New cam vs 1.7 ratio rockers

[Dave Garner]

I suspect this will have been raised many times so feel free to direct me towards the relevant threads!

I currently have 302 with original heads and cam. It is fed by a Holley 600cfm Quad Barrel carb and makes 280bhp on the rolling road. I am trying to decide which way to go with heads and cams. I am aiming for a 'balance' of around 80% fast road, 20% tack day capability.

My initial thought is to use a matched set of Edelbrock RPM Performer heads, intake and cam. If you have a similar set-up what sort of power is it actually making and how is the drivability?

However, whilst looking at the AFR web site I came across a article form 2003 where a stock 302 had been fitted with AFR heads and had retained the original cam which was 'hotted up' with 1.7 ratio roller rockers. The article claimed 400bhp (I assume dyno rather than rolling road).
Has anyone actually gone down the route of using good heads and 1.7 rockers (rather than a new cam) - and what was the actual outcome?

All the best

Dave

 

[Darrin]

AFR heads are some of the best in the business. They will flow well even without the higher ratio rockers.

A 302 with those heads, the right fuel setup and 1.7 rockers will make good power assuming the engine is good (compression up, etc.). Not sure if I'd buy the 400 HP as typical though, but I would bet with a good cam and intake it could get there easily enough.

 

[Cliff Beer]

Dave, I've studied the 1.6 v. 1.7 thing....and some very credible scientific (as close as possible at least) testing showed that the 1.7 rockers didn't add any horsepower at all, they just shifted the peak hp/tq point farther up the rpm band. Said another way, the 1.7's will open the valve farther off the seat, but not increase the duration or make the ramp up or down any more optimal. As between cam and 1.7's, a cam change has a lot more potential to improve hp/tq than the 1.7's.

Good quality heads can make a HUGE difference however. The right AFR's for your 302 can add a significant amount of power. Given the modest cost of good quality heads this is a major improvement for a little amount of money. Gas mileage may vary however.....

 

[Randy V]

........but not increase the duration......

are you certain about that?

 

[Jac Mac]

are you certain about that?

He probably was, but now that you have questioned it we can probably look forward to seven or eight pages of debate

 

[EGLITOM]

Jac Mac

here we go

Randy, i think CLiff is correct with that comment. Duration is defined by the cam ( no cam lift, no rocker lift) so if i´m not completly missing something the duration stays the same.
I think to remember that in a HOT ROD magazine of this year , they did exactly that test and only changed rocker ratios. Not quite shure of the outcome, but i think to remember that they measured some power gain. May be one can find that article ?

TOM

 

[Randy V]

Saw the article some time back.

Duration - baseline is measured / defined at the cam follower (usually at .050" lift) Effective duration is measured at the valve. If there is more lift at the valve because you have increased the rocker arm ratio, then it only stands to reason that the valve will start to open earlier and close later than it would with the standard ratio..

I've done the same thing on the dyno by tuning for peak power with the 1.6 arms and swapping for the 1.7 arms. There was a measurable difference between the two ratios - but it was negligble. If my memory serves me, it was only a few more pounds of torque, but it did move the torque peak a couple hundred RPM up the scale. This was on a 306 Ford with 10:1 compression, single 650 CFM Holley on a Performer RPM dual plane intake.

What I did not like was the pushrod angles. In order to achieve the additional .1 ratio, the pushrod cup was moved closer to the pivot. I have no evidence of proof, but I suspect that there is more stress to the rocker arm stud/pedastal.
I ended up changing both rockers back to 1.6 as that was what the rules called for and the gain was not worth the risk of being caught cheating.

 

[Dave Garner]

The article that started me thinking about this is on the address below:-

400 HP 302 ci AFR 165 cc(Stock Cam)

From their article changing to 1.7 ratio rockers only added 23bhp & 12 lb-ft to an engine producing 380bhp & 368 lb-ft at 6000rpm (with the AFR heads on) but moved that peak to 6200rpm, so not a massive percentage gain.

The attraction of course is the simplicity of the change, just meaning swapping the heads (with the 1.7 rockers in) and inlet manifold plus retaining the 'drivability' of the standard cam.

