MTFCA Forum

I have this mid-teens(?) Holley G on a spare engine that apparently powered a saw bench way back when.

My 98-year-old Model T friend was over the other day and opined that it was a desirable unit for my newly-rebuilt '26 Touring engine (Z head, bigger valves, advanced timing gear, TW timer, well-balanced, hi vol intake manifold).

I read up a bit on this carb in the MTFCA Carburetor book... it looks daunting to disassemble and rebuild and the airflow passages would appear to to stifle breathing rather than enhance it.

I would like to hear from the forum on this question. Many thanks!

In case you want to rebuild it. open the link https://modeltfordfix.com/rebuilding-a- ... arburetor/

That G is for a hand cranker as it only has a single arm choke lever but that can be remedied by using the later G arm that has both. I run one on our '14 that Corey Walker rebuilt for me - the gas level is critical on their proper operation - Corey saved me the grief !

Do some scratching around...

If a mid-teens, it's a bronze body carb which looks terrific redone and polished (it's the simple things in life :):)).

If it's cast iron body...it's a later carb, maybe '18 on to '20 or so as a Ford standard supply.

NH seem to be a lot easier to work on and totally bullet-proof...but I'd say that a well (re)built G seems to (imho) be able to idle actually lower than an NH?

The G flows less volume than a NH. You would be sacrificing some top speed. Fuel mixing may be improved.
As it is with your motor improvements, a NH isnt enough carb. Its still the weak link choke point in the system. You would be pleasantly surprised with the performance of a model A carb. Many folks like the tillerson A replacement. I run a stock iron zenith A carb with an adjustable main jet. With 3.07 gears she will run happy 65 all day with plenty of power on the hills.

This flow chart has been posted before. Shows the G is rather poor at velocity. Holley NH is better choice for performance.

Is the G cfm really only 7.49. Must be a typo. Yes mine runs out of steam at 40 but sure does hand crack easy and purrs right along at 30-35.

The chart that Dan posted pretty much tells the tale for stock carbs. If you want CFM, then a Stromberg OF (or similar) would be at the top of the list...if it was on the list. That said, those carbs including the OF were designed for a stock engine.

What you're describing is not a stock engine and may very well need more carburetor than any of those...a Stromberg OE-1 or OS-1, or Zenith S4BF would probably be more suited for that car. Now, all of these carbs have a venturi of a specifically designed size for the jets used and air volume expected. Do not be surprised if any or all of these carbs perform more poorly with a high volume intake manifold. An overly large increase in area beyond the venturi will reduce air speed of the mixture and may foul up what would have been an ideal or more ideal pressure for the fuel vapor to travel through. An example of this is the Zenith S4BF manifold which incorporates a 2nd venturi on each leg, at the block. The high volume intake can be a mixed bag and at best will improve a stock carb on a stock engine at higher speeds but is not in and of itself necessarily a good thing for all engines or combinations of carbs. Personally, I would run whatever carb I wanted with a stock manifold, in all driving conditions FIRST and then swap in a high volume manifold and determine through empirical evidence, which is better.

Finally, CFM is not the be-all and end-all for carburation...there is a lot more to it than that!

Mountainrider wrote: ↑

Sun May 08, 2022 3:40 pm

Is the G cfm really only 7.49. Must be a typo. Yes mine runs out of steam at 40 but sure does hand crack easy and purrs right along at 30-35.

I question this also. I would expect a CFM around 20, as in the 14 carb. holley. Arent the basically the same carb?

How does a higher CFM affect MPG?


George is right. The bronze ones sure do clean up pretty.

Depends on the mixing effency. I think we can conclude from other posts that the nh straight thru with the highest cfm is lousy on mileage. The advantage is @ top speeds where maximum demand cant be met by a choked down manifold or carb. Thats where another period carb. may perform better. A stromberg or zenith may be just better engineered. I strongly suspect, but have NO personal experience with them. I defer to those with that experience. I do have experience with VW carbs & A carbs & know they improve performance above a NH. A stock post 15 intake manifold is too choked down for hi speed, say above 50. However @ the lower speeds/demand they are fine. I believe Henry did this to slow T's down intentionally. Earlier T's had better cams & manifolds. Or maybe it was deemed necessary because of the poorer grade of gasoline.

