Silvertop VS Blacktop Quads, at what flow are Blacktop really needed?

[Jimmee1990]

I have been thinking this over for quite a while, as I'm tossing up which size throttles to use for my next engine. I don't want to buy a set of each to compare on the flowbench and/or dyno when the time comes, wondering if anyone has any real world experiences on their own cars as to whether it's worth spending the extra on a Blacktop set or not.

For discussions sake the engine will be around 12.6:1 or 12.7:1 compression, capable of doing 9,000 to 9,500rpm tops (will be a road engine so won't see those revs all the time), 294 degree total duration cams with 8.45mm lift but possibly weighing up going bigger again. Will be 1.5mm o/s inlet valves and 2mm o/s exhaust valves also.

Info on 20v engines would be relevant also, as the head has had significant work done so port flow isn't a problem.

[Sam-Q]

Going off the book "four stroke performance tuning" by something Bell it works out to be about a max of 220HP. However this is dependant on many things such as shaft size, transition and so on. For example I would be quite confident hat 43mm Silvertop throttles wouldn't have any trouble at all outflowing 45mm GTIR throttles because of how terrible they are designed.

Have also heard second hand reports of blacktop engines picking up power by going to silvertop throttles, but as usual for the internet no numbers or fine details so it can't be trusted.

That's all I know which doesn't happen to be much unfortunately.

[Frak]

James, I have flowed ST quads, individually they flowed 256cfm @ 28 inches H20, now I do have a set of AE111 quads at work and can flow one if you want, but be aware in both cases ST and BT that above about 70% throttle they do not make anymore power suggesting that the quads are actually too big, which again makes sense, for a 1600cc engine that is a lot of throttle! I have also Flowed ST(late) and BT heads and on the intake side there is not much in it at all, the BT exhaust does flow better, but it's also a bigger diam than the ST. In an NA engine used on the street or even competition the quads/ stock heads flow more on the intake than you will ever need. If I were going to use a late ST head I'd just put the head in the mill and make the exhaust diam same as BT then blend it, at lower lifts the ST head was actually better than the BT by little bits here and there.

These heads flow VERY good on the intake and run out of puff around the 8.00mm lift, I have seen a number of these heads modified/ported and they don't deliver the goods.

Recently I flowed some VQ35 Nissan heads, stock and modified with porting and bigger valves on intake and exhaust. On the intake side they were nearly line ball, nothing in it. On the exhaust side the std head flowed the same as the modded head but needed less lift to do it, engine builder is using stock heads(engine is turboed), I am waiting on a set of VQ35 heads modified by Cosworth USA to see how these compare.

Time and time again I see heads that are ported etc then they go on the bench and the flow is worse (much worse often) than standard. I have flowed enough 16v heads now to tell pretty much just by looking what will and won't work, there are some dead give aways, often I just have a laugh when someone posts up pics of their backyard port job.

We actually did a 16v ages ago where we had a number of heads and ported each port a little or differently to see what would actually work, was very interesting.

[Jimmee1990]

My head is still being finished, but ported by my dad who was porting professionally for over 30 years and has been flow benched after every step so far so. I will definitely post up all the figures once it's finished for reference. His silvertop head that he developed for Meridian ended up out flowing the highest spec TRD blacktop head that they got hold of, I'll try and find the figures for it for arguments sake also.

So far by the sounds of it Silvertop will be fine though.

[Frak]

James, that would be great to see if you can find the data, always interesting to see what's going on. Many years ago, the first ST head I did, I did slowly slowly, on and off the flow bench so many times you wouldn't believe, when on the bench I was checking velocity throughout the port to make sure I only ported/built up where I needed to, took ages and in the end I turbocharged it, so probably didn't need to spend so much effort on it! Just let the boost do the work

[tottacrolla]

Frak, could you post up some flow figures for a small port head if you have any please ?