Has anyone got 'actual' power figures for a 302 fitted with the edelbrock RPM Performer 'kit' (heads, inlet & cam)?

Dave

 

[Steve26]

I looked at doing this on my car and have just not gotten around to it yet. On my engine the current lift is .513" with the 1.6 rockers, with the 1.7" the lift would be .544". Everything else on the cam would stay the same. I'm not sure what benifit if any there would be?

Steve

 

[John Milmont]

You have to look up your head specs and see what they flow at a given lift. My afr 190cc heads (SBC) don't flow any more past .550" of lift, so putting in a cam or rockers that give more lift than that is just a waste and rougher on the springs.

 

[Jac Mac]

You have to look up your head specs and see what they flow at a given lift. My afr 190cc heads (SBC) don't flow any more past .550" of lift, so putting in a cam or rockers that give more lift than that is just a waste and rougher on the springs.

In a perfect world yes, but not necessarily in practice. If that was correct the cam lobe could simply be designed to take the valve up to .550 lift then have a 'flat' top before the closing ramp to allow the valve to close...since the cam follower cannot stay in contact with a lobe of that shape it is quite in order to use a lobe that opens the valve further than necessary to get some mechanical reliability, BUT, sometimes when you flow test certain port profiles you find that opening the valve past a certain point can cause an 'aerodynamic' road block that kills airflow...further work is then reqd to get the head to flow at the higher lift even though the rest of the intake runner will never require it.. nothing is easy, sometimes you get lucky, other times you accelerate your impending 'bald' hairstyle...

 

[Al Jones]

Dave - You were looking for performance figures - I can't say how the Edelbrock heads would perform but I do have the chart for my engine using the AFR165 heads with a mild cam. Carb is a 680 cfm Quickfuel. This engine has 1.6 Scorpion rockers and was built for primarily street usage. Don't know if this will help you but ......

 

[Al Jones]

forgot to add dyno results with airflow.

 

[Cliff Beer]

Yup. Quite certain.

Valve lift off the seat is increased, but duration stays the same.

This is the main reason why a cam change has a greater potential to affect performance than simply the rocker ratio change. A cam change can alter both duration and lift.

In Sports Car & Exotic a few months back they did a test of stock ratio (think it was 1.5) and 1.6 and 1.7. There was a negligible increase in hp/tq with the increased rocker ratios (something like 2hp) but the peak power points moved upwards in the rpm band considerably. I can look up the article and post it if anyone is interested. Nice to hear that Hot Rod may have gotten a better test result however.

 

[Doug S.]

The attraction of course is the simplicity of the change, just meaning swapping the heads (with the 1.7 rockers in) and inlet manifold plus retaining the 'drivability' of the standard cam.
Dave

It doesn't always work that way...the heads, cam and intake system should be chosen as a group so that they work in concert. While you would probably get a significant boost in power going to just new heads and intake manifold, what happens is that the potential in the heads is not realized because the stock cam is not designed for the power band to be in the same range as the heads. The 165's would really start to make power at about the point in the RPM range where the cam is starting to fall off...of the 3 the intake probably has the least impact on total power, IMHO, the cam and heads should be very closely matched and sometimes you have to just take whatever the intake can provide (like in my Cobra, where right now there isn't enough room for anything other than a Performer, not a Performer RPM and certainly not a high-rise 360* manifold). Displacement can have a huge effect on how cams and heads interact, too....

Cheers!

Doug

 

[Dave Garner]

Thanks for the graphs Al - exactly what I was looking for!

Hi Doug - I do appreciate the benefits of matching the key components for maximum output. I have an additional constraint (beyond my laziness in wanting to avoid pulling the engine and gearbox out again!) that I have not mentioned - which is the Gearbox (a Renault UN1 unit) which in its current format will not reliably take much more than approx 350bhp. So I can afford to 'leave some power on the table'.........

Hi Randy, I had not thought about the push rod angles (but I should have), your comments about that and the general comments about the (very limited) actual performance increase from the 1.7 ratio are steering me towards staying with the 1.6.

