because you cover different aspects of engine performance that while they can work together are actually very different and alone can have a great impact if everything else is appropriate to the engine.

I took some time to learn about my heads and camshaft theory.
I found a very nice u tube course on camshaft : very interesting !
Vortec cylinder heads max exhaust flow : 128 cfm 28 hg devided by max intake flows 195 cfm equal to 0,64. Under 0,75 it is recommend to select a cam with a dual pattern to help the exhaust side.
Also a simple formula provides approximate theoretic cylinder head max HP per cylinder, based on max intake flow : 195 x 0,26 , x 8 cylinders = 405 HP, to be multiplied by 0,64 = 260 HP.
So it could well be that my engine is not performing well above 4500 - 5000 RPM because of that wrong camshaft I have?

This is most likely the cause of your engines lack or high rpm performance. Most engines with stock or slightly modified heads will need a dual pattern cam (meaning more duration for exhaust compared to intake).

Another question I have here :
If I were to realize a camshaft swap (like the comp cam XE268 or a similar pattern) + modify my head with springs to deal with the additional lift + port my intake and remove the plate at the plenium --> to gain more HP output at all RPM range, would it be reasonable to do that keeping an impco 425 as mixer?

Yes.

I mean increasing engine performance will ask for more flow : a highest pressure drop created by passing more air flow (more that the advertised 450 cfm) through the mixer wouldn't hurt to much the engine performance?

If your engine is capable of flowing more than the carb/mixer/throttle body can provide you will be hurting performance because you will be making the engine work harder to achieve the same output. if you have a vacuum gauge or are using a datalogger system like Innovate Motorsports system you will see a gradual rise in vacuum as rpm increases. On a 4 barrel you do not want any more than 1.5" HG (HG = mercury) which is approximately 20" H2O (H2o = water). Anymore than that, e.g. 2" HG, means you are restricting airflow into the engine and performance will be lower than it should be.

What is finally the limiting factor and is it interesting to do the engine mods discussed above in my case for my engine?

I have thought long and hard about this exact question and I always come back to fuel flow. let me explain. If your engine is within acceptable limits of vacuum at maximum rpm yet you have a noticeable performance drop the problem must be lack of fuel. In NASCAR under the old carburettor rules 6 litre (366 cubic inch) v8 engines could be fitted with restrictor plates that restricted airflow (and made the engines pull huge vacuum numbers at maximum rpm) yet could still make 600 hp. From memory, and I'm happy to be proven wrong with verifiable evidence, the restrictor plates were flow tested to 350 cfm @ 1.5" HG so the restricted engines could only pull a maximum of 350 cfm @ 1.5" HG. Now they may, for example, be able to pull 600 cfm @ 3" HG but cannot pull more than 350 cfm @ 1.5" HG. So if the engines airflow is restricted and the engine can still make 600 hp they still need to burn a certain amount of fuel to achieve that. With the 425 the amount of air available is determined by the air valve, the amount of fuel available is determined by the air valve and regulator. If I had a 425 that wasn't the restriction, meaning it was within 1.5" HG at max rpm, I'd be making sure I was providing it with the amount of fuel my engine required. If 1 regulator is capable of supplying enough fuel for 350 hp and my engine should make 400 then my regulator is the restriction so I'd fit a regulator that could supply enough fuel or fit 2.

Lastly, I discussed with propane equipment dealer in the Nederland they develop very high efficient computer control (mapping) ignition system, and they advertise that the mapping + more spark intensity (3 x more), all together are adding + 15 % more power to the engine. The responsible guy explains that there is more to power than flow and the impco 425 will generate great power even above its cfm figures. he told me he reaches 370 - 400 HP on a 350 ci with a single impco and their ignition controlled system.
I want to go visit them and learn more.

Here's 3 links for you to look at.
CB Performance Programmable Ignition ModuleSimple
Haltech Elite VMS (Ignition only)Race engineered
Megasquirt (injection and ignition)Hobbiest
All these are capable of controlling ignition timing for a carburettor vehicle. There will be others I'm sure but these are the most well known and supported.

Already your thoughts will be appreciated as it is not clear to me to define the limiting factor and to know what to believe.
Usually with gasoline people (US) install at least a 600 cfm carb on a 327 ci and don't play around with ignition curve mapping. I have not yet understood when cfm will be restricting and how ignition system can really boost performance, considering I believed I have already a good system : MSD 2 baster coil, electronic distributor, very low resistance spark plug wires.

Some videos for you to watch regarding ignition timing and its effects on performance.
https://www.youtube.com/watch?v=jg14wdXg1_g
https://www.youtube.com/watch?v=Zzn3-ygH-v8
https://www.youtube.com/watch?v=CIDWTe4VphI
What you want when tuning the engine is to get the highest torque at the lowest ignition timing. So as an example if you can get 400 pound foot of torque at 25 degrees BTDC and you then try 26 degrees BTDC and only measure 385 pound foot of torque you know you have gone to far by 1 degree. You also have to consider detonation, and don't think it won't happen with LPG, as it is a sure fire way to kill an engine.