Well, thank you all for the informative replies. I feel both better-informed and more aware of my ignorance.

One thing for sure: the Holley G will stay put on the spare engine, no matter how pretty they look all tarted up.

Two bits of information inadvertently omitted from my OP: the current carb is a stock sway-back NH, and ignition is by mag and coils.

My other T is a '24 Speedster, with an engine of near-identical spec to the touring (and from the same builder) with the addition of a 280 cam, a period Warford under/over (50%/25%) transmission, lightened flywheel, shaved (Z) head and a Simons Superpower straight-through NH lookalike. Both cars have stock rear gearing. One of them has the earlier Ford larger intake manifold, the other has one of Chaffin's.

My thoughts now are to source a further upgraded carb for the Speedster and swap the Simmons straight-through into the Touring. I was surprised to read Scott's comments that a more restrictive intake could work better with certain carbs, and that is something I will experiment with when the time comes. A further experiment will be to try whatever whiz-bang carb I choose for the Speedster in the Touring vs the Simmons.

This topic is not closed for further discussion... really enjoying and appreciating the input

Chris Bamford wrote: ↑

Sun May 08, 2022 4:51 pm

Well, thank you all for the informative replies. I feel both better-informed and more aware of my ignorance.

One thing for sure: the Holley G will stay put on the spare engine, no matter how pretty they look all tarted up.

Two bits of information inadvertently omitted from my OP: the current carb is a stock sway-back NH, and ignition is by mag and coils.

My other T is a '24 Speedster, with an engine of near-identical spec to the touring (and from the same builder) with the addition of a 280 cam, a period Warford under/over (50%/25%) transmission, lightened flywheel, shaved (Z) head and a Simons Superpower straight-through NH lookalike. Both cars have stock rear gearing. One of them has the earlier Ford larger intake manifold, the other has one of Chaffin's.

My thoughts now are to source a further upgraded carb for the Speedster and swap the Simmons straight-through into the Touring. I was surprised to read Scott's comments that a more restrictive intake could work better with certain carbs, and that is something I will experiment with when the time comes. A further experiment will be to try whatever whiz-bang carb I choose for the Speedster in the Touring vs the Simmons.

This topic is not closed for further discussion... really enjoying and appreciating the input

It would be an interesting experiment to compare the performance of the simmons to a good nh along with manifold changes on your motor.

A recent engine job I did used a chaffin manifold & nh. I thought it ran quite well. The customer changed out the nh for a stromberg & is much happier with the performance. Acceleration & top speed very noticeably improved.

Are your coils ECCT tuned?. Now we are adding variables. Timer? Firing accuracy? Its possible that your ignition system is the weak link dragging down performance. At one point this was my problem. I almost gave into the idea of a distributor. I hate to say it but with your built motor it would be instructive to try a distributor to rule out your ignition system.

John, my coils are from Ron Patterson and the timers are TW carbon brush (both cars). Mag output voltages are excellent.

Chris Bamford wrote: ↑

Sun May 08, 2022 5:18 pm

John, my coils are from Ron Patterson and the timers are TW carbon brush (both cars). Mag output voltages are excellent.

My last set of coils was also. I love his work & price, but I checked them on an ECCT.
Not time wasted.

One thing you can do is set up a timing light mark & switch coils. Do all 4 coils fire on the same timing mark?

DanTreace wrote: ↑

Sun May 08, 2022 1:41 pm

This flow chart has been posted before. Shows the G is rather poor at velocity. Holley NH is better choice for performance.


6DCA503C-ED09-4D39-9624-E00A67071E0D.jpeg

Hi I'm new here. Mostly Model A and newer but trying to look back from the A now to the Model T-S-R-N.
Looking at the great CFM chart & wondering:
1. Is the small 7.49 CFM for the Holley G a typo?
2. Any idea what pressure drop these CFM were flowed at? So many standards have been used, 1.5" Hg=22.5" Water, 3.0" Hg=45" Water, also 21" water, 28" water, 41" water.
3. I've looked for the link discussing this chart but cant find it, any ideas?

Nowadays the standard is 1.5" Hg = 22.5" water for 4bbl carbs, and 3.0" Hg =45" Water for 2bbl or 1bbl carbs.But even now other standards get used.

RW

That chart posted came from older source, but couldn't find the ref.