Dave

 

[John Milmont]

In a perfect world yes, but not necessarily in practice. If that was correct the cam lobe could simply be designed to take the valve up to .550 lift then have a 'flat' top before the closing ramp to allow the valve to close...since the cam follower cannot stay in contact with a lobe of that shape it is quite in order to use a lobe that opens the valve further than necessary to get some mechanical reliability, BUT, sometimes when you flow test certain port profiles you find that opening the valve past a certain point can cause an 'aerodynamic' road block that kills airflow...further work is then reqd to get the head to flow at the higher lift even though the rest of the intake runner will never require it.. nothing is easy, sometimes you get lucky, other times you accelerate your impending 'bald' hairstyle...

Interesting. I think i get the picture. And here i was thinking I had too much lift for my heads. Thanks for the lesson.

 

[Jac Mac]

Saw the article some time back.

Duration - baseline is measured / defined at the cam follower (usually at .050" lift) Effective duration is measured at the valve. If there is more lift at the valve because you have increased the rocker arm ratio, then it only stands to reason that the valve will start to open earlier and close later than it would with the standard ratio..

I've done the same thing on the dyno by tuning for peak power with the 1.6 arms and swapping for the 1.7 arms. There was a measurable difference between the two ratios - but it was negligble. If my memory serves me, it was only a few more pounds of torque, but it did move the torque peak a couple hundred RPM up the scale. This was on a 306 Ford with 10:1 compression, single 650 CFM Holley on a Performer RPM dual plane intake.

What I did not like was the pushrod angles. In order to achieve the additional .1 ratio, the pushrod cup was moved closer to the pivot. I have no evidence of proof, but I suspect that there is more stress to the rocker arm stud/pedastal.
I ended up changing both rockers back to 1.6 as that was what the rules called for and the gain was not worth the risk of being caught cheating.

Splitting hairs here...Valve seat to Valve seat duration stays virtually the same if the valve lash is reconfigured to suit the ratio change...effective valve duration & lift are then increased throughout the open/close cycle apart from those initial open/close points.[ As a point of interest you can 'play' with valve lash settings by about 0.003" either way in most cases' also remember if you have alloy heads & or block that expansion rates of those two components might require a rethink of the valve lash, most manufacturers recommend a 'hot' clearance , I just warm the motor up after setting lash cold & measure the difference between hot & cold, then change my cold setting to suit- hate setting lash hot + my old hands are getting a bit temp sensitive these days:]
You should also check for pushrod clearance in the pushrod holes of the cyl head with the change to 1.7/1 rockers, not uncommon for some interference at that point. I prefer the 1.65/1 ratio setup, just seems easier to get the geometry correct in relation to stem tip & pushrod patterns with that.

 

[Jason Lycett]

Hi Dave.

The Edelbrock performer RPM package with your carb will make a real 325-340 bhp ish on an engine dyno. I went down the route of importing AFR 165 heads on the back of various US based mustang magazines that showed collosal horsepower from the AFR's. After further upgrading to a 331 stroker package i only achieved 348 bhp on Tim adams engine dyno . Tim really knows his stuff (he builds the engines for genuine 40's cars in the masters series and le mans classic).

The engine had perfect mixture and ignition but showed only a 20bhp gain. I was using the Holley 600, comp cams 280 magnum Roller cam / pertronix electronic ignition and 10:1 forged KB pistons with the 331 crank and rods. Seeing all these dyno plots showing over 400bhp is very surprising and Tim does have a strong view on some of the claims made . He has dynoed hundreds over the years and reckons 350bhp from a 10:1 engine with a fast road cam and a 600cfm carb is pretty good going.

i suggest you chat to people like Tim (tim addams racing services), as well as people like kenny coleman(EDA) and peter knight (Knight racing services). They can verify the numbers with you. Just dont get taken in by the numbers like i did!

 

[mark b.]

i did something similar to a 351 crate engine from fms and the cam was retarded 2 degrees, if yours is similar you can advance it to 4 degrees and pick up some performance. the difference in lift i have found negligible, but every instance could be different if it is lacking in the first instance, increasing cam advance will help the bottom end, while increasing lift might help the top. changing to the afr heads will likely increase compression also if you use 58cc chambers.