[Marc]

Thank you Storm it is all very interesting!

1° My 327 mods for efficiency and power
Ok it is very clear now, and with all information available on the web I feel ready to carry on these modifications (valve springs, camshaft, intake porting, vortec head minor porting as well). There are tons of good stuff available for the hobbiest and I learnt I can do everything in my garage

2° Flow questions and limiting factor
Basic physics related flow to pressure drop for a give pipe section. The Nascar example is a good example of this principle, as I understand it the engine is so powerful that it will suck air in creating more pressure drop through the restriction and sure having a negative effect on performance bu still it sucks this air...
In all case the flow can go through, until it reaches a certain speed where the noise and vibration can be a problem, so design helps you avoid these critical states.

For me now as I understand it all make sense, fuel indeed is more important as without is your engine dies. It also make sense that any engine, considered as an air pump, will gain from improvement in flow numbers through its components (measured at a given pressure).

3° Ignition mapping and improved coil spark intensity
I like the small video clip showing ignition is fixed for max HP, and I understand the concept of mapping. on old carburated V8 mapping can only be done for ignition. As I understand, installing a megasquirt with all additional sensors is exepensive (>600 $) and after that, it requires a lot of tuning and trial and errors IF you don't have a dyno stand. As I understand, the dyno would help directly provide an input signal about the HP result so the algotrithm could iterate to the best mapping for a paticular configuration. (my guess, I actually didn't find an example diy application for old carburated engines)
I also understand that my ignition, for the moment, has only been tuned on vacuum + checking AFR numbers being ok, and so sure it can be optimized.
I believe also that there is more to gain from a propane engine because of octane.

4° Next steps for me
First, I am getting soon parts for this winter project which is an complete electrical harness from Autowire and interior parts to rebuilt my interior... Also have some small body work I started already..
Second will be the engine component definition part purchase and installation starting with the mechanical part which I personally feel confident to carry over.
Third will be a visit to this shop in the Netherlands (I think it is best to do after the engine modificaitons) as I feel confident that they could help with a ECU + mapping and tune the engine as your video shows. Of course, in this case I would be dependent and not tune my self but I am not sure of the benefit to try it on my own neither the results I could get my self (point 3 above)... Maybe I need some help there... maybe megaskirt is not so difficult after all and I am wrong.

I may open another topic on camshaft (I have some confirmation to get) or I can continue on this topic however the subject will not relate anymore.

Marc

[storm]

Hi Marc
A little bit more information from IMPCO themselves to support what we are discussing.
In their master catalogue they rate the 425s flow at:
287 hp (214.0 kW) @ 460 CFM (217.1 L/s) and 1.5” Hg manifold depression
333 hp (248.0 kW) @ 533 CFM (252.0 L/s) and 2.0” Hg manifold depression
You can see the higher the depression (restriction across the mixer at WOT) the higher the potential airflow and power output.
If you look at what the regulators are capable of they rate the
Model L at 325 hp (242.4 kW) peak, 200 hp (149.1 kW) continuous
Model E at 325 hp (242.3 kW) LPG.
So 1 Model E will provide enough fuel for a 1.5" HG depression but not enough for a 2" HG depression. I suggest to get the full potential from your engine after you have finished changing it you will possibly need 2 Model E regulators.

[Marc]

Dear Storm

Thanks for the input, Indeed we will see what we get… and at last if fuel is a problem noticed upon tuning time this solution will remain to install a second vaporizer.

This cam swap job will be performed sooner that first anticipated I found all parts in stock at suprisingly low price, from the most famous online shopping platform we all know about, without spelling the name here, it is amazing to my eyes how these algorthims work to establish prices (incl shipping and import taxes)…

SO : I just ordered and will get all parts early February.
I ordered all gasket sets as well to complete the cam swap. (and looked for high quality fel pro gaskets)
I went for Comp Cam XE262H. I hesitated a while with the luniati Voodoo but their lift is too much and LSA is 112, so I kept with the well known LSA 110 for carburated application, with max lift below the 0.480.
Comparerd to HE268, the XE262H provide more lift, and more duration on the exhaust, which is the intention.
https://www.compcams.com/xtreme-energy- ... block.html

What I do not know is how to install camshaft, retarded or not ? I didn't inform yet on that...

I didn't want to go too big with the XE268H as my engine is a 327 I think it will match with my combo.

I will install the 787-16 retainers and a set of GM beehive springs on the cylinder heads.
I will minor port the cylinder head exhaust ports where the inserted seal ends with an angle; and I will remove the plate Inside the intake manifold.
I will make pictures and report if I succeed .

Regards,

Marc

[storm]

What I do not know is how to install camshaft, retarded or not ? I didn't inform yet on that...

Install it exactly how the diagram says.

I have to be honest and say that I feel the new cam still lacks exhaust duration compared to intake duration.