Low compression performance

[afterglow]

I'm looking to do an engine rebuild a few months from now and have started putting together a parts list. I'm still undecided whether to go with a 393 stroker or stick with an unstroked 351w. Either way, I plan to reuse some of the stuff I currently have: Weiand Stealth intake, Holley 670cfm and Compcams XE262H. I'll put the correct carb, cam and intake some time after the rebuild.

I've spoken to two engine builders and both of them have said that due to the poor quality of gas and specially because of the local conditions (year-round heat--I live in the Philippines), I should plan to have 9.5CR at most in order to remain streetable. That's quite a bit lower than the 10 or 10.5 I see around here.

With this limit in mind, I played around with the desktop dyno over the weekend and came to some conclusions:

1. It's a lot harder to get performance with lower compression. We all know that but I was quite surprised with how much was lost with just a little lower compression.

2. For some reason, the best builds I could come up with for the 393w and the 351w came out roughly even. Here are the builds:

393w:
6.2" rods
19.3cc dish pistons
9.5:1 CR
AFR185 61cc chambers

351w:
5.956" rods
15.9cc dish pistons
9.3:1 CR
AFR185 58cc chambers

In both cases, carb, cam and intake would be the same ones I'm currently using.

Both achieved similar 380hp/440ft.lbs numbers on the desktop dyno--in fact the 351w outperforms the stroker slightly. I would have thought that the stroker would be making better numbers. I have double checked my math and the numbers I plugged in to the dyno and they are correct. To verify, I changed the combos around to get 10:1 CR and the edge definitely goes to the stroker (~500hp/500ft.lbs).

Both buildups are comparable in cost since I plan to change all the internals. But since the 351w outperforms the stroker, should I go with that?? There's a certain elegance about getting the same output from less cubes. Or should I go with the stroker since nothing beats cubic inches??

And why the hell isn't the stroker thumping the 351w??

 

[rbohm]

it is on the low end. dont look at just the peak numbers, look at the torque and hp curves, everything happens at a lower rpm with the stroker, and the torque curve is much fatter.

 

[brianj5600]

Something is wrong. .5 more CR does not equal 120hp.

 

[SoCalCruising]

Yeah, a full point of compression, say from 9:1 to 10:1, should be worth 4-5%, that's about it.

rbolm is right. Your motor is an air pump. Tour carb/intake/cam will only support so much air. So, the motors make similar power, bu the power band will be shifted left-ward for the stroker. Swap in a bigger cam, carb and Vic. jr. intake on the stroker and see what happens - the power band will shift right-ward and the HP number will jump.

 

[Clarko]

The carb is a little small for sure.
The 670 works well on a 351 thats mostly stock. My friend has 69 Ranchero with a slightly warmed over Windsor. He has the same AFR heads you're looking at, along with an AirGap intake, and a mild cam but otherwise stock and the 670 he has is even a little too small for that.

 

[grego37]

if you want to go with aluminum heads you can usually go higher on the compression

also depending upon the cam you choose you can get away with more.
I have a buddy with 12:1 compression and cast heads and he runs 91 octane without any problems, he has a custom cam. Not sure how he gets away with it... he says its in the duration / overlap etc...??

However you should build for the future of gas, especially in your neck of the woods. The quality of gas is only going to get worse.

I would'nt go over 9.5:1 if you plan on running pump gas


good luck

 

[rbohm]

if you want to go with aluminum heads you can usually go higher on the compression

also depending upon the cam you choose you can get away with more.
I have a buddy with 12:1 compression and cast heads and he runs 91 octane without any problems, he has a custom cam. Not sure how he gets away with it... he says its in the duration / overlap etc...??

However you should build for the future of gas, especially in your neck of the woods. The quality of gas is only going to get worse.

I would'nt go over 9.5:1 if you plan on running pump gas


good luck

the key here is dynamic compression. when you increase the amount of valve overlap, either by using a longer duration cam, or closing up the lobe center angles, 110 to 108 for instance, you bleed off cylinder pressure at lower speeds, while maintaining cylinder pressure at higher speeds. since detonation is prevalent under load at low speeds far more than at high speeds, bleeding off that cylinder pressure reduced the tendency to detonate even with higher static compression ratios. and you dont even need aluminum heads to do it with. you do have to be careful though, as if you bleed too much low speed pressure of you run into drivability problems and loss of low speed torque.

 

[afterglow]

I just thought of something. Will I be able to reuse my Hooker long tubes if I go stroker?? They're 1-5/8". Or are 393 strokers just crying for 1-3/4" tubes?

 

[brianj5600]

With that cam I don't think you need 1.75 headers. More power is usually lost going too big vs too small.

 

[afterglow]

With that cam I don't think you need 1.75 headers. More power is usually lost going too big vs too small.

I may just decide to move up to a Compcams XE274H....will the 1-5/8" headers still be okay then??

 

[coupe5.0]

Wonder how much the stroker would pick up with afr 205s?

 

[calpolymustang]

i know of several 'long-rod' windsor motors that ran on pump gas, even with high static compression. you should check out the long rod mustangs plus put in one of their cars.

having a longer connecting rod can allow for high static compressions due to the fact that the rod length change affects the dynamics of how the piston compresses the gas. this changes when the gas detonates -> what octane level you need

 

[rbohm]

i know of several 'long-rod' windsor motors that ran on pump gas, even with high static compression. you should check out the long rod mustangs plus put in one of their cars.

having a longer connecting rod can allow for high static compressions due to the fact that the rod length change affects the dynamics of how the piston compresses the gas. this changes when the gas detonates -> what octane level you need

excellent point. a longer rod will also aid in reducing detonation propensity.

 

[afterglow]

excellent point. a longer rod will also aid in reducing detonation propensity.

How much higher?? Let's say that I'll do okay with a 5.956" stock-length rod at 9.5:1. How high a CR can I run with 6.2" rods?? 10:1?? 10.5:1??

And just how much power can I ram down my 2-5/8" headers??

 

[brianj5600]

What? Long rod cause the piston to hang around TDC longer making them more prone to detonation. You should rethink that one. I think you would have a hard time finding GOOD engine builder that would choose rod ratio over displacement. When displacement is limited by rules, they would choose a long rod, but not when stroking is an option. Long rods were used as bandaids for poor heads back in the day. Now that there are so many good heads the advantage of long rods is very small.