There was a rather detailed evaluation by Dr. Jim Cowart, U.S Naval Academy in the Model T Times, March/April 2014 issue,No. 390. A pub of MTFCI, there may be old copies avail from the club.

His article reports results of two variables tested, air-fuel ratio and torque at 1600 rpms, for the more common Model T carbs ('13-'26), and the Stromberg OF.

Below are some of the graphs of his study, the Holley G did perform good in flow bench (using a stock iron intake), but only midway in dynamometer testing.

I started with a Holley G on my '17. Started great and ran smooth. Got about 20 mpg
Replaced it with a swayback NH - takes a little more coaxing to start, runs smooth. Get about 20 mpg.

I had some engine work done at a very reputable shop and just for fun, while my engine was on a dyno we tried both a Holley G and a NH carb on it. Here are the results.

speedytinc wrote: ↑

Sun May 08, 2022 1:40 pm

The G flows less volume than a NH. You would be sacrificing some top speed. Fuel mixing may be improved.
As it is with your motor improvements, a NH isnt enough carb. Its still the weak link choke point in the system. You would be pleasantly surprised with the performance of a model A carb. Many folks like the tillerson A replacement. I run a stock iron zenith A carb with an adjustable main jet. With 3.07 gears she will run happy 65 all day with plenty of power on the hills.

"Sacrificing Top Speed", Gee, i didn't know I was missing any top speed on my 27 coupe. In 1970, My Dad and I finished the restoration of my Grandfather's 27 coupe. My Grandfather had ruined the original block and replaced it with a 22 block, using all the 27 internal and external engine parts, except the vaporizer. He used a conventional intake with a Wizard carburetor. We couldn't get the Wizard to run right, so somewhere I picked up a iron body Holley G. It ran well. So well, that decades later, I had convinced myself that we had rebuilt the rear axle using 3 to 1 gears. I remember, in 1970, wanting to use 3 to 1 gears, but couldn't remember if we actually did. Forty years later, a friend wanted a set of 3 to 1 gears for a speedster, so we pulled the axle and took it apart. to my surprise, it had stock 3.63 to 1 gears. I wouldn't have believed it if I hadn't seen them with my own eyes. I don't remember the top speed I ever achieved, but I could run off from other T's on a tour, except for those with modified engines.

Oldav8tor wrote: ↑

Thu Sep 05, 2024 10:43 am

I started with a Holley G on my '17. Started great and ran smooth. Got about 20 mpg
Replaced it with a swayback NH - takes a little more coaxing to start, runs smooth. Get about 20 mpg.

I had some engine work done at a very reputable shop and just for fun, while my engine was on a dyno we tried both a Holley G and a NH carb on it. Here are the results.
Carb_compare.jpg

Wow thanks for sharing that dyno data. Looks like quite an improvement with the NH Holley.
The Z head gives about 6.0 compression ratio right?

DanTreace wrote: ↑

Thu Sep 05, 2024 9:17 am

RW

That chart posted came from older source, but couldn't find the ref.

There was a rather detailed evaluation by Dr. Jim Cowart, U.S Naval Academy in the Model T Times, March/April 2014 issue,No. 390. A pub of MTFCI, there may be old copies avail from the club.

His article reports results of two variables tested, air-fuel ratio and torque at 1600 rpms, for the more common Model T carbs ('13-'26), and the Stromberg OF.

Below are some of the graphs of his study, the Holley G did perform good in flow bench (using a stock iron intake), but only midway in dynamometer testing.

IMG_7390.jpeg



IMG_7389.jpeg



IMG_7388.jpeg



IMG_7391.jpeg

Great excerpts thanks. Now I need to hunt for the whole article. I looked on the MTFCI web, they sell a flashdrive for $99 but I dont see a way to get just the one issue.
Do you know if the dyno was an otherwise stock Model T engine?
When I calculate engine Torque from BMEP as T=BMEP*CID/150.8 they come out awfully low but maybe that's explained in the article.

RW - the Z is called a "high compression" head but it's only a slight increase. The chamber design should result in more efficiency and as you pointed out, increase the compression ratio to around 6. Here is info on the "Z"
https://www.modeltcentral.com/accessories_zhead.html

Another big performance improvement is to install a Scott Conger full-flow valve in your NH. There are discussions about it on this forum if you do a search.