Alternative Fuels

hi guys,

its probably been covered, but cant find the info.

I have a bone stock 454 chevy mkiv big block. Only thing I changed on it is a RV style camshaft swap:
https://www.edelbrock.com/performer-plu ... -2162.html

I dont have the carb on it any more, just the IMPCO 300 mixer. It sits on top of a 454 TBI throttle body from wich I removed the gaoline injectors and placed the IMPCO300 directly on top using an adapter. My AFR readings are rich, around 13, under WOT up to 4500 rpm where the cam and springs are the limiting factor. So I always thought my mixer was big enough.

Now I looked it up and the IMPCO 300 supposedly flows only 428 cfm?

And these are the stock engine specs (before cam swap). Stock gasoline carbs supposedly are around 750cfm for these engines?

Question I have is how can I have rich mixtures when the IMPCO 300 flows allmost half of what the engine needs?

couple pictures of my setup.

A 454 only flows 450 CFM at 4500. Not an exact number. On over the autos the preference is to over carburate slightly. In other words the 425 mixer would be close to ideal unless your wot operation is over 5000 RPM and you say you top out at 4500. The 300 50 series is marginally small. Either way if you can achieve AFR's of 13 you can lean out the AFR by closing the power screw located at the vapour inlet on the top of the mixer. As long as your cruise and idle AFR's are ok the WOT AFR is a mute point since you can get rich enough to satisfy the engine. I would venture to suggest you are not reaching the 428 CFM mark considering you can reach 13:1 AFR. CFM is a funny thing. Many factors can change the CFM of an engine but in your case the 300 mixer seems adequate. Although I can't be sure I don't believe 750 cfm gas carbs were used on stock 454's unless they were high output versions. Secondly you had TB injectors. I'm not sure of the rating on the TB but that point is also mute as your CFM is based on the RPM rating in your case, not the TB size. In the older days the gas carbs had accelerator pumps built into the carbs so putting too big of a carb wasn't a major issue and many owners wanting more HP would put a big carb on with stocks exhausts. They always thought they had more power but really it was a waste of money. A stock 454 running 4500 RPM is rated at about 450 CFM max. Your 300 is very close to that and it is also one of the most reliable mixers ever made.

As advised you'll be able to trim the af ratio.

That cams no baby for a stocker, you're running a baby mixer with a half decent cam.

Cams costing you low rev power, the mid range and up is limited by your low horsepower mixer.

A top end manifold with a mid range cam and a bottom end mixer, missing out on lots everywhere.

thanks C3H8 thats a clear explenation.

I also found this website, I could have found myself...since its a simple calculation as you stated

https://www.widman.biz/English/Calculators/CFM.html

Tom68 I have a air-gap dual plane ready to be swapped on. They told me that a single plane big intake would be perfect for LPG. I cna see where you mean there is a mis match. the cam is supposed to be low and midrage. Why do you think different?

My old Trans Am with the Pontiac 400 had a 750 cfm Quadrajet on it, 455 engines had 800 cfm units. My 308 Holden has an 800 cfm unit on it. The Pontiac 455 SD had an 800 cfm unit on it. Not one of these engines "needs" 800 cfm nor do any of them use the full 800 cfm on performance street trim.

I have posted the formula in a couple of threads on how to work out how much cfm is required. The formula is generic and is mostly used for a 1.5" pressure drop (4 barrel carbs) and 3" (1 & 2 barrel carbs and throttle bodies). IMPCO uses 2" (for the air valve). The formula works quite well if the VE used is honest.

So lets do your engine. CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456. 454 x 4500 x 0.85 (I'm being generous with VE) ÷ 3456 = 502 cfm. Now what will happen, regardless of the induction system, is once you reach the maximum flow of the chosen unit the pressure drop will increase. So say you have a 500 cfm Holley it should theoretically max out at 1.5" HG pressure drop once you go passed 500 cfm @1.5" HG the pressure drop will increase to 2" HG and so on. The same thing will happen with your IMPCO 300 and a 425 as well. The pressure drop increases because the induction system has reached peak flow and even though the engine is still increasing in rpm it has to work harder to ingest the required amount of air.

Early VW boxer engines suffered with this badly to the extent they were speed limited by the induction system as the engines were not powerful enough increase rpm at higher pressure drops.

EDIT: The cam isn't all that big for an engine that size. Usually the larger the engine the larger the cam it can support and still get "decent" vacuum. Notice in this thread the 204@50 cam was consider an RV cam for a 4.9 litre engine but you have a 7.4 litre and the 214@50 is considered an RV cam. If you went to a 194@50 thou duration cam you'd probably get approximately 3 or so inches of vacuum more which would give you more low down torque and better low down throttle response. Using a "stock duration and lift" cam with higher ratio rockers would give you more lift without a change in duration which would give you a wider rpm range. Pontiac did a similar thing when they went from the Ram Air II to Ram Air IV engines, they went from 1.5 to 1.65 ratio rockers and gained 10% lift without increasing the duration or hurting vacuum.

thanks Storm.
So since my IMPCO 300 will only be my backup system once I get my propane vapour injection system going, I guess its fine. Ill tune it a little more for better AFR and just let it be.

kikkegek wrote: ↑

Mon Mar 16, 2020 3:46 am

thanks C3H8 thats a clear explenation.

I also found this website, I could have found myself...since its a simple calculation as you stated

https://www.widman.biz/English/Calculators/CFM.html

Tom68 I have a air-gap dual plane ready to be swapped on. They told me that a single plane big intake would be perfect for LPG. I cna see where you mean there is a mis match. the cam is supposed to be low and midrage. Why do you think different?

Single plane will be fine, short runners are just normally related to high rpm.

Cam is a great choice, it is mid range for a big, otherwise low revving, low comp street engine base, it will love extra airflow above what the 300a can provide, the cam shuts the intake valve fairly late which the big cubes can take but again will love extra airflow to take advantage of the late closing.

ok, thanks. I bought this Airp gap 2 years ago
Procomp PC23026 in want to install when I move to propane vapour injection. I was told the longer runners will be better for preventing cross feeding the cilinders.
And the runners have nice bosses that I can drill for the propane nozzles