Rear gear question

Stan Weiss · 10-08-2024, 11:33 AM

Lets look at Dynamic Compression Ratio a little closer. It is based on engine parameters and IVC. So unless you have done something to have these change once the engine is running it is really now static. People look at Cranking Compression PSI and running cylinder pressure and say see see the DCR has changed. No what has changed is the engines VE / mass air flow. There have been OEM manufactures that actually call this trapped CR and listed it as an engine spec.

Stan

1320racer · 10-08-2024, 12:32 PM

Let’s not! Start another thread about meaningless DCR. Meanwhile this thread is about rear gear, more of which the OP needs.

Stan Weiss · 10-08-2024, 12:50 PM

Quote:

Originally Posted by 1320racer

Let?s not! Start another thread about meaningless DCR. Meanwhile this thread is about rear gear, more of which the OP needs.

So peak HP is @ 6700 RPM and he said he is crossing the strip @ 7100 RPM. So just how much rear gear would you like him to run?

As I, and other have said he needs a 3 speed trans.

Stan

Bob F · 10-08-2024, 01:16 PM

800 rpm fallback.
Mast m311 Cyl head 273 runner

1320racer · 10-08-2024, 01:56 PM

Quote:

Originally Posted by Stan Weiss

As I, and other have said he needs a 3 speed trans.

He does not NEED a 3 speed trans, he NEEDS more gear!

Quote:

Originally Posted by Bob F

800 rpm fallback.

from what RPM/shift point?

Bob F · 10-08-2024, 03:15 PM

7000

1320racer · 10-08-2024, 03:32 PM

As I stated in post #5

Quote:

Originally Posted by 1320racer

For optimum ET, If it was mine, it would have a converter that flashes to 5800, I?d be shifting at 7200 and gear it so it?s crossing the stripe at 7700. I would also get a 30? tire under it if you can not go wider.

L.Fite · 10-08-2024, 05:13 PM

So...
Stupid me is going to throw this out there...

Peak torque is at 5300...
It makes plenty good torque for a 3.75" stroke...
If it falls to 6200 at the gear change I'm going to say you're leaving some power on the table...
I would think if you send dyno sheets to someone like Coan, they could dial you in...
May find you can tighten it a bit?

I think maybe play with the shift point...
As in higher... 7200? maybe more...
See where it slows down...

More gear would help, as would a 3 speed...
If it crosses 72+ it really doesn't matter if the motor doesn't self disassemble...
(don't know the parts or build)
I see things I might try if it were mine...

Depends on your budget really, converters/transmissions aren't cheap these days...

Any way just a few thoughts from a dumb ol' Texas boy...
Maybe some of this helps...
Maybe it don't...
As they say... "Your mileage may vary"

10-08-2024, 11:33 AM	#1
Stan Weiss Senior Member Join Date: Apr 2020 Location: Phila, PA Posts: 795 Likes: 0 Liked 734 Times in 385 Posts	Re: Rear gear question Lets look at Dynamic Compression Ratio a little closer. It is based on engine parameters and IVC. So unless you have done something to have these change once the engine is running it is really now static. People look at Cranking Compression PSI and running cylinder pressure and say see see the DCR has changed. No what has changed is the engines VE / mass air flow. There have been OEM manufactures that actually call this trapped CR and listed it as an engine spec. Stan

10-08-2024, 12:32 PM	#2
1320racer VIP Member Join Date: Nov 2007 Location: New Jersey Posts: 2,628 Likes: 160 Liked 759 Times in 379 Posts	Re: Rear gear question Let’s not! Start another thread about meaningless DCR. Meanwhile this thread is about rear gear, more of which the OP needs. Last edited by 1320racer; 10-08-2024 at 12:36 PM.

10-08-2024, 01:16 PM	#4
Bob F Junior Member Join Date: Sep 2007 Location: Bucks Co Pa Posts: 74 Likes: 17 Liked 16 Times in 11 Posts	Re: Rear gear question 800 rpm fallback. Mast m311 Cyl head 273 runner Last edited by Bob F; 10-08-2024 at 01:33 PM.

10-08-2024, 03:15 PM	#6
Bob F Junior Member Join Date: Sep 2007 Location: Bucks Co Pa Posts: 74 Likes: 17 Liked 16 Times in 11 Posts	Re: Rear gear question 7000

10-08-2024, 05:13 PM	#8
L.Fite Senior Member Join Date: May 2020 Posts: 517 Likes: 559 Liked 497 Times in 243 Posts	Re: Rear gear question So... Stupid me is going to throw this out there... Peak torque is at 5300... It makes plenty good torque for a 3.75" stroke... If it falls to 6200 at the gear change I'm going to say you're leaving some power on the table... I would think if you send dyno sheets to someone like Coan, they could dial you in... May find you can tighten it a bit? I think maybe play with the shift point... As in higher... 7200? maybe more... See where it slows down... More gear would help, as would a 3 speed... If it crosses 72+ it really doesn't matter if the motor doesn't self disassemble... (don't know the parts or build) I see things I might try if it were mine... Depends on your budget really, converters/transmissions aren't cheap these days... Any way just a few thoughts from a dumb ol' Texas boy... Maybe some of this helps... Maybe it don't... As they say... "Your mileage may vary"